Most helicopters use hydraulics to reduce the force needed to move the controls. Usually
there is still a mechanical connection. In some helicopters there might not be a mechanical
connection, or the control forces may simply be too high for the pilot to overcome with
muscle. Generally those sorts of helicopters will have redundant hydraulic systems.
Both the Bell JetRanger/LongRanger and the Robinson R44 have boosted controls with mechanical
backup. There are multiple ways such hydraulic systems can fail, but basically you can either
have a failure where the entire system stops working, or a failure where a part of the system
Total Hydraulic Failure
Of the two failure modes, this is probably the easier to deal with. When the failure occurs,
the pilot will notice that the control boost is gone, and he will have to fly the aircraft
using much more force to move the controls. In the case of the cyclic and collective, he will
also usually notice that they are connected, i.e. when he moves one control, the feedback
from the rotor system will cause the corresponding control to also try to move. This effect
is small and can easilly be overcome.
In a small helicopter such as the Bell or the Robiinson, the stick forces are low enough that
the pilot can fly the aircraft with little difficulty. If the pilot has to fly for a significant
amount of time before landing, he will probably be fatiqued and it may be difficult to avoid
overcontrolling the helicopter while attempting to hover. The aircraft manufacturers generally
recommend a running landing instead of attempting to hover in order to avoid this.
Partial Hydraulic Failure
A more difficult failure may be when one hydraulic servo fails, but the others continue to work.
This means that his controls are boosted in some parts of their movement, but not in others.
Such a failure could easilly result in an aircraft that is not flyable by the average pilot, so
Bell and Robinson both provide a switch to allow the pilot to disable the hydraulic system. When
the pilot activates the switch, the entire hydraulic system is defeated meaning that the pilot
still has to contend with a total hydraulic failure, but at least all the stick forces are
equally high, and the average pilot should be able to fly the aircraft with little trouble.
Both the Bell and Robinson hydraulic override systems are "failsafe". That is, it takes an
electrical circuit to hold the hydraulic system in the override condition. If the electical
system should fail, the hydraulics will continue to be boosted regardless of the position
of the hydraulic override switch. The prevents loss of the aircraft electrical system from
also causing a hydraulic systems failure.
Another failure mode of a hydraulically boosted control system is that one of the servos could
be driven to one extreme position or the other by a failure within the servo. This would result
in an unflyable aircraft. The pilot can recover from such a failure by turning off the hydraulic
system, assuming that the hardover failure did not cause him to lose control of the aircraft.
paul at copters.com
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