Lift and drag on a wing
Lift Augmentation Devices, II
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.
Previous article looked at trailing edge flaps, here we investigate leading edge flaps, slats and vortex generators.
Slats & Slots
This is a small wing like device installed ahead of the wing leading edge so that the air can flow in between the slat and wing (slot) at high angles of attack. Nowadays you will find automatically retractable slats as the fixed type creates a lot of drag at higher speeds.
Slats work by energizing the air over the wing at high AOA using kinetic energy from below the wing to the airflow on top so that separation is delayed with the result that the stall speed is lower and with a higher AOA compared to a wing without slats. CL is also higher.
Leading edge flaps
Airliners, regional jets are equipped with leading edge flaps increasing the camber of the wing, a higher angle of attack before the wing stalls and the center of pressure moves also forward when deployed.
There are two ways to accomplish this: either move the leading edge forward on tracks and creating a slat with slot or use a Kreuger flap which extends from beneath the leading edge.
These devices also energize the airflow so that the separation of the air during high angles of attack is much later than without them. The big advantage compared to slats is that there is almost no drag penalty at higher airspeeds. Some builders of slow aircraft even have removed their slats in favor of vortex generators.
How they work
A small vortex is created in the airflow over the wing and it energizes the boundary layer making it more resistant to separation.
The advantage is that the wing can fly with a higher angle of attack with much better aileron control and feel for the pilot.
For the pilot it is clear that VG's bring higher lift, shorter landing & take-off distances, better aircraft control at slow flight, gentler stalls and tighter turns (if desired). For bush pilots and those wishing to become one, its performance on another level.
Together with the discontinuous leading edge, these devices prevent the stall from progressing over the wing toward the tips and ailerons by placing a vortex over the wing.
The result is much better aileron control at higher angles of attack and even into the stall. Jets and other high performance aircraft (military fighters) are sometimes equipped with these devices.