LIFT AUGMENTATION DEVICES

by: dodi suroto

Lift Augmentation Devices 

The wing of an aircraft is designed for high or cruise speed where lift is mainly created by forward speed only. For slow flight we need to increase lift somehow, but there are limits to what can be done by increasing the angle of attack. Aerodynamicists have devised other ways of increasing the amount of lift generated. These are called lift augmentation devices. Pilots use these devices on almost every flight.

There are several devices that can be used on an aircraft wing to increase the lift. We all know them as leading and trailing edge flaps. Other techniques are also used to increase the angle of attack (coefficient lift): vortex generators, wing fences and discontinuous leading edges.

We will look at these devices on this page, exploring them and seeing how they operate.

Increasing Lift

An aircraft wing, designed for high or cruise speed, can be characterized by having low camber, low thickness/chord (T/C) ratio and you will see the maximum thickness somewhere in the middle of the chord. C_L and drag will also be low compared to high lift wings.

To be able to fly and maneuver safely at low airspeeds the wing must have a high lift aerofoil (high camber, T/C ratio and maximum thickness well forward).

Flaps

All these seemingly conflicting wing properties can be incorporated into one design. To accomplish this, the wing designer uses leading and trailing edge flaps. When deployed, these devices change certain properties of the wing: camber, wing area, etc.

If we revisit the lift formula: L = 1/2 ρ V² x S x C_L, we can see that we have several ways to increase the lift for a given wing. Speed (V), wing area (S) and the coefficient lift (C_L) can be varied for given angle of attack (AOA) to change lift. The use of flaps will change the wing area and coefficient lift.

Flaps

General Aviation aircraft normally use the plain flap, although some manufacturers (Dyn Aero) use the fowler flap on their aircraft. The image shows the types of flaps in use today. From top to bottom: the simple or plain flap, slotted flap, split and fowler flap.

Fowler flaps

Its easy to see that the fowler flap not only increases camber of the wing but also the wing area by moving toward the back, when they are deployed. They have the greatest increase in C_L with the lowest increase in drag, hence the use on high performance aircraft, airliners and so on.

Aerodynamic effects of flaps

The use of trailing edge flaps (lowering them) will have a number of effects aerodynamically. C_L increases for all angle of attacks (AOA), reducing stall speed, results in a higher C_D (with more drag airspeed stabilizes easily), rearward movement of the center of pressure (C_P), a lower stalling AOA, aircraft are more maneuverable and can land at lower speeds resulting in shorter landing distances.

Lift/Drag

Lift/drag ratio (L/D) is also reduced when full flaps are extended. Normally, flap settings between 0 - 25° will noticeably increase lift more than drag, ideal for take off. Flap settings beyond 25° will increase drag much more than lift, ideal for landing and steep approaches.

Flaps also increase the camber and wing area of the wing where they are installed, usually the inner part toward the wing root.