Tail Rotors
On a helicopter with a single main rotor, the main rotor is usually driven by means of a main rotor shaft. This shaft transmits torque from the engine to the main rotor. In this configuration an equal an opposite torque is felt about the main rotor shaft which if uncorrected will result in a yaw in the opposite direction to main rotor rotation.
In most light helicopters with this configuration we counter this problem with the installation of a tail rotor (a source of lateral thrust) mounted on the end of a lever arm (the tailboom). As we know torque can be defined by a force applied at a distance, if this torque is made equal and opposite to the main rotor torque no net yaw will result. If the force applied by the tail rotor to the tailboom is varied the torque about the main rotor mast can be varied and directional control about the main rotor mast is achieved. This is achieved by either allowing the helicopter to turn with the main rotor torque (opposite to main rotor rotation) or to turn against it, higher yaw rates can be obtained in the direction opposite to main rotor rotation, this is after all the direction the fuselage wants to turn naturally.
Effects Of The Tail Rotor
Tail Rotor Drift (also known as translating tendency) is a lateral motion of the helicopter over the ground, in the direction of tail rotor thrust (towards the main rotor's advancing side). It caused by the sideforce generated by the tail rotor, which prevents yaw about the main rotor shaft but brings about a sideways movement. To counter this tendency the main rotor thrust is displaced toward the retreating side and the helicopter in effect flies sideways in the opposite direction, at the same speed it would be translated by the tail rotor. This results in the helicopter holding its ground position, but rolling due to the main rotor thrust now pointing towards the retreating side. In our conventional anti-clockwise rotating helicopter this is sometimes referred to as the 'left skid low tendency'. It is important to realise that the lateral CofG of the helicopter will have a great influence on the roll and will cause the the helicopter to roll towards the lateral CofG.
Tail rotor roll is a tendency for the helicopter to rotate about its vertical center of gravity (CofG). The amount of roll is dependant upon the vertical offset of the tail rotor from the vertical CofG, and the magnitude of the tail rotor thrust applied. The direction of the roll is dependant upon whether or the not the tail rotor lies above or below the vertical CofG, above will result in a roll towards the advancing side, and below will result in a roll towards the retreating side. This is not the way this phenomenon is described in several books, it is however the correct one!
In summary a high vertical C. of G. will reduce the amount of roll brought about in correcting Tail Rotor Drift (see above).
Where a tail rotor is installed in a helicopter with a teetering main rotor head (R22, R44, B206). There is a phenomenon known as mast bumping, which can be brought about by an incorrect recovery from a Low-G condition. The vertical offset from CofG to tail rotor referred to above causes a roll usually (in the case of low CofG) in the direction of tail rotor thrust. If this roll is countered with cyclic, as would be normal in a positive (1 or more) G situation then rotor system merely flaps above the mast and contacts the mast (hence the term mast bumping).
The tail rotor is like a small main rotor turned on its side. As such it does gain the benefits of
effective translational lift just like a main rotor at the same kinds of forward speeds. This can be seen on transition to forward flight by a marked reduction in the amount of 'power pedal' required to hold heading at low (15kts) forward airspeeds.
Example Content
Most tail rotors are fitted with some form of flapping hinge to reduce bending stresses in the blades, hub and shaft. A side effect of the installation of these hinges is that the tail rotor will flapback in a similar manner to the main rotor when in forward flight. This acts to increase the power required for forward flight. During autorotation the application of pedal to counter the rotation caused by lack of torque reaction will reverse the airflow through the tail rotor and the tail rotor will once again flapback. It is mentioned here out of interest alone and is not required knowledge for the PPL.
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