Thin wings?

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skier

Well-Known Member
I was just looking around at the Quickie Q-1 and found that its MAC is around 1.4 ft. If you assume a 12% thick airfoil, this would make the wing only 2 inches thick. Is it feasible to make a sheet metal wing this thin?

WonderousMountain

Well-Known Member
Sure, go for it.

2 inches (5cm) is common tube size, you could go 3 inches with 18% airfoil. Easiest way would be to have a D-tube extrusion [large streamtube front half?] and ribs following.

Bending sheet metal is proportional to it's gauge. Double the gauge and the induced stress from airfoil curvature quadruples, doubles? This means you may reach a point where the leading edge radius is too much to provide structural support. This also has a benefit, with small distance between cells, and more curvature the wingskin will be less likely to buckle.

Have fun small wing = small problem

If you could build it with fewer rivets there might be a reduction in build time.

Jay Kempf

Curmudgeon in Training (CIT)
I was just looking around at the Quickie Q-1 and found that its MAC is around 1.4 ft. If you assume a 12% thick airfoil, this would make the wing only 2 inches thick. Is it feasible to make a sheet metal wing this thin?
Yup, there are some VERY high speed aircraft with VERY thin and stiff wings. You need to spend a lot of time stiffening the skin and put in a bunch of redundant spars. Which means a riveting or bonding strategy has to be worked out to make it possible.

bmcj

Well-Known Member
Yup, there are some VERY high speed aircraft with VERY thin and stiff wings. You need to spend a lot of time stiffening the skin and put in a bunch of redundant spars. Which means a riveting or bonding strategy has to be worked out to make it possible.
I.e. - the Cri Cri

Jay Kempf

Curmudgeon in Training (CIT)
I.e. - the Cri Cri
The Cri Cri is not what I meant. That has I think an 18% wing and I wouldn't consider it fast. I was talking about the F104 and that ilk with razor blade wings. Many spars and many top hat shaped skin reinforcements.

bmcj

Well-Known Member
The Cri Cri is not what I meant. That has I think an 18% wing and I wouldn't consider it fast. I was talking about the F104 and that ilk with razor blade wings. Many spars and many top hat shaped skin reinforcements.
Sorry, I meant to use the Cri Cri as an example of a narrow chord sheet aluminum wing in a similar size category as the Q-1.

wsimpso1

Super Moderator
Staff member
Log Member
Why are so many people hung up on thin wings? Remember that your drag is comprised of 2D wing drag, 3D effects, interference effects, landing gear, cooling drag, and excrescence drag. There is virtually zero pay value in subsonic airplanes for thin foils. First, 12% is the thickness where you quit getting any 2D drag benefits at all. Next, you get only tiny increases in 2D drag by going up as high as 18%, and your structure can be simpler and lighter, reducing induced drag which can largely compensate for the form drag increase, and will disappear in the rest of the effects in a finished airplane anyway. As an example, I will cite the Boomerang, with 17% thick wings from root to tip. One of the fastest 400 hp airplanes you will ever see.

Once you get into tiny birds, with small chords, it can get heavy indeed trying to get enough bending stiffness and strength into a 12% thick wing. 18% gives you roughly half the spar weight at the same strength, and is much simpler to build and assemble. Depending upon the chord and aspect ratio, you may even want to go as high as 21%.

Next up, build a spar out of something other than a tube. To be strong enough inboard, tubes have to be pretty beefy, and then you end up with a lot of useless weight outboard. Built up spars allow you to taper the webs and caps, and be much lighter. Really.

Billski

Himat

Well-Known Member
Possible but why?
Thin wings are for supersonic flight or model airplanes. At light airplane reynolds number a thicker wing work equally well or better.

cluttonfred

Well-Known Member
Wittman Tailwind and related Nesmith Cougar have quite thin wings, about 8-9%. The tradeoff in spar weight or complexity can be worth it--the Tailwind is a remarkably efficient design.

SVSUSteve

Well-Known Member
Wittman Tailwind and related Nesmith Cougar have quite thin wings, about 8-9%. The tradeoff in spar weight or complexity can be worth it--the Tailwind is a remarkably efficient design.
Correct me if I am wrong but didn't Wittman die in a crash due to flutter in a Tailwind?

SVSUSteve

Well-Known Member
Wittman Tailwind and related Nesmith Cougar have quite thin wings, about 8-9%. The tradeoff in spar weight or complexity can be worth it--the Tailwind is a remarkably efficient design.
Correct me if I am wrong but didn't Wittman die in a crash due to flutter in a Tailwind? Wouldn't a thin wing be more likely to flutter.

EDIT: Yes, but it was due to improper fabric bonding. My apologies. ATL95FA092

dcstrng

Well-Known Member
it was due to improper fabric bonding. My apologies. ATL95FA092
Yep – regrettably even the masters can get bit by inattentiveness…

That was Wittman’s O&O Special (Sort of a long-range Tailwind on steroids; O&O = Oshkosh to Ocala …). This topic was understandably batted around among the Tailwind folks – As best as I recall, by the time of the delamination, the fabric/wing attachment was almost 20-year old, so although the chain of events that led to catastrophic failure was real enough, it took years to actually develop… regrettably some of the old hands remember someone noting the start of the delamination (just a bubble at the time), and bringing it to Steve’s attention – he apparently said he’d fix it soon… The thin section of the wing had little or nothing to do with it and although the computer says the stall should be pretty exciting on the Tailwind section (see Schuck’s analysis on this…), the 3D combination of the Tailwind wing and the Tailwind fuselage is not noted for particularly breathtaking stalls…

Aviatorzaki77

Member
It'll stall like a guillotine who's lost it's guide rail, very, very bad.
Thanks For analyzing and I made some changes,Kindly advise on this,I would like to design a All metal single seat Aircraft high wing tricycle gear Tractor engine,I have got access to CNC machine,Aircraft will have wing span of 5.4mts and 1.3mts chord,To minimize stall it got 60% span flaps 30% of chord,Total aircraft weight 60Kgs Including ZDZ360 Quad engine drirect drive 36' 16' three balde carbon,Including Me and 20litrs fuel TOW will be 140kgs,wing loading is 20kgs sqm,

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cluttonfred

Well-Known Member
Short answer...while I applaud the spirit of wanting to design your own plane, I am not so sure that you yet have enough knowledge to risk your life or anyone else's life flying such a creation. There are quite a few proven designs that you could choose from to suit your needs. Some, like the TEAM MiniMAX series, are available as free plans from the manufacturer.

Longer answer...your weight numbers appear very unrealistic keeping in mind that the maxium empty weight of the American Part 103 ultralight regulations is 115 kg. Your engine (a 35 hp UAV/Giant Scale Model engine) is not cheap and I don't have any ideaof it's reliability. You could put together an ordinary ultralight with a proven engine for less. As for your airfoil, there are certainly some designs that have flown well on homemade airfoils, but you might want to look some airfoil reference materials to find a known airfoil similar to what you need. Also, such a thin section is probably a candidate for extensive wire bracing and you'll probably find that a thicker section would maker for an easier and cheaper build--turnbuckles are expensive!

BJC

Well-Known Member
Doc Brokow's Bullet is one of my favorite homebuilt designs. He used a 9% thick airfoil, but I don' remember which one it is.

http://www.airbum.com/pireps/PirepBJ-520.html

BJC

cluttonfred

Well-Known Member
86 mph stall speed, ouch, not quite LSA is it? I think that's higher than the cruising speed of a Volksplane. ;-)

BJC

Well-Known Member
86 mph stall speed, ouch, not quite LSA is it? I think that's higher than the cruising speed of a Volksplane. ;-)
Definitely not a LSA, with a wing loading of almost 39 pounds per square foot.

He went through several engines, with the final being a Walter Turbine. He flew the Bullet over 2,000 hours.

It is now in the museum at Sun-n-Fun.

BJC

BoKu

Pundit
Getting back to the OP, when I do a quick scaling off of the Quickie plans, I see that the main wing has a side-of-body chord of about 23" and a tip chord of about 15". For the section thickness, it looks like the main wing is about 15%. That gives it a root depth of 3.45" and a tip depth of about 2.25". So, yes, kind of shallow, but not exceptionally so. Given that the lift is distributed among four surfaces, the stresses in the extreme fibers are going to be reasonable.

As for whether is practical to make wings like that in sheet metal, I'd have to give a qualified yes. Since the bending stiffness of a beam scales with the cube of its depth, with a thin wing it becomes important to get as much structural meat as far from the neutral axis as practical. I-beam spars are well suited to that, as are box spars. There are good examples of both in the canon of Dick Schreder's HP series metal sailplanes.