Turns
A turn is a basic flight maneuver used to change or return to
the desired heading. It involves close coordination of all three flight controls
- aileron, rudder, and elevator. Since turns are a part of most other flight
maneuvers, it is important that the pilot thoroughly understand factors involved
and learn to perform them well.
When applying aileron to bank the airplane, the depressed or lowered aileron (on the rising wing) produces a greater drag than the raised aileron (on the lowering wing). This increased aileron drag tends to yaw the airplane toward the rising wing, or opposite to the desired direction of turn, while the banking action is taking effect. To counteract the yawing tendency, rudder pressure must be applied simultaneously in the desired direction of turn. This produces a coordinated turn. The adverse yaw produced by aileron drag has been a major consideration in design of the controls, and in most modern airplanes has been significantly reduced. This is accomplished by providing greater up travel than down travel of the ailerons, or by using Frise type, or slotted ailerons. After the bank has been established, in a theoretically perfect
medium turn in smooth air, all pressure on the aileron control may be relaxed.
The airplane will remain at the bank selected with no further tendency to yaw
since there is no longer a deflection of the ailerons. As a result, pressure may
also be relaxed on the rudder pedals, and the rudder allowed to streamline
itself with the direction of the air passing it.
In all turns in which a constant altitude is to be maintained, it is necessary to increase the angle of attack by applying back elevator pressure. This is required because lift that is equal to the weight of the airplane plus the centrifugal force caused by the turn must be obtained from the wing to maintain altitude. Producing sufficient lift for the turn could be done either by increasing the power and airspeed or by increasing the angle of attack. The force of lift must be further increased as the turn steepens and the load factor and the centrifugal force build up, but must be slowly decreased as the airplane is being rolled back to level flight when completing the turn. To stop the turn, the wings must be returned to laterally level flight by the use of the ailerons, and the resulting adverse yaw (now acting in the same direction as the turn), must be overcome by the coordinated application of rudder. The yaw effect will often be more apparent when rolling out of a turn than rolling into a turn, due to the higher angle of attack and wing loading, and the slower airspeed which exists when the rollout is started. A greater deflection of the ailerons is necessary to obtain the same control effect as when rolling into the turn from level flight. Due to the slower airspeed, this displacement of the ailerons requires less pressure on the aileron control, but it also imposes a greater drag on the "down" aileron since the wing is at a greater angle of attack. The greater drag on that aileron now tends to make the airplane yaw toward the direction of the turn rather than away from it; therefore, a greater rudder pressure must be used to counteract it. To understand the relationship between airspeed, bank, and radius of turn, it must be recalled that the rate of turn at any given airspeed depends on the amount of the sideward force causing the turn; that is, the horizontal lift component. The horizontal lift component varies in proportion to the amount of bank. Thus, the rate of turn at a given airspeed increases as the angle of bank is increased. On the other hand, when a turn is made at a higher airspeed at a given bank angle, the inertia is greater and the centrifugal force created by the turn becomes greater, causing the turning rate to become slower. It can be seen, then, that at a given angle of bank, a higher airspeed will make the radius of the turn larger because the airplane will be turning at a slower rate. With an airplane possessing positive lateral stability, a wing
which has lowered as the result of a gust of air will usually tend to return to
its original position. However, this inherent tendency should not be depended on
because other factors may overcome it. The pilot should take appropriate control
action to level the wing whenever necessary to prevent the airplane from
entering an unintentional spiral.
To establish the desired angle of bank, the pilot should use outside visual reference points as well as specific indications on the attitude indicator (Fig. 6-4). The best outside reference for establishing the degree of bank is the angle formed by the raised wing of low wing airplanes (the lowered wing of high wing airplanes) and the horizon, or the angle made by the top of the engine cowling and the horizon. Since on most light planes the engine cowling is fairly flat, its horizontal angle to the horizon will give some indication of the approximate degree of bank. Also, information obtained from the attitude indicator will show the angle of the wing in relation to the horizon. The pilot's posture while seated in the airplane is very important in all maneuvers, particularly during turns, since it will affect the alignment of outside visual references. At the beginning the student may lean to the side when rolling into the turn in an attempt to remain upright in relation to the ground rather than "ride" with the airplane. This tendency must be corrected at the outset, if the student is to learn to properly use visual references. This tendency is also characteristic in airplanes which have side by side seats because the pilot is seated to one side of the longitudinal axis about which the airplane rolls. This makes the nose appear to rise when making a correct left turn and to descend in correct right turns. As in all maneuvering, the pilot should form the habit of ensuring that the area toward which a turn is to be made is clear of other aircraft. This is especially vital in high wing airplanes since once the bank is started, the lowered wing may conceal other aircraft in that direction. If nearby aircraft are not detected before lowering the wing, it would be too late to avoid a collision. After the area has been cleared, the turn should be started by gradually and simultaneously applying pressure to both the aileron and rudder in the desired direction. This pressure will move the control surfaces out of their streamlined position and cause the airplane to bank and turn. The rate at which the airplane rolls into the turn is governed by the rapidity and amount of pressure applied. When entering turns, pressure should be applied on the ailerons and the rudder progressively; that is, applied smoothly, and gradually built up, then gradually relaxed just before the desired degree of bank is attained. The rotation of the airplane about its longitudinal axis when entering a turn should be accomplished in such manner that a pitch attitude also may be maintained which will produce flight at a constant altitude. Because of the decrease in the vertical lift component and the increase in load factor in the turn, the angle of attack required to maintain altitude will be slightly higher than in straight and level flight, if no power adjustment is made. Consequently, sufficient back pressure must be applied to the elevator control. The steeper the bank, of course, the greater the increase in required angle of attack; therefore, more back pressure is needed. As soon as the airplane 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. Any variation from this will be indicative of the particular control that is being misused. The following variations provide excellent guides: 1. If the nose starts to move before the bank
starts, rudder is being applied too soon. During all turns, the ailerons and rudder are used to correct minor variations just as they are in straight and level flight. However, during very steep turns, considerably more back elevator pressure is required to maintain altitude than in shallow and medium turns, and additional power may be needed to maintain a safe airspeed. Frequently, there is a tendency for the airplane's nose to lower, resulting in a loss of altitude. To recover from an unintentional nose low attitude during a
steep turn, the pilot should first reduce the angle of bank with coordinated
aileron and rudder pressure. Then back elevator pressure should be used to raise
the airplane's nose to the desired pitch attitude. After accomplishing this, the
desired angle of bank can be reestablished. Attempting to raise the nose first
by increasing back elevator pressure will usually cause a tight descending
spiral, and could lead to overstressing the airplane. Since the airplane will continue turning as long as there is any bank, the rollout must be started before reaching the desired heading. The amount required to lead the heading will depend on the rate of turn and the rate at which the rollout will be made. As the wings become level, the control pressures should be gradually and smoothly released so that the controls are neutralized as the airplane assumes straight and level flight. As the rollout is being completed, attention should be given to outside visual references as well as the attitude indicator and heading indicator to determine that the wings are being leveled precisely and the turn stopped. To make a precision 90 degree turn using outside references, the airplane should be aligned with a road or section line on the ground and then a turn made to where the airplane will be perpendicular to the road or line. In the absence of any ground reference, a point may be selected on the horizon directly off the wingtip, then the airplane turned until it is headed to that point. The selection of the reference should be made while visually scanning the area for other aircraft, obstacles, or nearby clouds.
|