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Tail Rotor Failure at a Hover
There are a couple ways a tail rotor can fail while at a hover. The tail rotor
can be stuck at a particular pitch setting, but still be producing thrust, or
the tail rotor can stop producing thrust. A slight variation on the second is
that if one blade fails to flat pitch, the tail rotor may be producing half
the normal thrust.
Stuck Pedals (fixed pitch failures)
A fixed pitch failure of the tail rotor is one kind of failure, but we tend
to charactersize it as stuck left, stuck right, or stuck neutral. This is
because depending on what pitch setting it is stuck at will determine what
kind of a recovery we will do.
Stuck neutral
This is probably the most likely fixed pitch failure to occur. It would
happen either because the pitch change mechanism failed while the tail
rotor was at flat pitch, or more likely that the pitch change mechanism
failed, and air loads drove the tail rotor to flat pitch. In either case,
the recovery is the same as if the tail rotor stopped producing thrust;
the pilot performs a hovering autorotation.
Stuck right
In this case, the tail rotor and the engine are both producing thrust to
the right. The spin will be quite rapid. The pilot should roll off the
throttle to get rid of engine torque. This will slow the spin, but not
stop it because the tail rotor thrust alone is capable of spinning the
helicopter. The next thing the pilot should do is his best hovering
autorotation, preferably over a flat piece of hard surface, although
he's going to have to take pretty much whatever was under him at the time.
One thing he can try to do is to hold the helicopter off the ground as
long as possible with collective. As RPM decays, the tail rotor thrust
will decrease and the spin rate should slow, but not stop. The helicopter
will contact the ground spinning (but hopefully slowly) and may roll over
if the ground is uneven, or the pilot did not hold the skids level. Chances
are fair, however, that the helicopter will remain upright on the landing
gear.
One thing I was shown by a friend who chopped his entire tailboom off with
a wire is that if a pilot is trained properly, he can maintain a helicopter
in a level attitude at a very rapid spin. However, most pilots are not
given this training which is a shame. Although you can get dizzy from
practicing this, it is not dangerous and any pilot can be quickly taught
how to keep the skids level even during a very rapid spin.
Stuck Left
(to be supplied)
Paul Cantrell
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)
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