The torque Q is a force multiplied by a length, and it follows that a rational expression for the torque is
Q = kQpn2Ds (9.19)
kQ being the torque coefficient which, like kj, depends on the airscrew design and on Re, M and J.
The power supplied to an airscrew is P;n where
Pin = 2imQ
whereas the useful power output is Pout where
Pout = TV
Therefore, the airscrew efficiency, jj, is given by
2nnQ к<2рп2052пп 1 kj V 1 kj 2-k kQ nD 2-k kQ
9.2.3 Power coefficient
The power required to drive an airscrew is
P = 2’KnQ = 2nn(kQpn2D5) = 2irkQpn3D5 The power coefficient, CP, is then defined by
P Cppn2D5 i. e.