Tail Rotor Povser
The power required by the tail rotor typically varies between 3 and 5% of the main rotor power in normal flight, and up to 20% of the main rotor power at the extremes of the flight envelope. It is calculated in a similar way to the main rotor power, with the thrust required being set equal to the value necessary to balance the main rotor torque reaction on the fuselage. The use of vertical tail surfaces to produce a side force in forward flight can help to reduce the power fraction required for the tail rotor, albeit at the expense of some increase in parasitic and induced drag. If the distance from the main rotor shaft to the tail rotor shaft is xtr, the tail rotor thrust required will be
where Q is the angular velocity of the main rotor. This assumes that there is no off-loading of the tail rotor by the fin. The interference between the main rotor and the tail rotor, and between the tail rotor and the vertical fin, is usually neglected in preliminary analysis. However, the effects of the main rotor wake may be accounted for by an increase in the induced power factor, к, to take into account the generally higher nonuniform inflow at the tail rotor location. The loss of tail rotor efficiency because of the vertical fin can be approximately accounted for by results discussed in Section 6.9.4. Although the tail rotor power consumption is relatively low, interference effects may increase the power required by up to 20%, depending on the tail rotor and fin configuration.
The tail rotor power required is initially high in hover, but quickly decreases as airspeed builds up and the main rotor torque requirements decrease. In high-speed forward flight, the tail rotor power required increases again as the main rotor torque increases to overcome parasitic drag. However, this can be offset to some extent by an aerodynamic side force that is produced on a vertical fin, such as by using a fixed incidence or using a cambered airfoil section. Because of the relatively low amount of power consumed by the tail rotor, for first estimates of performance the power required can be expressed as a fraction of the total main rotor power, with a good estimate being 5%.