THE VELOCITY RATIO VF/V
When the vertical tail is not in a propellor slipstream, VF/V is unity. When it is in a slipstream, the effective velocity increment may be dealt with as for a horizontal tail.
CONTRIBUTION OF PROPELLER NORMAL FORCE
The yawing moment produced by the normal force that acts on the yawed propeller is calculated in the same way as the pitching-moment increment dealt with in Sec. 3.4. The result is similar to (3.4,8):
= _ dCn„
Э/3 b S dap
This is known as the propeller fin effect and is negative (i. e., destabilizing) when the propeller is forward of the CG, but is usually positive for pusher propellers. There is a similar yawing moment effect for jet engines (see Exercise 3.7).