Updated: Dec 8, 2020
Airplane Flight Controls Lesson by wifiCFI
Flight Controls (PHAK C6)
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.
Aircraft flight control systems consist of primary and secondary systems.
Primary Flight Controls
Secondary Flight Controls
Leading Edge Devices
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 4 Types:
Coupled Ailerons and Rudder
With differential ailerons, one aileron is raised a greater distance than the other aileron and 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.
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.
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.
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.
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 the up-elevator position.
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.
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 nose of the aircraft.
Pushing the control column forward lowers the trailing edge of the stabilator and pitches the nose of the aircraft 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.
The canard design utilizes the concept of two lifting surfaces.
The canard functions 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.
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.
The rudder is controlled by the left and right rudder pedals.
When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction.
The V-Tail Design
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.
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.
Secondary Flight Controls
Secondary flight control systems may consist of:
Leading edge devices
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 4 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.
Slotted flaps increase the lift coefficient significantly more than plain or split flaps.
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.
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.
Leading Edge Devices
High-lift devices also can be applied to the leading edge of the airfoil.
The most common types are:
Leading Edge Flaps
Fixed Leading Edge 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 Leading Edge Devices
Movable slats consist of leading edge segments that 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.
Leading Edge Flaps
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.
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.
Found on some fixed-wing aircraft, high drag devices called spoilers are deployed from the wings to spoil the smooth airflow, reducing lift and increasing drag.
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.
Types of trim systems include:
Ground adjustable tabs
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.
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.
Servo tabs are very similar in operation and appearance to the trim tabs previously discussed.
A servo tab is a small portion of a flight control surface that deploys in such a way that it helps to move the entire flight control surface in the direction that the pilot wishes it to go.
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.
Ground Adjustable Tabs
Many small aircraft have a non-movable 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.
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 the use of a jack-screw mounted on the leading edge of the stabilator.
Auto-Pilot (PHAK C6)
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.
FAA Sources Used in this Lesson
Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 6