The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 155
disappears and the machine should fall back onto its gear. In the event of a sloping
landing, if the cyclic stick travel is becoming extreme, the landing should be rejected.
The collective lever is pulled up to get the uphill skid off the ground. Again the couple
is eliminated because there is no ground reaction. Even if the roll has started, a smart
and powerful application of collective will recover cyclic control as soon as the skid
leaves the ground.
Rollover is also possible in a hingeless rotor machine if the cyclic control is not
trimmed to neutral before collective is raised to take-off, although the mechanism is
not the same as that of dynamic rollover. Pilots like to tilt the disc around on the ground
to check the cyclic controls before flight. Hingeless rotor machines can have extremely
high cyclic power without much rotor tilt being present. The amount of tilt that would
be safe in an articulated machine might roll over a hingeless machine. Sometimes the
undercarriage squat or ground contact switch may be used to limit cyclic authority
when the machine is on the ground.
4.20 Some rotor head examples
The number of approaches used in rotor head design over the years has been subject to
wide variation. In the earliest days the theory of rotor dynamics was not well established
and most designs mixed theory with empiricism. As understanding increased, attention
turned first to optimization of design for a particular purpose and subsequently to such
refinements as the reduction of vibration and of build and maintenance cost.
Figure 4.34 shows the evolution of zero-offset heads. These are usually underslung so
that the hinge pin passes through the CM of the coned rotor. Supporting the rotor at its
CM allows the blades to conserve momentum as the disc tilts and dragging hinges can
be eliminated. This means that the rotor can be made supercritical in dragging. Ground
resonance cannot occur and no damping is needed in the head or in the landing gear.
This allows a cost and weight saving in small machines.
At (a) is the fully gimballed teetering head which essentially contains a Hooke joint
so that the rotor can tilt in any direction with respect to the mast. The blade grips
are mounted on feathering hinges. The feathering axis is parallel to one of the gimbal
axes and the attitude of the hub in that axis would be indeterminate were it not for an
equalizing linkage that makes the pitch of each blade equal with respect to the hub.
This type of head was used on early Bell and Hiller machines.
At (b) is the later type of teetering head where only a flapping hinge is fitted and
tilting in the other axis is accommodated by the feathering hinges. This type of head
can be seen on the Bell 206 JetRanger. It is not obvious how this arrangement can
act as a universal joint to allow the disc to tilt in any direction relative to the mast.
The answer is that the feathering bearings are used to provide the additional degree
of freedom. Figure 4.34(c) shows a helicopter with a teetering rotor hovering with an
offset CM. The control axis and the tip path axis are vertical, but the shaft axis is not.
As the rotor turns, the blades remain at a constant pitch relative to the tip path axis
and the control axis, but need to turn in the feathering bearings to do so. Thus in this
case it is the rotor head that is oscillating about the blade root and not vice versa.
The feathering bearings and the underslung teetering bearing act as a Hooke joint
to allow shaft axis to be different to the tip path axis. The Hooke joint is not ideal
because of the inclination of the feathering bearings at the coning angle, but for typical
shaft/tip path relationships the discrepancy is small. The Hooke joint is not a constant
velocity joint, and some torsional flexibility will be required in the transmission.