The Art of the Helicopter John Watkinson - Karatunov.net
The Art of the Helicopter John Watkinson - Karatunov.net
The Art of the Helicopter John Watkinson - Karatunov.net
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254 <strong>The</strong> <strong>Art</strong> <strong>of</strong> <strong>the</strong> <strong>Helicopter</strong><br />
In smaller helicopters <strong>the</strong> controls are power assisted and in <strong>the</strong> event <strong>of</strong> hydraulic<br />
failure <strong>the</strong> pilot can still operate <strong>the</strong> controls, albeit with greater effort. <strong>The</strong> JetRanger,<br />
for example, uses a single power assisted control system and can be flown without it,<br />
whereas <strong>the</strong> CH-47 has duplicated fully powered controls. This section first considers<br />
hydraulic power principles and uses <strong>the</strong> machines mentioned as examples <strong>of</strong> actual<br />
practice.<br />
<strong>The</strong>re are many ways <strong>of</strong> obtaining power operation, but <strong>the</strong> high pressure hydraulic<br />
system has <strong>the</strong> advantage that large forces can be developed in compact actuators. <strong>The</strong><br />
linear action <strong>of</strong> hydraulic rams is easy to integrate into real mechanisms. In hydraulic<br />
systems, losses are dominated by viscosity. <strong>The</strong> work done by an hydraulic ram is <strong>the</strong><br />
product <strong>of</strong> <strong>the</strong> pressure, <strong>the</strong> piston area and <strong>the</strong> travel, whereas <strong>the</strong> volume <strong>of</strong> fluid used<br />
is <strong>the</strong> product <strong>of</strong> <strong>the</strong> area and <strong>the</strong> travel. As <strong>the</strong> flow is inversely proportional to <strong>the</strong><br />
pressure, it follows that <strong>the</strong> smallest flow losses will be experienced when <strong>the</strong> highest<br />
practicable pressure is used. This is limited by <strong>the</strong> availability <strong>of</strong> sealing materials.<br />
A pressure <strong>of</strong> 3000 pounds per square inch is not unusual.<br />
Simple hydraulic pumps can be made with meshing gears, but higher flow hydraulic<br />
pumps use pistons. Figure 6.28 showed a swashplate fuel pump. <strong>The</strong> same approach<br />
can be used for hydraulic systems. <strong>The</strong> pistons are fitted with ball-jointed slippers that<br />
contact <strong>the</strong> angled swashplate. As <strong>the</strong> pump body turns, <strong>the</strong> pistons oscillate with<br />
amplitude controlled by <strong>the</strong> swashplate angle. <strong>The</strong> swashplate may be tilted with a<br />
pressure-sensitive actuator. As target system pressure is approached, <strong>the</strong> eccentricity<br />
<strong>of</strong> <strong>the</strong> swashplate is reduced. This is more efficient than a fixed delivery pump with a<br />
relief valve used in small systems.<br />
If hydraulic pressure is supplied to a pump <strong>of</strong> this kind it will act as a motor. <strong>The</strong><br />
swashplate can <strong>the</strong>n be fixed. As an alternative, hydraulic motors may have radial pistons<br />
acting on an eccentric. Hydraulic motors are used for winches and for engine<br />
starting.<br />
Fully powered systems operate with larger forces and <strong>the</strong> losses in <strong>the</strong> system result in<br />
heating <strong>of</strong> <strong>the</strong> fluid. Excessive fluid temperature may cause boiling and loss <strong>of</strong> control in<br />
addition to damaging <strong>the</strong> seals. <strong>The</strong> fluid reservoir may have cooling fins. Alternatively<br />
<strong>the</strong> fluid circuit incorporates an oil cooler which will typically have a fan driven from <strong>the</strong><br />
transmission. In most systems, fluid returning to <strong>the</strong> reservoir passes through a backpressure<br />
valve which maintains <strong>the</strong> return flow at a significant pressure. This prevents<br />
cavitation in <strong>the</strong> pump, reduces <strong>the</strong> probability <strong>of</strong> boiling and prevents dissolved air<br />
from causing frothing and airlocks in <strong>the</strong> system. Any dissolved air will come out <strong>of</strong><br />
solution in <strong>the</strong> low pressure region following <strong>the</strong> back-pressure valve and can escape<br />
into <strong>the</strong> reservoir.<br />
Filters are used to remove any foreign bodies from <strong>the</strong> oil as <strong>the</strong>se could cause seal<br />
damage and premature wear as well as malfunctioning <strong>of</strong> delicate parts such as valves.<br />
Many systems incorporate an accumulator or hydraulic energy reservoir. An accumulator<br />
consists <strong>of</strong> a reinforced cylinder or sphere in which a flexible membrane separates<br />
<strong>the</strong> hydraulic oil from pressurized gas which is typically nitrogen. <strong>The</strong> accumulator<br />
absorbs rapid fluctuations in oil pressure, allows peak flow in excess <strong>of</strong> <strong>the</strong> pump<br />
capacity and also provides continued operation for a time after a pump failure. In <strong>the</strong><br />
Chinook an hydraulic accumulator is used to start <strong>the</strong> APU. <strong>The</strong> starter motor acts as<br />
a pump when <strong>the</strong> APU is running.<br />
In almost all cases each hydraulic pump will feed a number <strong>of</strong> actuators. A single<br />
leaking pipe or actuator would allow all <strong>of</strong> <strong>the</strong> oil to be lost and cause all <strong>of</strong> <strong>the</strong> actuators<br />
to fail. A number <strong>of</strong> safety measures can be incorporated. First, <strong>the</strong> oil reservoir is fitted<br />
with a float switch to warn <strong>the</strong> pilot if <strong>the</strong> oil level is falling. This would happen if a pipe<br />
fitting was ‘weeping’ oil slowly. An oil temperature gauge may also be fitted. Oil loss will