Fuselage contribution to Nv

The body lift and drag forces produced by the fuselage of a helicopter during sideslip will result in a net sideforce. This sideforce will produce a yawing moment which will be stabilizing or destabilizing depending on the relative positions of the force and the CG of the aircraft. In forward flight with no sideslip, a symmetrical fuselage would produce no net sideforce. However, in the same manner as the fin, the fuselage in a sideslip presents an angle of attack to the relative airflow equal to the sideslip angle. Fuselage lift and drag forces will then be produced (although the lift will usually be very small) parallel with and perpendicular to the relative airflow. Components of
these forces will give rise to the net sideforce, Yfus, which will in turn produce a yawing moment, Nfus. The point of action of the force in relation to the CG will determine whether or not this moment is stabilizing. The moment is given by:

Nfus = Yfus lfus = [1p(U2 + V2 )Sfus CD sin P]lfus

Note that the component of the fuselage lift force has been neglected. The magnitude of the fuselage contribution to Nv for a given fuselage shape will increase with increasing forward speed, sideslip velocity and fuselage drag area.

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As the helicopter sideslips, the main rotor will flap away from the direction of the airflow as has been described above (a1 and bx effects). The tilting of the thrust vector will produce a horizontal thrust component that manifests itself as a sideforce at the rotor hub. If the main rotor is tilted forward with respect to the z-axis of the helicopter then a yawing moment will be generated. This moment will provide a destabilizing contribution to Nv but it will be small in relation to the others.

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