Trim Control and Level Turns

Trim Control

The use of trim systems relieves the pilot of the requirement to exert pressures for the desired flight condition. An improperly trimmed aircraft requires constant control pressures, produces pilot tension and fatigue, distracts the pilot from scanning, and contributes to abrupt and erratic aircraft control. Most WSC aircraft have a ground adjustable pitch/speed trim system that adjusts the carriage hang point on the wing keel that is set for the desired speed. Some WSC aircraft have additional pitch control systems that can adjust the trim speed in flight as described in Chapter 3, Components and Systems.

There is no yaw trim but the roll trim is usually adjusted on the ground for a wing that has a turn in it. Roll trim is usually adjusted so the wing flies straight in cruise flight. This is a balance between the full power torque of the engine wanting to turn it in one direction and minimum power when the WSC aircraft is in a glide. WSC pilots usually have to exert some pilot roll input for high power engines at full power climb to fly straight because of the engine turning effect.

Level Turns

A turn is made by banking the wings in the direction of the desired turn. A specific angle of bank is selected by the pilot, control pressures are applied to achieve the desired bank angle, and appropriate control pressures are exerted to maintain the desired bank angle once it is established.

Figure 6-9. Roll control into and out of turns.

Banking is performed with the following steps [Figure 6-9]:

Entering a Turn

Straight flight
Pilot applies sideways pressure to the control bar shifting the weight towards the direction of the desired turn initiating the bank.
Turn is established and maintained by moving the control bar back to the center position.

Exiting a Turn

Pilot is maintaining stabilized bank and a resultant turn.
Pilot shifts weight to opposite side to initiate exit out of the turn.
Straight flight is established and maintained by moving the control bar back to the center position.

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Coordinating the Controls

Flight controls are used in close coordination when making level turns. Their functions are:

The WSC is banked with side to side pressure with the control bar and the bank angle established determines the rate of turn at any given airspeed.
The throttle provides additional thrust used to maintain the WSC in level flight.
Pitch control moves the nose of the WSC aircraft up or down in relation to the pilot and perpendicular to the wings. Doing this sets the proper pitch attitude and speed in the turn.

Turns are classified to determine the bank angle as follows:

Shallow turns are those in which the bank is less than approximately 20°.
Medium turns are those resulting from a degree of bank that is approximately 20° to 45°.
Steep turns are those resulting from a degree of bank that is 45° or more.

Changing the direction of the wing’s lift toward one side or the other causes the aircraft to be pulled in that direction.

When an aircraft is flying straight and level, the total lift is acting perpendicular to the wings and to the Earth. [Figure 6-10] As the WSC is banked into a turn, the lift then becomes the resultant of two components. One, the vertical lift component, continues to act perpendicular to the Earth and opposes gravity. Second, the horizontal lift component (centripetal) acts parallel to the Earth’s surface and opposes inertia (apparent centrifugal force). These two lift components act at right angles to each other causing the resultant total lifting force to act perpendicular to the banked wing of the aircraft. It is the horizontal lift component that actually turns the WSC aircraft. [Figure 6-10]

Figure 6-10. WSC aircraft flying straight (left) and turning with the same lift and weight (right).

Shallow turns are accomplished by moving the control bar to the side slightly, waiting for the wing to roll the desired amount, and then releasing the side pressure on the control bar back to the center position. The WSC aircraft will stabilize in the turn with no control pressures required. During a shallow turn there is no significant increase in airspeed or G forces that can easily be noticed by the student. [Figure 6-11] Once a shallow turn is initiated, it is a good practice to be stabilized at a constant bank and then exit to a predetermined heading. To exit the shallow turn, opposite sideways pressure must be put on the control bar to bring the WSC aircraft back to level flight.

Figure 6-11. Pilot’s view of a shallow turn with a 20° bank.

For higher banked turns, the entry speed should be well above 1.3 times the stall speed, which increases significantly in higher banked turns. As an example, at least 1.5 times the stall speed should be the entry speed for a 40 degree banked turn to maintain the 1.3 times the stall speed safety margin. Wings with a trim speed of 1.3 times the stall speed require an increase in speed slightly. In all constant altitude, constant airspeed turns, it is necessary to increase the angle of attack of the wing when rolling into the turn by pushing out on the control bar. This is required because part of the vertical lift has been diverted to horizontal lift. Thus, the total lift must be increased to compensate for this loss. Similarly, the throttle must be increased to maintain the same altitude in steeper banks.

The additional load or G force in a medium banked turn is felt as the pilot is pushed down on the seat with enough force for this effect to be noticed. After the bank has been established in a medium turn, all side-to-side roll pressure applied may be relaxed, but forward pressure to maintain a higher angle of attack is still necessary in a steeper bank. The WSC aircraft remains at the selected bank with no further tendency to roll back to level since all the forces are equalized.

During the turn, roll, pitch, and throttle controls are adjusted to maintain the desired bank angle, speed, and level altitude. Coordinated flight is the coordination of the three controls to achieve a smooth turn to the desired bank angle while maintaining a constant speed and altitude.

The roll-out from a turn is similar to the roll-in, except flight controls are applied in the opposite direction. As the angle of bank decreases, the pitch pressure should be relaxed as necessary to maintain speed and the throttle decreased to maintain altitude.

Since the aircraft continues turning as long as there is any bank, the rollout must be started before reaching the desired heading. The amount of lead required to roll-out of the desired heading depends on the degree of bank used in the turn. Normally, the lead is one-half the degrees of bank. For example, if the bank is 30°, lead the rollout by 15°. As the wings become level, the control pressures should be smoothly relaxed so that the controls are neutralized as the aircraft returns to straight-and-level flight. As the rollout is being completed, attention should be given to outside visual references to determine that the wings are being leveled and the turn stopped.

To understand the relationship between airspeed, bank, and radius of turn, it should be noted that the rate of turn at any given true airspeed depends on the horizontal lift component. The horizontal lift component varies in proportion to the amount of bank. Therefore, the rate of turn at a given true airspeed increases as the angle of bank is increased. On the other hand, when a turn is made at a higher true airspeed at a given bank angle, the inertia is greater and the horizontal lift component required for the turn is greater causing the turning rate to become slower. Therefore, at a given angle of bank, a higher true airspeed makes the radius of turn larger because the aircraft is turning at a slower rate. [Figure 6-12]

Figure 6-12. Angle of airspeed and bank regulate rate and radius of turn.

When changing from a shallow bank to a medium bank, the airspeed of the wing on the outside of the turn increases in relation to the inside wing as the radius of turn decreases. The additional lift developed because of this increase in speed of the wing balances the inherent lateral stability of the aircraft. At any given airspeed, roll pressure is not required to maintain the bank. If the bank is allowed to increase from a medium to a steep bank, the radius of turn decreases further.

A steep bank is similar to a medium bank but all factors increase. Roll and pitch control pressures must increase, throttle must increase further to maintain altitude, and the G forces increase significantly. Students should build up to steep banked turns gradually after perfecting shallow and medium banked turns. Do not exceed the bank angle limitation in the Pilot’s Operating Handbook (POH).

The pilot’s posture while seated in the aircraft is very important, particularly during turns. It affects the interpretation of outside visual references. Pilots should not lean away from the turn in an attempt to remain upright in relation to the ground rather than ride with the aircraft. This should be a habit developed early so that the pilot can properly learn to use visual references.

Beginning students should not use large control applications because this produces a rapid roll rate and allows little time for corrections before the desired bank is reached. Slower (small control displacement) roll rates provide more time to make necessary pitch and bank corrections. As soon as the aircraft rolls from the wings-level attitude, the nose should also start to move along the horizon, increasing its rate of travel proportionately as the bank is increased.

The following variations provide excellent guides. If the nose moves up or down when entering a bank, excessive or insufficient pitch control is being applied. During all turns, the controls are used to correct minor variations as they are in straight-and-level flight.

Instruction in level turns should begin with changing attitude from level to bank, bank to level, and so on with a slight pause at the termination of each phase. This pause allows the WSC to free itself from the effects of any misuse of the controls and ensures a correct start for the next turn. During these exercises, the idea of control forces, rather than movement, should be emphasized by pointing out the resistance of the controls to varying forces applied to them.

Common errors in the performance of level turns are:

Failure to adequately clear the area before beginning the turn.
Attempting to sit up straight, in relation to the ground, during a turn, rather than riding with the aircraft.
Failure to maintain a constant bank angle during the turn.
Gaining proficiency in turns in only one direction.
Failure to coordinate the angle of attack to maintain the proper airspeed.
Failure to coordinate the use of throttle to maintain level flight.
Altitude gain/loss during the turn.

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