The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 255
result in the remaining oil circulating more often and this will result in a temperature
rise. If a seal fails inside an actuator there may be no net loss of oil, but there will be a
short circuit from the high pressure feed to the return and this will also result in heating.
Sometimes hydraulic systems fail catastrophically because of a burst hose or battle
damage. The solution here is to install the hydraulic equivalent of fuses. Each actuator
is fed by an independent pipe run from the pump manifold. This manifold is fitted with
flow-sensitive valves. These contain a spring-loaded ball that normally allows fluid
flow. In the case of a burst, the violent flow due to escaping fluid will push the ball
onto its seat causing it to seal off the leaking pipe. The pressure in the leaking pipe will
then be lost and the pressure differential will hold the ball valve shut indefinitely. The
back-pressure valves will prevent oil loss via the return pipes. In this way a burst pipe
or hose only affects one actuator and the others remain operational because total fluid
loss is prevented.
In the same way that electrical faults can be isolated by pulling circuit breakers, in
some cases the pilot can isolate parts of the hydraulic circuit with valves so that faulty
or damaged units are prevented from impairing the remainder of the system. These
valves may be remotely driven by electric actuators, but a manual operating lever will
be provided in case of electrical failure.
Figure 6.42 shows the basic components of a JetRanger hydraulic system having
power-assisted actuators. The machine can be flown manually in the event of hydraulic
failure. The hydraulic fluid is held in a finned reservoir that acts as a cooler. A sight
Fig. 6.42 Hydraulic system of the Bell 206 is not duplicated as the machine can be flown manually in the case
of failure.