On May 3, 9:33*am, soartech wrote:
Open up the wing, wrap the spar with Kevlar and carbon fiber/epoxy and
carbon rod top and bottom.

I once designed a wing that used a carbon rod wing spar and aluminum
skins. In order to make the spar stiff enough so that the wing skins
did not yield and buckle within the limit load envelope, the spar
needed four times the amount of carbon dictated by the strength
requirements. Basically, I would have had to make the carbon spar as
stiff as the aluminum spar it was intended to replace. That carbon
spar would still have been lighter than its aluminum equivalent, but
would have been much pricier. In the end, I went with composite skins
and only twice the amount of carbon rod dictated by strength, and it
worked out great.

Regardless, designing a retrofit that uses a carbon fiber spar but
retains the aluminum parts and detail design of the rest of the
structure would be a substantial undertaking with a lot of risk and
potential for heartache. I think it would be better to invest the time
and effort into a new training sailplane design more effectively
optimized for the current realities of soaring flight training.

Thanks, Bob K.