The Art of the Helicopter John Watkinson - Karatunov.net

284 The Art of the Helicopter
levers to move a large card painted with the earth at the bottom and the sky at the top.
In a dive the instrument is tipped nose down and this causes the horizon card to rise
with respect to the symbolic aircraft. In a climb the card falls. During a roll the outer
gimbal remains horizontal, as does the horizon card. The instrument effectively banks
around the card.
Clearly if a gyroscopic instrument is to work properly it must be fitted in the helicopter
with the correct orientation, which is usually with the instrument face vertical. The
artificial horizon is particularly sensitive to mounting attitude and if fitted to a sloping
instrument panel a wedge plate is used to keep the instrument level. Certain artificial
horizons can be adapted to work on a sloping panel. The card linkage is offset and
the caging cam is turned around the inner gimbal by the angle of tilt so that the gyro
remains vertical with the instrument tilted and the horizon in the centre of the display.
In a helicopter the attitude of the machine about the pitch axis depends on forward
speed. At high speed the nose down attitude would fool the instrument into thinking the
machine was diving. To overcome this, the symbolic aircraft is fitted with an adjusting
knob that allows it to be moved up or down slightly so that at the chosen airspeed it
can be set to the displayed horizon.
The gyroscope can only maintain its rotational axis if the gimbals allow it sufficient
freedom. This is true for moderate manoeuvres, but not for aerobatics. If a machine
equipped with an earth gyro performs a quarter loop so it is going straight up, the two
gimbals will become parallel and there will only be one degree of freedom. If, as this
condition of gimbal lock is approached, the machine also rolls, there will be violent
precession known as toppling as the gyro attempts to conserve momentum without the
necessary freedom. After toppling, the gyro will be useless until it has erected again. If
a gyroscopic instrument is expected to operate under aerobatic conditions, it will need
additional outer gimbals. These are servo driven from sensors on the inner gimbals so
that the latter are maintained at right angles at any attitude so that gimbal lock can
never occur.
7.15 The turn and slip indicator
The turn and slip indicator is actually two completely independent instruments but
combined in one housing as the two would be used together to make a turn. Figure 7.19
shows the mechanism of a turn indicator; actually a rate gyroscope. There is only one
gimbal pivoted on the fore-and-aft axis of the helicopter and the gyro shaft is transverse.
The gimbal is held central by a light spring and rocking of the gimbal moves a pointer
over a scale. If the helicopter turns, the gyroscope precesses and rocks the gimbal.
The faster the turn, the further the precessing gyro will be able to extend the centring
spring. A small dashpot is provided to damp the motion of the rocking gimbal. This
may consist of a piston sliding inside a cylinder having a small air bleed hole at the end.
This reduces pointer movement due to vibration or turbulence and displays the average
turn rate only. Helicopter turn indicators are generally designed to reach a scale mark
atarateof180 ◦ per minute, corresponding to the two-minute turn rate commonly
used in instrument flying. As the gyro is controlled by the centring spring, no action is
needed to compensate for earth rate and the instrument has no controls.
The slip indicator is no more than a weight in a fluid filled curved glass tube. In a
correctly banked turn, the apparent gravity should remain perpendicular to the cockpit
floor and the weight stays in the centre of the curved tube at the lowest point. If the
amount of yaw does not match the amount of bank, there is sideslip and this causes the
weight to slide away from the centre of the tube. Figure 7.20 shows that, in a fixed-wing