e=mc2

Hello all,

I am working on a simple low wing trainer design and had some questions about the stressed skin wing design that Andy Lenon describes in his design book.

I was very surprised to see that he does not mention a spar doubler anywhere. More so because most of his designs are rectangular planform, which will impose greater loads at the wing root than a tapered planform would.

Is the fully sheeted stressed skin design responsible for taking all of the bending loads?

I could see how the stressed skin design would help take some bending and torsion loads, but I am not totally convinced that it is a replacement for a spar doubler.

Could someone shed more light on this?



Thanks
e=mc2

Tall Paul

e, wing thickness alone can eliminate the need for doublers.
Much more than 12%, a decent set of spars and shear webs can handle most flight loads.
Andy uses thick wings.

e=mc2

Tall Paul (I almost called you TP),

Thanks for the reply.

I am using E197, which is a 13.5% section.

I think I might have to use a spar doubler purely due to the wingspan (68" wingspan). Andy's planes are smaller than that and he can probably get away with not having doublers.

Any thoughts?

e=mc2

Tall Paul

If the landing gear isn't in the wing, a normal doubler-less construction will work just fine.
However with landing gear in the wing, the landing loads at the wing root will be high, and a ply dihedral brace wouldn't be out of place.

CafeenMan

On wings less than 6' I almost never use spar doublers. Either I build one piece (flat) wings with shear webs or join the two halves with fiberglass cloth. I've never had a wing fold ever.

Now if the wing were exceptionally thin (less than 11% maybe) then I would consider a spar joiner.

Mike James

Just adding my agreement to the above answers...

Almost everything we build is overbuilt. Of course you can "prove this wrong", by yanking a plane around hard at full throttle, landing hard, handling it roughly, etc., but for typical flight loads, most of what we all do is way over-engineered.

The only real proof that a part meets your requirements is to test it to destruction, and this is a worthwhile test, if you plan on marketing your designs. On the other hand, if a 6 to 12-inch long spar doubler can give you piece of mind, then just do it. We're probably talking about less than an ounce here, so it's not that critical for most designs.

It's like the issue of glassing the wing center or not. Many of us will do that, even when it's not recommended by the manufacturer. Old habits (and old "bad experiences" ) die hard.

e=mc2

Thank you all for the replies.

I think I have a better understanding of this thing now.

Considering all of your responses I have decided to keep the spar doublers in addition to a ply dihedral brace. (maybe I will call it the flying tank [sm=lol.gif])

My reasoning:

1) I have the landing gear in the wing (not decided where yet, but it might turn to be more outboard than most designs (wider track and I think it looks better that way))
2) I am a horrible pilot and I would like this plane to survive some bad landings.

Once again, thank you all for the insight!

Regards,
e=mc2

kdheath

Aspect ratio has quite a bit to do with this, too. At 6:1 the doublers are probably that: extra weight for not much gain in bending strength. At 12:1, however, better add doublers and think about some spar caps.
Kelvin