The following movable surfaces allow pilots to control the attitude (position on all axis) of a fixed winged aircraft:

Elevators: This surface is usually found on the empennage of the aircraft. It is normally linked to a control in the cockpit called a yoke (it is sometimes, however, linked to a joystick). The yoke is one of the most noticeable controls in the cockpit. It looks like a shaft protruding from the panel (on most airliners, it stands from the floor) with a large handle on the end, very vaguely resembling an upside down oxen yoke, hence the name. When the yoke (or joystick) is pulled back, a surface on the trailing edge of the horizontal stabilizer, or the edge that, when a wing is moving forward through the air, will reach a point last, moves upward on a hinge at the leading edge of the surface. When the yoke is pushed forward, the surface angles in the opposite direction. When the surface moves up, air is deflected upward, forcing the tail of the aircraft down (Newton’s third law). The wings, as the primary surface of lift, become the “fulcrum,” therefore angling the nose of the aircraft up, and vice-versa when the yoke is pushed forward. When the horizontal stabilizer and elevator are on the nose of the aircraft, rather than the empennage, the surface is called a canard. Seems how canards are in front of the wing, they deflect air downward to establish nose up attitude. Also, sometimes the whole stabilizer is movable and works like an elevator. When this design is used, the control is called a stabilator.

nose up attitude nose down attitude

Ailerons: The ailerons are usually found on trailing edges of the wings of the aircraft. Most fixed winged aircraft have two control surfaces on the trailing edge of the wing. The aileron is usually the surface farthest away from the fuselage. The ailerons, like the elevators, are connected to the yoke. When the yoke is turned clockwise (from the pilot’s perspective) the ailerons will bank the aircraft clockwise (again, from pilot’s perspective). To do this the right aileron will angle to deflect air upward, forcing the right wing down; the left aileron will angle to deflect air upward, forcing the left wing up. To bank counterclockwise, the same things are done in the opposite direction.

bank left bank right

(counterclockwise) (clockwise)

Rudder: The rudder is usually found on the trailing edge of the vertical stabilizer. This, unlike the ailerons and the elevators, is not linked to the yoke. The rudder is controlled by two pedals on the floor of the cabin under the panel. When something is referred to as a “rudder” on most other vehicles, it is usually the primary or only means of turning. With most aircraft, this is not true. The ailerons are the primary means of turning. The rudder is normally used to coordinate the turn, not to make the turn (though it can be done in a very sloppy way). If the right rudder pedal is pressed, the rudder will deflect air to the right, forcing the tail to the left and the nose to the right. Vice-versa for nose left.

yawed left yawed right

Trim tabs: The trim tabs are essentially control surfaces on the control surfaces. They are found on many modem aircraft. Their purpose is to reduce the amount of force needed from the pilot to maintain a certain attitude at a certain airspeed. Though they can be found on any of the basic control surfaces (elevators, ailerons, and rudder) they are most commonly found on the elevators. Each surface’s trim is often manipulated by a switch that moves them in increments or by a wheel near the pedestal. In the case of elevator trim, when the control wheel is moved back, the trim tab is displaced downward and deflects air downward, forcing the control surface up and the nose of the aircraft up. The center of gravity in an aircraft should be slightly forward of the center of lift (the aircraft should be slightly nose-heavy).

To keep the pilot from having to apply much pressure to the controls, the trim tabs are usually deflecting the elevators upward to compensate for the difference in location of the center of lift and center of gravity. If power is increased and/or the airspeed acquires a tendency to increase; the air flowing faster past the elevator will exert more force on the elevator and force the the nose of the aircraft farther up (the increased airspeed and propwash also affect the trim tabs, forcing the elevator to deflect even more air up, but this is a slighter effect than the direct change in deflection from the speed of the air increasing over the elevators) and slowing the aircraft down. Because of this being prompted by more airflow over the surface, the aircraft probably has enough airspeed not to stall or change the attack angle dramatically because of a slightly higher nose attitude. If me attack angle does not change dramatically, but the pitch increases, the aircraft is in a climb. More of the power is now devoted to maintaining a climb and less to maintaining airspeed. The inverse effect occurs with a reduction in power or a tendency to lose airspeed; less force is applied to the elevator, the nose falls, and the wings deflect some air backward and also follow an effect similar to moving downhill, providing more forward propulsion. To make this explanation easier to understand, imagine riding a bike. Assume the bike does not have variable gears. You are on flat pavement using about the same amount of force per pedal revolution to maintain the same speed. You come to an uphill. To maintain the same amount of speed, you must increase the force application to the pedals. The same effect occurs with fixed winged aircraft, only in this case, with different causes and effects. Again speaking metaphorically and comparing an aircraft to a bike, if you are hiking on flat ground and you apply more force to the pedals, the trim tabs will allow the elevators keep your speed the same by generating an upward slope; if you apply less force to the pedals, the trim tabs will allow the elevators to generate a downward slope. Because the propwash is moving at a faster speed past the elevator than the surrounding air would be, any aircraft with the elevator directly in the propwash will have slightly more emphasized attitude changes due to power increases or decreases.

Elevator/stabilator trim tabs can also be combined with servo/antiservo tabs. Servo tabs are used to make a control surface easier to move. If a control surface has a servo tab. when the control surface moves, the servo tab will rotate in the opposite direction. Say, for instance, the control surface moves downward from an axis on its leading edge. The servo tab will move upward from the axis on its leading edge. This will apply downward force on the control surface, making the surface easier to move.

The inverse effect occurs with antiservo tabs. As you recall from previously m the article, a stabilator is a horizontal stabilizer that is movable and works like a set of elevators. Most stabilators are hinged close to their center of lift rather than at the leading edge. This gives them the ability to move too easily. Humans, when applying force to something (in this case, the yoke), do not respond well to simply the movement of the object they are applying force to, but respond better to the object returning an application of force when moved. Because of this, stabilators of the design specified above without antiservo tabs may give the pilots a tendency to be sloppy and imprecise in their control imputs. When the stabilator angles to deflect air upward, for instance, the antiservo tab will rotate in the same direction, but will rotate more than the stabilator, This will cause more air to be deflected upward from the trailing edge of the surface (behind the axis of the control surface) and the stabilator will have a tendency to neutralize.

Some information was acquired through flight instructor Paul McKeown.


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