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Nov 27, 2021, 01:15 PM
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Discussion

Airfoil integrity.


I'm some way off building my next model, but a kit with built up wings is a possibility.
If the airfoil shape has under camber, and I cover it with heat Shrink like Oracover, then the designated airfoil section is only true at the ribs.
Inbetween, a more flatter airfoil presides.
So I've got 2 questions relating to this.

1. Does this mean that the difference between 2 different airfoils with under camber would be not as it should be?

2 Has anybody removed heatshrink/tissue, replaced with balsa sheet, and found an improvement or otherwise?

The last time I covered using solarfilm was 40 years ago, when I was very young, so it might be that I wasn't good enough to get a perfect shape to keep the airfoil integrity, it's just that that's how I remember it.
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Nov 27, 2021, 01:48 PM
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mhodgson's Avatar
You might be surprised at how consistent the section is aft of the spar, whether undercambered or not, when film covered. Most 'distortion' occurs forward of the spar where the curvature is steeper.

As with anything in design, whether you sheet it with wood is a compromise. The sheeted section should be more accurate but it will be heavier. So for that model does the gain in one outweigh the loss in the other.

If the design you choose has an open structure behind the spar then I would suggest leaving it so. Sheeting it will gain you little if anything.
Nov 27, 2021, 02:01 PM
B for Bruce
BMatthews's Avatar
This applies to the top of the airfoil as well as the bottom. The amount of shrink related "tenting" depending on the amount of the curve and rib spacing.

Since airfoil sections with undercamber are typically not very curvy on the lower side the distortion between the ribs and any lower side spars is going to be pretty minimal. I'd worry a lot more about the "tenting" factor on the upper side. And that is certainly a place where having some supporting sheeting can pay off. You stand to gain benefits by being lower in drag for the high speed side of things and also to delay the stall at the lower speed side of things. I don't know of many that have altered a wing once built. But there is a fairly recent test where two particular examples might shed some light on what you're wondering.

For File See Post Below

In this article where actual wind tunnel tests were run we see a case where one airfoil, the AG35, shows up in both sheeted and unsheeted but with half ribs. The results of the test between these two options is highly interesting.

The sheeted version has a strong benefit at the higher lift coefficients provided it is flying fast enough to keep the Reynolds numbers up. At Rn = 100K and higher it's a very clear winner.

And even as things slow down and we go towards the Rn values below 100K the sheeted option is still the winner until the very low Rn value at 60K where I'm going to guess that the turbulence off the edges of the ribs is providing some sort of turbulation. I suspect that the sheeted AG35 could be given a turbulator strip and achieve better performance even at the 60K value where it shows some signs of separation bubbles causing the lumpy looking drag penalty on the 60K line.

So yeah, upper leading edge sheeting that extends back to around 40 to 45% of the chord is good. But no need to use extensive sheeting on the lower side of an undercambered airfoil.
Last edited by BMatthews; Nov 27, 2021 at 05:34 PM.
Nov 27, 2021, 02:31 PM
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Thread OP
Thanks for both your speedy replies.
From mhodgson, sounds as though the best thing to do is just to build as per the instructions. If sheeting provided any benefit, the designer would have thought of that.
Bmatthews, a very comprehensive reply, but I couldn't open your link. I think it's an answer to remember, and ensure that a future model has sheeting at the top.
Nov 27, 2021, 02:34 PM
Balsa breaks better
Thermaler's Avatar
Quote:
Originally Posted by BMatthews

file:///C:/Users/grapp/Downloads/Wind%20tunnel%20study%20for%202m%20RES%20class_v2% 20(2).pdf
Bruce,
If we could access your computer we could read this file . . . ;>)

Most sag or tenting happens forward the spar on wings that do not have a D-Tube section.
What happens behind the spar on a well constructed wing is of little importance or consequence on a built up wing.
The gains or penalties are negligible at best.


Joe


Balsa Breaks Better
Woodys Forever
Nov 27, 2021, 05:36 PM
B for Bruce
BMatthews's Avatar
Sorry, it's this most recent version of Edge. It downloads files and then opens them from the downloads directory in the new browser tab. Annoying it is....... I keep thinking I'm still looking at the online file.

You can download and see the PDF report yourself from the download link at the bottom in the first post in THIS RCG THREAD.
Nov 27, 2021, 08:20 PM
Balsa breaks better
Thermaler's Avatar
Quote:
Originally Posted by BMatthews
Sorry, it's this most recent version of Edge. It downloads files and then opens them from the downloads directory in the new browser tab. Annoying it is....... I keep thinking I'm still looking at the online file.

You can download and see the PDF report yourself from the download link at the bottom in the first post in THIS RCG THREAD.
Gad zooks! I do the download and by some miracle I put it in the folder I downloaded it Lord only knows when.
Then I read it and say gad zooks again! It is a great article.

Now about the Edge problem. Welcome to the wonderful world of some college kid knows what you are doing. ;>)


Joe

Balsa Breaks Better
Woodys Forever
Nov 28, 2021, 04:28 PM
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gliderguide's Avatar
Very interesting thoughts. Great article. Itís good to see a mind free range and then answers pop up. I really think this is the spirit of RC right there!!

I would say build as designed. If you chose to sheet, you would need to modify the ribs to keep integrity in the foil, add weight and increase build complexity. Plus, in my experience, unless itís scale with a HQ reflex or similar, undercamber generally indicates slow flight and high lift. Adding weight would be counterintuitive I think.

Whatís the design?
Nov 29, 2021, 03:28 AM
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Thread OP
Quote:
Originally Posted by gliderguide
Very interesting thoughts. Great article. Itís good to see a mind free range and then answers pop up. I really think this is the spirit of RC right there!!

I would say build as designed. If you chose to sheet, you would need to modify the ribs to keep integrity in the foil, add weight and increase build complexity. Plus, in my experience, unless itís scale with a HQ reflex or similar, undercamber generally indicates slow flight and high lift. Adding weight would be counterintuitive I think.

Whatís the design?
Thanks for your very kind comments.
The kit I'm looking at is the cad2cnc tango, with the arthobby adventure (all sheeted) as an equal contender.
Tango airfoil is HQ30/11, which has undercamber, but no D box.
My conclusion from the above, is that so long as you have a D box, sheeting will provide minimum benefits, and would add weight needlessly.
Nov 29, 2021, 04:41 AM
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mhodgson's Avatar
Sheeting becomes more important the faster you fly, both in terms of accuracy and strength (torsional as well as spanwise).

Either of the 2 models you name should fly well.
I suspect the Tango will be better in calmer conditions yet the Quabeck sections (which I like a lot) have a good speed range, even at that camber/thickness.
Nov 29, 2021, 08:51 AM
Registered User
Mr_SMO
It is worth pointing out that the amount of static sag between ribs is not quite the same when in flight. About 2/3 of the upper surface is below atmospheric pressure and most of the under side is above. It has to be to create the lift.
How much this might improve the aerodynamic section is open to question but at most model speeds any effect is going to be pretty minimal.
The only exception can be the first 1/3 of the upper surface where any static sag would be accentuated by the positive pressure resulting in a relatively sudden change in profile where the pressure changes to negative. This is why the wing leading edge may be sheeted in some way, particularly the upper surface.
Of course full size gliders now universally have moulded wings so the wing section can be very accurately controlled root to tip.
Nov 29, 2021, 12:45 PM
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Thread OP
Thanks Quorneg for another interesting contribution.
Bearing in mind all the replies, it seems to me that the order of preference would be :-

1. A fully sheeted wing. (just)
2. A built up wing with a D box
3. Totally unsheeted wing. (some way behind.)
Nov 29, 2021, 02:02 PM
B for Bruce
BMatthews's Avatar
If you're open to options other than balsa then I'd rank vacuum bagged foam as first. Then your three options.

If you haven't read the reasoning behind Mark Drela's Allegro Lite wing design then it's a worthwhile read. There's a very good reason for adding a small but significant amount of chord width to the upper D tube sheeting on a lot of wing designs.

One construction method I've wished for some time was tested in a real wind tunnel to obtain real world results is a multi spar or multiple "turbulator" spar wing that uses spanwise stringers to support the covering. So far no one has done such a test. I, for one, would be very willing to build the test wing for any such test. At first glance it's an easy to build wing which is also light. A graduated spacing of the stringers so spacing is reduced where the airfoil's curve is the greatest could limit the amount of geometric tenting due to the shrinking of the covering.

Would it be as good as the extended D tube option? I seriously doubt it. But I would love to see how it compares to the full and half rib combination used on some designs.
Nov 30, 2021, 03:37 AM
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mhodgson's Avatar
Quote:
Originally Posted by Mr_SMO
Thanks Quorneg for another interesting contribution.
Bearing in mind all the replies, it seems to me that the order of preference would be :-

1. A fully sheeted wing. (just)
2. A built up wing with a D box
3. Totally unsheeted wing. (some way behind.)
Yes, and no.
Really depends on the wing section chosen and the purpose/type of flying the model is expected to perform.
Dec 03, 2021, 09:31 AM
Registered User
On my second set of Sagitta 600 wings, I extended the top sheeting a little to reduce sag. While it might just be placebo effect, I thought it was better. Then again, I might have done something else right. I was poking around with Xfoil a while back, and it seemed that the E205 benefited from a trip at 50 0r 60 percent. Maybe the back of the sheeting acted as a trip. I forget whether I was using Reynolds numbers typical of the 600 or the 900. Your airfoil may vary.

If I was building an open structure wing with a high aspect ratio, I'd seriously consider something to stiffen up the inner panels in torsion. Maybe some diagonal bits in between the ribs to make a box with the spar. Or heavier covering. If I thought I wouldn't be going super fast at times, and wasn't going to winch launch it, maybe I wouldn't worry.


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