PPRuNe Forums - View Single Post - Tail Rotor Control Failure
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Old 24th Jan 2002, 08:44
  #9 (permalink)  
Nick Lappos
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With the symptoms as described, the tail rotor controls are producing too much thrust in the part power descent, so the nose swings left. For such a rare situation (often found in simulators, rarely anywhere else) the idea is to make an approach so that the speed is slowly reduced and power is slowly brought up until the speed/power combination for a nose straight ahead is found. If timed well, the touchdown occurs then. If the pilot allows the nose to get to the right because the power was allowed to get too high (and the speed was allowed to get too low) a gentle nose down will increase the descent, but will reaccelerate the helicopter until the nose swings back straight ahead. If you mess up and the nose begins to spin to the right, don't fully cut the throttle, but pull it back slowly a bit at a time while increasing the collective to arrest the descent. This will allow you to get more main rotor torque and less tail rotor thrust by reducing the rpm, and thus reducing the tail thrust and stopping the spin.

On experimental helos, I had the chance to practice these to touchdown on many occasions, and it is not too difficult. It is even possible to make a go-around, if you gently raise the collective to get a small but controllable left yaw, and concurrently lower the nose to begin a gentle climb.

About the wind, it depends on where the controls are stuck. If the tail thrust is more than that needed for hover, the right wind is helpful. If the tail thrust is less than hover thrust, a left wind is helpful. In both cases, your use of collective and speed dominate, and the wind contribution is small, unless the wind is hellacious, in which case you should land into the wind! Generally, controls that stuck do so where they are placed, and not beyond, so it is unlikely that the tail controls will stick to a position of hover thrust or greater (since one assumes that they stick in a forward flight setting). I believe the flight manual case cited assumes the controls stick to very high thrust.

For a tail-control-stuck-in-cruise case, if the runway has a wind from the left, this will add some anti-torque to the situation, and will allow the touchdown to be made slower (picture that a healthy helicopter will need some extra right pedal in the hover i.e. a little less tail rotor thrust, which is where the pedals are stuck).

After touchdown, lowering the collective will cause a sharp left yaw, and a possible ground-loop. One way to avoid it is to leave the collective pitch at the touchdown setting and while rolling out, pull back the throttles slowly, while using brakes to hold things straight. The throttle roll-back will reduce the main and tail rotor RPM exactly together (of course) and so will reduce their thrust concurrently, thus maintaining the anti-torque balance.

[ 24 January 2002: Message edited by: Nick Lappos ]</p>