High Lift Devices
Besides changing lift by simply pulling the control column in order to increase the angle of attack, wing design features may allow you to change the camber of the wing as well as enlarging wing surface through the extension of trailing-edge flaps and leading-edge high lift devices. Using these devices also delays air flow separation and lower stall speed.

Trailing Edge Flaps

The most common used high-lift device is the trailing-edge flap. It produces a large increase in airfoil camber as well as increasing both lift and drag. Trailing edge flaps are therefore often used to make steeper approach paths possible as they can add to your rate of descent without increasing airspeed. As flaps increase the coefficient of lift they also decrease stall speed. Although this may sound favorable, it should be something to remember before raising them at low airspeeds, for example after take-off, or high angles of attack. There are basically four types of flaps found on general aviation aircraft.

  • Plain flaps increase the coefficient of lift simply by increase the wing camber. In clean configuration the plain flap is in neutral position while in operation it is deflected downward.
Figure 1.1 - Plain flap
  • Split flaps increase the wing camber by deflecting part of the airfoil downward. Split flaps are commonly used on aircraft like the Douglas DC-3 and many small transport aircraft.
Figure 1.2 - Split flap
  • Fowler flaps also increase wing camber as well as wing area and help prevent airflow separation due to slots. Fowler flaps are both used on light aircraft and large commercial jet aircraft.
Figure 1.3 - Fowler flap
  • Similar as with the fowler flap, the slotted flap increases wing camber and prevent airflow separation due to slots in its design.
Figure 1.4 - Slotted flap

As you can see, plain, split and simple slotted flaps change the effective camber of the wing from a streamlined low drag profile to a more effective shape although having a greater drag penalty. On the contrary, flaps with slots also allow some of the high pressure air below the wing to flow over the upper surface of the wing, thus increasing lift by accelerating the air and decreasing local pressure. As said, slots also delay airflow separation, which results in a decrease of the stall speed. Likewise, fowler flaps increase wing area surface while allowing air to flow thru slots. Many large commercial transport aircraft are fitted with multi-slotted fowler flaps, further modifying airfoil camber, wing area, and airflow.

Besides flaps, some aircraft use small aileron deflections along with the flaps to further increase lift at low airspeeds. A special mechanism allows the ailerons to be used in this manner while still being able to control movement around the longitudinal axis. Due to large aerodynamic stresses on the flaps and its supporting structure, maximum operating speeds for different flap settings should be taken into account when planning to extent or retract them.

As mentioned earlier, many aircraft use flaps for take-off as well as landing. During landing, flaps obviously decrease approach and landing speed thus reducing ground roll. On take-off, flaps increase climb performance significant or reduces take-off run which could be used effectively on short runways.

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