CAMBER CHANGE
The most common method of increasing CLmax is increasing the camber of the airfoil. There are various types of high lift devices that increase the camber of the wing and increase CLmax. Trailing edge flaps are the most common type of high lift devices, but leading edge flaps are not unusual. Note the value of CL for this airfoil before and after flaps are deployed. Extending the flaps increases the airfoil’s positive camber, shifting its zero lift point to the left. Note that the stalling AOA (CLmax AOA) decreases.
Although stalling AOA decreases, visibility on takeoff and
landing improves due to flatter takeoff and landing attitudes
made possible by these devices. Since boundary layer control
devices increase stalling AOA, many modern designs utilize
BLC with camber change devices to maintain low pitch
attitudes during approach and landing. Flaps also increase
the drag on the airplane, enabling a steeper glide slope and
higher power setting during approach without increasing the
airspeed. This allows an airplane such as an EA-6B to carry
more thrust throughout the landing phase and not significantly
increase the approach speed (a higher throttle setting results
in less spool-up time in case of a wave-off or go-around). For
many airplanes, the first 50 percent of flap down movement produces most of the desired lift increase with less than half of the unwanted drag increase. Thus, raising flaps from 100 to 50 percent reduces drag significantly without a large loss of lift. This is especially important during engine failures on multi-engine airplanes.
A plain flap is a simple hinged portion of the trailing edge that is forced down into the airstream to in- crease the camber of the airfoil.
A split flap is a plate deflected from the lower surface of the airfoil. This type of flap creates a lot of drag because of the turbulent air between the wing and deflected surface.
A slotted flap is similar to the plain flap, but moves away from the wing to open a narrow slot between the flap and wing for boundary layer control. A slotted flap may cause a slight increase in wing area, but the increase is insignificant.
The fowler flap is used extensively on larger airplanes. When extend- ed, it moves down, increasing the camber, and aft, causing a significant increase in wing area as well as opening one or more slots for boundary layer control. Because of the larger area created on airfoils with fowler flaps, a large twisting moment is developed.
This requires a structurally stronger wing to withstand the increased twisting load and precludes their use on high speed, thin wings.
Leading edge flaps are devices that change the wing camber at the leading edge of the airfoil. They may be operated manually with a switch or automatically by computer. Leading edge plain flaps are similar to a trailing edge plain flap. Leading edge slotted flaps are similar to trailing edge slotted flaps, but are sometimes confused with automatic slots. Often the terms are interchangeable since many leading edge devices have some characteristics of both flaps and slats.
The exact stall speed for various airplane conditions are given in stall speed charts in an airplane’s flight manual. The directions on how to use the stall speed chart are on the chart itself and are self-explanatory.
