The Art of the Helicopter John Watkinson - Karatunov.net

246 The Art of the Helicopter
are mounted in a diamond-shaped pattern on the shaft and connected electrically as a
bridge. Power is applied to the ends of the bridge and the outputs are taken from the
centre to a differential amplifier. As the shaft temperature increases, all of the strain
gauges will increase in length but this will have no effect on the differential signal as all
four gauges are equally affected. However, if the shaft is twisted, two of the gauges will
contract and two will extend, producing a differential signal. The power and signals
are communicated using rotary transformers.
In the second, the end thrust on one of a pair of skew gears is measured. One of
the shafts is allowed to move axially. As it does so it reduces the area of a port that
allows oil to escape from a chamber at the end of the shaft. The oil is supplied under
pressure from the engine oil pump, through a restrictor. Engine torque will slide the
shaft into the chamber until the restriction of flow causes the oil pressure to rise and
oppose any further motion. The oil pressure is now proportional to the torque and can
be displayed on a gauge.
6.30 The helicopter revcounter
The rotor RPM must be maintained at the correct setting during all modes of flight
since the response to the controls and the available lift are both affected by it. Low rotor
RPM makes the controls sluggish and reduces tail rotor authority. Vibration control
techniques employing tuning only operate at one RRPM.
The rotor rev counter is one of the most vital instruments in the helicopter. It is generally
concentric with the engine rev counter. Each instrument is independent and has
its own scale and needle, but the scales are graduated so that when the engine is driving
the rotors the two needles overlap and move together. In a piston engine machine, the
engine rev counter is crankshaft driven, whereas in a turbine engine the RPM of the
power turbine (N2) is measured. The rotor rev counter is driven by the transmission
after the autorotation clutch.
Figure 6.37 shows some typical displays. At (a) both engine and rotors are stopped.
At (b) the clutch is disengaged, and the engine has been started and is warming up. At
(c) the engine is warm and the pilot is starting to engage the clutch. At (d) the clutch
is fully engaged. The needles are said to be married. At (e) the pilot has advanced the
throttle to flight RPM with collective fully down. At (f ) the throttle is closed from
flight RPM and the rotors continue turning whilst the engine slows down. This action
tests the autorotation clutch. At (g) the machine is in normal flight. At (h) the pilot
has let the revs drop and the bleeper has sounded. At (i) the pilot has initiated a
practice autorotation. The engine has dropped to idle, but the rotors maintain flight
RPM. This is called ‘splitting the needles’. At ( j) the machine flares in autorotation.
Rotor revs have gone up to maximum permissible. If the pilot wishes to abort the auto
and resume powered flight, he must not advance the throttle at this stage, because the
high rotor revs will allow the engine to overspeed. The correct procedure is to reduce
the rotor revs first, and then to bring the engine revs up slowly so that the needles meet
at flight RPM as shown at (k). At (l) is shown the result of an engine failure. The engine
has stopped and the rotors are slowing down. It is necessary to enter autorotation.
Since the rotors are turning at normal speed, an in-flight restart does not require the
clutch to be disengaged. Not shown is a condition resulting from transmission failure
between the engine and gearbox. The rotor slows down and the engine may overspeed
as the correlator attempts to maintain RRPM. It will be necessary to enter autorotation
and shut off the engine.