Controls 11

Speed Brakes

Speed brakes are devices to increase the drag of an aircraft when it is required to decelerate quickly or to descend rapidly. Rapid deceleration is required if turbulence is encountered at high speed, to slow down to the Rough-air Speed as quickly as possible. A high rate of descent may be required to conform to Air Traffic Control requirements, and particularly if an emergency descent is required.

Types of Speed Brake

Ideally, the speed brake should produce an increase in drag with no loss of lift or change in pitching moment. The fuselage mounted speed brake is best suited to meet these requirements,

Figure 11.21.

WING MOUNTED

SPEED BRAKES

FUSELAGE MOUNTED

SPEED BRAKE

Figure 11.21 Wing mounted & fuselage mounted speed brakes

However, as the wing mounted spoiler gives an increase in drag, it is convenient to use the spoiler surfaces as speed brakes in addition to their lateral control function. To operate as speed brakes they are controlled by a separate lever in the cockpit and activate symmetrically. There is no speed restriction for the operation of speed brakes, but they may “blow back” from the fully extended position at high speeds. Spoilers will still function as a roll control whilst being used as speed brakes, by moving asymmetrically from the selected speed brake position.

An example is illustrated in Figure 11.22. Speed brakes have been selected, and then a turn to the left is initiated. The spoiler surfaces on the wing with the up-going aileron will stay deployed, or modulate upwards, depending on the speed brake selection and the roll input. The spoiler surfaces on the wing with the down-going aileron will modulate towards the stowed position. The spoiler surfaces on the wing with the down-going aileron may partially or fully stow, again depending on the speed brake selection and the roll input.

SPEED BRAKES

SPEED BRAKE AND ROLL INPUT

Figure 11.22 Mixed speed brake & roll input

Effect of Speed Brakes on the Drag Curve

The drag resulting from the operation of speed brakes is profile drag, so it will not only increase the total drag but will also decrease VMD. This is an advantage at low speeds as the speed stability will be better than with the aircraft in the clean configuration.

Ground Spoilers ( Lift Dumpers)

During the landing run, the decelerating force is given by aerodynamic drag, reverse thrust and the wheel brakes. Wheel brake efficiency depends on the weight on the wheels, but this will be reduced by any lift that the wing is producing. Lift can be reduced by operating the speed brake lever to the lift dump position, Figure 11.19. Both the wheel brake drag and the aerodynamic drag are increased, and the landing run is reduced. On many aircraft types, additional spoiler surfaces are activated in the lift dumping selection than when airborne. These ground spoilers are made inoperative in flight by a switch on the undercarriage leg which is operated by the extension of the leg after take-off.

Controls 11

Controls 11

Directional Control

Control in yaw is obtained by the rudder. The rudder is required to:

•maintain directional control with asymmetric power.

•correct for crosswinds on take-off and landing.

•correct for adverse yaw.

•recover from a spin.

•correct for changes in propeller torque on single-engine aircraft.

Effect of Rudder Deflection

If the rudder is deflected to the left, the aircraft will begin to yaw to the left. This will create a sideslip to the right. The sideslip airflow from the right acting on the fixed part of the fin will cause a side load to the left, opposing the effect of the rudder. As the yaw increases, this damping force will increase until it balances the rudder force. The aircraft will then stop yawing and will maintain that angle of yaw, with the rudder deflected to its original position. If the rudder is returned to the neutral position, both the fin and the rudder will give a force to the left which will return the aircraft to its original position with zero yaw. A given rudder angle will therefore correspond to a given yaw displacement.

Fin Stall

The sideslip angle is effectively the angle of attack of the fin, and as for any aerofoil, there will be a critical angle at which it will stall. If the rudder is deflected in the direction to correct the sideslip, the stalling angle will be reduced.

DORSAL FIN

Figure 11.23

The stalling angle of an aerofoil is affected by its aspect ratio, and so the stalling angle of the fin could be increased by decreasing its aspect ratio. This can be done by fitting a dorsal fin,

Figure 11.23.