Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force

Fixed-Wing Aircraft Primary Controls

Schematic - the Three Axis of Control

In addition to moving forward, aircraft move about three axes in response to three forces: lift, drag, and side force. These axes can be visualized as three rods that pass through the aircraft so that each intersects the other two. The point of intersection is called the center of gravity. Each of these axes is also perpendicular to the other two.


The axis that extends crosswise from wingtip to wingtip is called the lateral axis. Rotation about this axis is called pitch. Side force acts along this axis.

 

Movements of the Elevators (solid black) cause changes in pitch.


The axis that extends lengthwise through the nose and tail is called the longitudinal axis. Rotation about this axis is called roll. Drag is the force that acts along this axis, but in the opposite direction of the flight path.

 

Movement of the Ailerons (solid black) produces changes in roll.


The axis that passes vertically through the center of gravity when the aircraft is in level flight is called the vertical axis. Rotation about this axis is called yaw. Lift acts along this axis.

 

Movements of the Rudder (solid black) produce changes in yaw.


A - Ailerons

B - Pilot's Control Stick and Rudder Pedals

C - Elevators

D - Rudder

Fixed-Wing Aircraft Secondary Controls

Seems complicated, right?  Let's take a look...


FLAPS

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.


SLATS AND SLOTS

 

Slats are auxiliary airfoils fitted to the leading edge of the wing. At high angles of attack, they automatically move out ahead of the wing. The angle of attack of the slat being less than that of the main-plane, there is a smooth airflow over the slat which tends to smooth out the eddies forming over the wing. Slats are usually fitted to the leading edge near the wing tips to improve lateral control.

 

Slots are passageways built into the wing a short distance from the leading edge in such a way that, at high angles of attack, the air flows through the slot and over the wing, tending to smooth out the turbulence due to eddies.


SPOILERS

 

 

Spoilers are devices fitted to the wing that increase drag and decrease lift. They usually consist of a long narrow strip of metal arranged span-wise along the top surface of the airfoil. In some airplanes, they are linked to the ailerons and work in unison with the ailerons for lateral control. As such, they open on the side of the upgoing aileron, spoil the lift on that wing and help drive the wing down and help the airplane to roll into a turn.


SPEED BRAKES

 

 

 Speed brakes are a feature on some high performance airplanes. They are a device designed to facilitate optimum descent without decreasing power enough to shock cool the engine and are especially advantageous in airplanes with high service ceilings. They are also of use in setting up the right approach speed and descent pattern in the landing configuration. The brakes, when extended, create drag without altering the curvature of the wing and are usually fitted far enough back along the chord so as not to disrupt too much lift and in a position laterally where they will not disturb the airflow over the tail-plane. They are usually small metal blades housed in a fitting concealed in the wing that, when activated from the cockpit, pivot up to form a plate. On some types of aircraft, speed brakes are incorporated into the rear fuselage and consist of two hinged doors that open into the slipstream.


TRIM TABS

 

 

Trim tabs are small surfaces connected to the trailing edge of a larger control  surface on an aircraft, used to control the trim of the controls, i.e. to counteract  aero-dynamic forces and stabilise the aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by adjusting the angle of the tab relative to the larger surface.

 

 

Changing the setting of a trim tab adjusts the neutral or resting position of a control surface (such as an elevator or rudder). As the desired position of a control surface changes (corresponding mainly to different speeds), an adjustable trim tab will allow the operator to reduce the manual force required to maintain that position—to zero, if used correctly. Thus the trim tab acts as a servo tab. Because the center of pressure of the trim tab is further away from the axis of rotation of the control surface than the center of pressure of the control surface, the movement generated by the tab can match the movement generated by the control surface. The position of the control surface on its axis will change until the movements from the control surface and the trim surface balance each other.

 

Elevator trim frees the pilot from exerting constant pressure on the pitch controls. Instead, the pilot adjusts a longitudinal trim control (often in the form of a wheel) to cancel out control forces for a given airspeed / weight distribution. Typically, when this trim control is rotated forward, the nose is held down; conversely, if the trim wheel is moved back, the tail becomes "heavy". Many newer aircraft, especially jet aircraft have electric trim controls.

 

 

Many airplanes also have rudder and/or aileron trim systems. On some of these, the rudder trim tab is rigid but adjustable on the ground by bending: it is angled slightly to the left to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of some prop-driven aircraft. Other aircraft have hinged rudder trim tabs that the pilot can adjust in flight.

 

When a trim tab is employed, it is moved into the slipstream opposite to the control surface's desired deflection. For example, in order to trim an elevator to hold the nose down, the elevator's trim tab will actually rise up into the slipstream. The increased pressure on top of the trim tab surface caused by raising it will then deflect the entire elevator slab down slightly, causing the tail to rise and the aircraft's nose to move down.  In the case of an aircraft where deployment of high-lift devices (flaps) would significantly alter the longitudinal trim, a supplementary trim tab is arranged to simultaneously deploy with the flaps so that pitch attitude is not markedly changed.

 

Beyond reducing pilot workload, proper trim also increases fuel efficiency by reducing drag. For example, propeller aircraft have a tendency to yaw when operating at high power, for instance when climbing: this increases parasite drag because the craft is not flying straight into the apparent wind. In such circumstances, the use of an adjustable rudder trim tab can reduce yaw.


 

VORTEX GENERATORS

 

 

 

A Vortex Generator is an aerodynamic surface, consisting of a small vane or bump that creates a vortex.  Vortex generators delay flow separation and aerodynamic stalling; they improve the effectiveness of control surfaces.

 

 

On aircraft they are installed on the front third of a wing in order to maintain steady airflow over the control surfaces at the rear of the wing. They are typically rectangular or triangular, about 80% as tall as the boundary layer, and run in spanwise lines near the thickest part of the wing.  They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.

 

A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by re-energizing the boundary layer.


WING FENCES/STALL FENCES

 

 

Wing fences are fin-like vertical surfaces attached to the upper surfaces of the wing that are used to control the airflow. On swept wing airplanes, they are located about two-thirds of the way out towards the wing tip and prevent the drifting of air toward the tip of the wing at high angles of attack. On straight wing airplanes, they control the airflow in the flap area. In both cases, they give better slow speed handling and stall characteristics.


WINGLETS AND WINGTIP FENCES

 

Wingtip devices increase the lift generated at the wingtip by smoothing the airflow across the upper wing near the tip, and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ration.

 

 

 

 

Winglet

 

A winglet is a near-vertical extension of the wing tips. The upward angle of the winglet, its inward or outward angle, as well as its size and shape are critical for correct performance and are unique in each application. The wingtip vortex, which rotates around from below the wing, strikes the cambered surface of the winglet, generating a force that angles inward and slightly forward.  The winglet also converts some of the otherwise-wasted energy in the wingtip vortex to an apparent thrust.

 

Blended Winglet

 

A blended winglet is intended to reduce interference drag at the wing/winglet junction. A sharp interior angle in this region can interact with the boundary layer flow causing a drag inducing vortex, negating some of the benefit of the winglet.

 

Wingtip Fence

 

Wingtip fence is a winglet variant, with surfaces extending both above and below the wingtip. Both surfaces are shorter than or equivalent to a winglet possessing similar aerodynamic benefits.

 

 

Blended Winglet in foreground, Wingtip Fence in background


SECONDARY CONTROLS - OVERVIEW

 

 

 

  1. Winglet
  2. Low Speed Aileron
  3. High Speed Aileron
  4. Flap track fairing
  5. Krüger flaps
  6. Slats
  7. Three slotted inner flaps
  8. Three slotted outer flaps
  9. Spoilers
  10. Spoilers Air-brakes

Cadet Flying


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Emergency Services GTM Training


Balloon Festival

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