S92 "unexpected control responses"

[JohnDixson]

T/R Pitch Centering Springs

Hello Mast Bumper. May I provide some more specific information on this design feature. BTW, this design feature is incorporated into the basic S-92 as well.

Actually, the nomenclature you applied is misleading. In the vertical tail structure of the S-70, and prior to the cables being attached to the tail rotor servos, the two tail rotor control cables are attached to opposite sides of a control quadrant. Each side also has a spring capsule. The idea is that in case one cable is shot away anywhere over the cable run from up forward in the machine, the other cable side of the quadrant remains in tension, against the spring force afforded by the spring capsule on the other side of the quadrant. The output side of the quadrant feeds the servo inputs. Full tail rotor authority remains. A switch in the failed cable side of the quadrant illuminates a cockpit warning capsule. The same basic arrangement is found in the basic S-92 design. It has been combat proven on the S-70.

I might add another design feature of the cross beam tail rotor design in both helicopters: As assembled, the cross beams are of course flat, BUT, the airfoils as attached to the beams do not have their zero lift lines coincident with the flat surface of the beams. The airfoils do incorporate twist, and the built in positive angle of attack of the design facilitates flight between 40 KIAS and 120 KIAS with a reasonable amount of sideslip, with no control input. We flight tested the S-70 with both stages of the tail rotor servos depressurized at the same time, and that was the result. We were almost able to come to a hover ( with rather a lot of force on the left pedal (that force is actuated thru the servo control follow-up link ), but not quite, hence the emergency procedure to perform a run on landing if that dual failure were to occur. For non S-70 pilots: the S-70 TR servos are pressurized by the No. 1 and the Backup Hydraulic Systems ( the backup being the same pump as the main No 1 and No 2 systems, but electrically driven ).

Anyway, as you can see, the tail rotor quadrant and spring capsules are probably not a factor in this event, at least as far as the information posted so far indicates, and I am not privy to any other.