The Art of the Helicopter John Watkinson - Karatunov.net

or without the AFCS. Systems of this type are intuitive and much more appropriate
for helicopters which are generally flown at lower altitude and in closer proximity to
potential hazards. Such autopilots can be left engaged at all times. If the pilot wishes
to perform an emergency manoeuvre, it is not necessary to disengage the autopilot;
an instinctive movement of the controls is all that is needed. The autopilot acts as an
AFCS and makes the machine execute the manoeuvre more accurately. However, if a
small dead-band is incorporated in the stick transducer, the autopilot can sense when
the pilot has released the stick and will then maintain the last attitude indefinitely.
An autopilot that can only maintain attitude is not enough. For prolonged use the
ability to maintain airspeed and altitude is also required. For instrument landing the
ability to maintain a specified rate of descent is essential, as is the ability to move
smoothly from altitude control to descent control as the glide slope is entered.
An encoding altimeter as described in section 7.10 produces an actual altitude signal
and this is subtracted from the desired altitude set on a dial by the pilot. The resulting
altitude error will be cancelled by climbing or descending using the collective control.
When cruising at a fixed altitude the necessary rates of climb and descent will be very
small, but if the pilot directs the system to a new altitude, a significant rate of climb
may result. In the absence of a stabilator, this will increase the downwash on the tail
plane and give an aft trim effect. However the vertical axis gyro will sense this as an
attitude error and try to cancel it with forward cyclic. This will produce undesirable
mast bending and vibration. A solution is to feed a small proportion of the collective
climb command across to the cyclic system to modify the pitch axis reference allowing
a slightly tail down attitude for the duration of the climb.
The climb will also require more torque and although the yaw control of the autopilot
would handle this a better result may be obtained if the measured torque is fed into the
yaw system.
Airspeed is easily obtained from the dynamic pressure at the pitot head. In a sophisticated
system, the static pressure may also be used to compensate the indicated airspeed
(IAS) to true airspeed (TAS). The pilot enters the desired airspeed on a dial and a subtraction
produces the airspeed error. An increase in airspeed requires the application
of forward cyclic to tilt the thrust vector, an increase in collective pitch to maintain
altitude, a change of lateral cyclic trim because of inflow and a change of tail rotor
pitch because of the increased torque.
In a simple system the airspeed control might simply apply a forward pitch command
to the vertical attitude reference. The resulting loss of height would then be sensed by the
altitude hold system that would increase collective to maintain height. The increased
torque would cause the yaw stabilizer to operate and the roll attitude would be held
against inflow changes by the roll axis stabilization. However, all of these changes
could take some time to stabilize. Instead of letting an error occur and then allowing
the automatic system to cancel it, it is more accurate to anticipate the error by issuing
an appropriate command. This is known as feedforward. For example, if the amount
of tail rotor pitch needed to cancel torque is known for all combinations of power and
airspeed, then if a power change is commanded, the appropriate pitch change can be
added directly to the tail rotor. In this way the tail would not yaw at all during a power
change unless the feedforward had not been correctly assessed. Part of the development
phase of an autopilot system is to map the feedforward parameters over the entire flight
envelope.
Feedforward of this kind may be used in manually controlled helicopters. For example,
the collective lever position can usefully be linked to the tail rotor pitch. An increase
in collective pitch will require more rotor drive torque and a corresponding increase
in tail rotor pitch. By passing the tail rotor cables over a system of pulleys operated
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