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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.

Elevator Angle with ‘g’

When the aircraft is performing a pitching manoeuvre, the tailplane angle of attack is increased by the effect of the rotational velocity and the aerodynamic damping is increased. This means that a larger elevator angle will be required than for the same conditions in 1g flight. The additional elevator angle required will be proportional to the ‘g’ being experienced. The elevator movement available should be sufficient to allow the design limit ‘g’ to be reached.

The most demanding requirement for elevator up authority will be when the aircraft is being flared for landing, in ground effect with most forward CG.

Effect of Ice on theTailplane

The tailplane is an aerofoil, usually symmetrical as it is required to produce both up and down loads. It is set at an angle of incidence which is less than that of the wing. This ensures that it will not stall before the wing, and so control can be maintained up to the stall. It is usually affected by the downwash from the wing and this reduces its effective angle of attack. Typically the tail will be at a negative angle of attack, producing a down load for balance. If ice forms on the tailplane leading edge, its aerofoil shape will be distorted, and its stalling angle reduced. This could cause the tailplane to stall, particularly if the downwash is increased as a result of lowering flaps. With the tailplane stalled its down load would be reduced, and the aircraft would pitch down and could not be recovered.

Lateral Control

Control in roll is usually obtained by ailerons or by spoilers, or by a combination of the two. The main requirement for lateral control is to achieve an adequate rate of roll.

On the ground with the control wheel in the neutral position both ailerons should be slightly below alignment with the wing trailing edge, or “drooped”. When airborne, the lower pressure on the top surface will “suck” both ailerons up into a position where they are perfectly aligned with the wing trailing edge, thus reducing drag.

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Effect of Aileron Deflection (Aerodynamic Damping)

In steady level flight with the ailerons neutral, the lift on the two wings will be equal. If the control wheel is turned to the left, the left aileron will move up and the right aileron down. The up aileron will decrease the lift of the left wing which will begin to ‘drop’. The downward movement of the wing creates a relative airflow upwards, which increases its effective angle of attack. The opposite effects will occur on the right (up-going) wing.

HIGH TAS

LOW TAS

RELATIVE AIRFLOW

FROM ANGULAR ROTATION

EQUAL WING TIP VELOCITY

INCREASE IN EFFECTIVE ANGLE OF ATTACK

DUE TO WING TIP DOWNWARDS VELOCITY

Figure 11.16 Aerodynamic damping in roll

The increased effective angle of attack of the down-going wing increases its lift, which opposes the roll. This is called aerodynamic damping. The greater the rate of roll, the greater the damping. It can also be seen from Figure 11.16 that the greater the TAS, the smaller the increase in effective angle of attack for a given roll rate.

The change in wing lift for a given aileron deflection depends on the IAS, but the change of effective angle of attack due to roll velocity depends on TAS. At high TAS (constant IAS, higher altitude) the change in effective angle of attack will be reduced and a higher rate of roll will be possible. Rate of roll therefore increases (aerodynamic damping decreases) with higher TAS for a given aileron deflection. The aileron is known as a rate control since a given aileron angle of deflection determines a rate of roll, not a roll displacement.

Effect of Wingspan on Rate of Roll

For a given rate of roll, the wing tip rolling velocity will increase with the wingspan. Aerodynamic damping will therefore be greater if the span is greater. Under the same conditions, a short span wing will have a greater rate of roll than a large span wing.

Adverse Aileron Yaw

The ailerons produce a rolling moment by increasing the lift on one wing and decreasing it on the other. The increased lift on the up-going wing gives an increase in the induced drag, whereas the reduced lift on the down-going wing gives a decease in induced drag. The difference in drag on the two wings produces a yawing moment which is opposite to the rolling moment, that is, a roll to the left produces a yawing moment to the right. This is known as adverse aileron yaw.

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Reducing Adverse Aileron Yaw

LARGE

UPWARD

MOVEMENT

SMALL

DOWNWARD

MOVEMENT

Figure 11.17 Differential ailerons

	• Differential ailerons: The aileron linkage causes the up-going aileron to move through a			11
	larger angle than the down-going aileron, Figure 11.17. This increases the drag on the up
	aileron and reduces it on the down aileron, and so reduces the difference in drag between			Controls
	the two wings.

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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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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Roll Control Spoilers

Spoilers may be used to give lateral control, in addition to, or instead of ailerons. Spoilers consist of movable panels on the upper wing surface, hinged at their forward edge, which can be raised hydraulically, as illustrated in Figure 11.20. A raised spoiler will disturb the airflow over the wing and reduce lift.

	OUTBOARD AILERONS	SPOILER SURFACES ASSISTING
		WITH ROLL CONTROL
	LOCKED - OUT AT
	HIGH SPEED

INBOARD AILERONS OPERATE

AT ALL SPEEDS

Figure 11.20 Roll control spoilers

To function as a lateral control, the spoilers rise on the wing with the up-going aileron (downgoing wing), proportional to aileron input. On the wing with the down-going aileron, they remain flush. Unlike ailerons, spoilers cannot give an increase of lift, so a roll manoeuvre controlled by spoilers will always give a net loss of lift. However, the spoiler has several advantages compared to the aileron:

•There is no adverse yaw: The raised spoiler increases drag on that wing, so the yaw is in the same direction as the roll.

•Wing twisting is reduced: The aerodynamic force on the spoilers acts further forward than is the case with ailerons, reducing the moment which tends to twist the wing.

•At transonic speed its effectiveness is not reduced by shock induced separation.

•It cannot develop flutter.

•Spoilers do not occupy the trailing edge, which can then be utilized for flaps.

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