Flight Controls: Secondary Flight Controls
Flight control surfaces are used to control an aircraft's direction in flight. The flight controls are separated into 2 classes: the primary flight controls and the secondary flight controls. In this article, the secondary flight controls are discussed.
The secondary flight controls are: flaps, trimming devices, spoilers, slats, slots and speed brakes. The secondary flight controls are not always all present on an aircraft.
Flaps
These are the most common secondary flight controls: you can find flaps on almost every aircraft wing. The flaps are deployed via a lever or digitally via the MFD (=Multi Function Display) in the cockpit.
Usually, the deployment is measured in 10 degrees increments from zero (not deployed) to 40 degrees (fully deployed). When the flaps are deployed, the lift and drag of the wing increases. By a setting of 10° flaps there is more lift than drag produced, so this setting is used for take-off. But when the flaps are more deployed, there is more drag produced than lift and it's possible to descent steeper without an increase in airspeed. So this flap setting is used in the landing phase.
Flaps also help to lower the stalling speed.
There are many types of flaps and their functioning can be difficult to understand, so I keep it by the most important sorts of flaps:
Plain flaps: this flap rotates on a simple hinge.
Split flaps: the upper and lower surfaces are separate, the lower surface operates like a plain flap, but the upper surface stays immobile or moves only slightly.
Fowler flaps: this flap slides backwards before hinging downwards, thereby increasing both camber and chord, creating a larger wing surface better tuned for lower speeds.
Slotted flaps: a gap between the flap and the wing enables high pressurized air from below the wing to re-energize the boundary layer over the flap. This helps the airflow to stay attached to the flap, creating a lower stall speed. There are also double slotted flaps and triple slotted flaps.
Left: A Boeing B747 from Singapore Airlines during landing with triple slotted flaps.
Right: Flaps in use on a BMI airlines Airbus.
(Click on image for bigger version)
Spoilers
Spoilers are plates on the top of a wing which can be extended upward into the airflow to reduce lift and speed. They are mostly used on landing, because they cause a dramatic loss of lift and hence the weight of the aircraft is transferred from the wings to the undercarriage, allowing the wheels to be mechanically braked with much less chance of skidding. The difference between airbrakes (speed brakes) and spoilers is that airbrakes are designed to increase drag while making little change to lift, while spoilers greatly reduce lift while making only a moderate increase in drag.
Under: Spoilers in use on a BMI airlines Airbus. (Click on image for bigger version)
Speed brakes
Speed brakes are not a common sight on civilian aircraft, although the Bae 146 has speed brakes. Some military jets have speed brakes (e.g. Blackburn Buccaneer, Su-30,...). The meaning of speed brakes is, as the name says, reduce speed during landing.
Under: Bae 146 speed brakes. (Click on image for bigger version)
Slats & Slots
Slats are aerodynamic surfaces on the leading edge of the wings. When deployed, there exists a hole between the wing and the slat, this allows the air to flow from below the wing to its upper surfaces. This way the wing still provides enough lift when flying with a higher angle of attack. So it's possible to fly the aircraft at a lower speed without stalling or land in a shorter distance. The slats are mostly used when landing or when doing operations at low speed. During normal flight, they are retracted. There are two types of slats:
-automatic: the slat lies flush with the wing leading edge until reduced aerodynamic forces allow it to extend by way of springs when needed. This type is typically used on light aircraft.
-powered: the slat extension can be controlled by the pilot. This is commonly used on airliners.
A slot is a span-wise gap in each wing, allowing air to flow from below the wing to its upper surface. This allows the wing to provide lift up to a higher angles of attack than would be possible without the slot. In this manner they allow flight at higher angles of attack and thus reduce the stall speed. But the slot produces more drag in normal flight. A way to overcome this, is making slots retractable and then you have slats.
Under: Slots in a Stinson 108-3.
Trim tabs
Trim tabs are small surfaces connected to the trailing edge of a larger control surface of an aircraft. They are used to stabilise the aircraft in a particular desired attitude without the need for the pilot to constantly apply control force. 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 during flight, the trim tab allows the pilot to reduce (to zero, when used correctly) the manual force required to maintain such position.
The vertical black wheel, here in the center console of an aircraft (can also be found on different places), controls the elevator trim tabs. Moving it upwards lowers the airplane nose and increases speed; moving it downwards raises the airplane nose and reduces speed.
Under: an elevator trim tab
