Design Requirements

As for the main rotor, the power required to drive the tail rotor depends on the disk loading. Whereas a larger diameter may be preferable for low induced power requirements,

this is outweighed by several factors. First, a larger diameter usually means a heavier design and this is undesirable, mainly because of adverse effects on the helicopter’s center of gravity location. Second, to meet certification requirements it is usually desirable that the tail rotor disk loading and induced velocities be high enough so that sideward flight of at least 35 kts is possible without the tail rotor entering into the vortex ring state. Both of these constraints dictate the use of a relatively small tail rotor with a high disk loading.

Tail rotors typically have two or four blades, and it has been shown in Section 6.4.4 that for a rotor there is no particular aerodynamic advantage of one number over the other. Only collective pitch is required because there is no need to control the orientation of the tail

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power requirements. However, the amount of blade twist is usually very small compared to the main rotor to avoid losses in efficiency and the possibilities of stall when the tail rotor is operating in an effective descent, such as when hovering in crosswinds or in sideward flight. Although some blade designs may use cambered airfoil sections, the tail rotor blades found on many helicopters use symmetric airfoils because of their reasonable overall performance and low pitching moments. While the higher maximum lift obtained with cambered airfoils can help reduce rotor solidity and thereby minimize tail rotor size and weight, this can be outweighed by their larger pitching moments (which lead to higher control forces) and poorer performance when operating at negative angles of attack. Generally, tail rotors are designed to operate at tip speeds that are comparable to those of the main rotor. Lower tip speeds are desirable to minimize noise. However, for a given thrust, tail rotors operating at lower tip speeds require higher solidity to prevent blade stall. A lower tip speed also increases the torque requirement. Both of these factors will increase the weight of the drive system. A secondary effect of the anti-torque side-force is the tendency for the helicopter to drift

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means of cyclic pitch inputs) so that a component of the main rotor thrust produces an equal and opposite side-force. This is the reason why it will be noticed that a helicopter will tend to hover with one wheel (or skid) lower than the other. On larger helicopters, the main rotor shaft is physically tilted slightly (as part of the design, thereby introducing a pretilt) so that the pilot does not require as much cyclic pitch input to counter the tail rotor side-force. The tail rotor thrust and the component of the main rotor side-force act together, producing a couple, and thereby, a rolling moment about the center of gravity (c. g.). To reduce this moment, the tail rotor is located vertically up on the tail structure at a location so that the line of action of its thrust vector is close to the c. g. of the helicopter.

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