YAW CONTROL

In most flight conditions it is desired to maintain the sideslip angle equal to zero. If the airplane has positive weathercock stability, and is truly

symmetrical, then it will tend to fly in this condition. However, yawing moments may act upon the airplane as a result of unsymmetrical thrust (e. g. one engine inoperative), slipstream rotation, or the unsymmetrical flow field associated with turning flight. Under these circumstances, (i can be kept zero only by the application of a control moment. The control that provides this is the rudder. Another condition requiring the use of the rudder is the steady side-slip, a maneuver sometimes used, particularly with light aircraft, to increase the drag and hence the glide path angle. From (8.1,2 and 5), the rate of change of yawing moment with rudder deflection is given by

This derivative is sometimes called the “rudder power.” It must be large enough to make it possible to maintain zero sideslip under the most extreme conditions of asymmetric thrust and turning flight.

A second useful index of the rudder control is the steady sideslip angle which could he maintained by a given rudder angle if the aileron angle, roll rate, and yaw rate were all zero. The total yawing moment would then be

+ Сщдг

For steady motion, Gn = 0, and hence the desired ratio is

»r СПй

The rudder hinge moment and control force are treated in a manner similar to that employed for the elevator. Let the rudder hinge-moment coefficient be given by

Cftr = 4" Ьфг (8.2,4)

The rudder pedal force will then be given by

P = G? V*Srcr(b1*F+b2dr)

A

= G? VSMhl-P + a) + 6A)] (8.2,5)

A.

where G is the rudder system gearing.

The effect of a free rudder on the yaw stiffness is found by setting Ghr = 0 in (8.2,4). Then the rudder floating angle is

^free = –r<*F

b2

The vertical-tail lift coefficient with rudder free is found from (8.1,2) to be

(8.2,7)

The free control factor for the rudder is thus seen to be of the same form as that for the elevator (see Sec. 6.6) and to have a similar effect.