Zero Lift Angle of Attack

The zero lift angle of attack of the vertical stabilizer is affected by built-in camber. Some helicopters are designed with cambered fins to unload the tail rotor in forward flight in order to reduce loads in the blades and tail rotor control system. Despite some wishful thinking, this feature seldom reduces the total engine power required to fly at high speed, since the induced drag of the cambered vertical stabilizer will generally be higher than that of the tail rotor. If the camber is achieved by using a conventional airfoil, the angle of zero lift can be obtained from such collections of airfoil data as appear in references 8.1, 8.15, or 8.16. If the camber is achieved with a deflected trailing edge like a rudder, a simplified version of a method given in reference 8.1 can be used. The equation for the change in the angle of zero lift is:

where ct and (a5 /a5 ) are functions of the geometric parameters of the

Sidewash Angle

The sidewash angle at the vertical stabilizer is produced by lateral velocities induced by the main rotor, by the tail rotor, and by the sidewash caused by the

Source: Hoak, "USAF Stability and Control Datcom,” 1960.

fuselage in sideslip. There has been less attention paid to sidewash at the empennage than to downwash, so analysis will have to rely primarily on estimates. One set of available test data from reference 8.7 is given in Figure 8.21 for the Hughes AH-64 (without wings). The top figure shows the rather chaotic pattern existing at the empennage. (Note: These vectors are based on averages of some

Source: Logan, Prouty, & Clark, “Wind Tunnel Tests of Large – and Small-Scale Rotor Hubs and Pylons,” USAAVRADCOM TR-80-D-21, 1981.

even more chaotic instantaneous measurements.) The tendency of the flow to move to the right at the higher survey locations is due to the swirl in the main rotor wake. The lowest row of measurements is probably reflecting the effects of vortices generated by the nacelles.

The sidewash angle induced by the tail rotor may be assumed to be a function of the momentum value of induced velocity. Unless there is an appreciable separation distance, the effect may be assumed to be the same as that of the main rotor on the fuselage; that is:

where

In most cases, the sidewash induced by a sideslipping fuselage will be small enough to ignore. This is another way of saying that there is not yet much data on this effect. For those analyses that must at least give the appearance of completeness, it may be assumed that:

where this value is taken from Figure 8.15 for the downwash effect of the body alone. The final approximation is thus:

rf= 0.06|3