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5-7
F
igure 5-8. When the aerodynamic efficiency of the horizontaltail surface is inadequate due to an aft center of gravity condition,an elevator down spring may be used to supply a mechanical load to lower the nose.ElevatorDown springPivot pointsBell crank
Figure 5-12. When the aerodynamic efficiency of the horizontal tail surface is inadequate due to an aft CG condition, an elevator down spring may be used to supply a mechanical load to lower the nose.
F
igure 5-9. One piece horizontal tail surface that pivots up and down about a central hinge point.Antiservo tabBalance weightStabilatorPivot point
Figure 5-13. The stabilator is a one-piece horizontal tail surface that pivots up and down about a central hinge point.
Figure 5-14. The Piaggio P180 includes a variable-sweep canard design, which provides longitudinal stability about the lateral axis.
from a central hinge point. When the control column is pulled back, it raises the stabilator’s trailing edge, pulling the airplane’s nose up. Pushing the control column forward lowers the trailing edge of the stabilator and pitches the nose of the airplane down.
Because stabilators pivot around a central hinge point, they are extremely sensitive to control inputs and aerodynamic loads. Antiservo tabs are incorporated on the trailing edge to decrease sensitivity. They deflect in the same direction as the stabilator. This results in an increase in the force required to move the stabilator, thus making it less prone to pilot-induced overcontrolling. In addition, a balance weight is usually incorporated in front of the main spar. The balance weight may project into the empennage or may be incorporated on the forward portion of the stabilator tips. [Figure 5-13]Canard
The canard design utilizes the concept of two lifting surfaces, the canard functioning as a horizontal stabilizer located in front of the main wings. In effect, the canard is an airfoil similar to the horizontal surface on a conventional aft-tail design. The difference is that the canard actually creates lift and holds the nose up, as opposed to the aft-tail design which exerts downward force on the tail to prevent the nose from rotating downward. [Figure 5-14]
The canard design dates back to the pioneer days of aviation, most notably used on the Wright Flyer. Recently, the canard configuration has regained popularity and is appearing on newer aircraft. Canard designs include two types–one with a horizontal surface of about the same size as a normal aft-tail design, and the other with a surface of the same approximate size and airfoil of the aft-mounted wing known as a tandem wing configuration. Theoretically, the canard is considered more efficient because using the horizontal surface to help lift the weight of the aircraft should result in less drag for a given amount of lift.Rudder
The rudder controls movement of the aircraft about its vertical axis. This motion is called yaw. Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface, in this case to the vertical stabilizer, or fin. Moving the left or right rudder pedal controls the rudder.
When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction. [Figure 5-15] By pushing the left pedal, the rudder moves left. This alters the airflow around the vertical stabilizer/rudder, and creates a
5-8
Aerodynamic force Figure 5-11. The effect of left rudder pressure.Left rudderCGLeft rudder forward Yaw
5-15. The effect of left rudder pressure.
Figure 5-16. Beechcraft Bonanza V35.
sideward lift that moves the tail to the right and yaws the nose of the airplane to the left. Rudder effectiveness increases with speed; therefore, large deflections at low speeds and small deflections at high speeds may be required to provide the desired reaction. In propeller-driven aircraft, any slipstream flowing over the rudder increases its effectiveness.V-Tail
The V-tail design utilizes two slanted tail surfaces to perform the same functions as the surfaces of a conventional elevator and rudder configuration. The fixed surfaces act as both horizontal and vertical stabilizers. [Figure 5-16]
The movable surfaces, which are usually called ruddervators, are connected through a special linkage that allows the control wheel to move both surfaces simultaneously. On the other hand, displacement of the rudder pedals moves the surfaces differentially, thereby providing directional control.
When both rudder and elevator controls are moved by the pilot, a control mixing mechanism moves each surface the appropriate amount. The control system for the V-tail is more complex than that required for a conventional tail. In addition, the V-tail design is more susceptible to Dutch roll tendencies than a conventional tail, and total reduction in drag is minimal.
Secondary Flight Controls
Secondary flight control systems may consist of wing flaps, leading edge devices, spoilers, and trim systems.Flaps
　
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