The Art of the Helicopter John Watkinson - Karatunov.net

8.10 Stability
Helicopter performance 341
An unconditionally stable aircraft is one that, if disturbed from trimmed flight, can be
placed in any attitude and then, if the controls are released, it will return to straight and
level flight on its own. The ability to return to straight and level flight is known as static
stability, whereas dynamic stability is concerned with the manner, or promptness, with
which the return occurs. Free flying model aeroplanes and some full-size aeroplanes are
unconditionally stable. In forward flight on the upward part of the power curve, many
helicopters are stable because the tail rotor acts like a fin and the tail surfaces have
sufficient pitch authority. However, as the goal of the helicopter is to hover, a great deal
of flying will take place on the back of the power curve where stability is impossible to
achieve without pilot intervention or some artificial mechanism. Thus to be fair, it is not
that helicopters are fundamentally less stable than aeroplanes, more that helicopters
can explore flight regimes that are not available to aeroplanes in which control is simply
harder.
Stability is a topic where there may be conflict between performance and safety and
between complexity and economy. It is important to consider the entire system in
order to form a balanced view. The airframe itself will have certain stability behaviour,
but this will be modified by the actions of the pilot. Technically a helicopter airframe
may be unstable, but piloting these unstable devices gives many of us a great deal of
pleasure. This is because the actions of the pilot turn the unstable airframe into a stable
system. In fact the unstable airframe with an intelligent pilot and/or control system
forms the most manoeuvrable combination because it is axiomatic that the more stable
the airframe is made, the less willing it is to deviate from its course.
Pilots very quickly become instinctive and fly at constant altitude with correct RRPM
and no slip without conscious thought. It doesn’t make much difference to the pilot if
the machine deviates from his intended course because of a gust or because it is has
a degree of instability. The result is the same; he operates the controls to bring it back on
course. In good conditions, the pilot has so little difficulty in doing this that he can fly
a machine with significant stability problems without necessarily realizing. However, if
visibility becomes poor so that the horizon is obscured, the pilot’s instinctive ability to
stabilize the machine is impaired. Now the machine that is fun to fly VFR because it is so
manoeuvrable becomes a menace in IFR because the workload involved in interpreting
the artificial horizon and other instruments to make constant course corrections may
be too great.
Today most helicopters are designed on the assumption that they will be used in
IFR, but this has not always been the case. In practice machines certified for VFR only
may have subsequently undergone a number of modifications in order to obtain IFR
certification. These modifications frequently involve stability and pilot workload. In
some cases modifications to stability can mean that a machine formerly requiring two
pilots for IFR can then be certified for single pilot IFR flight.
The stability of the airframe can also be augmented artificially. Several of these
techniques were shown in Chapter 7. In principle an unstable airframe with an
artificial stability system gives the best of both worlds because the pilot can retain
the manoeuvrability of the unstable airframe whilst enjoying the artificial stability. In
practice some thought has to be given to what will happen if the artificial stability
system fails in IFR. Either the artificial system has to be made redundant so that
it continues to function in the presence of a failure or the airframe has to be made
sufficiently stable that pilot workload remains reasonable if the augmentation system
fails. Aerodynamic stability improvements are very reliable because they usually