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Fluid Mechanics: Lift



Lift

As an object is moving through a fluid it will experience a net force from the pressure field surrounding it. What causes lift force is the difference in pressure. The difference in pressure is caused by the fluids velocity going over one of the surfaces of an object faster than the opposite surface of the body. Recall from Bernoulli's equation and continuity equation the faster the velocity of a fluid the lower the pressure and vise versa. As with drag, a lift coefficient can be used to determine the lift force; refer to equation 1.

equation to calculate the lift coefficient (1)

CL = Lift Coefficient

L = Lift Force

ρ = Fluid Density

A = Area


Recall from the drag section that there were a number of variables that could have an effect on drag. With lift, however, the most important aspect that will determine lift is the shape of the object in the fluid. The reason for this is because the shape will determine the pressure difference that causes lift.

In addition to shape, the angle of attack can change how much lift an object will experience. The angle of attack is based off of the orientation of an object, such as an aircraft wing, in respect to the direction of the fluids flow. By altering the angle of attack more lift can be generated, however, if the angle of attack is to great the aircraft could stall. Refer to the figures below.

angle of attack and its effect on lift

In addition to altering the angle of attack, the shape of the object or wing could be altered using flaps. For example an aircraft could extend its flaps to increase lift during takeoff, and slow speed landing. This in turn will increase drag on the aircraft, but for takeoff and landing it's not much of a concern. This configuration is known as the dirty configuration. Once an aircraft has reach the correct speed and altitude it will retract its flaps to decrease drag for fuel economy. This is known as the clean configuration. Refer to the data below to see how flaps can affect the lift coefficient in respect to the drag coefficient.

wing flaps and the effect they have on lift and drag



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