Here’s what Mr. Gessow and Mr. Myers have to say about this subject (Aerodynamics of the Helicopter):
“Control Sensitivity
The combination of control power and damping in roll (or pitch)
together determine an important flying-qualities characteristic. This
characteristic is called control sensitivity and may be defined as the
maximum rate of roll (or pitch) achieved by a unit displacement of the
controls. Control sensitivity may be defined in three alternate ways as
follows:
Physically, the manner in which control power and damping deter-
mine control sensitivity may be understood from the following argu-
ment. If the control stick is displaced (laterally, for example) and held,
the helicopter will initially accelerate angularly at a constant rate that
is inversely proportional to the moment of inertia of the helicopter
about its longitudinal axis. [This result follows from Newton’s law
M = I(d@/dt).] As the angular velocity builds up, the opposing damping-
in-roll moment increases in proportion until an angular velocity is
reached at which the damping moment is equal to the control moment.
The helicopter is therefore stabilized at that angular velocity, because
the resultant moment on the helicopter is zero. It is thus apparent that
if the rotor damping is large with respect to the control power, then the
maximum rate of roll reached by the helicopter by a given stick displace-
ment would be small, inasmuch as a sufficiently large damping moment
would be produced at a small rolling velocity to balance the control
moment. Alternately, it is clear that if rotor damping is small with
respect to the control power, then the maximum angular velocity
attained by a given stick displacement would be large. -
A Helicopters with conventional control systems are subject to high
control sensitivity. In fact, according to reference V146 (Appendix IIA)',
the maximum rate of roll achieved by a small, two-place helicopter may
be as great as those of some modern fighter airplanes at the speeds for
their maximum rates of roll. This is true not because of high control
power, but rather because of low damping, which, for the helicopter, is
a fraction of that for airplanes. This same_ reference goes on to state that
high control sensitivity can lead to overcontrolling, which in turn
results in a short-period, pilot-induced lateral oscillation. '
It is worth while to point out which of the physical characteristics of
the helicopter can be varied so as to reduce excessive control sensitivity.
The height of the rotor and the offset of the flapping hinges do not affect control sensitivity because they change control power and rotor damping in proportion. Design factors and devices which increase rotor
damping without affecting control power result in reduced control
sensitivity. Thus, large helicopters operating at low rotor speed,
helicopters with tip-jet units, and helicopters with devices such as the
Bell Stabilizer Bar and the Hiller Control Rotor will have more desirable
values of control sensitivity.
Although rotor height and fiapping—hinge oflset do not affect control
sensitivity, the ratio of these values to the fuselage moment of inertia
will determine to a large extent the time necessary to reach the maximum angular velocity.”