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Nov 18, 2019, 06:47 PM
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R2R's Avatar
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Effect of Airfoil Inaccuracies on Performance

Ever wonder how much of a difference it makes in how accurate you are (can be) with your airfoils when building a sailplane wing? Check out this issue of RCSD page 14.

As some on this forum have mentioned before, it doesn't take much of a divergence from spec to change the performance.

Also a great write-up of the Dodgson Saber 121" sailplane.
Last edited by R2R; Nov 18, 2019 at 06:54 PM.
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Nov 18, 2019, 07:36 PM
turn, turn, turn.
I never worried about it too much.
Nov 18, 2019, 08:42 PM
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Gratter's Avatar
Considering most kits come with “modified” airfoils worrying about an exact airfoil is a moot point most of the times.
Nov 18, 2019, 09:00 PM
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I remember reading about it when that article came out. There was a significant level of debate as to the effects. Then it was discovered a lot of kits flew just fine with airfoils that were "off".

Some designers have a knack for the total airplane being greater than the sum of the parts. Dodgson was certainly one of those.

Dodgson E214 airplanes benefited significantly from having a trip strip placed on them.
Nov 19, 2019, 01:57 AM
Registered User
I think it MIGHT be time to redo the covering on that wing. Or maybe it's just me? You might try to find some masking tape that's a better match. ;-)
Nov 19, 2019, 03:46 AM
Registered User
How do you get that many tears in a covering?
Is there a cactus on your on your landing strip,
Or is it just a brittle covering?

I suggest clear packing tape.
Nov 19, 2019, 06:48 AM
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WimH's Avatar
Originally Posted by kenny sharp
i never worried about it too much.
Nov 19, 2019, 08:09 AM
Barney Fife, Vigilante
tom43004's Avatar
There can be large performance degradation from not building correctly. Sadly a lot of pilots won't ever notice though because they just move on to an airplane that performs better.

Back in the days this was written, a lot of the "debate" was over rib and covering construction versus bagging and molding. Rib and covering does not do a good job at all of reproducing the actual sections. Bagging is better by an order of magnitude for many / most airfoils. Molding is better than bagging by a smaller but still noticeable margin.

"Modified" airfoils aren't necessarily inaccurate, they're just different than the original. Some modified airfoils are very accurately produced and perform better than the original for certain flight modes. Some not so much. It really depends on the designer and choices they make during the design process.

A bad example of modifying a section would be marrying the top polars of one airfoil to the bottom polars of another. Some designers did this to try to find more traveling ability or straight line speed back in the early days when many airfoils were mostly flat bottomed. *cough Antares* Those planes moved faster (design goal achieved) but did so at a HUGE sacrifice of thermal performance.

A good example of modifying a section would be taking a section like the 8020 as originally published, and parametrically re-cambering or changing thickness to eliminate deadband to use it as a horizontal stabilizer section. That made an otherwise not terribly useful section somewhat exceptional for a lot of applications.

Back in the early 90s, I was on a XC soaring team that built a new ship based on the SD4061. We thought it would be great. It wasn't. Honestly it was a total dog. In desperation, we sawed off some of the aft chord leaving a quite blunt trailing edge by most standards. That should have ruined the ship for good... and honestly we didn't care since that ship had been relegated to backup duties behind a much less sophisticated (and slower) airplane. That modification turned an otherwise terrible XC ship into a very good one. The plane moved better, carried energy better, and climbed better. Win Win Win. Obviously we had made mistakes in the original construction and what we ended up with probably didn't resemble ANY published sections, but it was a great flying airplane in the end.

Some sections are REALLY critical however, and building them correctly can be challenging, even for accomplished builders. The 214 as noted above was very easy to get wrong. The undercambered lower aft area could be accidentally flattened slightly or even worse could have a wavy distortion caused by bagging techniques... that pretty much ruined it. The SD4061 was very similar. Other airfoils in the modern era are even more critical because a large amount of their lifting capability comes from the very last couple percent of chord at the trailing edge. Molded construction is really the only way to extract the designed performance.
Nov 19, 2019, 10:35 AM
Registered User


Back in the long ago days. We built several Aquilas, (1970's) with stock airfoils, modified airfoils and crappy "snaggle-tooth" airfoils. We even launched two or three ships at the same time in same air. We traded ships and flyers to try to get similar sets of evidence, etc, etc!

We used airfoil patterns (like ramps for foam cutting) to keep as close as possible to the stock foil. Then it became very obvious that the covering between ribs sagged and we had a different set of airfoils between the ribs. So we sheeted farther back. Well so, it was obvious that a fully sheeted wing was needed, not just LE to spar.

The only mod that we found to really make an obvious difference ( YES, conditions were never the same exactly) was Phillips Entry. Raised LE with a more upswept LE rib bottom.

I suspect that the molded wings built in machined Aluminum molds come pretty close to the most accurate.

Oh I forgot about Reynold's Numbers... for such small chords....low aspect ratio,....scaling effects, etc

IMHO, Unless you are in the top 50 flyers around the world, slightly imperfect airfoils are not going to make much of a difference... Didn't a very famous flyer beat the crap out of VERY fancy and expensive ships/guys with a stock "gentle lady" type ship. It's the flyer and PRACTICE< PRACTICE, PRACTICE!!!

[QUOTE=R2R;43197865]Ever wonder how much of a difference it makes in how accurate you are (can be) with your airfoils when building a sailplane wing? Check out this issue of RCSD page 14.
Nov 19, 2019, 06:45 PM
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R2R's Avatar
Thread OP
I too do not worry about it too much when building!
One of my favorite birds (BOT) uses an improvised airfoil, IIRC.
I know I can't tell a difference between a 1 point difference in L/D.
Nov 19, 2019, 09:21 PM
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mlachow's Avatar
Even molded models have minor differences. It is easy to mess up a leading edge. Some trailing edges are a little fuzzy. Scoring at the hinge line can have some edges.

Over the years I have had 3 or 4 F3b models, all the same, three or four times. Each time after hundreds of flights, I would often find a favorite that just seemed to perform a little better. It wasn't the red one.

I did some tests flying in wet weather, testing different surface treatments. Nothing like a few rain drops to change an airfoil. BTW, a coat of Jet Dry dishwasher rinse agent was one of the better solutions. A thin sheet of water on the wing is a more desirable airfoil modification than nice big beads of water.
Nov 20, 2019, 11:59 AM
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Avaldes's Avatar
Might also remember at the time of writing this article, it was FAR more popular to build your own models than today. I used to love reading RCSD back in those days! That was fun looking at that scan. In fact I just saw my Selig wind tunnel test t-shirt the other day
Nov 21, 2019, 12:41 AM
Registered User
If the Antares airfoil was so terrible, I must have found the right mod. I cut 80 ribs traced from the Antares root rib* and made a two meter with them. It was ugly, of course, with that Hershey Bar wing, flat center section, and raised tip sections. However, it flew fairly well. I was complimented on how well it floated once. It was fairly good in the wind, too.

Somewhere, there's info on a test a guy did with F3-RES wings that seemed to indicate that, at moderately low Reynolds numbers, d-tube sheeting didn't really help. Of course, that might just mean that those airfoils had fat noses.

Fritz Bien once told me that he compared the recorded shape of the 7037 test model from the Princeton tests to the nominal 7037 and the 3021. He said that the model was closer to the 3021. (Maybe with a bit of flap? Can't remember for sure.) The nice results from the tests lead, at least in part, to the popularity of the 7037! If you look at the recorded coordinates from the test models and compared them to nominal airfoils, average differences of .015 inches and more were common. In cases where two models of the same airfoil were tested, drag differences of 10 percent weren't unusual. That would probably be hard to notice while flying except with a stopwatch. OTOH, those differences in drag might have been differences in the instrumentation or the wind tunnel ambient turbulence rather than differences in the airfoil.

I once built new wings for my Sagitta, with the sheeting extending a little further back. It seemed to me that it flew somewhat better. Ditto when I ironed out wrinkles in the covering. How much of that was real, and how much placebo effect, I don't know.

*I retrieved, or perhaps I should say de-treed an Antares for a guy. It was very banged up and I ended up with the root rib.
Nov 21, 2019, 08:19 AM
Mark LSF # 3792
I remember in the early eighties reading some articles written by Dick Johnson testing the performance of full size sailplanes. During their testing they discovered that even dead bugs splattered on the leading edge of the wings affected their glide test readings!
Nov 21, 2019, 09:07 AM
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whacker's Avatar

Start with a Good Set of Machine Cut Ribs

BUild wings accurate especially watch the symetry / mirror from left to right

That will outweigh most if not all inaccuracies when it comes to accurate airfoils.

Especially if you fly


in the Drizzy Waughs River Valley

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