J-3/Wag Aero Wing. Wood spar vs metal spar differences.

Hi all, first time poster here.

I have recently acquired a Cub project which has been previously worked on. The previous owner decided to do a direct substitution of the wood spars in the Wag Aero Sport Trainer plans with metal spars... leaving all of the compression struts and lift strut fittings the same but shimmed out with aluminum billet to the correct dimensions.

The main thing I am concerned about is the lack of an N brace between the lift strut fittings... as it is there is only the one standard compression strut and the extra one below it with the drag wire going through. I am figuring that this is N brace required because of the reduced torsional stiffness of the metal spar vs wood, and will especially be needed on this aircraft since the ends of the compression struts are the round style rather than having the long vertical ends of the metal spar style ones.

So what should I do? Should I buy or fabricate the full N brace? Should I weld a piece of 4130 between the two existing compression struts, or weld fittings on so I can put a removable one between them? It is that important? Any other differences between wood and metal spar assembly I should be aware of?

Thanks all!

I recently completed building a set of wings out of Wag Aero parts (Carlson spars and ribs). I followed a PA-18 drawing for the most part, 13 ribs and built an N-strut at the lift struts, which is basically what you described, welding in the extra brace between the two struts. I'm sure there are a lot of Wag cubs flying around with 12 ribs and no N-strut (per the Wag drawing) but I elected to stiffen mine up due to adding flaps and 150 HP.

I can send you a copy of the page of the N strut from my Wag 2+2 drawing set. The plans are for the wood wing but Wag's metal spar supplement has the shims like your wing has so not to worry about that. PM me with email address and I'll shoot you the page from the plans.
Marty

I would say that the rest of the wing is sound, just build the N strut. Seen a few T crafts redone this way with no problems.

Thanks for the replies... they pretty much confirm what I was thinking.