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Control Balancing

The aerodynamic force on the controls will depend on the area of the control surface, its angular displacement and the IAS. For large and fast aircraft the resulting aerodynamic force can give hinge moments / stick forces which are too high for easy operation of the controls.

The pilot will require assistance to move the controls in these conditions, and this can be done either by using (hydraulic) powered flying controls or by using some form of aerodynamic balance.

Aerodynamic Balance

Aerodynamic balance involves using the aerodynamic forces on the control surface to reduce the hinge moment / stick force and may be done in several ways:

HINGE SET - BACK
INTO SURFACE	F2
d

Figure 11.5 Inset hinge

Inset Hinge

If the distance (d) is reduced, the hinge moment will be reduced. The smaller the hinge moment, the smaller the stick force and the easier it will be for the pilot to move the controls. Setting the hinge back does not reduce the effectiveness of the control, only the hinge moment.

If the aerodynamic force (F2) were to move forward of the hinge, a condition known as “overbalance” would exist. As the force moved forward, a reduction then a reversal of the stick force would occur. This would be very dangerous and the designer must ensure the aerodynamic force can never move forward of the hinge.

AEROFOIL	HINGE
	LINE
	CONTROL
HORN	SURFACE

Figure 11.6 Horn balance

Horn Balance

The principle of the horn balance is similar to that of the inset hinge, in that part of the surface is forward of the hinge line, and forces on this part of the surface give hinge moments which are in the opposite direction to the moments on the main part of the surface. The overall moment is therefore reduced, but the control effectiveness is not.

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Internal Balance

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This balance works on the same principle as the inset hinge, but the aerodynamic balance area is inside the wing.

LOW PRESSURE

HIGH

PRESSURE

Figure 11.7 Internal balance

Movement of the control causes pressure changes on the aerofoil, and these pressure changes are felt on the balance area. For example, if the control surface is moved down, pressure above the aerofoil is reduced and pressure below it is increased. The reduced pressure is felt on the upper surface of the balance ‘panel’, and the increased pressure on the lower surface. The pressure difference on the balance therefore gives a hinge moment which is the opposite to the hinge moment on the main control surface, and the overall hinge moment is reduced.

See page 354 for a Tab Quick Reference Guide.

Balance Tab

The preceding types of aerodynamic balance work by causing some of the dynamic pressure on the control surface to act forward of the hinge line. The balance tab provides a force acting on the control surface trailing edge opposite to the force on the main control surface. The balance tab moves in the opposite direction to the control surface. The pilot moves the surface, the surface moves the tab.

TAB

FORCE

PILOT INPUT

CONTROL

FORCE

Figure 11.8 Balance tab

Unlike the previous types of balance, the balance tab will give some reduction in control effectiveness, as the tab force is opposite to the control force.

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Anti-balance Tab

The anti-balance tab moves in the same direction as the control surface and increases control effectiveness, but it will increase the hinge moment and give heavier stick forces. The pilot moves the surface, the surface moves the tab.

PILOT INPUT

TAB

FORCE

CONTROL

FORCE

Figure 11.9 Anti-balance tab

Servo Tab

Pilot control input deflects the servo tab only; the aerodynamic force on the tab then moves the control surface until an equilibrium position is reached. If external control locks are fitted to the control surface on the ground, the cockpit control will still be free to move; therefore, you must physically check any central locks have been removed before flight. Older types of high speed jet transport aircraft (B707) successfully used servo tab controls. The disadvantage of the servo tab is reduced control effectiveness at low IAS.

PILOT INPUT	CONTROL "HORN" FREE TO
	PIVOT ON HINGE AXIS

Figure 11.10 Servo tab

Spring Tab

The spring tab is a modification of the servo tab, such that tab movement is proportional to the applied stick force. Maximum tab assistance is obtained at high speed when the stick forces are greatest. High dynamic pressure will prevent the surface from moving, so the spring is compressed by the pilot input and the tab moves the surface. The spring is not compressed at low IAS, so the pilot input deflects the control surface and the tab, increasing the surface area and control effectiveness at low speed.

HORN FREE TO

PIVOT ON HINGE AXIS

PILOT INPUT

SPRING

Figure 11.11 Spring tab

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(Hydraulic) Powered Flying Controls

If the required assistance for the pilot to move the controls cannot be provided by the preceding types of aerodynamic balance, then power assisted or fully powered controls have to be used.

POWER FLYING

CONTROL UNIT (PFCU)

SERVO

VALVE

Figure 11.12 Power assisted flying control

Power Assisted Controls

With a power assisted flying control, Figure 11.12, only a certain proportion of the force required to oppose the hinge moment is provided by the pilot; the hydraulic system provides most of the force. Although the pilot does not have to provide all the force required, the natural ‘feel’ of the controls is retained and the stick force increases as the square of the IAS, just as in a completely manual control.

POWER FLYING

CONTROL UNIT (PFCU)

SERVO

VALVE

Figure 11.13 Fully powered flying control

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Fully Powered Controls

For bigger and/or faster aircraft, hinge moments are so large that fully powered controls must be used. In a fully powered control system, none of the force to move the control surface is supplied by the pilot. The only force the pilot supplies is that required to overcome system friction and to move the servo valve; all the necessary power to move the control surface is supplied by the aircraft’s hydraulic system.

Figure 11.13 shows that movement of the servo valve to the left allows hydraulic fluid to enter the left chamber of the PFCU. The body of the unit will move to the left, its movement being transferred to the control surface. As soon as the PFCU body reaches the position into which the pilot placed the servo valve, the PFCU body, and hence the control surface, stops moving. The unit is now locked in its new position by “incompressible” liquid trapped on both sides of the piston and will remain in that position until the servo valve is again moved by the pilot. Aerodynamic loads on the control surface are unable to move the cockpit controls, so powered flying controls are known as “irreversible” controls.

Artificial Feel (‘Q’ Feel)

POWER FLYING

CONTROL UNIT (PFCU)

STATIC
PITOT	SERVO
	VALVE

ARTIFICIAL FEEL UNIT ( 'Q ' FEEL )

Figure 11.14 Artificial feel (‘Q’ feel)

With a fully powered flying control the pilot is unaware of the aerodynamic force on the controls, so it is necessary to incorporate “artificial” feel to prevent the aircraft from being overstressed. As shown schematically in Figure 11.14, a device sensitive to dynamic pressure (½ ρ V2) or ‘Q’ is used.

Pitot pressure is fed to one side of a chamber and static pressure to the other, which moves a diaphragm under the influence of changing dynamic pressure with airspeed and causes “regulated” hydraulic pressure to provide a resistance or ‘feel’ on the pilot’s input controls proportional to IAS2, just as in a manual control. In addition, stick force should increase as stick displacement increases.

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