Admittance for the Moment

The yawing moment can be calculated around any axis a (Filippone and Siquier, 2003), and is written as:

#AMW= (a + 0 [МЩ – iJl(k)]C(k)

i 1 — A

– — (A + 2a)Ji(Afe)-— J2(Xk) (11)

Подпись: Hf (k) Подпись: A- 1 2A Подпись: J2( Ak) — iJ1 (Ak) Подпись: (12)

where J2 is a Bessel function of order 2. A number of simplifications can be made from Eq. 11. First, consider a body pitching or yawing around an axis a = -1/2. In this case, Eq. 11 can be simplified in at least two cases: If a = -1/2, then

Подпись: If A1 (stationary gust), then

Hjf=i(k) = (a+^Hx=1(k). (13)

Therefore, the admittance for the yawing moment is proportional to the admit­tance for the side force, and the two functions are in phase at all frequencies.

The latter equation can be normalized so as to have a unit value at k ~ 1. Also note that if a = -1/2 then the body is neutral at all frequencies with respect to the pitching moment.