RE: Twin Otter 125inch wingspan
A couple years ago I asked why they were using an inverted airfoil on The stab. I was given a complex answer most of which is way over my head but the simple parts I understood were the it is to add stability, and is used mainly because the thrust line is high and above the "Z" axis center of gravity which causes the nose to push down when thrust is applied and the extra propwash over the tail keeps the nose from pushing down. They have the airfoil figured out to provide a very pitch stable plane . it also apparently helps with the flaps being behind the center of lift and also aids tail stall recovery and other things beyond my understanding . I used the airfoils angles etc off the full size plane and so far the prototypes are extremly pitch stable. cheers John