DISCOVER

# How to Calculate Lift for Rotor Blades

Updated April 17, 2017

The study of lift force falls under fluid dynamics, in physics, and is primarily concerned with motion generated by airflow. When rotor blades turn in helicopters such that pressure of layered air below the blades is greater than pressure of the layered air above the blades, the rotorcraft will fly vertically. The difference in air pressure is directly proportional to upward-acting vertical force, which causes aircraft to lift off the ground into hover. The modern lift equation offers a means to compute the magnitude and units of lift force on aircrafts. A standard unit for measuring lift is newton (N). Another unit is pascal meter squared (Pa.m2) because pressure equals force per unit surface area.

Obtain the proper information to make the calculation. Whether you use data from homework or from experiment, use the lift equation. The modern lift equation states that lift (L) is equal to the lift coefficient (Cl) times the density of the air (r) times half of the aircraft velocity (V) times the reference area (A). This means you will need to know "r," "Cl," "V," and "A."

Convert the quantities to correct units. The final units of lift force determine the units of parameters required to calculate lift. If you expect lift to be in newton (N), then "r" will be in kilogram per meter cubed, "Cl" in radians, "V" in meter per second and "A" in meter squared.

Calculate user defined values. If the question provides all the parameters you need to compute "L" directly, then perform the calculation otherwise you will need to calculate them. You can obtain the value of "r" at a specified altitude from the appendix of an aerodynamic textbook or use an "Atmospheric Properties Calculator" like Aerospaceweb.org. "Cl" is approximately two times pi (3.14159) times the angle of attack expressed in radians, "V" equals altitude in meters divided by the corresponding travel time in seconds and "A" equals pi (3.14159) times blade radius (in meters) squared.

Perform the calculation. Multiply density of air by lift coefficient by half of the square of the velocity with the aid of your calculator to obtain the lift force.

Check your value and units to ensure that they are correct. If your final units is either kilogram-metre second squared (Kg.m/s2) or pascal meter squared (Pa.m2), that is fine because they are equivalent to "N."

#### Tip

Air density changes as a function of altitude, so the value of this variable depends on the height you want to find the lift at. The form of velocity applicable to the lift equation is the true airspeed, which is actual speed of the aircraft through the air. For simplicity and ignoring any centrifugal effects, assume the lift coefficient to be constant across the rotor disk. Modern lift equation is used to calculate lift force in aeroplanes.

#### Warning

For a helicopter, reference area is the rotor disk area. Assuming thin airfoil and small angle of attack, the disk area reduces to a circular area through which the rotor blades turn. The calculation is different for an aeroplane. The lift equation assumes a single blade rotorcraft. When you are given multiple blades, multiply the lift value by the number of blades to obtain the total lift force. Lift coefficient, containing all the complex dependencies, is usually determined experimentally. Assuming thin airfoils and small angles of attack, the lift coefficient is approximately two times pi (3.14159) times the angle of attack expressed in radians.