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

[rv8bldr]

Stuff deleted....

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

Nick

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.

[nborer]

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

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

[rv8bldr]

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....

[Frank]

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?

[rv8bldr]

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

[jaredyates]

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.

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.

[Mark Goldberg]

The ribs in the kit conform to the mylar. MG

[v.x]

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.

[davzLSA]

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.

[nborer]

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

[davzLSA]

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.

[Frank]

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

[nborer]

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.

[davzLSA]

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/ex...iation-trials/

[Frank]

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.