The relationship between the angle of attack and speed
The main lift aircraft with flight speed and angle of attack changes when flying at great speed, requires only a small angle of attack, the wings can generate enough lift to maintain flight. When a small flight speed, angle of attack requires a large wing to generate enough lift to sustain flight.
Why use lift devices
The method for increasing the lift coefficient to increase the angle of attack, thus reducing the speed is limited, the aircraft angle of attack can only be increased to the critical angle. Therefore, in order to ensure the aircraft during takeoff and landing, can still generate enough lift, it is necessary to increase the lift coefficient of the device mounted on the wing.
Lift means for increasing the maximum lift coefficient of the aircraft , thereby shortening the distance of the aircraft taxiing on the ground takeoff and landing phase.
The main lift apparatus comprising:
1, the slat;
2, the trailing edge flap;
3, the leading edge flap;
Slat
The wing leading edge slat, the slat opening at high angles of attack, can delay the flow separation on the surface, so that the maximum lift coefficient is increased and the critical angle of attack, at small opening angle of attack slat wing, wing lift will lead to poor performance.
The high pressure air flow through the airfoil slot, close to the airfoil flow. Reducing the adverse pressure gradient on the one hand, delaying the flow separation, increasing the maximum lift coefficient and the critical angle. On the other hand, reducing the pressure differential of the upper and lower surfaces, reducing the lift coefficient.
The figure is the influence of the slat Pressure Distribution : larger angle of attack, the use of slat lift coefficient can be increased .
Trailing edge flap
After the edge flaps down the lift coefficient and drag coefficient is increased at the same time. Accordingly, the discharge flaps small angle during takeoff, landing, the flap angle amplification. A trailing edge flap comprising:
1, the split flap; 2, plain flap; 3, slotted flaps; 4, flaps retracted; 5, slotted flaps retracted;
1, split flap
Split flap is a rear surface of the deflection downwardly from the lower wing airfoil split off, so that the maximum lift coefficient and lift coefficient increases, the critical angle of attack decreases.
After the split flaps down, the formation of vortices in the wedge between the wing and the flap, the pressure is reduced, increasing the surface attract the air velocity, pressure increase upper and lower surfaces, thereby increasing the lift coefficient, delayed flow separation.
Further, the split flaps down so that the airfoil camber is increased, the upper and lower airfoil pressure increase, thus increasing the lift coefficient.
2, simple flap
Ailerons and flaps similar to the shape of a simple, plain flap down, increasing the camber of the wing, the airfoil further increasing the pressure difference between the upper and lower, increasing the lift coefficient. However, the discharge pressure such that the flaps simply drag and induced drag increases, the resistance increases more than the lift, so that the lift-drag ratio is lowered.
Plain flap decentralized high angles of attack, due to the increase in camber, the upper airfoil adverse pressure gradient increases, flow separation in advance, leading to the critical angle of attack is reduced.
3, slotted flap
Slotted flap in the flap on the basis of a simple improvement, a partial slit, while the lower, and a simple comparison slat, the upper surface may further delay the flow separation, increasing the camber of the wing, so that more improved lift coefficient while the critical angle of attack was much lower.
The gas flows through the airfoil slotted airfoil flow
Slotted flaps flow map
4, back flap
Back flap in the flap on the basis of a simple improvement, while the lower slide rearwardly biased, and compared to plain flap, wing camber increases the wing area is increased, so that the maximum lift coefficient and lift coefficient more increases, reducing the critical angle of attack less.
5, Back slotted flap
Back combined slotted flap back flaps and slit opening common characteristics of the flaps, the best and most complex.
Large aircraft generally use the back double or triple slotted slotted form.
747 Back slotted flap
Leading edge flap
The wing leading edge slat, the slat can slow down the flow separation on the surface, it can increase the airfoil camber, so that the maximum lift coefficient and the critical angle is improved.
Leading edge flap boxes widely used in high subsonic aircraft supersonic aircraft.
B737-800 leading edge flap
Lift devices principle summarized:
Lift devices purpose is to increase the maximum lift coefficient.
Lift system is mainly achieved by three lift:
1, increasing the camber of the airfoil, the pressure difference between the upper and lower surfaces improve.
2, delaying the flow separation on the surface, to improve the critical angle and the maximum lift coefficient.
3, increasing the wing area.