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Secondary Flight Controls



"Flaps 25 set..." - Secondary Flight Controls
Secondary flight controls are used to influence the performance of the aircraft. These controls can influence the amount of lift or drag produced by a particular airfoil and thus can eiter stimulate or decrease the performance of the aircraft. Trailing edge (TE) devices and leading edge (LE) devices are examples which stimulate aircraft performance. Thereby, spoilers can stimulate, but also reduce the aircrafts performance. The trim assists the pilot during the flight and makes it easier for a pilot to control the aircraft.
Trailing Edge Devices
TE flaps are used during take-off, landing and low speed flight. They increase camber, drag and decrease stall speed. Therefore the take-off or landing of the aircraft can be made much safer because the chance of stalling the aircraft is much smaller. Also because the take-off and landing speed are much lower. There are several types of TE devices, these are:

Plain Flap

  • This is the simplest type of flap (figure 1.1). The flap can deflect down which increases the camber.
Plain Flap
Figure 1.1 - Plain Flap

Split Flap

  • Consists of a hinged section on the TE of the lower surface of the wing (figure 1.2).
Split Flap
Figure 1.2 - Split Flap

Slotted Flap

  • This flap looks like the plain flap but it has a gap between the wing and the flap (figure 1.3). This gap has the same effect as a venture tube. The air flows from the lower surface to the upper surface of the wing, adding energy to the boundary layer. This prevents separation of the airflow.
Slotted Flap
Figure 1.3 - Slotted Flap

Fowler Flap

  • The fowler flap is used in modern aircraft (figure 1.4). Like all other flaps it increases the cam-ber. When it slides backwards it has a similar gap as a slotted flap it also increases the wing area. This causes the most significant change in lift and is used on most commercial jet aircraft.
Fowler Flap
Figure 1.4 - Fowler Flap
Leading Edge Devices
LE devices are together with TE devices high lift devices, which increase wing lift by extending wing surface. An increase in lift results in a decrease of stall speed during take-off, low speed flight and landing. At high angles-of-attack the air flow could separate from the wing’s upper sur-face and become turbulent. This results in a stalled condition were much of the wing lifting ca-pability is destroyed. To prevent the airflow to separate from the wing, LE devices allow some of the high energy air from beneath the wing to flow through a slot and over the upper surface of the wing. This allows the aircraft to fly at higher angles-of-attack at lower airspeeds. There are several types of LE devices:

• Hinged nose
Works exactly the same as the plain flap.

• Fixed slot
The fixed slot cannot extend, it’s a permanent slot in the wing. The slot adds more airflow to the top of the wing, thus adding energy to the boundary layer. The gap works like a venture tube.


• Krueger flap
The surface under the wing at the LE extends. Apart from increasing the camber, it also increases the wing area.

• Slat
The slat works just like the slotted flap. It acts like a venture tube and adds energy to the boundary layer.
Spoilers

Spoilers are used to increase drag and decrease lift. When the spoilers are deployed the airflow on the upper surface of the wing is disrupted. The pressure difference between the upper and lower surface of the wing is much smaller and causes the increase in drag and decrease in lift. There are three kinds of spoilers:

Flight spoilers

Spoilers are called flight spoilers when they are used asymmetrically. Flight spoilers are used on modern jet aircraft. When these aircraft are flying at cruise speed, near the speed of sound, they are influenced by shock waves. Because of these shockwaves, the ailerons do not work prop-erly and therefore spoilers are needed to keep proper control of the airplane. When the control wheel is turned, the spoilers will open automatically. In a right turn, the right spoiler will open and in a left turn the left spoilers.

Ground spoilers

Ground spoilers are used to stop an airplane on the runway as soon as possible. To stop an aircraft as soon as possible, it has to lose any form of lift. After the touch down, airplanes still have a lot of lift because of the high landing speed. The ground spoilers eliminate any form of lift. These spoilers also increase drag, which decreases the speed of the airplane. Modern air-planes have an automatic spoiler extension system. This will only work if the spoiler lever is put in the ‘armed’ position before takeoff or landing. If the engine power levers are pushed back, the spoilers will open automatically, when they are armed.

Speed brakes

Spoilers can also deploy simultaneously, thus symmetrically. This does not happen because of the turning of the control wheel, but with a spoiler lever or speed brake handle. They decrease lift on both wings of the airplane and this causes a rapid descent of the airplane. The speed brakes also slow down the aircraft, because of the increased drag.
Trim

Trim is used to balance the airplane in flight and reduces the force that the pilot has to exert on the control column. There are three different kinds of trim systems:

Trim tabs

A trim tab is a small, adjustable hinged surface. It is used to maintain balance in straight and level flight without the pilot having to exert pressure on the controls. This is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface must be held. The force of the airflow striking the tab causes the main control surface to be deflected to a position, which will correct the unbalanced condition of the aircraft.

Balance tabs

Balancing tabs look like trim tabs and are hinged in approximately the same places as trim tabs would be. The difference between the two is that the balancing tab is coupled to the control sur-face by a rod, so that when the primary control surface is moved in any direction the tab auto-matically is moved in the opposite direction. These tabs also act as trim tabs as described above.

Servo tabs

Servo tabs are very similar in operation and appearance to the trim tabs. Servo tabs are primar-ily used on large aircraft. They help the pilot in moving the control surface and holding it in the desired position. Only the servo tab, unlike the trim and balance tab, moves in response to movement of the control column. The force of airflow on the servo tab then moves the primary control surface.