RE: Viper /Great Planes
It seems the weak spot on the Viper is the area just in front of the stabilizer. My second Viper nearly crasched due to a crack just there (se picture below) and I have seen other Vipers go down because of the stabilizer leaving the fuselage just there. It's just the 3mm balsa sides that's supports the stabilizer, which isn't enough when the you use a Nelson or Jett engine.
The remedy is to support the fuselage inside with thin 0,4mm plywood or some more 3mm balsa and hard balsa stringers or something like that and /or adding 25g glassfiber/laminating epoxy on the out side.
I did both when I repared mine.