How To Calculate Lift For Rotor Blades

Lift is the aerodynamic force generated by airfoils—such as propellers, rotor blades and wings—that occurs at a 90-degree angle to the oncoming air. With respect to rotor blades—such as those found on a helicopter—when the leading edge of the blade strikes the oncoming wind, the shape of the airfoil generates an area of high pressure directly below and an area of low pressure above the blade, resulting in lift. To determine the amount of lift generated by a rotor blade, we will use the lift equation

\(L=\frac{1}{2}\rho v^2AC_L\)

Step 1

Understand each element of the lift equation. L signifies lift force, measured in Newtons; ρ signifies air density, measured in kilograms per cubic meter; v² signifies true airspeed squared, which is the square of the speed of the helicopter relative to the oncoming air, expressed in meters per second. In the equation, A signifies rotor disk area, which is simply the area of the rotor blade, expressed in meters squared. C_L signifies the dimensionless lift coefficient at a specific angle of attack, which is the angle between the chord line of the rotor blade—an imaginary line drawn through the middle of an airfoil extending from the leading edge to the trailing edge—and the oncoming air. C_L is dimensionless, in that no units are attached to it; it is simply displayed as a number.

Step 2

Identify the values for each element of the lift equation. In the example of a small helicopter with two blades, the rotor disk travels at 70 meters per second (v). The coefficient of lift for the blades is 0.4 (C_L). The planform area of the rotor disk is 50 meters squared (A). Assume international standard atmosphere, in which the density of air at sea level and 15 degrees Celsius is 1.275 kilograms per cubic meter (ρ).

Step 3

Plug the values you have determined into the life equation and solve for L. In the helicopter example, the value for L should be 62,475 Newtons.

The value for C_L is typically determined experimentally, and cannot be determined unless you first know the value of L. The equation for the lift coefficient is as follows:

\(C_L=\frac{2L}{\rho v^2A}\)