I’ve often been asked why helicopters with one skid fly low. Helicopters with a counter-clockwise rotating rotor system will hover skid-deep to the left and helicopters with a clockwise rotating rotor system will hover skid-deep to the right.
Now two factors must be considered to explain this phenomenon. The first is what we call gear bias or tail rotor drift. This factor creates a lateral pull on the fuselage of the helicopter and in the case of a counter-clockwise rotating rotor system, this pull will be to the right. Very briefly explained, it is the result of the tail rotor force acting at the tip of the tail boom, counteracting the torque reaction trying to rotate the fuselage clockwise.
To counteract this lateral pull, the second factor is tail rotor roll. To counteract the sideways motion to the right, the gyro must be positioned slightly to the left, which tilts the total rotor thrust to the left, inducing a leftward horizontal component of the total rotor thrust that acts in the same plane as the plane of rotation.
Normally, in small helicopters, the tail rotor, which generates a force to the right when viewed from behind on the helicopter, acts below the plane of rotation. This force, opposing the torque reaction, pairs with the horizontal leftward force that was the result of the cyclic (total rotor thrust) tilting to the left. To put it a little more simply, imagine a horizontal line with one force acting to the left and above the line and another force acting to the right and below the line. The two forces acting against each other and not in the same line create a rolling moment.
As I mentioned earlier, this is just a brief explanation of a not-so-simple aerodynamic principle.
A final comment on tail rotor roll is that it is most noticeable when hovering off course as it is related to the torque the helicopter is developing. For this reason, a counter-clockwise rotating helicopter tends to drift to the right during the transition from forward flight back to hover because power or torque is applied slowly as speed decreases.
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