Shaft Tilt

For best performance in forward flight, the fuselage angle of attack should correspond to its minimum drag condition. Since the tip path plane angle of attack must be nose-down to produce the proper balance of forces, a fuselage whose longitudinal axis is parallel to the rotor may be generating more drag than it would if designed to fly more level. To minimize fuselage drag, many helicopters are built with nose-down shaft tilt. A typical value. is —5°. Higher values might be desirable for very high speed helicopters, but the designer must consider the effects on hover attitude where the rotor must be horizontal. A helicopter with high shaft tilt would hover with the fuselage tilted far nose up, with possible effects on field of view and seat comfort.

The aerodynamicist and the weights engineer could save themselves considerable later work if they could convince the designer to make his layout drawings with the waterlines perpendicular to the rotor shaft. Drag optimization would then be done with fuselage tilt with respect to the waterlines instead of shaft tilt. This would simplify analyses in which both vertical and longitudinal positions of the center of gravity are used.

Flare Angle

A parameter that is affected by the nose-down shaft (or nose-up fuselage) tilt is the maximum fuselage angle that can be achieved during a landing flare without doing structural damage by striking the ground with the fuselage aft end or the tail rotor. Unless special provisions are made, such as a sturdy tail boom and a shock­absorbing tail landing gear, the maximum shaft flare angle is that reached as the main landing gear and the aft end of the tailboom or tail wheel/skid touch simultaneously. The higher this angle, the better will be the final deceleration capability. A survey of many helicopter side views such as those in Appendix В indicates that designers believe that a flare angle of at least 8° is desirable. Note that some designers—such as those of the Hughes AH-64—have preferred simply to make their tail booms sturdy enough to withstand a significant landing impact.