What is Lift ?

Lift

What is lift?

Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.

Note: An aerofoil is cambered on its topside and flat on its bottom side. Therefore, the airflow over the top of the aerofoil has to travel farther and thus faster than the airflow below the aerofoil. This causes the pressure below the aerofoil to be greater than above, creating a pressure difference, which results in an upward lift force.

What if the formula for lift?

Lift: 1/2R + V2 + S + CL
%R = half the value of the air density V2 = airflow velocity squared S = wing span area CL = coefficient of lift The combined values of these properties determine the amount of lift produced.

What is coefficient of lift (CL)? Coefficient of lift (Ct) is the lifting ability of a particular wing. It depends on both the shape of the wing section (fixed design feature) and the angle of attack.

Center of pressure: The center of pressure is represented as a single point acting on the wing chord line at a right angle to the relative airflow, through which the wing’s lifting force is produced. The position of the center of pressure is not a fixed point but depends on the distribution of pressure along the chord, which itself depends on the angle of attack. Thus, for a greater angle of attack, the point of highest suction (highest air pressure value) moves toward the leading edge. The distribution of pressure and center of pressure point thus will be further forward the higher the angle of attack and further aft the lower the angle of attack.

the lift-weight pitching moments

If the forces of lift and weight are not acting through the same point (line), then they will set up a moment causing either a nose-up or nosedown pitch depending on whether the lift is acting in front of or behind the center of gravity point.

Note: A center of gravity forward of the center of pitch has a nosedown pitching moment. A center of gravity aft of the center of pitch has a nose-up pitching moment. The center of pitch moves if the angle of attack changes, and the center of gravity moves as the weight changes (mainly due to fuel being used). Therefore, their positions will vary during a flight.

aspect ratio

Aspect ratio is the ratio of the wing’s span to its geometric chord, e.g., 4:1. High aspect ratio = high lift (gliders) Low aspect ratio = lower lift but capable of higher speeds.

During what phase of flight is lift the greatest?

In general, the takeoff.

Note: Lift is caused by a pressure difference above and below the wing, and the size of the difference determines the amount of lift produced.

The difference in pressure experienced is affected by the functions of lift, which are;

1. Configuration (flap setting)

2. Speed of airflow over the wing

3. Angle of attack (which is optimized during the takeoff stage of flight) plus

4. Air density

What is direct lift control?

The elevator/stabilizer provides the direct lift control. The elevator and stabilizer are aerofoils that by their positions create an upward or downward balancing force that controls the dircct lift forcc from the main aerofoils (wings), thus determining the attitude of the aircraft around the lateral axis.

What are high lift devices?

The following devices increase the lift force produced by the wings:

1. Trailing edge flaps (Fowler flaps) increase lift at lower angles of deflection.

2. Leading edge flaps (Krueger flaps) and slats increase lift by creating a longer wing chord line, chamber, and area.

3. Slots (boundary layer control) prevent/delays the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.

 

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