11 Controls

Secondary Effects of Controls

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The controls are designed to give a moment around a particular axis but may additionally give a moment around a second axis. This coupling occurs particularly with the rolling and yawing moments.

Yawing Moment Due to Roll

•A rolling moment is normally produced by deflecting the ailerons, and it has been seen that they can also produce an adverse yawing moment due to the difference in drag on the two ailerons. Induced drag is increased on the wing with the down-going aileron, making the aircraft, for instance, roll left and at the same time, yaw right.

•If the aircraft is rolling, the down-going wing experiences an increased angle of attack and the up-going wing a decreased angle of attack, increasing the adverse yawing moment.

Rolling Moment Due to Yaw

•If the aircraft is yawing to the left, the right wing has a higher velocity than the left wing and so will give more lift. The difference in lift will give a rolling moment to the left.

•If the rudder is deflected to the left (to give yaw to the left) the force on the fin is to the right. This will give a small rolling moment to the right because the fin CP is above the aircraft CG. This effect is usually very small, but a high fin may give an adverse roll.

One way to counteract this effect is to interconnect the ailerons and rudder so that when the rudder is moved, the ailerons move automatically to correct the adverse roll.

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Trimming

An aeroplane is trimmed when it will maintain its attitude and speed without the pilot having to apply any load to the cockpit controls. If it is necessary for a control surface to be deflected to maintain balance of the aircraft, the pilot will need to apply a force to the cockpit control to hold the surface in its deflected position. This force may be reduced to zero by operation of the trim controls.

The aircraft may need to be trimmed in pitch as a result of:

•changes of speed.

•changes of power.

•varying CG positions.

•changes of configuration.

Trimming in yaw will be needed:

•on a multi-engine aircraft if there is asymmetric power.

•as a result of changes in propeller torque.

Trimming in roll is less likely to be needed but could be required if the configuration is asymmetric, or if there is a lateral displacement of the CG.

Methods of Trimming

Various methods of trimming are in use. The main ones are:

•the trimming tab.

•variable incidence (trimming) tailplane.

•spring bias.

•CG adjustment.

•adjustment of the artificial feel unit.

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11 Controls

Trim Tab

A trim tab is a small adjustable surface set into the trailing edge of a main control surface. Its deflection is controlled by a trim wheel or electrical switch in the cockpit, usually arranged to operate in an instinctive sense. To maintain the primary control surface in its required position, the tab is moved in the opposite direction to the control surface until the tab moment balances the control surface hinge moment.

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d F

D

f

Figure 11.25

Figure 11.25 shows (f × D) from tab opposes (F × d) from control surface. If the two moments are equal, the control will be trimmed, i.e. the stick force will be zero. Operation of the trim tab will slightly reduce the force being produced by the main control surface.

Fixed Tabs

Some trim tabs are not adjustable in flight but can be adjusted on the ground, to correct a permanent out of trim condition. They are usually found on ailerons and rudder. They operate in the same manner as the adjustable trim tab.

Variable Incidence (Trimming) Tailplane

This system of trimming may be used on manually operated and power operated controls. To trim, the tailplane incidence is adjusted by the trim wheel until the tailplane load is equal to the previous elevator balancing load required, Figure 11.26. Stick force is now zero.

The main advantages of a variable incidence (trimming) tailplane are:

•the drag is less in the trimmed state as the aerofoil is more streamlined.

•trimming does not reduce the effective range of pitch control as the elevator remains approximately neutral when the aircraft is trimmed.

•it is very powerful and gives an increased ability to trim for larger CG and speed range.

The disadvantage of a variable incidence (trimming) tailplane is that it is more complex and is heavier than a conventional trim tab system.

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	Controls	11
	ELEVATOR POSITIONED TO TRIM
A / C STRUCTURE	SCREW JACK
	AFTER TRIM INPUT

Figure 11.26 Variable incidence (trimming) tailplane

The amount of trim required will depend on the CG position, and recommended stabilizer takeoff settings will be given in the aircraft Flight Manual. It is important that these are correctly set before take-off as incorrect settings could give either an excessive rate of pitch when the aircraft is rotated, leading to possible tail strikes, or very heavy stick forces on rotation, leading to increased take-off distances required.

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10
20
10
	5
15	20

Figure 11.27 Reduced aircraft nose-up pitch authority

The disadvantage of a ”conventional” elevator and trim tab, Figure 11.27, is that the aircraft nose-up pitch authority reduces with forward CG movement. Forward CG positions will require the elevator to be trimmed more aircraft nose-up. The illustration shows up elevator authority reduced from 10° to 5°.

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Controls

Spring Bias

In the spring bias trim system, an adjustable spring force is used to decrease the stick force. No tab is required for this system.

CG Adjustment

If the flying controls are used for trimming, this results in an increase of drag due to the deflected surfaces. The out of balance pitching moment can be reduced by moving the CG, thus reducing the balancing load required and therefore the drag associated with it. This will give an increase of cruise range. CG movement is usually achieved by transferring fuel between tanks at the nose and tail of the aircraft.

Artificial Feel Trim

If the flying controls are power operated, there is no feedback of the load on the control surface to the cockpit control. The feel on the controls has to be created artificially. When a control surface is moved, the artificial feel unit provides a force to resist the movement of the cockpit control. To remove this force (i.e. to trim) the datum of the feel unit can be adjusted so that it no longer gives any load on the flight deck controls.

TABS - Quick Reference Guide

	Type of Tab	Operated	Movement Relative	Stick	Control
		by	to Control Surface	Force	Effectiveness
	Balance	Control	Opposite	Less	Reduced
		Surface
	Anti-balance	Control	Same	More	Increased
		Surface
	Servo	Pilot	Opposite	Less	Reduced
	Spring	Pilot at High	Opposite at	Less at	Reduced at
		Speed	High Speed	High Speed	High Speed
	Trim	Trim Control	Opposite	Zeroed	Reduced
		ONLY

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