bkdavy

My Feedback: (1)

I'm bringing my own design to life, and am looking for advice on which material would be best suited for my wing ribs. Its a symmetrical tapered wing, 15.25 inch chord at the root, 8.25 inch at the tip, with a 1.75 inch taper on the leading edge. The plane is a mid wing design, and I'll be using a wing tube. The wing is designed with 12 ribs over a 30 inch span on each side. The inner four ribs will be 1/8" plywood to take the stress and contain the wing tube. For the remaining 8 ribs, my plan is to use 1/8 balsa, but I'm wondering if 3/32 or even 1/16 balsa would be sufficient? The wing will have 1/4" square hardwood spars top and bottom, with 3/32" shear webs between each rib. The wing tube will be 1/2" OD aircraft aluminum.

Overall the plane is designed for a .61 two stroke or a .91 Four stroke, with a final weight target in the 6-7 lbs range.

Any suggestions?

Thanks
Brad

Teachu2

My Feedback: (133)

Cap strips on the ribs?

bkdavy

My Feedback: (1)

Hadn't thought about cap strips, but that might not be a bad idea with 3/32 or 1/16 ribs. With 1/8, I'd skip the cap strips.

John 38

I have an aerobatic 60" plus with a 90 4 stroke up front and it has a 1.25" alloy wing tube bought independantly of the kit, which came without the tube. I have heard that the 1.25" tube as supplied with the model could not take the strain and was bending. Any models I have seen with wing tubes have all been greater than 1/2"
If your model is aerobatic at 6/7 lbs, you may need to rethink 1/2" ally
john

aerowoof

basically there are no loads in the ribs their only purpose is to give shape to the wing..there are designs that use a solid spar with cap strip type material bent over it and attached to the l.e and t.e .

bkdavy

My Feedback: (1)

The wing tube has already been load tested on an 8 lbs aerobatic plane. When that plane pancaked into the ground, the landing gear completely flattened, the fuselage snapped in half, but the wings could be reused, and the tube is in pristine condition. The wings on that were foam cores (self cut from pink fan-fold). I'm basing this design on that plane, but trying to cut back on the weight by using balsa construction.

I may think eventually about replacing the aluminum with a CF tube.

Sounds like I can easily get away with the 3/32 ribs. I'll post picks in another thread once I actually start construction.

Thanks,
Brad

flipstart

3/32" ribs should be fine for that size wing-with cap strips-even with 1/8" ribs I would use cap strips just for the neater appearance.
As for wing tube diameter, you might get away with 1/2" diameter if the aluminum is one of the T6 type alloys, otherwise I would go with at least 5/8"-even 3/4"-I know what you said about the crash and maybe I overbuild, but 1/2" just seems a little on the light side.
Don't have any computations to back that up, just the experience of building. Good luck.

BMatthews

I'd shoot for 3/32 ribs with cap strips as well. There IS a load on the ribs but so often we can forget about it other than on larger models. But this one qualifies in my estimation. Also cap strips on the ribs will help them to avoid buckling from the forces in the covering material when it shrinks up. All in all well worth the small weight penalty.

A word on wing tubes. The wing tube should be located at the same location as the main spar and the structure designed to conduct the spar loads directly into the slip tube and joiner tube within. If you do it that way then your ribs do not need to be made from plywood at all other than the first rib that should have a full airfoil shaped "liner" added at the root. This liner will better support the airfoil's shape at the root and also serve as a strengthener for the panel's anti rotation indexing pin that is needed along with the main joiner. Sheeting the first rib bay is a good idea both for covering adhesion as well as helping to further spread out any local bending forces by boxing in the first rib bay.

John 38

Brad,
the foam wings would spread the loading -aerobatic or pancaking- onto the tube. Ribs will point load onto the tube increasing any bending forces, particularly at root.

I also agree with facing root rib with thin ply and I also slide on little ply formers onto tube when finally fitting and these formers than get glued to balsa ribs to absorb/transfer load from wing via ribs onto tube/fuselage.

On a couple of wings I built, I had room to fit "tubes" between top and bottom spars, blocked in the rest of space and then faced the lot including spars with vertical balsa/thin ply sandwich to give a solid tube box. No need for ply reinforcing of ribs except for facing root rib. With this set up, there is no need for wing tubes as the box space will both guide and absorb loads onto rod.

If lack of space for this, then consider carbon rods for wing joiners.
John

bkdavy

My Feedback: (1)

Note on the wing tube - its T-6. Light, but very strong.

Wing tube location is planned immediately behind the spar. The concern was compromising the strength of the spar. The spar is located at 25% of the chord, which is where the inital CG will be located, but I expect to move the CG back from there. The front and back of the wing will be pinned to the fuselage to help prevent rotation. I had already planned to sheet the front, and completely box in the first rib bay. Using ply for the first 4 ribs should be adequate to transfer the loads between the wing-tube and the spar. I'm using 1/8" aircraft ply, so there shouldn't be too much of a weight penalty. I may go ahead and move the wing tube onto the spar line, and completely box in the tube. I haven't cut those ribs yet, so changes are still possible.

Brad

John 38

Ok Brad,

wings can be prevented from rotation and held into fuselage by nylon bolts from inside fus thru into wing captive nylon nuts.
wings I have, have bolts with thumb " tags" which can be finger tightened ( instead of screw slots etc )

john

bkdavy

My Feedback: (1)

I had thought about a similar arrangement with nylon bolts or a hex head screws for the "pinning". I was thinking about either using a blind nut or just threading the plywood. If I use a nylon screw, I'll just include an extra plywood piece on the inner rib, and thread that. I have one plane that holds the wings into the fuselage with a rubber band, and its been pretty reliable as well.

Brad