Chapter 5 - Flight Controls
This chapter focuses on the ﬂight control systems a pilot uses to control the forces of ﬂight, and the aircraft’s direction and attitude. It should be noted that ﬂight control systems and characteristics can vary greatly depending on the type of aircraft ﬂown. The most basic ﬂight control system designs are mechanical and date back to early aircraft. They operate with a collection of mechanical parts such as rods, cables, pulleys, and sometimes chains to transmit the forces of the ﬂight deck controls to the control surfaces. Mechanical ﬂight control systems are still used today in small general and sport category aircraft where the aerodynamic forces are not excessive. [Figure 5-1]
Figure 5-1. Mechanical flight control system.
As aviation matured and aircraft designers learned more about aerodynamics, the industry produced larger and faster aircraft. Therefore, the aerodynamic forces acting upon the control surfaces increased exponentially. To make the control force required by pilots manageable, aircraft engineers designed more complex systems. At ﬁrst, hydromechanical designs, consisting of a mechanical circuit and a hydraulic circuit, were used to reduce the complexity, weight, and limitations of mechanical ﬂight controls systems. [Figure 5-2]
Figure 5-2. Hydromechanical flight control system.
As aircraft became more sophisticated, the control surfaces were actuated by electric motors, digital computers, or ﬁber optic cables. Called “ﬂy-by-wire,” this ﬂight control system replaces the physical connection between pilot controls and the ﬂight control surfaces with an electrical interface. In addition, in some large and fast aircraft, controls are boosted by hydraulically or electrically actuated systems. In both the ﬂy-by-wire and boosted controls, the feel of the control reaction is fed back to the pilot by simulated means.
Current research at the National Aeronautics and Space Administration (NASA) Dryden Flight Research Center involves Intelligent Flight Control Systems (IFCS). The goal of this project is to develop an adaptive neural network-based ﬂight control system. Applied directly to ﬂight control system feedback errors, IFCS provides adjustments to improve aircraft performance in normal ﬂight as well as with system failures. With IFCS, a pilot is able to maintain control and safely land an aircraft that has suffered a failure to a control surface or damage to the airframe. It also improves mission capability, increases the reliability and safety of ﬂight, and eases the pilot workload.
Today’s aircraft employ a variety of ﬂight control systems. For example, some aircraft in the sport pilot category rely on weight-shift control to ﬂy while balloons use a standard burn technique. Helicopters utilize a cyclic to tilt the rotor in the desired direction along with a collective to manipulate rotor pitch and anti-torque pedals to control yaw. [Figure 5-3]
Figure 5-3. Helicopter flight control system.
For additional information on ﬂight control systems, refer to the appropriate handbook for information related to the ﬂight control systems and characteristics of speciﬁc types of aircraft.
Flight Control Systems
Aircraft flight control systems consist of primary and secondary systems. The ailerons, elevator (or stabilator), and rudder constitute the primary control system and are required to control an aircraft safely during ﬂight. Wing ﬂaps, leading edge devices, spoilers, and trim systems constitute the secondary control system and improve the performance characteristics of the airplane or relieve the pilot of excessive control forces.
Primary Flight Controls
Aircraft control systems are carefully designed to provide adequate responsiveness to control inputs while allowing a natural feel. At low airspeeds, the controls usually feel soft and sluggish, and the aircraft responds slowly to control applications. At higher airspeeds, the controls become increasingly ﬁrm and aircraft response is more rapid.
Movement of any of the three primary ﬂight control surfaces (ailerons, elevator or stabilator, or rudder), changes the airﬂow and pressure distribution over and around the airfoil. These changes affect the lift and drag produced by the airfoil/control surface combination, and allow a pilot to control the aircraft about its three axes of rotation.
Design features limit the amount of deﬂection of ﬂight control surfaces. For example, control-stop mechanisms may be incorporated into the ﬂight control linkages, or movement of the control column and/or rudder pedals may be limited. The purpose of these design limits is to prevent the pilot from inadvertently overcontrolling and overstressing the aircraft during normal maneuvers.
A properly designed airplane is stable and easily controlled during normal maneuvering. Control surface inputs cause movement about the three axes of rotation. The types of stability an airplane exhibits also relate to the three axes of rotation. [Figure 5-4]
Figure 5-4. Airplane controls, movement, axes of rotation, and type of stability.
Ailerons control roll about the longitudinal axis. The ailerons are attached to the outboard trailing edge of each wing and move in the opposite direction from each other. Ailerons are connected by cables, bellcranks, pulleys and/or push-pull tubes to a control wheel or control stick.
Moving the control wheel or control stick to the right causes the right aileron to deﬂect upward and the left aileron to deﬂect downward. The upward deﬂection of the right aileron decreases the camber resulting in decreased lift on the right wing. The corresponding downward deﬂection of the left aileron increases the camber resulting in increased lift on the left wing. Thus, the increased lift on the left wing and the decreased lift on the right wing causes the airplane to roll to the right.
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). From the pilot’s perspective, the yaw is opposite the direction of the bank. The adverse yaw is a result of differential drag and the slight difference in the velocity of the left and right wings. [Figure 5-5]
Figure 5-5. Adverse yaw is caused by higher drag on the outside wing, which is producing more lift.
Adverse yaw becomes more pronounced at low airspeeds. At these slower airspeeds aerodynamic pressure on control surfaces are low and larger control inputs are required to effectively maneuver the airplane. As a result, the increase in aileron deﬂection causes an increase in adverse yaw. The yaw is especially evident in aircraft with long wing spans.
Application of rudder is used to counteract adverse yaw. The amount of rudder control required is greatest at low airspeeds, high angles of attack, and with large aileron deﬂections. Like all control surfaces at lower airspeeds, the vertical stabilizer/rudder becomes less effective, and magniﬁes the control problems associated with adverse yaw.
All turns are coordinated by use of ailerons, rudder, and elevator. Applying aileron pressure is necessary to place the aircraft in the desired angle of bank, while simultaneous application of rudder pressure is necessary to counteract the resultant adverse yaw. Additionally, because more lift is required during a turn than when in straight-and-level ﬂight, the angle of attack (AOA) must be increased by applying elevator back pressure. The steeper the turn, the more elevator back pressure is needed.
As the desired angle of bank is established, aileron and rudder pressures should be relaxed. This stops the angle of bank from increasing, because the aileron and rudder control surfaces are in a neutral and streamlined position. Elevator back pressure should be held constant to maintain altitude. The roll-out from a turn is similar to the roll-in, except the ﬂight controls are applied in the opposite direction. Aileron and rudder are applied in the direction of the roll-out or toward the high wing. As the angle of bank decreases, the elevator back pressure should be relaxed as necessary to maintain altitude.
In an attempt to reduce the effects of adverse yaw, manufacturers have engineered four systems: differential ailerons, frise-type ailerons, coupled ailerons and rudder, and ﬂaperons.
With differential ailerons, one aileron is raised a greater distance than the other aileron is lowered for a given movement of the control wheel or control stick. This produces an increase in drag on the descending wing. The greater drag results from deﬂecting the up aileron on the descending wing to a greater angle than the down aileron on the rising wing. While adverse yaw is reduced, it is not eliminated completely. [Figure 5-6]
Figure 5-6. Differential ailerons.
With a frise-type aileron, when pressure is applied to the control wheel or control stick, the aileron that is being raised pivots on an offset hinge. This projects the leading edge of the aileron into the airﬂow and creates drag. It helps equalize the drag created by the lowered aileron on the opposite wing and reduces adverse yaw. [Figure 5-7]
Figure 5-7. Frise-type ailerons.
The frise-type aileron also forms a slot so air ﬂows smoothly over the lowered aileron, making it more effective at high angles of attack. Frise-type ailerons may also be designed to function differentially. Like the differential aileron, the frise-type aileron does not eliminate adverse yaw entirely. Coordinated rudder application is still needed wherever ailerons are applied.
Coupled Ailerons and Rudder
Coupled ailerons and rudder are linked controls. This is accomplished with rudder-aileron interconnect springs, which help correct for aileron drag by automatically deﬂecting the rudder at the same time the ailerons are deﬂected. For example, when the control wheel or control stick is moved to produce a left roll, the interconnect cable and spring pulls forward on the left rudder pedal just enough to prevent the nose of the aircraft from yawing to the right. The force applied to the rudder by the springs can be overridden if it becomes necessary to slip the aircraft. [Figure 5-8]
Figure 5-8. Coupled ailerons and rudder.
Flaperons combine both aspects of ﬂaps and ailerons. In addition to controlling the bank angle of an aircraft like conventional ailerons, ﬂaperons can be lowered together to function much the same as a dedicated set of ﬂaps. The pilot retains separate controls for ailerons and ﬂaps. A mixer is used to combine the separate pilot inputs into this single set of control surfaces called ﬂaperons. Many designs that incorporate ﬂaperons mount the control surfaces away from the wing to provide undisturbed airﬂow at high angles of attack and/or low airspeeds. [Figure 5-9]
Figure 5-9. Flaperons on a Skystar Kitfox MK 7.
The elevator controls pitch about the lateral axis. Like the ailerons on small aircraft, the elevator is connected to the control column in the ﬂight deck by a series of mechanical linkages. Aft movement of the control column deﬂects the trailing edge of the elevator surface up. This is usually referred to as up “elevator.” [Figure 5-10]
Figure 5-10. The elevator is the primary control for changing the pitch attitude of an airplane.
The up-elevator position decreases the camber of the elevator and creates a downward aerodynamic force, which is greater than the normal tail-down force that exists in straight-and-level ﬂight. The overall effect causes the tail of the aircraft to move down and the nose to pitch up. The pitching moment occurs about the center of gravity (CG). The strength of the pitching moment is determined by the distance between the CG and the horizontal tail surface, as well as by the aerodynamic effectiveness of the horizontal tail surface. Moving the control column forward has the opposite effect. In this case, elevator camber increases, creating more lift (less tail-down force) on the horizontal stabilizer/elevator. This moves the tail upward and pitches the nose down. Again, the pitching moment occurs about the CG.
As mentioned earlier in the coverage on stability, power, thrustline, and the position of the horizontal tail surfaces on the empennage are factors in elevator effectiveness controlling pitch. For example, the horizontal tail surfaces may be attached near the lower part of the vertical stabilizer, at the midpoint, or at the high point, as in the T-tail design.
In a T-tail conﬁguration, the elevator is above most of the effects of downwash from the propeller as well as airﬂow around the fuselage and/or wings during normal flight conditions. Operation of the elevators in this undisturbed air allows control movements that are consistent throughout most ﬂight regimes. T-tail designs have become popular on many light and large aircraft, especially those with aft fuselage-mounted engines because the T-tail conﬁguration removes the tail from the exhaust blast of the engines. Seaplanes and amphibians often have T-tails in order to keep the horizontal surfaces as far from the water as possible. An additional beneﬁt is reduced vibration and noise inside the aircraft.
At slow speeds, the elevator on a T-tail aircraft must be moved through a larger number of degrees of travel to raise the nose a given amount than on a conventional-tail aircraft. This is because the conventional-tail aircraft has the downwash from the propeller pushing down on the tail to assist in raising the nose.
Since controls on aircraft are rigged so that increasing control forces are required for increased control travel, the forces required to raise the nose of a T-tail aircraft are greater than those for a conventional-tail aircraft. Longitudinal stability of a trimmed aircraft is the same for both types of conﬁguration, but the pilot must be aware that the required control forces are greater at slow speeds during takeoffs, landings, or stalls than for similar size aircraft equipped with conventional tails.
T-tail airplanes also require additional design considerations to counter the problem of ﬂutter. Since the weight of the horizontal surfaces is at the top of the vertical stabilizer, the moment arm created causes high loads on the vertical stabilizer which can result in flutter. Engineers must compensate for this by increasing the design stiffness of the vertical stabilizer, usually resulting in a weight penalty over conventional tail designs.
When ﬂying at a very high AOA with a low airspeed and an aft CG, the T-tail aircraft may be susceptible to a deep stall. In a deep stall, the airﬂow over the horizontal tail is blanketed by the disturbed airﬂow from the wings and fuselage. In these circumstances, elevator or stabilator control could be diminished, making it difﬁcult to recover from the stall. It should be noted that an aft CG is often a contributing factor in these incidents, since similar recovery problems are also found with conventional tail aircraft with an aft CG. [Figure 5-11]
Figure 5-11. Airplane with a T-tail design at a high AOA and an aft CG.
Since ﬂight at a high AOA with a low airspeed and an aft CG position can be dangerous, many aircraft have systems to compensate for this situation. The systems range from control stops to elevator down springs. An elevator down spring assists in lowering the nose of the aircraft to prevent a stall caused by the aft CG position. The stall occurs because the properly trimmed airplane is ﬂying with the elevator in a trailing edge down position, forcing the tail up and the nose down. In this unstable condition, if the aircraft encounters turbulence and slows down further, the trim tab no longer positions the elevator in the nose-down position. The elevator then streamlines, and the nose of the aircraft pitches upward, possibly resulting in a stall.
The elevator down spring produces a mechanical load on the elevator, causing it to move toward the nose-down position if not otherwise balanced. The elevator trim tab balances the elevator down spring to position the elevator in a trimmed position. When the trim tab becomes ineffective, the down spring drives the elevator to a nose-down position. The nose of the aircraft lowers, speed builds up, and a stall is prevented. [Figure 5-12]
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.
The elevator must also have sufﬁcient authority to hold the nose of the aircraft up during the roundout for a landing. In this case, a forward CG may cause a problem. During the landing ﬂare, power is usually reduced, which decreases the airﬂow over the empennage. This, coupled with the reduced landing speed, makes the elevator less effective.
As this discussion demonstrates, pilots must understand and follow proper loading procedures, particularly with regard to the CG position. More information on aircraft loading, as well as weight and balance, is included in Chapter 9, Weight and Balance.
As mentioned in Chapter 2, Aircraft Structure, a stabilator is essentially a one-piece horizontal stabilizer that pivots 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]
Figure 5-13. The stabilator is a one-piece horizontal tail surface that pivots up and down about a central hinge point.
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]
Figure 5-14. The Piaggio P180 includes a variable-sweep canard design, which provides longitudinal stability about the lateral axis.
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.
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 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.
Figure 5-15. The effect of left rudder pressure.
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]
Figure 5-16. Beechcraft Bonanza V35.
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 are the most common high-lift devices used on aircraft. These surfaces, which are attached to the trailing edge of the wing, increase both lift and induced drag for any given AOA. Flaps allow a compromise between high cruising speed and low landing speed, because they may be extended when needed, and retracted into the wing’s structure when not needed. There are four common types of ﬂaps: plain, split, slotted, and Fowler ﬂaps. [Figure 5-17]
Figure 5-17. Five common types of flaps.
The plain ﬂap is the simplest of the four types. It increases the airfoil camber, resulting in a signiﬁcant increase in the coefﬁcient of lift (CL) at a given AOA. At the same time, it greatly increases drag and moves the center of pressure (CP) aft on the airfoil, resulting in a nose-down pitching moment.
The split ﬂap is deﬂected from the lower surface of the airfoil and produces a slightly greater increase in lift than the plain ﬂap. More drag is created because of the turbulent air pattern produced behind the airfoil. When fully extended, both plain and split ﬂaps produce high drag with little additional lift.
The most popular ﬂap on aircraft today is the slotted ﬂap. Variations of this design are used for small aircraft, as well as for large ones. Slotted ﬂaps increase the lift coefﬁcient signiﬁcantly more than plain or split ﬂaps. On small aircraft, the hinge is located below the lower surface of the ﬂap, and when the ﬂap is lowered, a duct forms between the ﬂap well in the wing and the leading edge of the ﬂap. When the slotted ﬂap is lowered, high energy air from the lower surface is ducted to the ﬂap’s upper surface. The high energy air from the slot accelerates the upper surface boundary layer and delays airﬂow separation, providing a higher CL. Thus, the slotted ﬂap produces much greater increases in maximum coefﬁcient of lift (CL-MAX) than the plain or split ﬂap. While there are many types of slotted ﬂaps, large aircraft often have double- and even triple-slotted ﬂaps. These allow the maximum increase in drag without the airﬂow over the ﬂaps separating and destroying the lift they produce.
Fowler ﬂaps are a type of slotted ﬂap. This ﬂap design not only changes the camber of the wing, it also increases the wing area. Instead of rotating down on a hinge, it slides backwards on tracks. In the ﬁrst portion of its extension, it increases the drag very little, but increases the lift a great deal as it increases both the area and camber. As the extension continues, the ﬂap deﬂects downward. During the last portion of its travel, the ﬂap increases the drag with little additional increase in lift.
Leading Edge Devices
High-lift devices also can be applied to the leading edge of the airfoil. The most common types are ﬁxed slots, movable slats, leading edge ﬂaps, and cuffs. [Figure 5-18]
Figure 5-18. Leading edge high lift devices.
Fixed slots direct airﬂow to the upper wing surface and delay airﬂow separation at higher angles of attack. The slot does not increase the wing camber, but allows a higher maximum CL because the stall is delayed until the wing reaches a greater AOA.
Movable slats consist of leading edge segments, which move on tracks. At low angles of attack, each slat is held ﬂush against the wing’s leading edge by the high pressure that forms at the wing’s leading edge. As the AOA increases, the high-pressure area moves aft below the lower surface of the wing, allowing the slats to move forward. Some slats, however, are pilot operated and can be deployed at any AOA. Opening a slat allows the air below the wing to ﬂow over the wing’s upper surface, delaying airﬂow separation.
Leading edge ﬂaps, like trailing edge ﬂaps, are used to increase both CL-MAX and the camber of the wings. This type of leading edge device is frequently used in conjunction with trailing edge ﬂaps and can reduce the nose-down pitching movement produced by the latter. As is true with trailing edge ﬂaps, a small increment of leading edge ﬂaps increases lift to a much greater extent than drag. As greater amounts of ﬂaps are extended, drag increases at a greater rate than lift.
Leading edge cuffs, like leading edge ﬂaps and trailing edge ﬂaps are used to increase both CL-MAX and the camber of the wings. Unlike leading edge ﬂaps and trailing edge ﬂaps, leading edge cuffs are ﬁxed aerodynamic devices. In most cases leading edge cuffs extend the leading edge down and forward. This causes the airﬂow to attach better to the upper surface of the wing at higher angles of attack, thus lowering an aircraft’s stall speed. The ﬁxed nature of leading edge cuffs extracts a penalty in maximum cruise airspeed, but recent advances in design and technology have reduced this penalty.
Found on many gliders and some aircraft, high drag devices called spoilers are deployed from the wings to spoil the smooth airﬂow, reducing lift and increasing drag. On gliders, spoilers are most often used to control rate of descent for accurate landings. On other aircraft, spoilers are often used for roll control, an advantage of which is the elimination of adverse yaw. To turn right, for example, the spoiler on the right wing is raised, destroying some of the lift and creating more drag on the right. The right wing drops, and the aircraft banks and yaws to the right. Deploying spoilers on both wings at the same time allows the aircraft to descend without gaining speed. Spoilers are also deployed to help reduce ground roll after landing. By destroying lift, they transfer weight to the wheels, improving braking effectiveness. [Figure 5-19]
Figure 5-19. Spoilers reduce lift and increase drag during descent and landing.
Although an aircraft can be operated throughout a wide range of attitudes, airspeeds, and power settings, it can be designed to ﬂy hands-off within only a very limited combination of these variables. Trim systems are used to relieve the pilot of the need to maintain constant pressure on the ﬂight controls, and usually consist of ﬂight deck controls and small hinged devices attached to the trailing edge of one or more of the primary ﬂight control surfaces. Designed to help minimize a pilot’s workload, trim systems aerodynamically assist movement and position of the ﬂight control surface to which they are attached. Common types of trim systems include trim tabs, balance tabs, antiservo tabs, ground adjustable tabs, and an adjustable stabilizer.
The most common installation on small aircraft is a single trim tab attached to the trailing edge of the elevator. Most trim tabs are manually operated by a small, vertically mounted control wheel. However, a trim crank may be found in some aircraft. The ﬂight deck control includes a trim tab position indicator. Placing the trim control in the full nose-down position moves the trim tab to its full up position. With the trim tab up and into the airstream, the airﬂow over the horizontal tail surface tends to force the trailing edge of the elevator down. This causes the tail of the airplane to move up, and the nose to move down. [Figure 5-20]
Figure 5-20. The movement of the elevator is opposite to the direction of movement of the elevator trim tab.
If the trim tab is set to the full nose-up position, the tab moves to its full down position. In this case, the air ﬂowing under the horizontal tail surface hits the tab and forces the trailing edge of the elevator up, reducing the elevator’s AOA. This causes the tail of the airplane to move down, and the nose to move up.
In spite of the opposing directional movement of the trim tab and the elevator, control of trim is natural to a pilot. If the pilot needs to exert constant back pressure on a control column, the need for nose-up trim is indicated. The normal trim procedure is to continue trimming until the aircraft is balanced and the nose-heavy condition is no longer apparent. Pilots normally establish the desired power, pitch attitude, and conﬁguration ﬁrst, and then trim the aircraft to relieve control pressures that may exist for that ﬂight condition. Any time power, pitch attitude, or conﬁguration is changed, expect that retrimming will be necessary to relieve the control pressures for the new ﬂight condition.
The control forces may be excessively high in some aircraft, and, in order to decrease them, the manufacturer may use balance tabs. They look like trim tabs and are hinged in approximately the same places as trim tabs. The essential difference between the two is that the balancing tab is coupled to the control surface rod so that when the primary control surface is moved in any direction, the tab automatically moves in the opposite direction. The airﬂow striking the tab counterbalances some of the air pressure against the primary control surface, and enables the pilot to move more easily and hold the control surface in position.
If the linkage between the balance tab and the ﬁxed surface is adjustable from the ﬂight deck, the tab acts as a combination trim and balance tab that can be adjusted to any desired deﬂection.
Antiservo tabs work in the same manner as balance tabs except, instead of moving in the opposite direction, they move in the same direction as the trailing edge of the stabilator. In addition to decreasing the sensitivity of the stabilator, an antiservo tab also functions as a trim device to relieve control pressure and maintain the stabilator in the desired position. The ﬁxed end of the linkage is on the opposite side of the surface from the horn on the tab; when the trailing edge of the stabilator moves up, the linkage forces the trailing edge of the tab up. When the stabilator moves down, the tab also moves down. Conversely, trim tabs on elevators move opposite of the control surface. [Figure 5-21]
Figure 5-21. An antiservo tab attempts to streamline the control surface and is used to make the stabilator less sensitive by opposing the force exerted by the pilot.
Ground Adjustable Tabs
Many small aircraft have a nonmovable metal trim tab on the rudder. This tab is bent in one direction or the other while on the ground to apply a trim force to the rudder. The correct displacement is determined by trial and error. Usually, small adjustments are necessary until the aircraft no longer skids left or right during normal cruising ﬂight. [Figure 5-22]
Figure 5-22. A ground adjustable tab is used on the rudder of many small airplanes to correct for a tendency to fly with the fuselage slightly misaligned with the relative wind.
Rather than using a movable tab on the trailing edge of the elevator, some aircraft have an adjustable stabilizer. With this arrangement, linkages pivot the horizontal stabilizer about its rear spar. This is accomplished by use of a jackscrew mounted on the leading edge of the stabilator. [Figure 5-23]
Figure 5-23. Some airplanes, including most jet transports, use an adjustable stabilizer to provide the required pitch trim forces.
On small aircraft, the jackscrew is cable operated with a trim wheel or crank. On larger aircraft, it is motor driven. The trimming effect and ﬂight deck indications for an adjustable stabilizer are similar to those of a trim tab.
Autopilot is an automatic ﬂight control system that keeps an aircraft in level ﬂight or on a set course. It can be directed by the pilot, or it may be coupled to a radio navigation signal. Autopilot reduces the physical and mental demands on a pilot and increases safety. The common features available on an autopilot are altitude and heading hold.
The simplest systems use gyroscopic attitude indicators and magnetic compasses to control servos connected to the ﬂight control system. [Figure 5-24] The number and location of these servos depends on the complexity of the system. For example, a single-axis autopilot controls the aircraft about the longitudinal axis and a servo actuates the ailerons. A three-axis autopilot controls the aircraft about the longitudinal, lateral, and vertical axes. Three different servos actuate ailerons, elevator, and rudder. More advanced systems often include a vertical speed and/or indicated airspeed hold mode. Advanced autopilot systems are coupled to navigational aids through a ﬂight director.
Figure 5-24. Basic autopilot system integrated into the flight control system.
The autopilot system also incorporates a disconnect safety feature to disengage the system automatically or manually. These autopilots work with inertial navigation systems, global positioning systems (GPS), and ﬂight computers to control the aircraft. In ﬂy-by-wire systems, the autopilot is an integrated component.
Additionally, autopilots can be manually overridden. Because autopilot systems differ widely in their operation, refer to the autopilot operating instructions in the Airplane Flight Manual (AFM) or the Pilot’s Operating Handbook (POH).
Because ﬂight control systems and aerodynamic characteristics vary greatly between aircraft, it is essential that a pilot become familiar with the primary and secondary flight control systems of the aircraft being ﬂown. The primary source of this information is the AFM or the POH. Various manufacturer and owner group websites can also be a valuable source of additional information.