astrohog

My Feedback: (1)

I am building an old Sig Skybolt bipe. I am want to modify the bottom wing to accept one servo in each wing half three or four ribs out from the root. I know how to build a removable servo "tray" out of plywood (ala Four-star sixty), but do I need to reinforce the balsa ribs on either side of the servo bay? If so, should I just double them up with balsa? plywood? or do they need reinforcing at all? by the way, the wing halves are already assembled. If any of you all have any favorite ways of doing this, your input would be greatly appreciated!!

Thanks,

Astrohog

Woodpile

What is is the rib material? If it's 1/8", you probably don't need to do anything, but if you want to be safe, just sister the area where the tray will mount with whatever the original material is. You'll make that area twice as strong as anywhere else and the glue will help stiffen it.

Ed

JimCasey

My Feedback: (1)

If you are mounting the servo right next to the spar and the servo mount transfers loads directly to the spar, that's good.
ANother thing to worry about is supporting the covering around the servo hole. Unreinforced ribs will be pulled sideways by the covering shrinkage, and you'll get crinkles next to the servo hole.

This is why it's so simple to put a one-bay wide plywood skin on the bottom of the wing, punch a hole for the servo, and mount it from the outside. It's ugly, but it's structurally efficient. It would work to put the ply skin touching the spar, rib-to-rib, and if there's enough room between your ribs, put the servo mounting hatch in the ply surface. Just make sure that the sides of it are at least 1/4" wide. It doesn't have to go all the way to the trailing edge.