Dyhedral/Anhedral/Polyhedral

[DX Rick]

True or False.
An airplane MUST have either one to be able to fly.

 

[Singlecoil]

I don't know what polyhedral is, but I'm guessing false. Dihedral and anhedral relate to stability, not whether the airplane will fly. Can the airplane be certified without one of the three? That's a different question.

 

[transampilot]

Dihedral, anhedral, and polyhedral are all designed into an aircraft to change the roll stability of an aircraft. Stability has a tradeoff with manueverability. The more stable the less manueverable an aircraft is. Dihedral adds stability to airplanes which need more stability for their designed mission, for example low wing trainers. High wing airplanes already have a bunch of roll stability using the pendulum effect so many high wings use anhedral to lessen their stability and increase manuverability, for example the AV-8 Harrier. Or to lessen the dutch roll effect in airplanes with high or swept wings. Polyhedral is when the dihedral/anhedral angle changes along the wingspan. The F-4 Phantom is one such example, unique among fighters for having dihedral wingtips. This was added after prototype flight testing (the original prototype of the F-4 had a flat wing) showed the need to correct some unanticipated roll instability - angling the wingtips, which were already designed to fold up for carrier operations, was a far more practical solution than re-engineering the entire wing. Bottom line dihedral has an effect on stability not on whether the aircraft will fly. Some airplanes are designed with a negative stability like modern fighters. They only fly because of their fly by wire control systems and related computers but never the less they still do fly.

 

[goahead]

Great info transampilot, thanks