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Wing Structural Design

Old 02-16-2004, 04:48 PM
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Default Wing Structural Design

Hello, my friends.

Where and what kind of stresses can be expected on the wings of those (SAE) weight-lifting airplanes?

Thanks. Souza.
Old 02-16-2004, 04:53 PM
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CafeenMan
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Default RE: Wing Structural Design

The same stresses as any other wing. What specifically do you want to know?

If you want advice, then build a thick wing with the spars located at the thickest point of the wing. Probably 2 pairs of spars will be necessary. Use Shear webs.

What are the contest parameters?
Old 02-16-2004, 07:44 PM
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Default RE: Wing Structural Design

Off the top of my head probably the static weight of about 30 lbs for model and ballast times the maximum G load of maybe 4 or 5 G's during the emergency pullout from a dive that comes after a stall. So figure the wing has to support 150 lbs of max load spread out over the whole area.

Note that this does not mean you put blocks under the tips and then stand a 150lb kid on the center. The load is spread out over the whole span evenly. If you want to test such a load on a structure the old fashioned way was to mount the wing upside down and then evenly add sandbags over the entire surface to a thickness dependent on the local wing chord width. Keep adding until it explodes then back up one layer....
Old 02-16-2004, 07:59 PM
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Default RE: Wing Structural Design

to simulate the 150 lb total force on the wing in a static test... (for the 30 lb airplane)

support the wing on padded rails, 1/3 of the distance from fuselage to wingtip (close to the fuselage) load the center with 100 lbs of sandbags spread over the center 1/3 of the wing.

Its not anywhere near precice... but if it won't hold that... it will break in the 5 G pull.

You can calculate the way to load the wing... hanging the plane inverted by a cradle.. and distribute sandbags for a more accurate load distribution on the wing. The above is just a "quick and dirty" 3.3 G test. Without properly distributing the load thats as far as you can go on a 5G rated structure.
Old 02-16-2004, 10:09 PM
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Tall Paul
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Default RE: Wing Structural Design

I've worked at 4 of the SAE events. I can't recall seeing a wing fail in flight.. The two I do recall occurred on takeoff, but these wings had probably been damaged earlier and ineffectively repaired. (photo 1)
Normal model construction techniques with balsa, spruce, and plywood and advanced techniques with foam and composites are used equally well.
The planes NEVER manuver, their flight is so close to falling out of the sky anyway just doing the 2 180° turns a normally stressed structure will be just fine.
I suppose if you want to beat yourself up you can test the load limit, but as Ben mentions, that takes two wings. One to find the limit and the next one, don't load that high!
I seriously doubt there's any need to test the strength of a normally constructed wing, it just isn't loaded to the g-level a manuvering airplane would need.
Get together with an experienced flier and builder, and do the normal stress computations for the materials you'll be using, and it work out just fine.
The new wingspan limit places more priority on torsional stiffness relative to overall strength relative to merely not breaking under extreme load. A wing can be built to take the stress, but twist into flutter. I've seen that also. (photo 2)
The third photo shows an unlimited wing after a crash, with 400 pounds on it..It took more than that to collapse.
Foam and f.g. cloth and CF construction.
I'm presuming this is your first effort.. be a bit cautious and overdesign it. Next year, you can make changes based on your experience.
Getting a flight in at ALL can be the difference between a good placing and nothing.
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Old 02-17-2004, 12:11 AM
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Default RE: Wing Structural Design

Actually, this is going to be my second year on the Brazilian SAE competition. 2004 rules: cargo bay 4x5x6in, max wing span 6ft, t.o. within 200ft...
I'll graduate this year on Mechanical Engineering and I'm familiar with the structures fundamentals - moments of inertia, etc. I did not want to make my first question too long ...sorry!
Rephrased: a real example, our airplane last year (photo 1) had a big all-moving horizontal stabilizer (E423 inverted airfoil, foam-balsa contruction) - and it pivoted around its aluminum tube spar at 1/4 mac. With all the rush, we simply did not provide it with a means to transfer the airloads from the skin to the spar. After the first flight the foam was crushed where it contacted the spar, causing a lot of play there...
That's what I mean - I mean, ask... on this example, if the stab. was of an all balsa rib-spar contructution, how many ribs and how thick would the ribs have to be? What about the wood grain orientation? How far would the spar have to go each way?
Another perspective: If I was to make a wing skin out of fiber glass, for example, what would be the best orientation of the fibers? Should they be layered 0-90, 45, mixed? Why? What about the ribs - are their spacing dictated only by skin buckling? What kind of load do they have to carry? I have heard of wings that collapsed up and foward under extreme high angle of attack and lift coeficients...
Thanks.
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Old 02-17-2004, 12:12 AM
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Default RE: Wing Structural Design

Ooops,
a better picture...
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Old 02-17-2004, 11:17 AM
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Default RE: Wing Structural Design

BMatthews,

If you're designing kind of close to the limit, a uniform load distribution might be a bit harsh. Ideally, the spanwise lift distribution will be elliptical, and since the wing will be highly loaded, the designers should strive for that to reduce induced drag. In order to have some safety margin, a distribution between uniform and elliptical might be prudent.

banktoturn
Old 02-17-2004, 12:50 PM
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Tall Paul
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Default RE: Wing Structural Design

On your flying tail, I've used 1/4" aluminum tube 12" long for the axle, with a 3/16" wood dowel inside the tube for the full span of the stabilizer.
You need something to continue carrying the load past the end of the axle tubing, otherwise the surface might break right there. I had that happen on my Eindecker.
For my 2003 plane, made just to make it, it uses this type of structure...
and in flight..
The axle stops where the triangular gussets are on the vertical spar.
It's a little loose in the mounting, but that hasn't been a bother.
Uses a standard Sullivan snake to push-pull on the horn which is inside the vertical.
With the 6 foot span.. SAE changed to 120" for 2004 here in the states, ordinary construction practices work well, because there's no need, or even any way, to have a high-aspect ratio wing with sufficient area unless... you go biplane!
The 2003 winner used balsa all the way in the wing with CF spar reinforcement.
Ribs and spars notched for each other to make a very rigid (but fragile)wing. This picture shows the internals... AFTER the winning flight, when another attempt with more weight failed due to the extremely poor landing area away from the runway..
The innovative winner was a biplane, span loader. I don't know the construction details, but the ballast was 1/8' x4"x36" steel plate.
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Old 02-17-2004, 12:58 PM
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Tall Paul
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Default RE: Wing Structural Design

The horizontal on the 2003 SAE Aero Design West winner.. simple balsa structure, inverted airfoil.
The second photo shows the first takeoff attempt by the innovative winner.. note the boom flex, AND the horizontal flex!
The plane hit the catch fence and was not damaged.. the boom was wrapped with CF tape to stiffen it, I believe and the plane behaved properly after.
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Old 02-17-2004, 03:44 PM
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Default RE: Wing Structural Design

ORIGINAL: Souza

.... With all the rush, we simply did not provide it with a means to transfer the airloads from the skin to the spar. After the first flight the foam was crushed where it contacted the spar, causing a lot of play there...
I suspect that the failure to provide vertical webs to join the inner tubing to the skins was your only error and a good example of how much loading there was.

For a new composite wing I would say that 0-90 would probably be fine but ideally the orientation of the fibers should be 45-45 to best provide torsional rigidity from a minimum of glass and epoxy. But such will not have great load carrying capacity so there should be a tapered carbon or other material spar out to the 3/4 wing position. The tips can then depend on the skin for the loading from that point. The carbon upper and lower spars must be boxed or webbed together to prevent the foam from being loaded too far. The wing joiners can be located between the upper and lower spars but need to be bonded properley to the spar caps so that the bending loads of the joinder can properley be transffered to the tesion and compression loadings of the I or box beam spar system. For sailplanes and other projects that I have worked on this has been done using a mix of epoxy and microballoon filler to create a strong but light putty that I pack in between the spars and joiner slip tube. The putty acts and the webbing at these root points but I then box it in with front and back webs to keep the putty in place as it cures.

B2B, I may have been a bit over the top with my 150 lbs of sandbags but I can assure you that if the wing survived that it wouldn't break in flight... I chose 5 G's as a figure because I suspect that if one of these models stalls and noses in the pilot would need to pull full back to avoid crashing in the low altitude these models generally fly at. A full up pullout would probably generate something in the order of 3 or 4 G's?????? So 5G's would provide some safety factor????? It was all a guess, an EDUCATED guess I would hope but a guess all the same.
Old 02-17-2004, 11:13 PM
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Default RE: Wing Structural Design

Just for kicks, the new spar for our 24 foot unlimited SAE heavy lifter weighs 8 ounces, tip to tip! 5 g load factor carrying 60 lbs plus 12 lb emptyweight. The secret is Graphlite! Amzing stuff. Cheers,

Daniel

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