The discussion so far has only been concerned with determining the power required by a single main rotor. Such a rotor system will generate a torque reaction in the fuselage and without some form of anti-torque device the fuselage will spin uncontrollably. The most common anti-torque system is a small rotor affixed to a tail boom and orientated so that it generates a moment as it rotates which acts in opposition to the torque reaction. Alternative systems based on ducted fans, or fenestrons, and pressurized air (the NOTAR) have been used with great success. It should be remembered that tail rotors are required mainly in low speed flight when there is insufficient airflow over the fin to generate a side force that can oppose the torque reaction.

The tail rotor power can be estimated by considering the torque it must oppose. The main rotor torque is responsible for the torque reaction and thus the tail rotor thrust can be found by a simple balance of moments. Therefore:

T і — q — PMR 1 TR »TR — QMR — 0


where, in forward flight:

Подпись: 1 f 1 L2XCT + 2 p3 A + 8 sCD(1 + 4.3p2 ) + Cpm + CPS

Подпись: MR

pmr — pAMR ()R)Mr or, in axial flight:

Подпись: 1.151 ^+1 j CT+8 sCD + CPm + CPsPMR — pAMR ()R)Mr


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