Hinge Offset

Once a hub design with hinge offset has been selected, the question becomes, "How much?” In most cases, the decision is based on mechanical or structural considerations rather than on performance or handling qualities. There is, however, some advice that can be given to the designer in this matter. Large hinge offsets produce penalties in the form of large, draggy hubs and the increased ability to accidentally roll the helicopter over on the ground. Small offsets, on the other hand, may produce marginal control power especially when the rotor is unloaded

such as in pushovers to low load factors. A minimum offset can be defined to satisfy a flying quality requirement in the form of: "The control power during flight at zero load factor shall be no less than one half (or some other fraction) of the control power in level flight.” A requirement such as this will help avoid "mast bumping” or "droop stop pounding” when the pilot tries to control the helicopter under conditions of low’g’s”.

In Chapter 7, it was shown that the control power is:

where the first term is the hub moment due to hinge offset:

dMM _ 3 e AbpR(ClRya dah 4 R у

If the control power at zero load factor is to be a fraction, l/l + K, of what it is in level flight, then:

( dMC G

da l J о 1 1

dMCG 1 + К ^ + ЮЖ.

da s J icvci dMM

da і

LJ

With a little algebraic manipulation, the required hinge offset ratio is:

/ 2D CT/o lcvd

/ e _ 3 R a

{r/щ k

But both in hover and in forward flight, the coning is approximately:

2 у

“o = ~~ Ct/gicvei, radians

3 a

Thus the required hinge offset ratio can be related to the coning in level flight and to the rotor height above the center of gravity:

The main rotor, of course, is the dominant component of the helicopter and therefore receives most of the designer’s attention. As several projects have shown, however, unfortunate decisions about tail rotors can jeopardize the operational success of the aircraft. Tail rotor parameters of some current helicopters are listed in Appendix B.