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

[NCLSA183]

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 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…


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

[Sir Newton]

I for one enjoyed your analysis. It never took a half hour to read either.

[NCLSA183]

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.

[NCLSA183]

Darn... I wish I had said "Riblett Hole"...

[Frank]

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! 😀

[Frank]

Still to decide whether to joggle or not.

[NCLSA183]

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

[Frank]

fullsizeoutput_10ed.jpeg

[NCLSA183]

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.

[davzLSA]

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.

[NCLSA183]

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

[rodsmith]

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.

[NCLSA183]

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

[NCLSA183]

You never know. Stranger things have happened...

[Bcone1381]

Dave read my mind.

[NCLSA183]

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.

[svyolo]

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.

[NCLSA183]

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

[NCLSA183]

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

[Frank]

Go for it Mitch! I look forward to seeing your work! What tools and methods do you plan for forming the ribs?

[NCLSA183]

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

[NCLSA183]

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>

[NCLSA183]

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

[Bcone1381]

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.

[NCLSA183]

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

[Mark Goldberg]

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

[NCLSA183]

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

[svyolo]

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.

[NCLSA183]

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.

[Mark Goldberg]

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

[NCLSA183]

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

[jaredyates]

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.

[NCLSA183]

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