Torque coefficient

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.

9.2.2 Efficiency

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

TV kTpn2D4V

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.

p

CP =

Cp.