Fin contribution to Nv
In forward flight with no sideslip or rotor wake interference a symmetrical fin produces no net sideforce. However, should the helicopter sideslip to the right, say, the relative airflow is such that the fin now has an effective angle of attack equal to the sideslip angle. The net sideforce, YF, produced by the fin comprises components of both the fin lift, Lf , and drag DF. This force will give rise to a yawing moment, NF, given by:
Nf = Yf lF
where lF is the moment arm. If the fin lift and drag coefficients and the effective fin area, SF, are known, then:
Nf = [2(U2 + v2)pSF (CL cos p + CD sin p)]lF
The direction of this moment is such that it will always tend to yaw the helicopter into the direction of the sideslip, thus it is stabilizing. The size of the moment for a given fin design will depend on the forward speed, Ue, and lateral velocity, v.