Tail

Aerodynamics of Tail Rotors The vast majority of helicopters in production are of the "conventional" single main rotor and tail rotor configuration. The purpose of the tail rotor is threefold: 1. To provide an anti-torque side force to counter the torque reaction of the main rotor on the fuselage. 2. To provide the pilot with directional control about the yaw axis. 3. To give directional (weathercock) stability. While at first glance this might seem a straightforward enough task for the tail rotor, it must be recognized that the tail rotor operates in a complex aerodynamic environment and this requires some considerable care in the design. It is actually quite difficult to design a tail rotor that has completely acceptable characteristics. The tail rotor is also mounted in proximity to a fin, tail assembly, and operates in fairly "dirty" interference flow field generated by the main rotor hub and fuselage wakes and also the energetic main rotor wake itself. This adverse environment means the aerodynamic design requirements for the tail rotor are similar but also different in many respects from those of the main rotor. There are two types of tail rotors: 1. Tractors: These tail rotors are mounted on the side of the vertical fin where the rotor slipstream is directed toward the fin. This design experiences a strong interaction between the tail rotor and the fin - a small increment in tail rotor thrust is produced, but this is offset by a large side force on the vertical fin in the opposite direction to the tail rotor thrust. 2. Pushers: These tail rotors are mounted on the side of the vertical fin where the rotor slipstream is directed away from the vertical fin. For this design there are only mild aerodynamic interactions, the main effect being a distortion to the inflow caused by the fin resulting in a mild loss of rotor efficiency. This is the most efficient tail rotor design and the most common to find on modern helicopters. The photo below shows a pusher tail rotor design (Sikorsky CH-53D).

This next photo (below) shows a tractor tail rotor design (Sikorsky UH-60). Note that in this case the tail rotor is canted relative to the vertical fin. This provides a small vertical component of thrust to allow for a larger center of gravity envelope, and without much loss of side force.

Note also that each of the tail rotors shown above turns in the 'preferred' direction, which means that the upper blade moves away from the main rotor. This provides the best compromise in terms of maintaining an overall aerodynamic environment that provides maximum thrust. The next example (below) is from a AH-64 Apache. Note the interesting 'non-orthogonal' blade design. The design consists of two teetering rotors, that are mounted one over the other. Mechanical considerations determined the non-orthogonal alignment of the two rotors, although some acoustic benefits are realized as well.

This example (below) shows the bearingless design used on the UH-60. There are no mechanical flap hinges or feathering bearings. The 'spider' assembly allows for collective pitch changes. Look carefully to see the 'pitch-flap' coupling or 'delta-3', which is a kinematic coupling between flapping and pitching, and minimizes blade flapping in response to the changing aerodynamic loads.

The simplicity of this tail rotor design (BK-117) is readily apparent.It is a teetering rotor assembly, which provides the lightest form of tail rotor design. Note that 'delta-3' pitch/flap coupling is introduced into a teetering rotor assembly by orienting the flapping (teeter) axis at an angle (in this case 45-degrees) to the blade axis.

Alternative anti-torque devices are also in wide-spread use. An alternative to the traditional tail rotor is a fenestron or fan-in-fin or fantail design. The fenestron is used on a number of helicopters, including the Eurocopter Dolphin (see below). The performance benefits realised from a fenestron allows for a smaller and lighter design to meet the same anti-torque requirements.