Laurie:
Good to see you back and active again!
Can you describe for us rotorless pilots how you might accommodate a tail rotor failure and keep flying? I had always assumed that your only choice was to go to a no-power (ie, autorotation) mode, to avoid becoming a centrifuge.
Hey Spike! I'm probably not as active as I ought to be, but slowly coming out from under my rock
Basically you have two kinds of tail rotor failures: A drive failure in which the tail rotor no longer turns, i.e. the drive shaft breaks or seizes, or a control failure where the tail rotor is still spinning thus creating thrust, but you are unable to control or change that thrust, i.e., it's 'stuck' or fixed.
If I have a drive failure and the tail rotor stops spinning creating the thrust to counteract the torque effect of the main rotor, I may be able to continue provided that I have enough forward airspeed to essentially streamline the aircraft. As an earlier post said, it will fly 'cockeyed' but is controllable. This depends heavily on the design of the vertical fin and tail section. The vertical fin is an airfoil, however on the 92 it is called a vertical 'pylon' because it really has no aerodynamic benefit; it is there simply to hold the tail rotor. Thus, on the 92 when I have a drive failure, I cannot stop the right hand rotation even in forward flight until I enter autorotation. In a hover or with little or no forward airspeed, that vertical airfoil isn't producing any or enough 'lift' to counteract the torque of the main rotor, so unless you have enough altitude to get airspeed while 'flying' the aircraft in the right spin, it's unlikely you will be able to recover and will have to accept a landing, using whatever rotor speed an inertia you have to cushion the landing.
Fixed pitch on the other hand, is controllable under most all conditions. For example, the aircraft I currently fly, the S92, has a positive 10.5 degrees of pitch on the tail rotor blades in cruise flight. For simplicity sake, we'll call this neutral because in the cockpit what you see is that the pedals are centered. If during cruise, both of my control cables break, I am unable to control the tail rotor, but it's still turning and providing anti-torque 'lift' at that 10.5 degree pitch setting. Now if I continue in cruise flight, the aircraft flies straight as an arrow, BUT, when I make a power change or airspeed change, thus changing the demand for anti-torque, the aircraft will begin to fly out of trim. If I get too slow, the rotation will increase to the right and will be uncontrollable, so in order to land I cannot come to a hover, but must land with forward airspeed. In this aircraft in this situation, the touchdown speed to remain aligned with the centerline of the runway is about 45-50 knots. As you can see, this will vary from aircraft to aircraft.
Now if I'm taking off from a hover say, I have a high power demand, thus a high anti-torque demand so I have left pedal applied, increasing the pitch on the tail rotor and increasing lift. If say something jams my pedals and they are stuck right there, when I reduce power the aircraft will yaw left in the direction of the pedal applied because the tail rotor is producing all that lift to the right. My options for landing this one is to land with high power, thus I will need a slower forward airspeed than the situation above. This will vary based on just how high of a power setting I had when the tail rotor fixed, but as an example, in the 92 it's in the neighborhood of 20 knots under most atmospheric and loading conditions.
If in cruise I decide to descend, I'll reduce power (lower collective), thus reducing the demand for anti-torque, therefore I may have to apply right pedal. If a torque tube breaks, cable jams, or pedals jam here, now I am going to have to land with more forward airspeed than both situations above. Forward airspeed will be critical in maintaing airflow over the tail surfaces to provide a streamlining effect in order to land the aircraft aligned with the centerline. Even though the tail rotor is still turning and producing lift, it's a lower setting than that required for cruise flight. In the 92, again as an example, this can be an airspeed in the 70-80 knot range. This scenario can be especially tricky because as the nose yaws right it tends to pitch up, reducing airspeed further. The airspeed that was keeping you controlled can go away in a real hurry if you don't anticipate and adjust pitch attitude accordingly. That's why in the helicopter world, we say lucky left and rotten right. A stuck left is lucky because it's easier to deal with and requires less runway and forward speed to land than a rotten stuck right. It also applies to wind, in that I would rather have a slight left crosswind for landing (lucky) than a straight on wind. A right crosswind would be downright rotten and make my job a lot harder. But that's another aerodynamics lesson.
I hope this wasn't too long-winded and answered your question.
Writers are so ignorant.
