Pilot's Handbook of Aeronautical Knowledge
Chapter 6: Flight Controls
Flight control systems and characteristics can vary greatly depending on the type of aircraft flown. Mechanical flight control systems, which date back to early aircraft, are still used today in small general and sport category aircraft where the aerodynamic forces are not excessive. Fly-by-wire and boosted controls are used in newer, more sophisticated aircraft.
The ailerons, elevator (or stabilator), and rudder constitute the primary control system and are required to control an aircraft safely during flight.
Control surface inputs cause movement about the three axes of rotation. The types of stability an aircraft exhibits also relate to the three axes of rotation. [Figure 6-4].
Primary Control Systems
Ailerons
Ailerons control roll about the longitudinal axis. Moving the control wheel, or control stick, to the right causes the right aileron to deflect upward and the left aileron to deflect downward.
Since the downward deflected aileron produces more lift as evidenced by the wing raising, it also produces more drag. This added drag causes the wing to slow down slightly. This results in the aircraft yawing toward the wing which had experienced an increase in lift and drag. This adverse yaw is opposite the direction of bank. It is a result of differential drag, as well as the slight difference in the velocity of the left and right wings.
Adverse yaw becomes more pronounced at low airspeeds, where aerodynamic pressure on control surfaces are low and larger control inputs are required. The increase in aileron deflection causes an increase in adverse yaw.
Application of the rudder is used to counteract adverse yaw. As with all control surfaces, the vertical stabilizer/rudder becomes less effective at lower airspeeds.
Four systems have been designed to reduce adverse yaw:
- Differential Ailerons: For a given control input, one aileron is raised a greater distance than the other aileron is lowered, producing more drag on the descending wing.
- Frise-Type Ailerons: The aileron that is being raised pivots on an offset hinge, projecting the leading edge of the aileron into the relative wind, which creates drag. A slot allows air to move more smoothly over the lowered aileron.
- Coupled Ailerons and Rudder: The rudder is deflected automatically via linked controls when the ailerons are deflected. Slips are still possible.
- Flaperons. These can control bank angle and also be lowered to function like flaps. Typically mounted separate from the wing.
Elevator
The elevator controls pitch about the lateral axis. The up-elevator position decreases the camber of the elevator and creates a downward aerodynamic force [6-10]. Conversely, the down-elevator position creates an upward force. The longitudinal axis pivots around the CG.
In a T-tail configuration, the elevator is above most of the effects of downwash from the propeller, fuselage, and wings. Operation of the elevator is more consistent throughout all phases of flight. Because the force of propeller downwash is not applied, the T-tail elevator must have a greater range of travel than low-mounted elevators/stabilators. Additionally, the forces required to raise the nose of a T-tail aircraft are greater than the forces required to raise the nose of a conventional-tail aircraft
Because the weight of the T-tail horizontal surfaces has an increased moment arm, the design can induce flutter. The vertical stabilizer must be strengthened to counter this, which results in a weight penalty.
A T-tail may experience a deep stall at high AOA, since the wake of the wing can make the tail surface almost ineffective. It can be difficult to recover from this type of stall [6-11]. Aft CG can be a contributing factor. Many aircraft have compensatory systems to address this, such as control stops, elevator down springs, and stick pushers. The elevator down spring produces a mechanical load on the elevator, causing it to move toward the nose-down position if not otherwise balanced
Forward CG in a T-tail aircraft may create an issue during roundout for landing, since elevator effectiveness may be reduced with minimal airspeed in the flare.
Stabilator
A stabilator is a one-piece horizontal stabilizer that pivots from a central hinge point. Because of this, they are extremely sensitive to control inputs and aerodynamic loads. An anti-servo tab on the trailing edge is typically installed to decrease sensitivity. The anti-servo tab deflects in the same direction as the stabilator, requiring an increase in control force and reducing the risk of the pilot over-controlling the input. Additionally, a balance weight is usually incorporated in front of the main spar.
Canard
A canard is a horizontal stabilizer in front of the main wings. However, unlike a horizontal stabilizer on an empennage, a canard creates lift, holding the nose up — whereas an aft horizontal stabilizer balances the aircraft via downforce applied by the airflow from the wings. The Wright Flyer used a canard, and it was common in the earliest days of aviation. Canards are theoretically more efficient because they contribute to less total drag compared to a conventional aircraft.
Rudder
The rudder controls movement around the vertical axis, known as yaw. When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction. Rudder effectiveness increases with speed. Propeller slipstream flowing over the rudder increases its effectiveness.
V-tail
The V-tail design performs the same functions as the surfaces of a conventional elevator and rudder configuration. The control wheel can move both surfaces ("ruddervators") simultaneously, while rudder inputs control the surfaces independently. The V-tail design is more susceptible to Dutch roll, with minimal reduction of total reduction.
Secondary Control Systems.
Flaps
Flaps increase both lift and induced drag for any given AOA. When extended, they increase the camber of the wing. Flap extension may cause a nose-up or down pitching moment, which should be compensated for by the pilot. The four common types are plain, split, slotted, and Fowler flaps.
Plain flaps are the simplest type, and increase both lift and drag with a nose-down pitching moment.
Split flaps deflect from the lower portion of the airfoil and provide slightly more lift than plain flaps. However, both plan and split flaps provided minimal additional lift.
Slotted flaps create more lift than plain or split flaps, via airflow that is created in a duct between the flap well and the leading edge of the flap. Large aircraft have double- and triple-slotted flaps.
Fowler flaps are a subtype of slotted flaps. They provide the same function of slotted flaps, but with a rearward motion that increases both the camber and the larger wing area.
Leading-Edge Devices
Leading-edge devices are also known as high-lift devices, and are attached to the front of the wing. The four most common types are fixed slots, movable slats, leading-edge flaps, and cuffs.
Fixed slots direct airflow to the upper wing surface and delay airflow separation at higher angles of attack.
Movable slats consist of leading edge segments that move on tracks. Slats are held flush at low angles of attack, and move forward at high angles of attack.
Leading-edge flaps usually work in tandem with trailing-edge flaps, also increasing lift and drag, but with a reduction in nose-down pitching moment.
Leading-edge cuffs are similar to leading-edge flaps, but they are fixed. They cause air to attach better to the the upper surface of the wing at slow speeds, but they can create a penalty at cruise speed.
Spoilers
Spoilers are deployed from the wings to spoil the smooth airflow, reducing lift and increasing drag. They are common on gliders, and they can be used to limit adverse yaw. They also reduce ground roll after landing, destroying lift and transferring weight to the wheels for braking effectiveness.
Trim Systems
Trim systems relieve the pilot of the need to maintain constant pressure on the flight controls. Common types include trim tabs, balance tabs, antiservo tabs, ground adjustable tabs, and an adjustable stabilizer
Trim Tabs can be operated manually, by a wheel or a crank. Placing the elevator trim control in the full nose-down position moves the trim tab to its full up position. However, the installation is intuitive, since moving the trim wheel forward will move the nose down. Pilots should establish the desired power, pitch attitude, and configuration before trimming the aircraft to relieve control pressures.
Balance Tabs decrease excessively high control forces. They automatically move opposite the control surfaces.
Servo Tabs are similar to balance tabs. The help the pilot move the control surface in the desired direction. The pilot moves the servo tab, and the servo tab then moves the control surface. They are sometimes referred to as flight tabs, and they are found on large aircraft.
Antiservo Tabs have the opposite function of servo tabs, dampening the pilot's input to reduce the risk of over-controlling.
Ground Adjustable Tabs are only adjustable on the ground. The most common type is the rudder trim tab, which applies a constant trim setting to the rudder, ideally to prevent the aircraft skidding left or right in cruising flight.
Adjustable Stabilizers are similar to elevator trim, but in this case the entire stabilizer can be adjusted for trim effect.
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