11 Controls

Mass Balance

Mass balance is a WEIGHT attached to the control surface forward of the hinge. Most control surfaces are mass balanced. The purpose of this is to prevent control surface flutter. Flutter is an oscillation of the control surface which can occur due to the bending and twisting of the structure under load. If the control surface CG is behind the hinge line, inertia will cause the surface to oscillate about its hinge line. The oscillations can be divergent and cause structural failure. A detailed explanation of flutter is given in Chapter 14.

HINGE LINE

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Figure 11.15 Mass balance weights

Flutter may be prevented by adding weight to the control surface in front of the hinge line. This brings the centre of gravity of the control forward to a position on, or slightly in front of the hinge, but always to the point required by the designers . This reduces the inertia moments about the hinge and prevents flutter developing. Figure 11.15 illustrates some common methods of mass balancing.

Longitudinal Control

Control in pitch is usually obtained by elevators or by an all moving (slab) tailplane, and the controls must be adequate to balance the aircraft throughout its speed range at all permitted CG positions and configurations and to give an adequate rate of pitch for manoeuvres.

Effect of Elevator Deflection

Suppose that the aircraft is flying in balance at a steady speed with the elevator neutral. If the elevator is deflected upwards, the tail will develop a down load which will begin to pitch the aircraft nose upwards. As the angle of attack increases, the tailplane down load decreases and the aircraft will reach an equilibrium pitch position. It will then remain in that pitch position with the elevator kept at the selected angle. If the elevator is returned to neutral, the tail will develop an upload which will begin to pitch the aeroplane down again. At a given CG position there will be a given pitch attitude for each elevator position.

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Direction of theTailplane Load

The elevator angle required to give balance depends on IAS and the CG position. At normal cruising speeds and CG positions, the elevator should ideally be approximately neutral. The tailplane will be giving a down load and, consequently, a nose-up pitching moment. This will balance the nose-down moment created by the wing with its centre of pressure fairly well aft. At higher than normal speeds, the CP will move further rearwards giving a stronger nosedown pitch and needing a larger down-load from the tailplane. However, at higher speed the aircraft’s angle of attack will be decreased, requiring some down elevator to provide the correct tail-load.

At lower than normal speeds, the CP will move forward and the wing and fuselage may cause a nose-up pitching moment. The tailplane will be required to give an up-load for balance. At low speed, the aircraft will be at a high angle of attack, and to reach this attitude, the elevator will have been moved up.

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Reducing Adverse Aileron Yaw

LARGE

UPWARD

MOVEMENT

SMALL

DOWNWARD

MOVEMENT

Figure 11.17 Differential ailerons

Figure 11.18 Frise ailerons

•Frise ailerons: These have an asymmetric leading edge, as illustrated in Figure 11.18. The leading edge of the up-going aileron protrudes below the lower surface of the wing, causing high drag. The leading edge of the down-going aileron remains shrouded and causes less drag.

•Aileron-rudder coupling: In this system the aileron and rudder controls are interconnected, so that when the ailerons are deflected, the rudder automatically moves to counter the adverse yaw.

•Roll control spoilers: If roll spoilers are used to augment the roll rate obtained from the ailerons, they will reduce the adverse yaw, as the down-going wing will have an increase in drag due to the raised spoiler.

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11 Controls

Inboard Ailerons

Ailerons are normally situated at the wing tips to give the greatest rolling moment for the force produced. However, this means they are also able to generate the maximum twisting loads on the wing. For instance, a down-going aileron will twist the wing tip and decrease wing tip incidence. The wing is not a rigid structure and any twist will cause a decrease of aileron effectiveness. As IAS increases, a down-going aileron will give more wing twist (decreased wing tip incidence). Eventually, an IAS will be reached at which the decrease in tip incidence will give a larger downforce than the upforce produced by the aileron. This is called high speed “aileron reversal”; the wing will go down, rather than up as the pilot intended. To reduce this effect, the ailerons could be mounted further inboard. Unfortunately, this would reduce aileron effectiveness at low speed.

SPOILER SURFACES

(LIFT DUMP POSITION)

OUTBOARD AILERONS

(LOW SPEED ONLY)

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INBOARD AILERONS

(HIGH SPEED AND LOW SPEED)

Figure 11.19 Inboard & outboard ailerons & spoiler surfaces

Alternatively, two sets of ailerons may be fitted, as illustrated in Figure 11.19: one set at the wing tips for use only at low speeds when the forces involved are low, and one set inboard for use at high speeds when the forces are greater and could cause greater structural distortion. Outboard (low speed) ailerons are “locked-out” as the flaps retract. At low speed both sets of ailerons work, but at high speed only the inboard ailerons respond to control input.

Flaperons

The flaps and the ailerons both occupy part of the wing trailing edge. For good take-off and landing performance, the flaps need to be as large as possible, and for a good rate of roll, the ailerons need to be as large as possible. However, the space available is limited, and one solution is to droop the ailerons symmetrically to augment the flap area. They then move differentially from the drooped position to give lateral control. Another system is to use the trailing edge moveable surfaces to perform the operation of both flaps and ailerons.

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