Hi I´m designing a scale amphibian-airplane, Osprey2, escale 1/5, Wingspan 60" , weight 6-7 Lbs average, to .56 or .60 four stroke engine. I don´t know how main spars to use, 2 spars of...1/4 x 3/8 balsa or 1/4 x 1/2 balsa or hardwood. have examples plans, but ihave doubt.I am afraid that it is fragile. Anyone know any standard form for that weight?

Thanks and best regards vansenair

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On most of my scratch built 60 size warbirds, I use 1/4" X 5/16" or 1/4" X 3/8" hardwood. I cut my spars from spruce 2 X 4s, as I am not a fan of balsa spars and most of basswood spars you get are crooked. If you can find some straight basswood spars in this dimension, you should be fine, just be sure to put shear webs in to make the D type construction.
I have a scratch built FW with 54" wing span using 1/4" X 1/4 spars that has been put thru some severe muanuvers, plus it flies about 80+ MPH. Never had a wing fold yet, so I must be over building, but I like it that way. Good luck.

Spruce sticks are usually sold in LHS's that have balsa. They know that lots of scratch builders used to use spruce for spars for lots of models. That said, with a lot of the R/C models your size, you don't actually need to worry about using balsa if you have decently sized spars and webbing.

Thank you very much for your experience, is likely to do so with 2 to 1 / 4 x 1 / 2 balsa and sheet webs of 3 / 32 balsa and 2 aluminum tubes 1 / 2 from fuselage at the the third rib because the wings are detachable. an the gear landing set into wing.