Lift and drag on a wing
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 its 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.
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. CL 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.
It is possible to combine all these seemingly conflicting properties into one wing design. To accomplish this, the designer uses leading and trailing edge flaps. When deployed, these devices influence and or change certain properties of the wing: CL, CD, camber, wing area, etc.
If we revisit the formula: L = 1/2 ρ V2 x S x CL, 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 (CL) can be varied for given angle of attack (AOA) to change lift. The use of flaps will change the wing area and coefficient lift.
General aviation aircraft normally use the plain flap, although some manufacturers (DynAero) 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.
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 during deployment. They have the greatest increase in CL with the lowest increase in drag, hence the common use on high performance aircraft, like airliners, turboprops and so on.
Aerodynamic effects of flaps
The use of trailing edge flaps (lowering them) will have a number of effects aerodynamically. CL increases for all angle of attacks (AOA), reducing stall speed, results in a higher CD (with more drag airspeed stabilizes easily), rearward movement of the center of pressure (CP), and keep in mind: a lower stalling AOA. To complete the list: aircraft are more maneuverable and can land at lower speeds resulting in shorter landing distances and less airframe loads.
This 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 to runways when there are obstacles in the approach path.
When flaps have their hinge point below the wing (instead of at the rear wing spar) they also increase the camber and wing area of the wing where they are installed, it is usually the inner part toward the wing root.