Dukester

My Feedback: (31)

I am scratch building a 32" Sopwith Pup from a set of plans. While building the lower wings, I failed to add the 1/32 lite ply reinforcement which goes on either side of the spar. The lite ply should cover both sides of the spar on the two bays at the root of the wing. Unfortunately, I sheeted the inner bay before I realized I had missed the lite ply. I don't really want to remove the sheeting as I've already sanded it smooth so here are my questions.

1. As far as strength goes, I used spruce spars rather than the balsa called for, so do you think I should remove the sheeting and add them?

2. Does the lite ply reinforcement serve any other function other than to strengthen the spar at the root? I can't see any other reason to have them, but someone else might have an idea.

Thanks,
Duke

Flypaper 2

Iwould put on a layer or two of 6 oz glass cloth about 2 in. wide with finishing resin. If your going to do aerobatics with it, use two layers. Would be stronger than the litply spar doublers.

Tall Paul

The lower wing on those things is subject to tip strikes on takeoffs and landings.. The spar invariably breaks right outside the usual sheeting, if it's not reinforced with a doubler, and even then it can break.
I'd saw a slot in the bottom sheeting 1/16" wide, and add a 1/16" real ply doubler out to the 2nd rib bay.

Dsegal

> Does the lite ply reinforcement serve any other function other than to strengthen the spar at the root? <

This is a non-trivial matter as the dihedral joint bears the greatest loads on the wing. Note that wood of 1/32" thickness is not LitePly, it is real plywood.

SST

My Feedback: (2)

Since you have replaced the balsa spars with spruce, I wouldn't worry about it.

MikeSell

My Feedback: (2)

If there is dihedral it is not a doubler but a dihedral brace. As Dsegal says the joint in the center is the area of most stress in th airframe. Unless that area is reinforced the wing will fold in the middle when under stress. That glassing idea has merit.
Your wing is so short and the stress is spread over two wings you might slip by. Think why you used spruce instead of balsa. That caution should be mirrored in the reinforcing of the dihedral joint.

BMatthews

A simple end grain to end grain butt joint is the weakest form of woodworking joint you can get. This is why we use joint doublers otherwise known as dihedral braces to join spars. I'm with Tall Paul. Cut that slot in the sheeting and through the ribs on the bottom side and slide a proper plywood doubler up into place. In fact you should probably do one on the front side and another on the rear. Just don't end them at the same spot outside the sheeting. Run one of them about an inch or more beyond the other so the stress tapers out rather than concentrates where the doublers end.

Dukester

My Feedback: (31)

Well at the end of the day, I am just going to remove the lower sheeting and add the reinforcement. The wing is actually split and each piece mounts to the fuse, so I was already worried about strength of the joints. I think I would get by without the ply with the stronger spars, but its not that much trouble to add the ply and re-sheet the bottoms.

Duke

BMatthews

I think that is a very wise move. Sure you put in spruce spars but without any proper structure to carry the spar loads through the fuselage from one wing to the other there would be no strength at all in the lower wings. All your loading would then be transfered to the upper wing through the interplane struts and it would have to do double duty.

Where the spars meet in the center of the wing(s) is the most critical joint in any airplane.