Rolling Moments

[rotorque]

G'day guys,

A while ago there was a thread refering to loss of tail rotor thrust and how it effects helicopters in general. At one stage there was a comment made by myself and others regarding the rolling moment when a helicopter starts to spin in the hover. I attributed this to the drag created by the vertical stabiliser being below the level of the rotor head, causing the fuselage to tuck under and roll the helicopter over to the point where rotor thrust will not be able to support the 'weight' of the helicopter etc etc.

I think it was Nick who mentioned that the rolling moment is made around the Centre of Gravity. I am after clarification of this as I can't get my head around it.


Any help on this ?

[Nick Lappos]

Rotorque asked:
.....regarding the rolling moment when a helicopter starts to spin in the hover. I attributed this to the drag created by the vertical stabiliser being below the level of the rotor head, causing the fuselage to tuck under and roll the helicopter over to the point where rotor thrust will not be able to support the 'weight' of the helicopter etc I think it was Nick who mentioned that the rolling moment is made around the Centre of Gravity. I am after clarification of this as I can't get my head around it.

Nick sez:

Whatever rolling or pitching that might occur in a loss of tail thrust event is probably not purely aerodynamically induced, as the velocities developed by the vertical tail surfaces are not high enough to generate much roll or pitch. If the aircraft spins at 1 turn every 3 seconds or so, a small helicopter will generate about 20 mph of side velocity on the fin, a small amount relative to all the other stuff that is happening.

I have discussed such maneuvers with two pilots who rode through the loss of tail thrust in a hover event, and actually held control over the situation, lowered the collective and waited to get to about 10 feet wheel height before cutting the throttles. The trick is to spot the horizon, and keep the cyclic going so that the disk stays somewhatlevel against the horizon. The aircraft will turn around the center of gravity, which might be a few inches from the mast, and this could cause the machine to gradually dish out and begin a sloppy spiral where pitch or roll might start getting out of hand. The cause is quite likely pilot disorientation, and perhaps reluctance to trim the cyclic quickly around and around to keep the spin a one axis event. In a strong wind, the amount of cyclic needed might be an inch or two.

Imagine that you have trimmed into the wind, and the cyclic is perhaps one inch forward of trim to hold the steady hover. If you slowly turm to the right, you must move the stick to the left and aft so that you had perhaps 1 inch of left stick when cross wind, and then one inch of aft stick when down wind. If you don't move the cyclic this way, the aircraft will roll and then pitch away from the wind and the maneuver will approximate a swinging path probably nose down and rolling opposite the yaw direction. After a bit, the attitudes could get quite large.

I have done dozens of rapid turns in Comanche and also in the Fantail demonstrator aircraft with yaw rates of about 4 seconds for 360 degrees, and this cyclic technique does work, but I doubt anyone will learn it in the unlikely event of a tail rotor thrust loss.

In any hovering tail rotor thrust loss case, from a low hover, cut the throttle, from a high hover, try to keep cool, spot the horizon, keep things generally level (green on the bottom half of the field of view, blue on the top half), lower the lever a bit, and when the ground comes up, cut the throttle cushion with collective, and then kiss the ground.

[heedm]

Nick, that's how I was taught on my wings course. When I went on a Mountain Flying Course my instructor (giga-hours on helicopters) showed how you can use timing and collective to land smoothly in a helicopter with pedals stuck in a right yaw hover rather than chopping the throttle.

I think it's playing with the fact that the airframe responds slowly to the inputs. Also, wind or a little translation helps.

He demonstrated it, talked me through one, and then I was doing them on my own. If I could pick it up like that, I'm sure it could be taught ab initio.

This instructor is also the guy who landed an auto from the left seat of a 206 with his left hand on his cyclic and his right hand on the student's collective. That's another story.

Matthew.

[ 11 August 2001: Message edited by: heedm ]