The empennage on a helicopter consists of the vertical and horizontal stabilizer
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namic lifting force (positive or negative). The primary purpose of a stabilizer is to enhance stability about a particular axis (pitch, yaw), although there are secondary aerodynamic characteristics of stabilizers that are important design considerations for helicopters. These problems are often associated with component interaction aerodynamics – see Chapter 11.
6.7.1 Horizontal Stabilizer
The primary purpose of a horizontal stabilizer is to give the helicopter stability in pitch. Both the helicopter rotor and its fuselage have an inherent negative stability derivative in pitch and the stabilizer helps to give the helicopter better overall handling qualities. The fuselage itself has a powerful negative stability because of the typically large surface area of the airframe forward of the center of gravity – see Prouty (1985). The selection of the size and position of the horizontal stabilizer on the tail has proven to be one of the most difficult challenges facing helicopter designers. In addition to aerodynamic, structural, weight and stability and control trade-offs, there are important aerodynamic interactional effects between the rotor wake and the tail region – see Section 11.3.1.
Prouty & Amer (1982) describe how there are basically three types of horizontal stabilizer designs used on helicopters: a forward mounted stabilizer, an aft mounted low stabilizer, and a T-tail design. Using a forward fixed stabilizer will generally avoid any sudden changes in download caused by wake impingement because it will remain inside the rotor wake boundary from hover up until a fairly high forward flight speed is reached. Many Bell helicopter designs use this forward stabilizer position. However, because the reduced moment arm, the surface must be larger (and heavier) compared to a stabilizer mounted further back along the tailboom. The stabilizer, however, may have a capability of being used for trim augmentation in that the pitch angle may be linked into the longitudinal cyclic. In hovering flight the rotor wake produces a vertical download on forward mounted horizontal stabilizers, which usually represents a significant performance penalty. The Bell designs also use an inverted airfoil for the stabilizer, which creates a download in forward flight to keep the fuselage at an angle of attack for lowest parasitic drag. It is also designed to stall when the helicopter is in steep autorotation to avoid producing an upthrust and an undesirable nose-down pitching moment on the fuselage.
A stabilizer that is mounted low down near the end of the tail has good structural efficiency, with all the loads being carried directly into the tail boom. However, this stabilizer design tends to produce interactional aerodynamic issues and trim during transition from low-speed flight into hover or vice versa, where the main rotor wake may suddenly move forward over the empennage location and so produce a nose-down pitching moment on the helicopter – see Section 11.3.1. Also, the unsteady separated flow from the upper fuselage and rotor hub tends to reduce the efficiency of this type of stabilizer design so that the lifting area needs to be greater than for one that could be located away from the wake. On military helicopters, clearance issues between the tail and the ground may be important and can preclude this particular design choice. Nevertheless, as evidenced by the large number of helicopters with this low mounted horizontal stabilizer configuration, it is a popular design choice for small to medium size helicopters.
In the T-tail design, the horizontal stabilizer is mounted at the top of the vertical fin. This moves the stabilizer away from the rotor wake for most flight conditions, and so it can be smaller in area to give the same overall stability. However, the design is structurally inefficient because of the higher overall weight of the vertical fin required to carry the stabilizer loads, and also because of various low-frequency structural vibration modes that can be excited by the main and/or tail rotors. On a low or forward set stabilizer, it may be necessary to have different incidence (pitch) settings for the port and starboard sides to account for the gradients in downwash in the rotor wake. With a T-tail design, there are often twisting moments that may limit the maximum area of the stabilizer. Often a compromise is drawn by using a stabilizer mounted to only one side of the fin, such as used on many of the Sikorsky machines. See Prouty & Amer (1982), Prouty (1983), Hansen (1988), and Main & Mussi (1990) for further information on stabilizer design.
A stabilator is a stabilizer that has a variable incidence (pitch) capability. It can help give better overall handling qualities over a wider range of flight conditions than is possible with a stabilizer of fixed pitch, including hovering flight. The stabilator is set to large positive pitch angles in hovering flight to help reduce download effects and is reduced to low pitch values in cruise flight. The pitch angle is set automatically by a flight control computer using airspeed and other measurements, although manual override can be used to give the pilot control of the stabilator below certain airspeeds. A stabilator is mechanically relatively complicated compared to a stabilizer because it requires a pitch change mechanism, and is also a structurally inefficient design choice because of its higher weight. However, sometimes it can be the best choice to meet the demanding flight envelope of military helicopters. For example, a stabilator design is used on both the AH-64 and UH-60 helicopters.