Building wing strength at dihedral

[ifrichard]

I recently had the wing of a powered electric powered glider conversion fail. It failed at the dihedral joint – bottom side. This original plans for this glider had 12 inches of balsa sheeting in the center on the top and none on the bottom. When I converted it to a powered glider. I did not add any strength to the center wing joint. It broke in a steep diving fast turn just after I had turned the power on to regain elevation. It fell 200 feet. The fuselage and radio were totaled. The wing, however, can be rebuilt again. As soon as it happened, I realized how foolish this design was.

The plane has a wing area of 530 square inches and a weight of 46 ounces. It is heavy because it carries a camera. It also carries a speed 500 motor 1600 mAH 9.6 NiMH and a gearbox.

My question: How should I strengthen a dihedral wing joint? I was thinking about this combination:
1) 1/32 balsa sheet 6 inches over each side of the wing on the bottom. Grain in line with the wing’s long direction. 12 inches of balsa sheeting total.
2) 1/32 balsa sheet 4 inches over each side of the wing on the top. Besides strengthen the joint this is to allow the rubber bands to hold the wing on.
3) ¾ ounce glass cloth 4 inches over each side of the wing on the bottom over the balsa. 8 inches total.
4) ¾ ounce glass cloth 2 inches over each side of the wing on the bottom over the first layer of glass. 4 inches total.
5) ¾ ounce glass cloth 2 inches over each side of the wing on the top. 4 inches total.

Should I use CA or pure epoxy (System 3) for the glass?

What are the tried solutions to this problem?

Thanks, Richard

[Viper Pilot]

Richard,

Sounds like overkill to me. I use 0.56 oz. glass cloth, about 4" wide at the center joint. I use exremely fast CyA over the whole thing, then re-cover.

I've never tried a sailplane, but I've done this with glow'ers up to 86" WS.

VP

[Vonbaron]

I just use a ply dihedral brace. If the wing breaks it's usually somewhere else.

[Dereck]

Hi Richard
Suspect the major culprits here are things like 'increased weight', and 'steep diving fast turn'. However, that structure you describe doesn't sound very sound.

Firstly - did the wood part company, or did the joint fail? Too many models with balsa spars should really have spruce spars, as they aren't that much heavier and far stronger. Your wing failed in tension - either the spars didn't have the structural integrity to keep together or the wood in one of them let go under the pull exerted by the diving turn, weight of the model, etc.

You might be able to repair it. HOWEVER! There'll be a lot of work involved in making sure the joint is solid and you have to build a new fuselage anyway. Might be safer on the new radio you seem to in the market for, if you buy a new kit and start from scratch.

FWIW - my favourite model has mainspars that are 1/4" square spruce at the centre (they taper off to 1/4 x 1/8 halfway out). These butt together in the middle, with a single 1/8" Birch ply joiner about 5" long across the centre join. There's a single piece of 1/16" ply about 5" long across the wing TE - takes the wing bolts - and a 1" GF bandage around the centre joint. It's a D box LE section, with a C channel TE and one rib bay of full sheet top and bottom (1/16") either side of the fuselage. This model has 600 squares, 6-1/4lb, 20 cells and sometimes does aerobatics ;).

Yes, there are a lot of marginal designs around. I've seen what you describe on far too many occassions. OTOH, I once did a magazine review on Avoex's "Thermic Traveler" electric soarer - with a wing structure pretty much like my aerobatic models. Very re-assuring. If you are going to haul a camera and need maneuvrability when hauling, your average glider structure might well not be adequate. A sports type model with aerobatic capabilities or at least a decently designed soarer will cost you much less in the long run here.

[Megowcoupe]

The old standby it to use some plywood for a dihedral brace. On a 3 lb airplane, I suspect that 1/8" of ply (don't use the lite stuff) should work. (This can also be 2 pieces of 1/16" ply on either side of the main spar. You want to run it several inches past the fuselage and taper it. Your idea of sheet also works, but I'd use 1/16" on a plane that size. You can wrap the center section with glass and epoxy- odds are it won't fail again. Just be careful how you transition the load from the center to the rest of the wing- too much stiffness ending abruptly creates a likely spot for failure.

Sam

[ifrichard]

The wing broke at the joint. It does have 2 spruce spars that are spliced to balsa about 10 inches from the wing root. There was also one 1/8-inch ply joiner across the 2 wing panels. This design was made before fiberglass was so easy and popular, so I am willing to forgive some of its shortcomings.

If there is balsa sheeting on both the top and bottom wing chords, I doubt that the plywood joiner splice is required. However, good contact in the middle of the wing joint is important to allow the fiberglass to only be stressed in tension. I also agree that I do not want to just move the failure outboard to the end of the center balsa sheeting, so good spars going beyond the joint and sheeting seem critical.

The wing really did survive well, so I will most likely rebuild it, unless someone has a better idea. See below.

I think my first plan was an overreaction and the VP is correct with one layer of 4 inch wide glass top and bottom over balsa sheeting.

I have never liked this wing. It has never worked well in a glider. Some of the glider problems were not enough area and it was too heavy. At present my glider interests are high tech discus launch gliders and Zagi slope soaring. I am not considering making another 2-meter glider. I purchase the wings for the discus launch gliders.

I am afraid that I have just moved the curse of this bad designed glider into my powered rudder- elevator camera plane. Unfortunately this wing is a survivor and can be easily repaired. On the other hand, I would be happy to build from suggested new plans and start over. The only requirements are that it will still weigh or carry about 40 ounces all up, fit the camera width of about 2 inches and be easy to spot and control at long distances. It would be also be nice to stick with my speed 500 motor and 1:2.9 gearbox. Although the gearbox bore the brunt of the crash, I think it can be salvaged.

Thanks, Richard

[PeterH]

I think Derek's responses best address the physics involved. I think 'dihedral braces' do their job because they act as shear webs and in doing so keep the spars from collapsing. And I think that is the center of the problem. For years and years r/c 'pattern' aircraft used sheeted foam wings that had no spars and were butt-joined with a fiberglass bandage. They held up just fine. But they were thick and that in itself helps. Also solid white foam doesn't crush that easily, and the wings were free to flex along their span, however little that might be. There were no stress risers built in.

[ifrichard]

I reviewed Derek's response again and I now believe that my 1/8 ply joiner failed because it was not boxed in with sheeting. Some twisting occurred in the open channel next to the dihedral joint and allowed the tearing apart of the ply joiner. The wood damage to the wing seems to support this theory. Also the forces in the turn would have caused this torsional force to the wing.

This wing has adequate spruce spars. However, as I have discussed this problem with others, I have more respect for using the spruce spars. Spruce is the best way to avoid just moving the breaking failure point out 4 to 6 inches from the center. The spruce spars are also much stronger in compression than the balsa and therefore increase the strength of the center joint even if they are only butted together.

Richard