Airfoils, Ordinates, Fat Wings, Harry Riblett and other musings…

NCLSA183

NCLSA183

LSA Plans Builder
Sup!

I received my LSA plans about 3 years ago while knowing that it would be some time before I could start building. This year is beginning to look promising so I decided it was time to start getting my head back into the game. I'm sharing my thought process for starting the wings in hopes that someone might find it helpful, or at least entertaining.

After receiving the plans, I spent a good amount of time reviewing them in order to get an idea of what I was up against, although I do have some background in building. One of the first things that caught my attention was the "as measured" size of the airfoil on Dwg 4 compared to the rib and spar dimensions shown on Dwg’s 3 & 5. A search of this forum, at that time, revealed that I was certainly not the first to question this discrepancy, if you will. I read the thoughts regarding cutting the mylar drawing to the outside, middle and inside of the perimeter line. Maria Barrows Harris recommended that the middle was the correct technique. Others reminded us that we were building a custom aircraft and not a Swiss timepiece. As for me, I don't disagree with any of those statements as long as I am willing to accept that Dwg 4 is the gold standard for wing rib construction. I wasn't quite convinced...

In the context of the LSA wing only, although not limited to such, it has always been accepted, to my knowledge, that an airfoil designs shape and dimensions, as calculated and modeled, are inclusive of the wing skin. Therefore, it stands to reason that any bending form for the wing ribs would have to be of a smaller size than the full size of the airfoil design. For simplicity, if we assume that the LSA wing consist of only 0.020 ribs with partial flanges and non-overlapped 0.020 skins, then we must subtract 2 x 0.020 for the flanged areas and 1 x 0.020 for the un-flanged nose area from the full airfoil shape in order to make a bending form for the ribs that will result in a finished wing surface that matches, closely, with the airfoil designers’ intentions. No allowances need to be made for the section of the rib that joins the spars. That will be trimmed to length when assembling. Since the majority of the wing uses 0.020 ribs and skins, this assumption would result in a wing with the majority of its airfoil shape being more closely representative of the design criteria.

OK then… if you experienced builders are still awake, I hope that my musings will become more interesting from this point forward. Or maybe it’s just old news by a new reporter.

Looking at Dwg 5, you see that Bob Barrows, our most favorite and respected aircraft designer, list the main spar height as 7 5/8 inches, 7.625 decimal. For reference, I checked that dimension with a digital caliper and my readings were within 0.013. I also checked the main spar centerline height on Dwg 3, in two separate places, with similar results. This would seem to indicate that Bob is really good at the drafting table! Now, I will make an assumption and suggest that 7.625 is the number that Bob used for the spar height in his wing load calculations. So, at this point I'm asking myself… why would I want to use Dwg 4 to make my ribs when the thickness of the airfoil on Dwg 4, at the main spar centerline chord station, without flanges included, is already 0.125 inches thicker than the same location on Dwg 3 with flanges included? That’s 7.75 inches, carefully measured with a engineering scale, on Dwg 4 and 7.625 inches, per Bob, on Dwg 3 and Dwg 5 and verified with digital calipers. I know what some of you are thinking, it’s only a tenth and a quarter over a 60” chord. But that isn’t the whole story…

It’s a very fortuitous time for me, since my legs are falling asleep, that I have to get up and walk over to my aviation bookcase. Once there, I pull out my faded and discolored copy of GA Airfoils, Sixth Edition by Mr. Harry Riblett. As I dust off the spider webs, literally, I think back in time and wonder why I never got around to reading this classic. Once I opened it up and saw the content, my memories came flooding back and I remembered why. No offense intended to the late Mr. Riblett, aeronautical engineers or aficionados. With shaky fingers, I turn to page 71 of this airfoil bible for the lowdown on a genuine GA30-613.5 airfoil. I take a quick glance, a deep breath and then start looking frantically for my Excel shortcut. I now crunch the numbers and use them to create a Selig formatted DAT file, that can be scaled to any chord length, for importation into Fusion 360 via the Airfoil DAT to Spline add-on, kudo’s to Scorpio9999. Even with my first glance at the plotted output on my computer screen, I can see a slight difference compared to LSA Dwg 4. The plotted output looks just like the sketch on page 71 of Mr. Ribletts book and not quite so much like Dwg 4. Using Fusion 360 tools, I confirmed that the ordinate dimensions that were plotted were the same as calculated from Harry’s book, down to 4 decimal places. I later re-verified the plot and dimensions with QCAD and with the exact same results. Sidenote: The spline curves of both CAD applications seem to indicate they use the same algorithm for plotting.

As I don’t own a wide format printer or plotter, and I didn’t relish the thought of assembling 8 sheets of paper, I had to use the ruler/caliper method of comparing the LSA drawings to the computer-generated airfoil shape and dimensions. I was not surprised to find that the thickness of the airfoil on Dwg 4 was consistently ~0.100 thicker, over about 80% of its chord, than the Riblett design. That extra thickness, almost exclusively, was in the lower section, -y, below the chord line. That is why I was able to spot the distinct difference between the two airfoil profiles when I first saw the computer design. At this point, I really didn’t know if my calculated and plotted profile is actually an accurate representation of Harrys’ design. This was my first ever attempt at doing so. And then, in a cloud of dust, Bob, unknowingly, rides in on a white horse to rescue me, via an idea. I now start comparing my calculated dimensions, +y, -y and full thickness at every x chord station that Harry had provided and with my painstakingly, manually measured dimensions of the rib profiles on Dwg 3. Voila!! We have a near perfect match!! Why am I so excited, you ask? In the Bearhawk forums, when people were discussing the size and shape of Dwg 4, there was some input from seasoned posters that Bob had, or might have, used a “modified” version of the 30-613.5 airfoil and this, if true, might be what accounted for the thicker lower section of Dwg 4 that I had measured. Also, if true, that would mean that none of us could calculate and plot that “custom” airfoil without the ordinates. For builders like me, this discovery was a relief. The LSA wing was indeed designed around the standard GA30-613.5 as Harry originally created. Now I can continue my work.

Getting back to the calorie unconscious Dwg 4 - Why was it much thicker, relatively speaking, than the Riblett design and Bobs’ dimensions on Dwg 3 & 5? We have already established that Bob is a literal Ninja at the drafting table so we can’t pin this on him. Could it be “shop gremlins”, as I’m sure that his shop building has megaton’s of incredible history and maybe one of the ideas that he never pursued has come back to seek revenge? Maybe we can blame it on an old (like me) employee whose French curve has very stiff joints??? Ahh, come on… that was really funny!! More realistically, I suppose, would be a plotter in need of calibration. But hey, I enjoyed the speculation, even without the laughs.

In conclusion. You hope! Yes… there’s more… What have we (me) learned from all this? Personally, unless someone convinces me otherwise, building the LSA wing by using Dwg 4 will result in a thicker, heavier, less optimized wing than a true 30-613.5 design. As I calculated it, if building with an un-modified Dwg 4, the overall thickness of the wing with 0.020 ribs and skin would result in it being 0.180 thicker than when using an as-designed 30-613.5 airfoil. The difference in thickness and where that increase is located, will likely to also have some effect on the lift, camber profile and air flow over the lower surface. Using the previously mentioned 0.020 rib and skin wing section, building with Dwg 4 will result in a 13.8% – 13.9% incorrectly proportioned wing instead of a 13.5% proportioned wing. Yes, the LSA wing with different rib and wing skin thicknesses and overlapping sections does prevent one from optimizing to the fullest extent but, as you can see, the majority of the wing is 20/20. What effect, if any, this thinner more design-accurate wing, that I’m proposing, will have on the flight characteristics of the LSA is something that I cannot possibly answer. Actually, an opinion from an aeronautical engineer might only be experienced-based conjecture unless someone were to do wind tunnel testing. Regardless, I’m going with the skinny jeans, (I meant wings). Sadly, it will be a bit before we can compare notes…

Let me say here that I have the upmost respect and admiration for the LSA designer, Bob Barrows, and for the wonderful flying machines that he has designed, built and helped others to build for themselves. And to all the new and experienced builders in this forum, I say, your time, expertise and assistance is invaluable. Regrettably, I have not yet had the pleasure of flying in any of the Bearhawk aircraft. I have, however, read many, many articles and comments from builders and qualified experts as to the flying and handling characteristics of each of them. I am certainly not so arrogant or presumptuous to think that simply correcting some tolerance abnormally is going to improve the wing performance anywhere except, possibly, the wind tunnel. I also know that others have modified the rib forms in order to allow for the wing skin thickness, which would make the wing thinner and lighter, but I’m not sure if they were also aware of the deviation from the original design that exist in the lower section of the wing. To get to a flying LSA, I have to build a wing. So, why not give the skinny-wing a go… Isn’t that what homebuilding is all about?

I sincerely welcome any thoughts, comments, corrections, ideas and even questions that I’m not qualified to answer. To those of you who might request to get back the last 30 minutes of your life, I regret to say that I can currently only offer Bitcoin credit based upon your normal hourly rate. For those two forum members that would like more information, I will include some [FONT=Tahoma, Geneva, sans-serif]links to files I created that might assist anyone in, not only verifying my ramblings, but also to create and/or modify other airfoil designs as well. I’m looking forward to building and sharing. But just not so verbose next time…[/FONT]


Mitch



Page 71 ordinate data table. Redacted for copyright reasons: http://rf-tech.us/lsa/GA Airfoils_Page 71_redacted.pdf
Airfoil with camber profile from QCAD converted to PDF: http://rf-tech.us/lsa/Riblett G30-6135_Camber_QCAD.pdf
Fusion 360 airfoil plot with all dimensions called out: http://rf-tech.us/lsa/Riblett GA30-613.5 v3.f3d
Airfoil profile to compare with Dwg 4 if you choose: http://rf-tech.us/lsa/Riblett G30-6135_QCAD.dxf
Airfoil profile with camber profile and some dimensions called out: http://rf-tech.us/lsa/Riblett G30-6135_Camber_QCAD.dxf
Rib bending form trimmed for skin, top, bottom and nose. Not yet tested for CNC compatibility: http://rf-tech.us/lsa/Riblett G30-6135_QCAD_Trimmed.dxf
Selig formatted DAT, scalable chord, for importing into Fusion 360 as described in post: http://rf-tech.us/lsa/Riblett GA30-6135_Selig.dat
DAT file for importing into QCAD. Fixed chord - 60 inches: http://rf-tech.us/lsa/Riblett GA30-6135_QCAD.dat
All data from my calculations. This might not be self explanatory. I did it in a hurry: http://rf-tech.us/lsa/Riblett GA30-6135.xlsx
 
Thanks Mitch that was fun... but yikes! Good thing I am past that stage of decision-making. For me the die is cast. Not qualified to comment on your analysis. But this is something only a scratch builder can appreciate, and I do. For others it's "another boring Mylar drawing post; I have my quick build wings so who cares?"

My thinking was as follows. The Mylar drawing was made by running a pen around a template back in Fincastle. I assume that template was the master template of the LSA prototype. My job was to reproduce that template. Therefore I cut to the inside of the line. And I suppose that will result in lighter wings than if I cut to the outside of the line.

Can you do us a favor and calculate the difference in weight - inside versus outside the line?

Regarding the spar dimensions, I think the critical notation on drawing 5 is "check for fit with ribs". I feel the plans require leaps of faith to make reasonable adjustments and to infer some details. And I proceed with trepidation! 😀
 
Thank you. I thoroughly enjoyed my journey down that rabbit hole. Some things are much clearer now. For one, I no longer have to be concerned about that pesky line on Dwg 4. :)
 
Are you being frank?? Just kidding. I do appreciate your comments and want you to know that reading your postings was a major motivator for all that research and computer time that I invested last week. Anyone who has started building ribs are certainly not going to change course due to my research, nor would I. As I hoped to be clear in my post, I am not qualified to give advice but only to present what I have learned to be facts and not just my opinions. Being from the south, my friends and I have, at times, referred to ourselves as "redneck engineers". I have no degree. Several years ago the great Budd Davisson used that term to describe Bob Barrows and himself. I can only wish to be in that class.

As I have read, Bob is strictly a manual draftsman. Therefore, he plotted the stations and ordinates on paper. The airfoil shape of the ribs on Dwg 3 are extremely accurate in matching the Riblett design so he definitely used the same ordinates from Harrys book that I did. But with Dwg 4 being so different, I felt the need to verify for myself. There are possibilities other than the ones I mentioned as to why. It really doesn't matter.

I do admire the effort you put in to get your bending form to a more acceptable size. I just wasn't satisfied that Dwg 4 was the way I wanted to go. If people accept, as fact, that my airfoil plot is a true GA30-613.5 vs Dwg 4 then builders wouldn't need to be concerned with where to cut the line. They could do as I did. Just plot the true airfoil, subtract whatever value they choose from the perimeter, print and cutout that profile for the bending form. Or if they choose to spend a few more dollars, have it laser or waterjet cut from their choice of material and start bending. There are other obvious advantages, as well.

I have always planned to joggle. Even more so now. As for weight savings, to be of any real value it would have to be based upon some standard value. For example, if everyone were building their ribs from a standard accepted 613.5 profile that had an accepted offset for thickness, the ribs could be easily calculated. But you would also need to calculate the weight savings from the spar, skins and possibly a few more parts. Even then, It wouldn't be a tremendous savings, I would speculate.

Mitch
 
Hey Mitch, thanks for sharing your hard gleaned research and thoughts. That is what this forum is about. And that is what EXPERIMENTAL aircraft building is about. I have the spar and rib kit from Avipro so I will not be banging out any ribs. I enjoyed the read and it did not take 30 minutes, it was very interesting. The number quoted in the specification on the LSA are what made me decide to go with the LSA and I have to believe that these numbers were obtained from the prototype that was built by Bob and every other LSA hence forth. So what ever that airfoil ended up being in reality will serve my purposes . I wish you luck in your build and look forward to your further post here.
 
Hello davzLSA. Thank you for taking your time to comment. It is appreciated. I have been quietly following you and all the others for about 3 years with much enjoyment. I've read every post, word for word, in the LSA and Patrol groups and most of the 4-place, over the months.

Mine was a long post but I wanted to give enough detail to hopefully make it usable by anyone. I was concerned about how some of it might be interpreted, so I tried to be very careful with my wording and inject a little humor to ward off boredom. I never meant to imply that there was anything wrong with the LSA at all. In fact I dedicated a whole paragraph trying to convey that. With all that wording, the only real point that I was trying to make, and I wanted to show to others how I came about it, was that Dwg 4 is not a true 30-613.5 airfoil but yet the rib profiles on Dwg 3 were. And of course, speculate and get feedback from others as to if it would make any difference in flight characteristics. Something happened to Dwg 4, evidently, and I was curious. As I said, I'm certainly interested in giving it a try. Just wish it didn't take soooooo long...

Sincerely hope that I haven't offended anyone in any way. As I'm sure you are aware, phone calls are much better than text messages when expressing ones self.

Looking forward to your next progress report and especially regarding the engine.

Mitch
 
Hi Mitch, I believe you are way over thinking this. Reading Harry's book, you may have noticed that he prefers thicker airfoils and determined that if designed correctly they don't cause a loss of speed. The difference between 13.5 and 13.8 is negligable. Accuracy of the airfoil would be more important if it was a laminar flow wing. Bob worked with Harry Riblett to modify the airfoil for the Bearhawk and presumably LSA. I have no idea what those modifications entailed. I admire your effort at accuracy, I went down a pretty similar road. When I started my Bearhawk project I asked Bob about using a Riblett airfoil. To my surprise, instead of being offended, he said let me know how it turns out. I've always wondered if that conversation is what led to him using Riblett airfoils on the rest of his designs. I got as far as making form blocks for the GA 30-613.5. That would have been more camber than necessary for the flapped Bearhawk. In the interest of eventually flying I abandoned that idea and bought the quick build wings.
 
Rod, thanks for your comment. I have especially enjoyed reading your more technical based posts over the years.

I've never gotten around to actually reading the book. I thumbed through it last week in search of the data table for the 30-613.5 and read some portions along the way. I was a bit surprised to find the 613.5 as I had read previously that it was not included in other peoples editions. Maybe they meant the 614.5 from the Patrol. Anyway, in hindsight, I probably should have began this topic with a shorter overview of what I was searching for. I now believe that I might have unintentionally hidden my signal inside the noise.

Yes, I am one of those people who prefer accuracy, in certain situations. I have worked in different fields where one would require accuracy and another where 1/4 inch was acceptable. I'm perfectly comfortable with both. But, despite my rambling's, this search had a different purpose that maybe wasn't as obvious as I had assumed or hoped. In this case, it was "consistency", instead. So, where you suggest "overthinking", would you accept "attention to detail"?

The details that I refer to are, and I'll try to be brief. (Not my strong suite) :(

In video interviews and in the LSA builder book supplied by Bob, he states that the LSA uses the Riblett 30-613.5 airfoil and goes so far as to give its specifications. I felt that if Bob had modified that airfoil, then he most certainly would have taken credit for that in the interview or book, and rightfully so. But since other people had suggested otherwise, then I couldn't be sure.

Without quoting measurements, a rib built from Dwg 4 was never going to fit into the Dwg 5 spar. Why should it be so? The wing load calculation's were most likely performed using the 7.625 dimension. Why build a thicker, heavier wing and then have to build a taller, heavier spar to attach it too? There began my quest for the lack of "consistency". All the details are in the original post. Bottom line... the rib profiles on Dwg 3 "are" of a stock 30-613.5 airfoil, no modification's, but without the skins included, 2 x 0.020, but accurate enough for me. See, I'm flexible.:) The profile on Dwg 4, if flanges and skins were included, would be 0.180 thicker, 3/16 inch, at the 30% chord station than a stock 30-613.5 airfoil. Not really accurate enough for me. Again, why? Was it a drafting error on Dwg 4 or plotter calibration, as I mentioned. It shouldn't be thermal expansion, being on mylar. It can't just be my copy as many others have discussed it. Your thoughts on any of this is highly regarded. Please don't hesitate to comment.

I never made any claims about there being wing performance improvement's with a more accurate profile but only the possibility of such. That was never my goal, though. I'm familiar with the work that race teams put into just an extra ounce of downforce. I don't have the time or brain capacity for that. I really can't imagine that the Dwg 4 profile inconsistency was intentional. But if it was, I would surely love to know the explanation straight from the horses' mouth.;) And no, I would be too embarrassed to call.:(


Mitch
 
Hi Mitch, I have spoken to Bob many times and he is always been willing to listen to and calmly discuss any hair brained idea I would throw at him, (not not say your ideas are hair brained.) I think his reaction would be surprising to you, You should call him and discuss your issues.
 
Two current threads sort of overlap. The one about wings skin thickness and this one. Fabric covered wings and thin skinned wings aren't exactly perfect airfoils. At the speeds we fly it doesn't seem to be a show stopper. High speed aircraft it is a show stopper. Laminar flow airfoils are made for higher speed aircraft where the airflow naturally wants to be turbulent. Any imperfections allow the turbulence to take over from laminar. In a body traveling through a free stream of fluid, there is a non-dimensional calculation called Reynolds Number, which is most often misused. Basically the faster, and longer the free body (chord of the wing), the higher the Reynolds number, and the more likely the boundary layer goes turbulent. The slower and smaller, the boundary layer tends to be laminar. The size and speeds we fly we are on the lower end of what used to be called the transition area. Sometimes inducing turbulence is good, like VG's.

At our speeds small imperfections don't have a huge effect. At 500 mph, they would be a design or quality control catastrophe.
 
Hello svyolo, Thank you for the comment. Yes, there is some overlap. I am aware of the things that you are pointing out although my knowledge of airfoils and aerodynamics is very, very basic. I did notice in Harrys book that the 30 series of airfoils are considered "turbulent" airfoils and, as you pointed out, they have features are are better suited to slow flying aircraft, relativity speaking, like the Bearhawk LSA. But again, it is not my intent to build a better wing with different flight characteristics but only to make the LSA wing profile to match, as near as possible with manual techniques, the 30-613.5 profile as drawn by Bob on Dwg 3 and verified with my CAD generated profile that is based on the data table in Harrys book. That is my only goal. See my next post.

Best regards,
Mitch
 
After a very long conversation with Mr. Sandman last night, he has advised me that it is time to put this topic to bed. Pun intended. I feel that I'm taking up your time with something that is not important enough to do so but is only to satisfy my curiosity.

I will be building my wing ribs per the profile as shown on Dwg 3 and that has been confirmed with my CAD generated profile from the data in Mr. Ribletts book. They both are an accurate profile of the GA30-613.5 airfoil. Being built with that profile and calculated dimensions will allow the ribs to fit within the 7 5/8 spar as shown on Dwg 3. I absolutely will not allow the spar to be any least in size than the dimensions that Bob has specified. I don't view this as a modification of the design but rather a slight correction of the profile on Dwg 4. I will most likely run this by Bob after I have developed my patterns and formed a rib or two.

Once I have the bending form sized properly, and verified with Bob, I will post a copy of the CAD file should anyone be interested. There doesn't seem to be much LSA scratch building going on but hey, you never know. I am still hoping to get started building later this year but I might find some time to work on this idea before then and I will post my results here. I will still be reading and posting on the forum, as well. Thank you all for indulging me and, more importantly, giving me encouragement to pursue my ideas.

Sincere regards,
Mitch
 
Hello Brooks. Thanks for stopping by. Your username has also become very familiar to me over the past months/years. I must say, however, I am a bit disappointed in your comment. It wouldn't have required much more of your time to just say, "Hey Dave, tell Mitch what Bob is thinking and save him the embarrassment. :) :)
 
Go for it Mitch! I look forward to seeing your work! What tools and methods do you plan for forming the ribs?
 
Hey Frank. I hadn't seen that photo on here until today. Must have been stuck in the system somewhere. You're down there real close to the dimensions on Dwg 5. Obviously, you removed more material from your bending form that most, it would seem. What about banging out a few dozens for me?? :) Thanks for the photo.
 
Frank, all you guys, and that includes both sexes, that have come before me have pretty much worked out the best techniques for all the different facets of Bearhawk construction. I am seeing some new, imaginative ideas coming from the members. Sir Newton is one that comes to mind.

I will most likely use the rubber press method as I already own a 20 ton press. I've seen great looking results from those that have used it. I also have access to a 60 ton press so I might even try bending the center rib flanges. I might have to go visit some of my neighbors who have horse and cow farms to pickup some used rubber mats like Chris in Milwaukee over in the Patrol forum. New rubber pads can be expensive, especially if you get the really good ones from McMaster-Carr like on N3UW's videos. And of course, the old Bob-Stick and the flange straightener from the RV guys. Pretty much just the old tried-and-true tools and methods that have become standard.

I hope you are making progress every day and thanks for all the sharing of photos and building updates.

MItch
 
I wasn't expecting to be back this soon. Let's blame rodsmith and svyolo.;) And by my own admission, I'm definitely on the borderline of "over thinking"!!

Seriously though, I took time to read Harry's book this weekend. Great bedtime reading.. NOT. I might understand 10%, maybe less, but things I read and things that the two aforementioned members had written certainly kept me thinking. No long details are coming. You're welcome.

I didn't pay enough attention to Rod's reference to "modifications". My apologies to Rod. Yes, I stated that I had read about possible modifications but after verifying that the rib profiles on Dwg 3 were the same as my calculated profile, I guess I kinda stopped considering that to be a possibility. Add that to some of the aerodynamic statements that svyolo had mentioned and the result was me not getting much sleep this weekend. (That was a bit exaggerated but you know what I mean.)

OK... my current useless thoughts are that: Bob designed and created the LSA drawings based on what I'll call the "stock" GA30-613.5 airfoil. After the drawings were finalized and, either during or after construction of the prototype, before or after flight testing, he had reason to think differently and, either on his own or in consultation with Harry, he made a modification to the lower wing section. Possibly to remove a portion of the cusp, which is beyond my basic airfoil knowledge and just a guess, and there was no need to change the LSA rib drawings.

Hopefully, for my sanity, I will know the answer to these questions sometime soon. I have asked Mr. Ed (I mean Bob) and if I get a reply then it will be straight from the horses mouth. No disrespect meant to Bob. I really loved Mr. Ed. :)

<Mitch>
 
Hello Builders. I had a little free time tonight so I reverse-engineered LSA Dwg 4 and overlaid it on the GA30-613.5 drawing that I made last week. I would appreciate any input as to what your feelings might be regarding the differences in the profiles, especially you more seasoned builders. We could all gain from your input. Is there enough difference to really matter? Do you think that it would have an effect on the flight characteristics, etc.? I'm just asking for opinions based on the knowledge and experience you've gained over the years. I have very little regarding airfoils and their flight characteristics. I have attached a link to the file. Thanks.

Mitch

http://rf-tech.us/lsa/Riblett G30-6135_QCAD_Compare.pdf
 
Mitch;

Thanks so much for the overlay. It really helps me see what you are talking about.

I like data, and I have no data to give. My opinion is worth nothing. How I feel about it proves nothing. I use to have the Lift formula memorized, and could do calculations when in the 1980's in my collage aerodynamics class. But I've forgotten a lot, have never touched a wind tunnel. All my experience is flying. Maybe some of the Ice I've collected when flying in the Great Lakes Region (arguably the best place to collect ice in the world) in small piston twins and turbo-props for a decade or so is the best aerodynamics experience I can claim, but thats pretty sketchy and a far reach for real data.

However, after reviewing your PDF, my opinion is that if you had two set of wings and could change from the Red profile to the black, go back and forth, and test and collect real life numbers, then I believe that no measurable difference would have been experienced.

However, you are now the expert. I want you to pick the one that is easier to build, rationalize why that one is superior any way you can....faster, stronger, lighter, superior stall....you know what I mean, then commit and don't look back. You gotta get this done and the biggest hurdle is running out of energy. If after you finish that first set of wings you still have a good level of energy remaining, then build the other set, so you can the collect data and prove my opinion was right or wrong.

But I think you will be having so fun flying your LSA that wont get the second set done and thats a good thing too.
 
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I do not quite understand why you are asking the forum to comment on this when likely the only person to be able to give you a good answer is Bob Barrows. I suggest you call Bob. Mark
 
Thank you Brooks. Your experienced opinion is worth a lot to me and I'm sure to others. You may have never touched a wind tunnel but you were certainly inside of one while testing all the different airfoils that the icy weather was giving you. Too bad that you couldn't harness that situation, for example, "Hey Mother Nature! Give me a NACA 0015 for 10 minutes and then switch out to a GA30-615 and don't forget to log that data! :)

After making the overlay, the difference in the profiles didn't seem as great as when I only had a few notes of some of the chord station dimensions and, because of that, my opinions from Post #10 have changed to be more inline with yours. Seriously, I'm not looking for a profile that would meet any personal goals. Although I'm still, likely, a couple months away from starting any meaningful building, I was just wanting to get started on my cutting and bending templates. Without clarification from Bob, I'm still in limbo. My questions to Bob was: 1- Is the slightly larger size of Dwg 4, in relation to Dwg 3, a printing error, drafting error or an intentional modification of the GA30-613.5 airfoil? 2- If Dwg 4 is unintentionally slightly larger then would he have any objections to me building with the profile as in Dwg 3, which is the same as my CAD drawing. While waiting to hear from him, I wanted to hear experienced opinion's, like your own.

Again, thanks for your comments. I also enjoyed hearing about your life experiences, aircraft and otherwise. I'm looking forward to sharing thoughts and ideas with you and many of the other members.

Mitch
 
Thank you Mark. I understand your comment. I started with asking the forum because I was expecting that one of the more experienced builders with a close working relationship with Bob might already know the answer, as it has been in question for years on the forum. I was also thinking that you might have the answer as your long relationship with Bob has including working directly and in depth with the LSA drawings. The opinions, so far, from the comments and myself, are that regardless of the reasons for the discrepancy, it would likely make little difference. But, as you pointed out, we are not the experts. I have presented my questions to Bob but I was just looking for experienced opinions why waiting for a reply.

Best regards,
Mitch
 
I think I remember reading a couple years ago that Bob used a "modified" Riblet airfoil. My memory, though, is not what it used to be.
 
I read the same. That is part of my motivation for getting this cleared up. I'm sure it doesn't matter to most and the difference in size is not very much but I don't have the expertise to say which profile is correct. Thanks svyolo.
 
I asked Bob about this when we were speaking a little while ago. He says the mylar is what he wants and it flies fine. He said that to his recollection - Harry Riblett never published info for the 13.5% airfoil like is on the LSA. Just the 12 & 15. He said Harry Riblett called and gave him the coordinates which he plotted. He recommends to get to work building the plane. Mark
 
Hello Mark,

Thank you for presenting my question to Bob. I had sent him an email Sunday evening that I considered to be very courteous, short, to-the-point and included supporting documentation. Despite that, I think that Bob's email filter sent me straight to the "crackpots" folder. With Bob's reputation, I can't help believing that he might have reacted differently if I could have explained my position, or aybe not. Although his response, as you relayed, could be viewed as a bit condescending, I take no offense. Honestly.

You stated in your previous comment that “I do not quite understand why you are asking the forum to comment on this”. I replied that I understand the reason for your comment, and I do. But, to add a bit more, isn’t the purpose of this forum to freely exchange ideas, thought’s, opinions and to share information that has previously been asked of Bob and yourself in order to help lower the demand on your time? That is what I will always try to do.

As I wrote in my email to Bob, though in a shorter format, and also in my original post, Harry Riblett published the ordinates for his GA30-613.5 airfoil profile in the 6th edition of “GA Airfoils” dated February 1996, Page 71, that is available to anyone. A redacted copy of that page was linked in Post #1, since removed. But regardless of that fact, scaling to other thickness distributions is nothing more than simple division and multiplication that I, due to my curiosity, wanted to learn for my own education. And to my knowledge, the Patrol airfoil, GA30-413.5 was never published by Mr. Riblett but my very basic Excel spreadsheet will spit-out the ordinates in milliseconds or it could easily be calculated manually, with time. The question of discrepancies regarding the mylar drawing have been on this forum for years. Why has it been so difficult to get answers to those dimensional discrepancies, Dwg 3 vs Dwg 4? I postulated some possibilities in Post #1 but was just wanting to hear it from Bob. Again, mostly curiosity. I never thought that it might be seen as an attack on Bob's design, as it seems to be. I regret that.

The good news is, at least for me, Bob has now, indirectly, verified that Dwg 4 is intended to be an unmodified GA30-613.5 profile regardless of its dimensional accuracy. Believe it or not, that is “ALL” that I was really wanting. The other things that I learned from the forum members is additional knowledge that I might not have, had I not asked questions. Now, with this additional, new knowledge, we can all make an informed, personal decision as whether to use Dwg 4 or a more dimensionally accurate profile as I and others have plotted from Harrys published ordinates. My personal feelings are that a first-time builder would benefit from cutting out the profile on Dwg 4 and making the bending forms, etc. For old geezers like me, I have spent enough time tracing and cutting templates and might want to move into the 21st century. Now, we can use our own CAD generated profiles to CNC cut the master template, should we want. We might even go so far as to CAD create and CNC cut separate bending forms with allowances made for the 3 different material thicknesses, as MDF is very cost effective. After that… who knows? Isn’t that what we are expected to do, as aircraft home builders?? Obviously, we stay within engineering boundaries unless we have obtained consultation from Bob. Once more, in different words, I am NOT trying to modify or customize Bob's design and thinking it would somehow be a better design. I couldn't if I wanted too. But if better accuracy is easily available to us, why would we not want to utilize it? And why would Bob, or you, not encourage it? Many of the forum members have made things easier and sometimes more accurate for us all by sharing their ideas via words, photos and videos. I, for one, will not stop trying to do the same, whenever possible.

Sincere thanks to everyone that has contributed to this conversation, including those that only pressed the thumbs up. :)

I also must add that nowhere in my postings do I recall having ever stated that a more accurate Dwg 4 would result in a better wing. But there "IS" that possibility…

Mitch
 
Mitch, we don't typically think of Bob as contactable by email. He's usually available by phone but these past few days he's been on a long-distance adventure in one of his very old cars that included some unexpected maintenance needs. In my head I can hear his voice saying the words about building the airplane, and they aren't coming from a place of condescension (is that a word?). He has built very many airplanes including his own designs and others, and his experience helps guide his ability to triage minutia. Sometimes the tiny details are life and death, and sometimes they are just a resource black hole.
 
Hey Jared,

I literally smiled when I read your choice of words in the last couple sentences. Thanks! :) (Not the life and death part!!)

Yes, I was aware of Bob's preference for phone communications and I wasn't wanting to be disrespectful of his wishes but the email was the best way to get my supporting documentation to him. And I was going to follow-up with a phone call on Monday. If you read some of my other post, then I'm sure you know that the comment about the "crackpots folder" was just an attempt at levity. I was hoping for a folder at least one level up, so some disappointment...

I first learned of Bob and the Bearhawk from an article in the early 90's. I don't remember where. Since then, it's possible that I might have read, listened or watched everything that has been published, in regard to first-hand personal interviews. My perception was developed years ago and has not changed, regardless of how it may have seemed in my previous comments. I'm truly looking forward to someday verifying that perception. Maybe while talking about an engine purchase.

I appreciate you taking time to comment. Especially with the things that you are having to deal with currently. I would guess that I'm about 2 hours from your hanger location and will gladly make myself available to help with cleanup, errands, material pickup or any other ways that you might need some assistance. Please don't hesitate. Seriously!

Mitch
 
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I have obtained some additional information since my last post that takes a bit of air from under my wings and leaves me a little less excited about the path that I was planning to take in forming the ribs.

It turns out that the dimensional discrepancies in Dwg 4 are not printing or drafting errors but rather an intentional modification to the wing profile. I have learned that the modification, in some form, also exist on the mylar of the Patrol, 4-Place and possibly the Model 5, as they share the same profile and chord dimension. I don’t have the mylar for any of those aircraft so I can’t verify what I learned from other sources. I have calculated and plotted the GA30-413.5 airfoil with a 65.125 inch chord should anyone want to do the comparison. Link below. So, the answer to the obvious question seems to be completely unknown to anyone except Bob, and he appears to be deflecting or obfuscating, based upon my research and the answer obtained recently by Mark, on my behalf.

If the modification was done to improve performance or flight characteristics, then why did previous members not receive that information as an answer to their questions years ago? I’ve also learned that some aircraft designers, including Steve Wittman, would sometimes attempt to smooth-out the concave portion of a high camber wing in order to make rib forming a bit easier. If that was Bob’s reason for the modification then why not tell us? I can also speculate on a couple more possibilities but sometimes speculations can be interpreted as accusations and I certainly don’t mean to infer anything negative.

If the modification was made in order to make rib forming easier for plans builders, then what about the factory ribs? They were formed in the -O condition, thus much easier to form, so do they have the correct 613.5/413.5 profile or are the ribs formed to their respective mylar profile? The LSA is marketed as having the Riblett GA30-613.5 rib but that is only partially correct as it only applies to the plans set drawings. If the factory ribs are formed to the Dwg 4 dimensions, that marketing statement would not be correct as the Dwg 4 ordinates are not obtainable by calculations using Harry’s GA30-015 basic thickness form data with the GA-6 camber data. It might, then, be more correct to refer to the wing rib profile as a Bob Barrows modified version of the GA30-613.5 instead.

Having purchased our chosen aircraft plans set, does that cost not include the information that we might want or need that would further our knowledge and understanding of that aircraft and, specifically, what modifications were made to the airfoil that technically changed it from a true 613.5?

I might be the only current forum member that would like an answer to these questions but, based upon my research, there have been many more over the years. There are, likely, only a handful of people on this forum that could ask these questions to Bob with the persistency possibly needed to get an answer without creating animosity. And none of those are me…

Best regards to all,
Mitch



P.S. Until a few days ago, I was assuming that I would be forming my ribs to the true 613.5 profile. Maybe still. Although I planned to have the master template CNC cut, I still wanted to have a mylar drawing to use as a reference at times and to maybe hang up on the shop wall. Duncan-Parnell was my source for topographical maps back in the late 70’s and I knew that they were still in business and now had a location in Concord. Still a 45 minute drive but a nicer drive than to Charlotte. They only had 24” mylar so I put two different profiles onto one 65” length. It was $19.00 plus tax. Not too bad, I don’t think. I have included a link to the PDF that I had printed should anyone like to have it for any reason. It might make a good conversation piece for your workshop. It cost about $6.00 on standard paper.


Riblett GA30-413.5 Airfoil Profile - http://rf-tech.us/lsa/Riblett_GA30-4135_Base_AftStart_Points.pdf

Table of Chord Stations and Ordinates for the 413.5 Airfoil - http://rf-tech.us/lsa/Riblett_GA30-4..._Ordinates.txt

Riblett GA30-613.5 Airfoil Profile and Chord Station Dimensions - http://rf-tech.us/lsa/Riblett_GA30-6135_Final_Dual_Profiles.pdf
 
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"The LSA is marketed as having the Riblett GA30-613.5 rib..."
I didn't know that. Where do you see that?
Not that it matters, if you have the plans and you want to build the wings...
You'll be missing a lot of fun if you don't start banging out some ribs pretty soon.
But thanks for stirring the pot and keeping things interesting!
 
Hello Frank. Good to hear from you. I hope your build is progressing nicely. I do hope to get started banging ribs before the end of the year. Still getting my ducks lined up and just stirring the pot while I'm waiting.;) Have you ever cooked a big cast iron pot of chili beans over an open fire? You got to keep stirring or those suckers will stick to the bottom and create a big mess!! I promise you, it is not my intention to stir the airfoil pot and, more seriously, not to offend anyone. You may remember from my original post, I learned about the Dwg 4 issue, or whatever we may call it, about 4 years ago and I can not find any resolution that is documented. I'm pretty darn sure that there is a reason behind it but I can't find an answer. I don't mind calling Bob but based on his response to the question over the years, I'm relatively certain that he wouldn't be forthcoming. I did send an email 2-3 weeks ago, no response, but I will not rule out that phone call, eventually. Maybe even a physical visit. He isn't' too many hours away and seems like a nice guy to chew-the-fat with.

Frank, the Riblett airfoil profile GA30-613.5 is mentioned many times in print and videos and also Page 26 of the LSA Builder Manual that came with the plans. I'm confident that the aircraft is and would be a great performer with the 613.5 profile or the Dwg 4 profile. I also wonder if the 613.5 was flight tested before making the Dwg 4 change. Dang, another question waiting for an answer. The LSA plans were drawn up and the loads were calculated using the true 613.5 profile but Dwg 4 departs from that profile and there is what has stimulated my profound curiosity. I suppose if Bob were to tell us that is was none of our business as to why he modified the profile, then I would likely stop stirring but I would still be pretty-darn-curious.;)

Take care and keep banging!!

Mitch
 
Ha! Shows how much I know! My excuse, just getting to page 10: "Once the ribs are done you can start to work on the spars." When I get back from short vacation I'll download your attachments for my files.
 
Hope you have a wonderful vacation, Frank.

I just want to be clear to all members that the profile drawings that I have linked are not meant as a replacement for the Dwg 4 mylar sheet. They may be nothing more than a worthless souvenir from my recent Bearhawk safari.

I do have a set of ordinates I arrived at mathematically that will duplicate the profile of Dwg 4. I created it as an educational project, and for my own use, but I would like to share with other LSA builders who, like you and me, might prefer to preserve their mylar and just use a paper copy to create their master template. While I see no legal reason that I cannot share this data with licensed plans owners, there is an ethical issue that I can't overlook.

Mitch
 
I've been inactive or lurking a bit (remodeling the house instead of building airplane parts after a flood in the kitchen), but found this thread intriguing. I too found that my estimate of the Riblett 13.5% thick airfoil from the description of how they were built from his book vs. the as-built to be different. I took a very low-tech method of measuring the Mylar drawing by hand and comparing it to my estimates based on Mr. Riblett's descriptions of how he modified the NACA mean line and thickness forms (I never did use any of the calculators you can find online, but I was able to replicate his published thickness forms, so I think I got it right. I'm attaching an image of my analysis - showing the theoretical Riblett 13.5% thickness form, mean line, and overall airfoil ordinates vs. my measured values. You'll see that they're different.

Now... at the risk of a little hubris, I am an aerospace engineer, I work in the field of aircraft design and performance analysis, I've taken airfoil design and analysis classes, and I use aerodynamics every day as part of my day job (okay, when I'm not answering e-mails, writing reports or PowerPoint charts, preparing budgets... I guess I do less analysis lately than I wish I could do these days). I've analyzed the Riblett and Bearhawk airfoils with better computational tools than Riblett used, and when I get the time, hope to run full Navier-Stokes simulations on the airfoils. I even have visions of a slotted flap design that uses the same hinge point or a drooped hinge point, but that takes time that I don't currently have for what is likely a dubious return on investment. The best thing the Bearhawk has going for it is a solid history of good performance. I'll take the factory form of the Bearhawk airfoil any day over the theoretical form of the 13.5% airfoil. I do recall chatting with Bob when I picked up my plans, and we spoke a bit about the airfoil selections and aerodynamics when he heard of my day job. I believe he mentioned that Mr. Riblett had sent him the airfoil ordinates, and I believe had even suggested some modifications (I don't know if it was Bob or Riblett that suggested the mods, but it does appear they were made).

At the end of the day, I'll trust the scoreboard - Bob's design clearly does well, even if it doesn't match the theoretical ordinates implied by the Riblett text. I don't think it matters that it doesn't match the Riblett ordinate descriptions/formulas - it clearly works. Besides, I suspect that Mr. Riblett and I would not see eye-to-eye on everything, and I find some of his assertions a bit dubious, but I do agree with him more than I disagree. Sadly, I'll never get the chance to make my case to him on this earthly plane.

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Thanks for confirming Mitch as per discrepancies. Do all the BH models have the same airfoil? Are airfoils scalable, I mean do the different BH models have different cord lengths? How do the BH models' wings differ? When it comes down to variations discoverable only by careful informed analysis, it would be interesting to make other comparisons as well. Compare the mylar drawings to as-built results (factory and homebuilt) and to each other.
 
I believe the 4-place "A" model uses a NACA 4412. The modified Riblett 13.5% was introduced on the Patrol, and then used for the 4-place "B" model. I don't know what is used in the LSA, Companion, or Five, but I think it's reasonable to believe that they all use the same airfoil as the Patrol and the B.
 
Hello nborer. I’m sorry to hear of your recent misfortune with flooding but I’m glad that you were intrigued enough to offer your analysis and/or opinions. Your “low-tech” method would hardly seem as such to many of us. I did enjoy the time that I spent in deciphering the graph of your comparisons. It’s a bit above my paygrade, though.

It seems that everyone, including myself, agrees that the Beakhawk aircraft have great flying qualities, although I have never flown in any of them. You didn’t state your experience, either way. But, that was never the intended point of this conversation and would surely be very subjective, besides. I certainly can’t say that I would accept the scoreboard when there’s only one team on the court. I will attempt to explain…

Let me start by respectably disagreeing that the 30-613.5 airfoil is “theoretical” as it has been flight tested on several different aircraft, since first published by Mr. Riblett, including the very popular Highlander STOL. While not being an apple-to-apple comparison with the LSA, all these aircraft have great flying qualities.

This leads me to restate my intended point of this conversation, with modifications and bullet points:

* The Bearhawk LSA wing was designed, drawn and load calculations performed using the true 30-613.5 profile. This can be easily verified by using the printed ordinate data from Mr. Ribletts book or by calculating the ordinate data, oneself, using his 30-015 basic symmetrical form, GA-6 camber data, scaling to a 60 inch chord and comparing these data dimensions to LSA Dwg 3.

* The Bearhawk LSA wing was never built and flight tested with the as-designed, true 30-613.5 profile. Therefore, no flight data was available to be recorded, compared or shown on the metaphorical scoreboard. No cheerleaders, either...

* Without building and flight testing the as-designed, 30-613.5 wing, a modification was made to the airfoil profile for reasons unknown except to Bob or Harry, as far as I know. Without data to compare, no one can really know if the result of the modifications were as intended. It’s possible that the modified wing could be less desirable than the 30-613.5 wing would have been. Especially, considering how well the 30-613.5 has performed on other aircraft.

* While we don’t know if the Dwg 4 wing is any better than the 30-613.5 wing, we do know that it is thicker, heavier and, most likely, has more aerodynamic drag. The Dwg 4 wing probably has slightly less camber that would, in my opinion, result in a marginal speed increase but so might the speed be higher with the lighter, thinner and less aero drag 30-613.5 wing. The higher camber 30-613.5 should have a slower stall, steeper climb than the Dwg 4 wing. Again, my opinion from my research.

* Since we don’t know the intended purpose for the modification and we don’t have data to compare it to the true 30-613.5 wing then we, I, can’t make an informed decision as to which airfoil I want to build. I’m relatively sure that Bob never meant for us to have a choice but, yet, has unwittingly given us one by not being forthcoming with the information that many of us have asked for, over the years. While he isn't wrong, his recent response to the question was, effectively, "use drawing 4, it flies just fine, stop asking questions". I took no offense but was a bit disappointed.

I, for one, would love to have someone with your expertise and experience to analyze the two different profiles of the LSA. From my understanding, it may also apply to the airfoil used on the Patrol, BH4B and BH5 due to a similar modification of the true 30-413.5 profile. Not that you would need my assistance, but I would be glad to furnish you with the ordinates for both the 413.5 and 613.5 profiles in their respective chord lengths, as well as, the ordinates for the LSA Dwg 4 mylar profile. Or any other info than I may be of help with. The Navier-Stokes simulation now has me very intrigued.

Thanks again for your input!
Mitch
 
Frank,

The Patrol, BH4B and BH5 use the GA30-413.5 airfoil, modified I'm told, with a 65.125 inch chord. The LSA uses the GA30-613.5, modified, airfoil with a 60 inch chord. The Riblett airfoils are scalable to different thickness distributions and chord length within the different series. I have created a spreadsheet that makes those calculation's very simple by only inputting the thickness distribution percentage, i.e.13.5%, and a chord length, i.e. 60 inch. I have been meaning to share a link to the file but I wanted to create some basic documentation so that it would be usable by most anyone regardless of their knowledge of Excel. I'll try to get that completed and posted ASAP. Hopefully, nborer might find some time to help with the analysis that you speak of. We have to keep stirring...

Mitch
 
Mitch:

Thank you for the consideration. I'll pose a few responses to your assertions that I hope will motivate some additional thoughts and perhaps help you in your quest for the best way to move forward.

You mention that the 30-613.5 airfoil is not theoretical, "as it has been flight tested on several different aircraft, since first published by Mr. Riblett, including the very popular Highlander STOL." I agree that the airfoil characteristics have been "demonstrated" in flight. However, I have yet to see actual *test* data for that airfoil, or, frankly, for any of Mr. Riblett's airfoils. When I work on a project and have a requirement with a stated verification method of "test," I am expected to conduct a highly controlled experiment to determine if the requirement is met. A verification method of "demonstration" indicates that far more subjectivity is involved. I've seen dozens of "demonstrations" of the effectiveness of Riblett airfoils - pilot testimonials, reports, even quotations of numbers associated with changes in stall speed, stall characteristics, cruise performance, etc. However, I have yet to see a highly controlled set of experimental data associated with any Riblett airfoil - can those providing those testimonials show me a data sheet indicating the density altitude, weight, CG, and all other variables associated with the aircraft prior to the modification are the same? Can they provide how they calibrated their equipment? Did they use best practices for evaluated handling qualities or task completion, like Cooper-Harper ratings? Please, if you are aware of any, please send my way - this is not snark, but I have not seen anything about these airfoils tested in a controlled environment. A "test" should try to reduce all uncertainty - discuss ways in which the environment is controlled and/or measured, the manner in which the test article is controlled (including the tolerances and verifications associated with those tolerances), the approach for test equipment calibration, and even how the objectives of the experiment are designed. If not, it is a "demonstration" - something that can be very powerful (and is used very often in engineering!), but something with far less rigor.

I know that sounds snooty, and that is not my intent. I have read Riblett's book - in whole or in part three times - and I largely *agree* with his overall assertions regarding airfoil design practices. Unfortunately, he disparages a lot of good research conducted by honest folks over many decades, and I'm happy to offer some alternate explanations as to why things ended up the way they did that are less... conspiratorial. I personally know two of the individuals that have worked on the airfoils or GA projects that he ends up maligning in his book, and I think his narrative does not provide the appropriate context. That said, it's his book, not mine, and he's free to his thoughts and opinions. Beyond that, I won't go into the weeds here and I have no wish to argue with someone who is not around to defend himself.

Going back to *test* data... from my (admittedly limited) searches, and from reading Mr. Riblett's book (I have the sixth edition), all of the airfoil profiles were generated per a (sound) theory, and the performance was estimated for these airfoils with a dated, but reliable, analysis code. However, I have not seen reliable, repeatable, calibrated *test* data associated with any of these airfoils. Again, I'm happy to be shown otherwise, but all I've seen thus far are very positive results from demonstrations, not from tests. In the absence of this data, any discussion that the 30-613.5 airfoil is somehow tested and vetted is moot - it has been demonstrated (very!) favorably, but I have not seen calibrated data that validates performance claims - or, even, a thorough description of those detailed performance claims. I'm not referring to "gentle stall," I'm referring to "stall at an angle of attack of 19 degrees with a pitching moment about the quarter chord of -0.026 when corrected to a standard atmosphere at a Reynolds number of 3,000,000."

You state that the "the Bearhawk LSA wing was designed, drawn and load calculations performed using the true 30-613.5 profile" and claim that this can be verified by inspecting LSA drawing 3. I don't know LSA drawing 3, but if it's similar to my model B plans, drawing 3 shows various wing ribs (nose, center, etc.). I would not consider those to be a definitive source on rib geometry - I would consider the formblock master, the Mylar sheet that is drawing 4 in the model B plans, as the source for the rib shapes. That is the intent of the Mylar sheet - in fact, it is on Mylar because, as a medium, it is subject to less deformation than the paper drawings. My first internship was as a drafting/CAD monkey, and I had to edit the official drawings in our computers (and some older hand-drawn line drawings) to what the guys on the shop floor had marked up and were actually using. I learned quickly that some drawings and dimensions "controlled" others, beyond the standard that we learned in class regarding controlling dimensions on an individual drawing. I can offer you no objective proof that the Mylar drawing is the controlling drawing, other than it makes the most sense and fits anecdotally with what I have read on this site.

Regarding the load calculations, Riblett never provides loads in his book for the 30-613.5 airfoil. He provides a scaled picture of its shape and an estimate from his computer code for minimum drag, maximum lift, and pitching moment about the quarter chord at some selected Reynolds numbers in Table I and Figure III-19 on page 71 of my edition, but the only way to estimate lift, drag, and moment coefficient per the data he provides for the GA30-613.5 airfoil can come from interpolating the plots on page 77. By my own estimates from XFOIL, the small changes in lift and drag from the Bearhawk Model B drawing 4 (Mylar) profile and the published Riblett method for defining that airfoil geometry for the GA30-413.5 are so small as to not warrant further consideration. The only larger difference I have is in moment coefficient, and I suspect that lies in the discretization of the Riblett book ordinate definition vs. the finer discretization (and associated numerical smoothing necessary) to eliminate numerical roughness that I applied to my personal measurements but did not apply to the Riblett book measurements (the variations in moment coefficient otherwise defy conventional logic). I won't get into such nuances here. We should also recall that the 4-place wing structure was designed for the thinner, more highly loaded NACA 4412 profile, so that would imply to me that applying the same skin and spar cap thicknesses would result in higher structural margins, since the Model B airfoil is thicker. I'm attaching a PDF of my XFOIL estimates for the "book" Riblett GA30-413.5 airfoil, my Kulfan-smoothed estimates from the Model B drawing 4 (Mylar) airfoil, as well a stock NACA 4412 (similar to Model A?) airfoil to this message. I'm also providing a revised estimate of the camber line and forms for the book vs. smoothed drawing 4 airfoils - my previous post was a bit confusing and based on data I hadn't looked at in 18 months. I've removed the x/c and y/c axis values and made sure the scaling in x and y isn't square so I don't run afoul of any copyrights.

Bringing it home... I don't know how many Bearhawk LSAs have been built and flown, either scratchbuilt or from the factory. But I'll bet you a beer that the ones that have been built and flown have used the Mylar drawing as the formblock, and not the published approach to the Riblett 30-613.5 ordinate generation (let's recall that Riblett told us how to generate thicknesses in between the 12%, 15%, and 18% forms in his book, but he did not explicitly publish the ordinates for anything in between - though I suppose you could scale up the picture he provides in Figure III-19). By that standard, they fit the "demonstration" criteria that they fly well - just as much as other "testimonials" that the published approach to generating the airfoil ordinates produces good flying characteristics. I'll bet you another beer that whatever ordinates show up in the Mylar drawing for the LSA will perform within 5% of the lift and drag estimates that one would reasonably expect using modern-day analysis techniques on both sets of ordinates.

If you wish to send me your measurements of the LSA, I can compare those to my ordinates for the GA 30-613.5 airfoil. It would probably also help to have your ordinates based on Riblett's book, to make sure we're both working from the same sheet of music. I can run a similar comparison as I've done here for the GA 30-413.5 airfoil and the Bearhawk Model B drawing 4 airfoil. PM me if it's more straightforward or if you're concerned with copyrights.

Nick
 

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How about building an LSA or Patrol with the right wing Riblett and the left wing Bob Bearhawk? Then you can have a really good real world test! : )

Kevin D
KCHD
 
Now that is an example of why this forum is so important to it's members. So many great and unique ideas are presented. Sadly, I would have absolutely no idea how to connect the test instrument's in this scenario. :(

Thanks Kevin!! :)
 
Nick… Dang!… Where do I start??... I have “information overload”!! It would appear that you must have missed a previous post where I placed the disclaimer “I am not an engineer”!! LOL!! Unfortunately for you, I do still possess that “yearn to learn” mentality and, as time permits, I’ll be spending some of that time with Mr. Google. The “unfortunate” part of that statement will likely be the volume of questions that I’ll have for you at a later time. LOL, again!

We are working from the same sheet of music, in regard to Mr. Ribletts’ book, but your apparent interpretation of some of my statements would seem to be a page or two off from what I had intended or hoped. I have never considered myself to be an articulate communicator and that may have contributed to a possible misunderstanding or, at the least, suggest some clarification.

When I disagreed that the 30-613.5 airfoil was “theoretical” I was only referring to the fact that it was no longer a theory in someone’s head or a sketch on a napkin but rather had reached the point of practical application on several highly regarded aircraft. I believe that to be a valid and adequate reason to un-characterize the airfoil as theoretical.

You may have taken some issue with my use of the term “fight tested”. In hindsight, and by trying to look at it from your perspective, I can understand if you did. I only meant to convey my thinking in that the 30-613.5 airfoil was flight tested in the same manner as all the Bearhawk airfoils and most other homebuilt or low production aircraft, i.e., pilot testimonials, and that the subjective data from the 30-613.5 testing were equally as compelling as the Bearhawk airfoil data. That is why I said that I couldn’t accept a scoreboard with only one team competing. From an engineering standpoint, I completely agree that, demonstrated characteristics, would be a better term to use and that a controlled flight-testing regime, as you presented, would certainly be ideal. But for now, I suppose our best option is to hope for comprehensive, unbiased flight test reports from seasoned and respected test pilots.

Yes, Mr. Riblett could have been much more respectful of the oftentimes invisible but yet very talented and hardworking individual’s that make up a large government entity such as NACA/NASA. I make no excuses for him on that point. Throughout this topic of discussion, I have attempted to not misled anyone into thinking that I have expertise in any aeronautical discipline or that I am qualified to offer any arguments/counterpoints to Mr. Ribletts design theory, methods or performance analysis. I would like to learn more about this interesting topic but I don’t currently have the time to invest. If I unknowing implied that I had seen or heard of any reliable, repeatable, calibrated test data of the 30-613.5 airfoil, it was completely unintentional. I, too, wish that it existed. But it has been tested/vetted to the generally-same extent as the modified airfoils of the Bearhawk line and that was my only available method for comparison.

I am guilty of assuming that a contributor to this topic/discussion has read and remembered the, subjectively, important bits. Because of that, I tend to not go into complete detail when I attempt to explain or make points of my findings, opinions or speculations. That is meant as a courtesy to anyone that is sacrificing their time to read my often, wordy musings. But, it can, and likely does, sometimes result in our message not being transmitted clearly and/or received without overriding background interference.

A couple months back, after having decided that I might be able to get started on my LSA by years-end, I started thinking about this well-known question regarding the airfoil for the LSA and others. I had a thought process already in my head so I pulled out Mr. Ribletts book and started some heavy reading. I didn’t understand a great deal of the data graphs and, due to a very blurry copy of the book, I couldn’t read the numbers and text within many of the graphs so I didn’t spend a lot of time trying. Nick, unless I completely misunderstand your comment about there not being any published ordinates for a thickness distribution that fall within the 12%, 15% and 18% ordinates, my copy, 6th edition, has the 13.5% ordinate table on Page 71 just above Figure III-19 that you reference in your most recent post. I used those published ordinates, scaled to a 60” chord length, to compare with the rib profile drawings of LSA plans sheet Dwg 3 and the Main/Rear/Aileron spar dimensions of Dwg 3 & Dwg 5. I made very careful and repeated measurements of these drawn wing components with a digital caliper. The rib profiles on Dwg 3 include the flanges and match up near perfectly with the published ordinate dimensions from Riblett’s book and at each station point along the chord line, allowing for manual drafting tolerances. The accuracy was so near perfect that I even made a comment in one of my earlier postings that Bob was a virtual Ninja at the drafting table. In addition, the measured thickness of the center rib at the main spar attach point was within a few 1/100’s of the call-out dimensions of the main spar height as shown on Dwg 5. This is what I considered and described as verifiable data to confirm that the LSA wing was indeed designed around the true as-published and scaled GA30-613.5 airfoil profile.

Regarding the wing load calculations, I could have done a better job in my wording from my last post. I seem to remember that I described it a bit differently at an earlier time but that point is mute. So, after I was convinced that the as-designed wing profile was a verifiable 30-613.5 airfoil, I then postulated that Bob would have also, most likely, used the spar dimensions from Dwg 5 for his wing load calculations. Verifiable only by Bob, I suppose.

Soon afterward, I used Harry’s guidance and created a spreadsheet that would allow me to calculate, scale and create a DAT file for any combination of thickness percentage and chord length within a series. What range of combinations would be useful, I haven’t a clue. I tested the spreadsheets accuracy with Harry’s procedure of using his basic, symmetrical thickness form, GA30-015, along with the camber profile ordinates GA-4 and GA-6. By using the appropriate inputs for percentage, chord and camber, the spreadsheet will output ordinates that match all published ordinate tables for the chosen airfoil series. Satisfied with this, I then plotted and printed the airfoil profile for the GA30-613.5 with a 60-inch chord. Overlaying this printout with the Dwg 4 mylar shows a perfect match in all areas except for the lower most perimeter line. It differs in that it is a relatively consistent 0.100 thicker from the 12” chord station to the aft most point. The thickness tapers back down from the 12” chord to the leading edge. I have an overlay drawing and others that I will submit whenever we determine what is most useful.

I’ve done some simple testing of my profiles with paper and mylar and the current paper being used by Duncan-Parnell is surprising stable when cycled in and out of my building but it can’t substitute for the mylar, for the reasons that you pointed out.

The Dwg 4 mylar is indeed the master that Bob would like for us to use and the one that has been used by all previous LSA builders, AFAIK. But it is, from all my research and measurements, a modified version of the 30-613.5, as briefly described above. Since I started this discussion, and currently, I have no reason to choose the 30-613.5 over the Dwg 4 profile. My decision will be made after learning the intended purpose of the modification or, with your help, the performance characteristics of each profile, combined with your expertise, that would assist in making the decision. We have no test data, demonstrated or otherwise, that would allow a comparison between the two. Should the differences appear negligible, then I would choose the thinner, lighter wing and trust that the load calculations were performed with that airfoil and spar dimensions, as I believe it was.

The 30-413.5 ordinates were never published but I recently calculated, plotted and linked to the file that, if printed and overlaid with your Dwg 4 mylar, would likely match except for the lower perimeter as it does with the 30-613.5. The profile drawing and ordinates table files for the 30-413.5 are linked in Post# 16. You will also find there, the 30-613.5 drawing profile with all ordinate dimensions included, for your reference if interested.

I have lost hope that I might soon be enjoying one of your cold beers. Your wagers were also things that I consider to be accurate or, at least, believable. Maybe next time… (insert sad face)

Best Regards,
Mitch
 
Below is a link to the Excel workbook that I created and used while exploring the new and exciting world of airfoils and ordinates. I have enjoyed my foray into this strange world, immensely. I’m sharing this with our members because there may be someone out there in Bearhawk land that enjoys learning new things as much as I. Maybe even the next generation Harry Riblett is lurking in the shadows.

I spent some time recently reading over the guidelines for “Fair Use” of copyrighted material. Some of our major universities have great information on their websites. After careful thought, I have included two data tables from Mr. Ribletts airfoil book. I had originally planned to only include a small sample which would require that you fill in the rest of the data from YOUR own copy of the book. Although that is no longer required, I would still like to encourage you to purchase a copy if you haven’t already. The Harry Riblett family donated the copyright to the EAA.

I have included some basic instructions on the ReadMe worksheet that I hope will be enough to get you started. If you have any questions, please don’t hesitate to ask.

Have fun!

Mitch


View attachment Riblett_Airfoil_Thickness_Calculator.xlsx
 
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Well, there's one beer back to you... in my reads of the Riblett book, I never noticed that the ordinates on page 71 just above Table I and Figure III-19 didn't follow the format of the previous pages (columns of station, 15% thickness, camber line, and then upper/lower ordinates for 12, 15, and 18% thick airfoils). So it's all right there in terms of the 13.5% airfoil with the 30-6 camber line. I guess I reconstructed it appropriately because my calculations are within 10^-4 accuracy of what he has listed, and I figure we both are just using slightly different criteria for round-off error.

That said, I suggest we don't go for "net beers" but instead trade rounds. It's more fun that way.

I've gone through the files you posted... I believe the drawings you have are for the Riblett-defined 30-631.5 airfoil, scaled to a chord length of 60 inches. In your first post are some .dxf and .pdf files and others that I'm unable to download - do those have your as-measured drawing #4 airfoil? That's what I'll need to use for the comparison.

I also see that you had placed the chord line outside of the airfoil shape. Is that so you can get a blunt trailing edge? I'd recommend scaling the airfoil slightly larger so you end up with a blunt trailing edge in the formblock. Riblett mentions this on page 14, where he says to "cut it off square" at 99% of the chord. Presuming you want to preserve your chord length, just build the template but multiply the x station by (1/0.99 = 1.010101...). That is, scale your drawing as if your chord length were 60.61 (60/0.99) rather than 60 inches. I notice that the blunt trailing edge in my Model B formblock drawing is more equivalent to cutting off at 98% rather than 99%. Note that this may also be a part of the source of the slightly larger thickness, but doesn't account for the whole discrepancy (that is, it'll increase the apparent thickness of the blunt airfoil from 13.5% to 13.63%, or even 13.78% if you go with the 98% rather than 99% chord length). It'll also push back the apparent point of max thickness, camber, etc. by the same relative amount.

Blunt trailing edges can be hard to implement in vortex panel codes because they need to enforce the Kutta condition - basically, the velocity on the upper and lower surface must be parallel and depart the trailing edge in order for the code to calculate any lift. These codes neglect viscosity and compressibility when calculating lift (and drag), but, paradoxically, no lift and drag is possible without viscosity or compressibility. These terms are very small for low-speed flight, but (viscosity in particular at low speeds) is critically important. If you feel like Googling, look up D'Alembert's paradox. We use inviscid, incompressible assumptions with the Kutta condition in potential-flow codes because they are fast and reasonably accurate (and prior to full-blown CFD, were the best we had), but like all engineering models, they are an approximation of reality.

In reality, sharp trailing edges are at best unnecessary, and at worst can cause even more problems - both with the flow and especially with structures (and even operations - imagine hitting your head on a razor-sharp trailing edge - it hurts enough with a blunt one!). A little bit of thickness with a reasonably sharp corner acts to force flow separation if it has not happened, sort of "enforcing" the Kutta condition in reality. Also, you can imagine that you quickly get to zero bend radius with trailing edge pieces, which is of course a no-no. So, you can view this as cutting off the theoretical trailing edge from your chord length, as Riblett recommends, or by extending your theoretical airfoil beyond the trailing edge, as I wrote about above. The real question is designer's intent... when Bob put a 65-inch chord line on my Model B drawing 4, that was 65 inches to a blunt trailing edge, and judging by the thickness, it appears to be around 98% of the theoretical chord length - putting the imaginary chord line out to 66.33 inches, if one were to scale airfoil drawings with a sharp trailing edge.

This thread has caused me to dig back into some work I did almost two years ago, when I had plans but was ordering materials and messing around with formblocks instead of building ribs. Given that my current circumstance makes it impossible to build, it's been fun - I haven't thought about any of this for a while. I even found one of my old large-format scans of drawing 4 (I did this prior to cutting the Mylar to lay over the formblock), and will try to correlate those measurements to my hand measurements. Correcting large-format scans is always a bit of an art... they never come out perfectly perpendicular or scaled, which is why I abandoned using that approach when I first started messing with it. I have a few more tricks that I'll try. It sure beats installing crown moulding... though if I can get that done, and a dozen other jobs, I may finally be able to get my shop back!
 
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Nick,

That is very considerate of you to cancel the beer wager as we both know that it would be me making all the runs to the 7/11. I’m sure that I discovered the 13.5% ordinates only because it was my single-minded goal. I could not have reconstructed them as you did. My criteria for round-off was just whatever Excel suggested.

I must take a moment to thank you for your time and patience. You surely have more important things to do than to teach me a foreign language. I will make every effort to ensure that it is not a complete waste of your time. Feel free to just give me Cliffs Notes so I won’t feel as guilty about occupying so much of your time.

The files from my first post were only left up for a few days because I was questioning myself regarding copyrights, in the beginning. Also, I did some review of them and re-plotted a few different times to gain more experience and try a few different things.

The chord line was placed outside of the airfoil only because Bob did it that way on Dwg 4. I will attach a scan of the trailing edge of LSA Dwg 4 for reference. I will also spend some time in trying to understand your suggestions regarding the blunt edge and I’m always interested in a paradox of any kind. I once owned a Cessna 172 and I’m just over 6’2” so I have had a love/hate relationship with the pointy end of the ailerons many times. Especially if I happened to be wearing a ball cap that obstructed my vision. Yikes!!

It pleases me that you are having fun with this. It certainly is fun for me. As I alluded to earlier, don’t let me prevent you from doing the more important things so that you can get back to your shop. I can relate closely to your crown moulding project. I spent my high school summers and then, after the Army, all of my free time during college working as a framing and finish carpenter. My brother had a construction business and I was usually the sawman. Probably because I enjoyed math and I was a bit OCD regarding accuracy. Interior trim work was usually our rainy-day projects. I enjoyed it and the experience allowed me to build my home several years later.

Thanks,
Mitch


This is a scan of the LSA Dwg 4 trailing edge. The tick mark that Bob made near the end of the chord line is the 60” chord station. Maybe you can speculate on the designers intent.
Dwg4_Aft_Profile.jpg
This is my most up-to-date DXF of the GA30-613.5 profile. I put the control points on a separate layer so you can hide or show as needed. I hope that feature is universal with other CAD software.
View attachment Riblett_GA30-6135_Base.dxf

This is my latest DXF of the LSA Dwg 4 with control points. I believe it to be a modified 30-613.5 rather than a completely different airfoil. I experimented with several different thickness distributions in an attempt to find something that matched, but no luck. It just seems to be an added thickness to the true 30-613.5 airfoil profile. You will understand better when seeing the overlay.
View attachment Riblett_GA30-6135_Dwg4.dxf

This is LSA Dwg 4 overlaid on the true GA30-613.5 profile. The only place it differs is the red lower perimeter line as I mentioned in my last post. This is what it looks like when I place the Dwg 4 mylar over a printout of the 30-613.5 profile. I didn’t include the blunt end, which I measured to be 0.625” back from the 60” chord station. The red line is not completely parallel to the 30-613.5 either by design or just small plotting error.
View attachment Riblett_GA30-6135_Dwg4_Overlay.dxf

Let me know if you would like these profiles in PDF format.
 
Nick,

As you are probably aware by now, Bob did as Harry suggested and scaled the airfoil chord to 60” and then squared it off at 99%. He closed off the gap as in the attached drawing, below. Possibly the same as on your BH4B.

I honestly don’t remember having ever known about the Cooper-Harper ratings but it’s possible that I did at some point. Regardless, I do now. Thanks.

I had read mention of XFOIL a few weeks ago but haven’t had time to read in-depth or watch a video. Probably more than I want to get involved with, at this time. I’m not planning on trying to design an airfoil but I would like to know more regarding the science and engineering.

D’Alembert’s paradox will also have to wait a few more days, as well.

It seems that when I was doing my careful measurements and comparisons of the LSA plans vs the 30-613.5 airfoil profile, a couple month ago, I missed something. It’s still true that I measured and verified that the rib profile and the spars matched with the 30-613.5 airfoil as-drawn. But, a few minutes ago I was flipping though the plans sheets when I realized that I had not measured the components on Dwg 10, Aileron Spar & Rib, although I had measured the rear spar on Dwg 9 but not the aileron spar on Dwg 9. That area was very congested and hard to accurately measure so I skipped it since I had already verified the airfoil itself. My bad. Turns out that Dwg 10 is drawn and dimensioned with reference to the profile of Dwg 4 instead of the 30-613.5 as was used for Dwg 3, 5 and, and part of 9. The rear spar and the pocket rib template do reference the 30-613.5 profile but the 1/2 scale aileron matches the Dwg 4 profile. This can of worms are really wiggling.

I’m not really sure, at this point, how much that is going to affect/complicate things for me should I decide to build the 30-613.5 wing. Going to need some time to study that area of the plans. Bob’s instruction’s regarding the ribs and spars are basically, build the ribs to Dwg 4 and then bend the spars to match. So, pocket ribs whose template would result in being formed to match the 30-613.5 profile and then fitting to the aileron nose that match the Dwg 4 profile. I suppose the aft radius of the pocket rib fits good enough to the nose of the aileron to have little, if any, effect. Darn worms, stoppit!! Gonna take a good nights sleep to sort this one out…

Mitch

Click image for larger version Name: img014.jpg Views: 0 Size: 402.8 KB ID: 68750

In this scan, The rear spar and pocket rib are 30-613.5 but the aileron is Dwg 4. Of course, once you build rib and spar to Dwg 4, the rear spar will be 0.100 thicker, or more.
Click image for larger version Name: img015.jpg Views: 0 Size: 811.2 KB ID: 68751
 
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I'm not up to speed with you guys by any means but I have been pondering what might be the ramifications if you depart from drawing 4, the Mylar. If it were me I would be worrying about unforeseen consequences. Also (and I may be missing the point regarding your aileron issue) my understanding is the aileron has its own airfoil shape independent of the wing and the two surfaces are not "flush". Also, when forming my pocket ribs I left the single flange "tab" unbent so that I could bend it to fit the rear spar. Just a few comments from the peanut gallery. Carry on, gentlemen!
 
Mitch:

Thanks for the updates. I haven't had a chance to import the new drawings and finalize my comparisons; that could take a bit as it's a busy few weeks here (I'm treasurer for my kids' PTA, and it's tax form season, so that means my spare evening cycles are taken up with PTA stuff). I did resurrect my smoothing codes that I wrote in Python; long story, but the only reason I code in Python for personal projects is that the stuff I use for work requires licenses that cost way too much, and I'm not allowed to double-dip on those machines. So, I'm re-learning how I implemented the airfoil smoothing technique I used when I did these computations 22 months ago from raw data. I use Python a little, but I don't use it for work, so I'm a little rusty at the moment. I'll get there.

Why smooth, you ask? Any numerical analysis, in particular the vortex panel code I'm using (and the predecessor code that Riblett used) really don't like noise (based on our measurement tolerances) in the inputs - this noise appears as artificial variations when solving the system of equations used to get the answers for lift, drag, etc. In "real life," those little variations (like flush rivet dimples) are, essentially, "filled" with stagnated air, but in an inviscid panel code, there is nothing to "fill" it, so it introduces numerical noise that looks like variations and lift and drag that aren't physical. Riblett actually alludes to this in his book; in the first paragraph on page 12 (6th ed.) he laments some issues with the "improved" version of the Eppler code, including "the results appear to be overly sensitive to data point (ordinate) accuracy, thus I still favor the old program." When plotting Riblett's raw ordinates, I can see that the data in the book does not appear to be smooth to the precision that most numerical solvers love to see. That doesn't make the airfoils bad; it just makes the numerical predictions of their performance challenging, especially as you use more accurate methods. My current process smooths the airfoils by fitting the points to functions from Kulfan's CST method; I won't belabor them here but if you want a heady read here ya go: https://brendakulfan.com/docs/CST2.pdf. The short version is that it makes airfoil ordinates smooth in ways that make numerical solvers behave better.

My plan is to try to extract the points from your Drawing 4 .dxf file by first converting it over to an image and then sending it over to one of my data grabber programs - I use Engauge or more recently, Graph Grabber. That process itself introduces error. Since you have the native CAD file, I could eliminate this step if you could send an Excel or even just a CSV or some other delimited text file of the drawing x, y coordinates for the airfoil ordinates and chord line (I don't have AutoCAD, so I'll need to download a translator to get the .dxf as an image). If not, no worries, it doesn't take much longer... it's just a step that introduces a little more error.

Once I have them, and I have smoothed ordinates, it'll be pretty straightforward to run the cases. I've already run the Riblett-defined 30-613.5 airfoil in XFOIL, so it's a matter of getting the other airfoil from drawing 4 into the system. I've also run a few validation cases, since a few folks have (rightly) called into question the accuracy of what I've been presenting. I found some good data on the NACA 4412 and 4415 airfoils from wind tunnel tests (Abbot & von Doenhoff and NACA report 563, though the latter does not include drag information) that I digitized as "truth" data and ran against XFOIL. I'll present those validation cases as well so we can get an understanding of the tool accuracy. I'm including a teaser of the validation data below for the NACA 4412 at a Reynolds number of 3 million - not awesome, but certainly pretty good, at least until we get near stall.

XFOIL_comparsion_NACA_4412.png
 
Hey Frank. I’ll never be up-to-speed with Nick but I’m having fun accelerating. My brain lost its ability to impersonate a sponge many years ago. But, I am doing my best to avoid anything unforeseen and I welcome your input and advice on things that I might need to avoid or to give special consideration.

It is also my understanding that the aileron is a different airfoil and I had verified that it was different than the aft section of the wing profile. I incorrectly implied in my previous post that the aileron matched the Dwg 4 profile when I only meant that it matched in regard to the Dwg 4 aileron spar height and then, based upon that, the aileron was designed to be a thicker airfoil to complement the Dwg 4 wing profile rather than the thinner 30-613.5 profile. I’m only trying to relate what I see and measure into, hopefully, useful information for Nick. Thank you for noticing that my explanation was clear as mud, maybe still so. LOL. When it comes to understanding airfoils, I AM the peanut gallery.

When I was posting Tuesday evening, I had wondered to myself, what were builders doing to make the pocket ribs fit the thicker Dwg 4 rear spar. I like your simple solution. At what point were you aware that the full scale, pocket rib template would only fit the 30-613.5 spar and not accurately fit the Dwg 4 rear spar that you were building/had built? Was it something you had read on the forum or after you had formed the inaccurate pocket ribs? These discrepancies on Dwg 9 combined with the others that I’ve brought up in this discussion has only reinforced my stated belief that the modification of the 30-613.5 profile was an afterthought in Bob’s design process. At what point and for what reason, I can’t say without speculation. Only the reason is of interest to me.

Mitch
 
Nick,

If I wasn’t completely clear in a previous post, then please hear this… Post #21 and #22 were only meant to provide you with information that may assist you in some way with this “project”. I do not require or expect a prompt response. If you would like to continue working with this, then please only do so when and if you have the time and desire. I still plan to talk to Bob at some point before starting my project with the hope of learning his design goal for the airfoil modification. The information and experience that you are providing will go a long way in helping with my decision and the things that I am learning on this journey will always be invaluable regardless of what decision I make. In order to get started building in this calendar year, my backup plan has always been Dwg 4.

I think Python is great and I’ve dabbled a bit over the years. I read recently that it is rated #1 this year by the IEEE. Most of my client work the past 5-10 years have been either VBA or C++ (as used with the Arduino line of hardware and clones) and a hodgepodge of others languages over the years, including low-level assembly on an 8748H microcontroller circa 1991, I think. Yeah, you’re right… that was TMI. At least I didn't go back into more ancient history, so consider yourself lucky!

Yes, smoothing. You read my mind. LOL. More acceleration!! (referring to my reply to Frank) I can actually visualize those lazy, stagnated molecule’s being shooed away by those geeky numerical noisy molecule’s that refuse to get physical.

I wasn’t sure what all you needed from me so I apologize for leaving you short of data. I’m sure that is considered to be cruel and unusual punishment to an engineer. So, to brighten your day, I am attaching an .xlsx with the ordinates for Dwg 4 in the Riblett format. If I understand correctly, you won’t need an image file as long as you have the ordinates. But if needed, I can supply most image formats or a PDF. I haven’t used AutoCAD since the late ‘90’s but have gotten by with free or opensource alternatives until recently when I tested and purchased QCAD Pro for $39.00, (available free with limitations). I think it will serve me well in the foreseeable future.

As always, I’m looking forward to reading, or at least trying to read, the Kulfan’s CST method and learning the results and data from the cases mentioned in your last paragraph.

Good luck with the tax forms. Always been my most favorite part of life…

Mitch


Ordinates data in Riblett style format
View attachment Bearhawk_LSA_Dwg4_Ordinates.xlsx

DAT file for importing control points into QCAD
View attachment Bearhawk_LSA_Dwg4_DAT.txt
 
Mitch:

Thanks for the ordinates. I just took them without 3rd party smoothing and just used XFOIL's smoothing routine (the PANE command, if you're deep into XFOIL menus and you want to make it work). So, these results are without Kulfan smoothing, and a comparison of the Riblett 30-613.5 airfoil to the LSA drawing 4 coordinates you provided.

Looking at the ordinates, I see that the lower surface is translated slightly downward in the LSA drawing 4 vs. the Riblett coordinates. I believe this is to help with the blunt trailing edge. I still see some waviness in the lower surface that doesn't seem intuitive, but that's present in both forms - the LSA drawing and Riblett coordinates.

Overall... both airfoils perform very similarly. Not surprisingly, the slightly thicker drawing #4 airfoil with the thick trailing edge does slightly better at higher angles of attack. That may be an artifact of the analysis code more than anything else. From the validation results, I would call this performance "essentially identical" if I were giving a design review.

I'm biased, but I say go with drawing 4. You'll be in good company with other LSA builders, and the performance difference is at worst nil, and at best my eek out a small amount more of high-lift (though I don't really believe those results, for reasons I won't belabor).

Build away, my friend!

Low-speed comparison

LSA_comparison_Re_3e6.png
High-speed comparison
LSA_comparison_Re_6e6.png
Geometry comparison - scale and labels distorted for copyright reasons
LSA_vs_30-613p5_geom.png
Nick
 
Hi Mitch, my thinking on the pocket ribs was this: the center and back ribs are formed and the spars are bent to fit; but it might be best to fit the pocket ribs to the rear spar after, not before. Not noting any "discrepancy" in the plans but noting the general "instruction" to make the parts fit to each other. Maybe I will succeed.
 
Fellas, I am not expert or particularly knowledgeable about aerodynamics but I like to think I am open minded enough to never look a gift horse in the mouth. I chose the LSA for many reasons, among them is that it is an easy airplane to fly, it uses a relatively inexpensive engine, it has great performance. It ticked all the boxes that were important for me. The type of airfoil type is a moot point for me. So I wanted to bring up a point about the designer of the air foil that not many people may consider. What was the motivation of the designer in the first place. I am in no way saying or implying here that this airfoil is unsafe because it has proven its self to be perfectly safe and a high performance airfoil. I only wanted to share a story about several designers, I believe it is a very insightful look at the motivations that make some people claim to be experts in their chosen fields and I wanted to share it with you guys just as food for thought. If you read William Wynne's website you have probably already read this article. If not and you would like a little more insight to Mr. Ribletts motivations please read this article. I offer it for your edification. https://flycorvair.net/2013/01/28/expert-witnesses-in-civil-aviation-trials/
 
Insightful piece. I’ve been an EAA member since 2003 but didn’t actually read about Riblett much prior to getting my Bearhawk plans. After I got my plans, I bought his book and was… shall we say surprised with the tone. I agree with many of the principles, but don’t buy the conspiracies. I currently work with the designer of a few of the airfoils that Riblett disparages, and he (and his wife) are among the sharpest aerodynamicists I know. They didn’t “screw up” - their airfoils do exactly what they’re supposed to do.

My basic message in this thread, which I’ve tried to back up with analysis, is that there is likely little if any perceptible difference in airfoil performance between whatever profile Riblett published and the one that exists in the Mylar drawings.
 
Interesting reading, Dave. My takeaway is there is a danger in knowing too much (but not everything) about a topic, or about people in general. Once the door is opened a crack on various personalities it's hard to close it again. I agree with you: it's the LSA specs (and esthetic appeal) which brings me hereI​​​​​​​​​​​​​​
 
HI Frank and nborer please don't get me wrong, I am a fan of the airfoil on the LSA regardless of who designed it but I like to play devil advocate in most discussions i participate in and try to bring up several points of view. My friend William seems to be more than just an engine builder but a philosopher and he puts ideas into words that I would have a difficult time doing. I totally agree with his philosophy and his out look on life and the way we treat other people. I think it is a fair philosophy and most of all very objective based on experience and empirical data. . I believe William is a guy who would never sell out his morals and principles for profit. Im my personal dealing with him he has never done me wrong and has been more than generous to me and always treated me with the utmost respect.
 
davsLSA: I'm in agreement with you and with the article you posted, at least about the Riblett discussion. When I said "I don't buy the conspiracies" in my post above, I was referring to the ones in Riblett's book about NACA, criminal negligence, etc. - not Mr. Wynne's article. Sorry if I didn't make that clearer. I'm actually in full agreement with Mr. Wynne - the aerodynamic principals Riblett proposes in his book for "good" airfoils are, in fact, good, but otherwise the tone of "GA Airfoils" is a bit dramatic and overstated. I've tried to indicate that in my posts, but also don't wish to offend, since I do know that many people think very highly of Mr. Riblett. Dave Lednicer provides a much less-nuanced takedown here: https://yarchive.net/air/airfoils_riblett.html

The introduction to "GA Airfoils" starts by stating incredulity that NACA airfoils were not designed for actual airplane use, but rather were wind tunnel test specimens used to investigate effects of different geometry changes on airfoil performance. This was the first thing I learned in my aerodynamics class when we were introduced to NACA airfoils and the seminal tome of Abbott & von Doenhoff. Yes, people have used those airfoils in a variety of designs, but the intent of the research wasn't to create a catalog of airfoils - it was to investigate different geometries. Maybe it's not well-known outside of aerospace engineering, but it's common knowledge in the field.

Another "surprise" for some folks is that airfoils actually don't matter a whole lot on their own - they need to be mated with an appropriate 3D wing design and tail planform. A "good" airfoil can perform very poorly on a "bad" wing and tail, and vice versa. I learned this firsthand, at least computationally, when I started down the path of rotor and propeller design several years ago. I randomly generated thousands of airfoil shapes (using the Kulfan method linked above, so they would be "airfoil-shaped") and created minimum-induced loss propellers (an analytical method for optimum chord and twist distribution for a "design point" - a specified thrust/power/torque at a specified RPM and airspeed) with those airfoils. There were a lot of dogs, but what surprised me were how many of them got with 1-2% of the maximum propeller efficiency seen in the whole lot. The airfoil design mattered far more for good off-design performance than for best on-design performance. So, pick your favorite airfoil, then tailor it to a wing and tail planform to it for good performance. That's what Bob has done here with the LSA and others.

Overall, I'm trying to help the OP make a choice about what shape he uses... with gentle nudges towards the as-drawn #4 profile on Mylar. But it's up to him. From an airfoil performance perspective, my admittedly simplified analysis tools indicate very little, if any, performance difference.

Nick
 
HI Nick, Your posts are very educational and I enjoy reading them, As any pilot the study of aerodynamic is a subject I'm always interested in and knowing more about. You are certainly not offending me with your posts. And besides God knows I could use more education.
 
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Would it be illuminating to this question to derive the coordinates of the ribs being produced and supplied with the LSA Kit? Perhaps someone here with the kit could take sufficient measurements of a rib (and the associated other components that connect to the rib if need be) to deduce if the kit follows the mylar rib shape (drawing 4 rib shape) or if it follows Riblett's published coordinates.

It seems that this information could help LSA scratch builders who encounter this apparent discrepancy in the plans and find it problematic for their project.
 
v.x said:
Would it be illuminating to this question to derive the coordinates of the ribs being produced and supplied with the LSA Kit? Perhaps someone here with the kit could take sufficient measurements of a rib (and the associated other components that connect to the rib if need be) to deduce if the kit follows the mylar rib shape (drawing 4 rib shape) or if it follows Riblett's published coordinates.

It seems that this information could help LSA scratch builders who encounter this apparent discrepancy in the plans and find it problematic for their project.
Click to expand...

There is no need for this to be problematic problem. I have enjoyed this discussion from the academic perspective but realize the drawback of hosting it may be confusion about whether there is something wrong with the airplane. I hope that folks won't arrive at that conclusion. Build the airplane to match the plans and you will have a strong and light, fantastic, proven performer.
 
What Jared said...

While this discussion has been educational, we aren't building an SR-71 or an F-104. In fact, our VNE is lower than the stall speed of both of those :-)

Also, keep in mind that many of us are scratch building and are hammering our ribs out over a wooden template: there is no way in, well.... you know, that all of our ribs are *exactly* the same as the kit ribs. I would argue that not all of the ribs in a wing are *exactly* the same, although you probably can't tell that by eye. The kit ribs, however, are as close to perfection as you can get WRT the plans/mylar template.

There are many Bearhawks happily speeding and STOLing around and they couldn't care less about a line width on the plans here or there. The airplanes are wonderful to fly, have great performance, and have no nasty gotchas.

By all means, carry on with the academic discussions (I'll admit my eyes glazed over a bit after a while...I'm a software guy...) but for those that are concerned/confused, don't be. Bob knows what he is doing and designing. The proof is in the flying (both scratch and kit built) and there is a LOT of proof out there....

My $0.02
 
Factory nose ribs (LSA) are different from scratch built ribs in an important way. The former have a flange all the way around the nose while the latter do not. In fact, scratch noses get trimmed (LSA) while factory noses do not - or am I wrong?
 
Sorry, Frank, I can't answer that question as I am scratch building 4pl A model. I don't think I've even seen an LSA is the flesh. I'm sure someone will be able to answer it for you. The point I was trying to make was to not get all wrapped around the axle about the minor differences between the "book" and the plans on these type of aircraft. Unless you are Bob Hoover or some other highly qualified and experienced test pilot you aren't going to be able to tell the difference. And even then....
 
rv8bldr said:
What Jared said...

While this discussion has been educational, we aren't building an SR-71 or an F-104. In fact, our VNE is lower than the stall speed of both of those :-)

Also, keep in mind that many of us are scratch building and are hammering our ribs out over a wooden template: there is no way in, well.... you know, that all of our ribs are *exactly* the same as the kit ribs. I would argue that not all of the ribs in a wing are *exactly* the same, although you probably can't tell that by eye. The kit ribs, however, are as close to perfection as you can get WRT the plans/mylar template.

There are many Bearhawks happily speeding and STOLing around and they couldn't care less about a line width on the plans here or there. The airplanes are wonderful to fly, have great performance, and have no nasty gotchas.

By all means, carry on with the academic discussions (I'll admit my eyes glazed over a bit after a while...I'm a software guy...) but for those that are concerned/confused, don't be. Bob knows what he is doing and designing. The proof is in the flying (both scratch and kit built) and there is a LOT of proof out there....

My $0.02
Click to expand...

Hear, hear. I do NOT want to steer anyone away from the plans. I was about to start forming my aileron spar web prior to my inadvertent shop "closure," and the plans very clearly say "check for fit with rib" right next to the dimensioned spar height. In order to fit my aileron ribs (without joggling - another religious argument that I won't dive into), my aileron spar web needs to be about 0.1" taller than the dimension showed in the plans. Lack of joggling will only account for half of that variation. Yet, when I lay my ribs on my formblock and compare to the Mylar, they look pretty darn close to me. I guess I went a little more to the outside of the line. I'm not sweating it. When my build comes in heavy, this will be one of the 1000 reasons.

But seriously, a software guy? That's where all the problems originate! (*Ducks shoe thrown in my general direction.*)

Nick
 
nborer said:
Stuff deleted....

But seriously, a software guy? That's where all the problems originate! (*Ducks shoe thrown in my general direction.*)

Nick
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At one time them would have been fightin' words.... However, I'm less than 2 years from pulling the plug on working for a living so I find that aspersions don't stick anymore.
 
Said in jest, of course. My brother is a software engineer, so we've been trading barbs for the last several decades.
 
I did not cut my nose ribs , I wrapped Duck tape around the tip two layers . I think it turned out well. Yes wing ribs may be different however my LAS fly's quite good .I have 21" tires, Cont. 0200 with a top speed of 135 mph at red line of 2750 rpm. Cruise 2450 rpm around 110mph. Stinger
 
Nick, so sorry for being AWOL for a few weeks! My dang ducks would not stay in a neat row as I had instructed them. A couple of them just had to get out-of-line and take up most of my free time the past few weeks. I am, of course, referring to clients. My plan was to take down my shingle at years end but that date might slip a bit. As may my LSA start date. But I still have hope.

I suspect, that after you read my post #22, you likely saw the writing on my wall. In the beginning, I either stated, or alluded to, the fact that I had no intentions of attempting to redesign, reengineer or otherwise modify the design of the LSA. I couldn’t if I wanted. I only wanted it to be a true Riblett 30-613.5, as the plans partially were and as it was advertised. So, when I discovered that I had not verified the aileron dimensions on Dwg 10, and subsequently learned that it matched with Dwg 4 instead of Dwg’s 3, 5 & 9, then my quest to build a true Riblett 30-613.5 wing was over. Even if I could, I had/have no desire to try designing and building a new aileron that would complement the 30-613.5 wing. Case closed…

What have I learned from all this? Well, quite frankly, some things that I am greatly disappointed with. Was this only an exercise in futility? Depends on one’s outlook. Was it avoidable? Yes. But I don’t regret the time and knowledge that I gained. And, I truly enjoyed the interaction with the members of this forum. What a great bunch of generous and knowledgeable people. Thanks to you all.

Nick, again, thank you for your patience, expertise and, most of all, your time. I can’t say that I’m surprised by the results of your comparisons but it’s really nice to see it confirmed by your analysis and calculation’s. And, of course, I always knew that you were biased, from the git-go. LOL Who wouldn’t be! You also understood where I was coming from with my questions and curiosity and, instead of writing me off, you made extreme efforts to help me understand and to answer my questions. You are, mi amigo, a great asset to this forum and will always have my sincere appreciation and respect.

Sincere regards,
Mitch
 
About 10% of my work over the years has been the design, prototyping and building of custom electronic projects for clients’ niche applications. I would design the hardware and code the software and/or firmware and not once have I ever thrown a shoe, of any kind, at my alter ego. You guys should grow up and stop clowning around before someone loses an eye!! Just learn to get along and play nice!! LOL

Mitch
 
stinger. You are wrapping “duck” tape, nborer is getting attacked by “duck” shoes while I’m having trouble keeping my “ducks” in a row… Maybe we need a new topic. LOL Nice performance numbers on your LSA. I truly enjoyed following your progress. Congrats!!
 
Thank you, v.x, for your comment. You succeeded in getting an answer to a question some of us have asked but failed to obtain. It does confirm my thoughts from an earlier post.

Discrepancy, like beauty, appears to be in the eyes of the beholder. It doesn’t seem that the designer or the kit manufacturer consider it to be a discrepancy, or at least one worth noting. Possibly because it ##doesn’t affect the safety## of this aircraft. By definition, the differences that I have outlined in this conversation are, indeed, discrepancies. Subjectively speaking, the last place that I want to find discrepancies would be in the plans for an airplane that I was building and/or flying, regardless of its proven flight record. And, any unexplained and undocumented discrepancy, as is in the LSA plans, would, in my opinion, be problematic enough to warrant the questions that I have presented in this topic. NOT problematic in regard to any safety reasons. I have never said or meant to imply such. But rather, problematic, as defined in being “difficult to solve or decide”. My reasons are outlined throughout this conversation.
 
rv8bldr. Mark, thank you for adding your 2 cents. I’m glad this conversation got you up from that very comfortable looking chair with the beautiful trees in the background.

As someone who has endured some very long coding sessions over the years, I submit that your eyes may likely have been glazed over long before you starting reading this topic. (wink)

I certainly agree with you in regard to the inconsistency of the hammered ribs. But that is an area where we have a bit of control without adding very much to the build time. The airfoil mylar drawing represents the size and shape of the completed wing, cross section. Allowing for rib flange and skin thickness when making the rib form blocks would result in a completed wing that would better match the mylar and, therefore, more likely have the flight characteristics that were originally intended by the designer. And remember, we don’t know the intentions of the Dwg 4 modification.

Based upon the analysis by nborer, a few line widths don’t appear to make any truly noticeable difference, but yet, Bob considered it important enough to make that small change to the Riblett 30-613.5 airfoil without any flight testing to quantify the design or verify and compare the results. A very talented, current builder of a large wing Bearhawk recently told me that his completed wing came in at 0.210 inch thicker than the mylar drawing. Will this person be just as happy flying around with that extra weight, drag and likely loss of performance or will he be wondering “what if” as he questions whether or not to load that trophy moose into the aircraft? To borrow a slogan from the Army, my little LSA wants to “be all you can be” so I’ll be making every effort to assure that. Due to the reasons I wrote in some other posts, I will now be using LSA Dwg 4 as the profile for my build.

Mitch
 
Regardless of how carefully words may be chosen, there is always room for personal interpretation. A reader should not hesitate to question any statement that may seem vague, misleading or differ significantly from generally accepted norms. I hope, as well, that arriving at an unintentional conclusion was not the case in this conversation.

I respectfully suggest that a bit less vagueness from the designer would be greatly appreciated by those who wish to understand the reasons and rational behind mid-design modifications and especially in regards to drawings that don't factually represent the designers published specifications. This has been somewhat problematic for years without resolution. Unless, of course, one was sufficiently openminded and receptive to data from sources other than the designer.(wink)

Thanks for allowing this topic. Ironically, and in reference to my original post, it is I that wishes for a refund of his time...
Mitch
 
Hi guys

I have a question if someone can answer it and maybe somebody already did or talk about it somewhere : The Bearhawk Patrol have a wingspan of 33 feets, 66 inches of chord and 180
square feets of wing area, ( lets put the wing spar out of the equation for the moment) if i take the LSA ribs (60 inches chord) and build a the wings for a 36 foots wingspan wich will also have
180 square feets of wings area, longer flaps and ailerons, wich one will theorically be better?

Franky
 
I'm a builder and not a designer, so for me the best wing is the one drawn on the plans. When it comes to deciding on the wing chord and span, Bob knows how to make each one longer and shorter, but the final design was the optimum. He didn't have the luxury of disregarding the spar, of course.
 
Franky139 said:
( lets put the wing spar out of the equation for the moment)
Click to expand...

As Jared points out, Bob didn't disregard the spar. The longer wing will be more efficient in all modes of flight, but it will be heavier for the same strength.

The narrower cord results in a spar that is less "tall". The less tall spar is weaker. ...but because of the larger moment put on it by the longer wing, it needs to be stronger.

The longer wing also won't clear trees on narrow strips as well. :p
 
Everything is a trade off, you win on one side and you loose on the other, i did ask the question because i have a set of beef up spars for a super cub, have a look
below, the main spar is 20 lbs and a bit and the rear spar is 16 lbs, they are supposed to be good for 3500 lbs but if i use them, my limit will be 2500 lbs,
I'm not sure yet if i will use theses spars but i'm exploring the idea.

For the trees, if i dont have at least 20 to 30 feets per side, it's going to be a no-no for me, i will mostly operate on floats.

Thank's for yours opinions, it's highly appreciated.

Franky
 

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How about a clipped wing patrol/supercub? Make it a cruiser and my guess is you can put on any engine and prop and still stol as much as you want. That's my contribution to the fantasy!
 
Frank

When you are talking about a clipped wing patrol supercub, i'm guessing about the standard patrol wing at 33 feets, for engine i already have a 150 hp Lycoming 0-320 with 1000 hrs tt, i'm
planning to take it up to 170 or 175 hp by having the cylinders sent to Lycon for a porting job and putting 8.5 to 1 pistons in it,

I'm not sure yet wich direction i will go for the wings, it will definitely be the Riblett GA 30.615.5 profile, for the spars, i will have an aircraft engineer ( university level) to look at it.

David

My fantasy is to be in my garage and building it. i've been a pilot for 46 years.
 
That's where you'll find me most afternoons. For a while I was getting in 20 hours a week. Lately it's been more like 15. Still requires a lot of imagination to look at the parts so far and see an airplane. Quite enjoyable though. And as I fabricate and assemble I am in increasing awe of the engineering savvy that went into the design (LSA). Not that I understand it.

I look forward to seeing your project when it gets going! Keep us posted!
 
Frank

I dont think i will really begin building before the fall, i still fly choppers and i will be busy this summer, after serious negociations with my finances minister (wife) for the project, the deal
is that i will not dig in our bank account for it, i need to get fresh money by working, anyway, yes i will post every once in while somes photos, on the other hand, i can not call this project
a Bearhawk, it will have the wings but that stop there, the end result will be more like a look alike 2 places Aeronca Sedan with a Bearhawk wing so some peoples might say that i dont have
much business putting this project on this forum, i just came to it because of some interest in extending the wingspan of the Bearhawk Patrol, i will see how it goes.
 
the end result will be more like a look alike 2 places Aeronca Sedan with a Bearhawk wing
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Hi Frank. I think most of us here have a real interest in aviation, building, and experimental aspects. I'd be real keen to follow your progress, even if it's not a "classic Bearhawk." There's also a wealth of knowledge lurking in these forums, previous builders, aeronautical designers, career pilots etc. Best of luck with the project. The only advice I can give at this stage is to finish the project with the same finances minister that you started with :rolleyes:
 
Franky139 said:
I don't think i will really begin building before the fall, i still fly choppers and i will be busy this summer, after serious negociations with my finances minister (wife) for the project....
Click to expand...

So, You decided on an airfoil. I think you chose a (the LSA airfoil) good one. What do you think your next step is? If you got a set of LSA plans you would get a full size airfoil printed on mylar, a book that gives broad instructions for fabricating wing and fuselage parts. If you got a set of 4B plans could see the spar web design for a higher loaded wing.

It would also give some ideas for the spar web. Depend on an aeronautical engineer for the spar design though. A longer wing will neeed more strength built into its spar web.

Two sets of plans....an engineer.... I am curious how you satisfy your Minister of Finance to overcome the hurdles this special build might require..... which takes me back to Post #36 and echoing what NEV said in #41. Life is web of interconnections! Btw, when I reserved my N-number our anniversary date was available. That was a good move! It definitely made my wife feel like it our build, not my build.
 
I'm not yet decided on the LSA airfoil, i'm looking at it because they would fit better with the spars that i have and i can always cut them down to the standard 33 feets, for the moment, i just want
to find out if a 36 feets wing would have a lower stall speed due to the longer flaps, if it's not worth it, i will probably stay with the standard Bravo wing at 33 feets.

I definitely agree with Post no 36, there's no points in rewriting the book, the thing is that i have that set of spars, they are brand new, and i would like to used them as much as possible, it would be a cost and time saving, they are more than strong enough, it is not a big deal to make the ribs to fit, no change in the cost of materiel, i just need an engineer to check the idea, theses spars were
designed for a fabric wing, there is no doubt that making an all aluminium wing with it, will turn into a much stronger wing, however, i will get it checked,
 
Becone 1381

I forgot to say, my best buddy's son is an aeronautical engineer in the Canadian forces, he will look into my stuff for a friend price.
 
Franky139 said:
Becone 1381

I forgot to say, my best buddy's son is an aeronautical engineer in the Canadian forces, he will look into my stuff for a friend price.
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I'm glad to hear you are involving an engineer. You might want to be sure he considers that the fabric covered wing would probably have had drag/anti-drag wires, which yours might not. There is much complexity to these things and the stresses of the spars, ribs, skins, and struts are all related in a system. They are not modular plug and play components.
 
Franky139 said:
I'm not yet decided on the LSA airfoil, i'm looking at it because they would fit better with the spars that i have and i can always cut them down to the standard 33 feets, for the moment, i just want
to find out if a 36 feets wing would have a lower stall speed due to the longer flaps, if it's not worth it, i will probably stay with the standard Bravo wing at 33 feets.

I definitely agree with Post no 36, there's no points in rewriting the book, the thing is that i have that set of spars, they are brand new, and i would like to used them as much as possible, it would be a cost and time saving, they are more than strong enough, it is not a big deal to make the ribs to fit, no change in the cost of materiel, i just need an engineer to check the idea, theses spars were
designed for a fabric wing, there is no doubt that making an all aluminium wing with it, will turn into a much stronger wing, however, i will get it checked,
Click to expand...

Just to let you know from the point of view of a person that scratch built the wings, the spars are only about 1-5% of the effort so far. I would think that trying to fit the ribs to a different spar might take more time than simply building the spar. The spar for the Bearhawk was optimized with both weight and strength and most likely any other spar would be some form of compromise.

if you are trying to save time and/or money you should look closely if using your existing spar would actually save you anything.
 
This has been my thought... The reason for using the spars is questionable. Just having them seems like the wrong reason.
 
N3UW

Your statement would be true if i already had a set of Patrol ribs, that is not the case.

Until my engineer come up with his evaluation, i see only good points in using my spars: 1) i have thems,
2) single piece extrusion spar (no splices)
3) time and money saving, no need to build new spars, no materiel needed
4) outboard of the gas tanks, no need to make any noses ribs, it will be single piece ribs
all the way to the rear spar ( because my spars are less taller than the Patrol ones)
5) As my spars were designed for a fabric wing, they are stronger than the Patrol ones
designed for an aluminum skin wing, definitely can't take the Patrol spars and put thems in
fabric wing as they are.
 
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