A body exposed to moving air or wind experiences an aerodynamic lift force. This force acts in a direction perpendicular to the direction of the wind flow. The amount of lift generated by the wind can be determined by multiplying four physical characteristics of the wind and the surface undergoing the lift. The air density, wind speed, area of the surface, and lift coefficient combine to determine the lift force. These characteristics can also be adjusted if an application requires a certain amount of lift force.
Determine the air density. Density is the mass per unit volume of air. It can be calculated based on the air temperature and pressure. The air density in pounds per cubic foot is equal to:
For this formula, the air pressure is measured in inches of mercury and the temperature is measured in degrees Fahrenheit.
Determine the wind speed. An anemometer will display the wind speed. A car speedometer can also show the velocity of the vehicle moving through the air.
Measure the area of the surface. If the surface is a rectangle, the area is defined by its length multiplied by its width.
Determine the lift coefficient for the surface. The angle of the surface to the wind affects the value of this coefficient. The actual shape also affects the lift coefficient. For a flat plate, a good approximation at small angles is:
Lift coefficient= 2(pi)(angle to wind in radians)
Since 1 radian equals about 57.3 degrees, and pi is approximately 3.14159, the lift coefficient is therefore:
Lift coefficient=0.1097(angle to wind in degrees)
Calculate the lift force.
Lift=1/2(air density)(wind velocity)^2(area of surface)(lift coefficient)
Be sure to use consistent units throughout the calculations. Do not mix meters and feet, or pounds and Newtons.
The lift coefficient approximation is only valid up to 8 degrees.
Tips and warnings
- Be sure to use consistent units throughout the calculations. Do not mix meters and feet, or pounds and Newtons.
- The lift coefficient approximation is only valid up to 8 degrees.