What is the lift formula or equation?

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The lift formula or equation is CL ½ p V2 S.

This formula is used to quantify the factors or components that influence lift production.  The factors are coefficient of lift, air density, velocity, and surface area.  Not all factors of the equation are equal.

CL is the coefficient of lift.  In general, this is the angle of attack on the rotor blade.  Until the stalling angle is reached, an increase in the CL will produce more lift.

½ p V2 This section of the formula is Dynamic Energy or Kinetic Energy.  Basically, dynamic/kinetic energy is derived from the movement of air.  The p is for pressure or air density.*  The greater the density (lower pressure altitude) the more lift produced.

V2 is for velocity or the rotor RPM with regards to helicopter flight.  As referenced by the squared component, velocity is a major factor in lift production.  A slight change in velocity can have a significant impact on lift.  This fact is one reason that low rotor RPM is a significant issue with helicopters.

S stands for surface area.  In helicopter flight, the surface area of the rotor blades does not change.  Unlike fixed-wing aircraft, rotor systems do not have flaps that can increase or decrease the surface area.**

* The p is m for mass in some equations.  With reference to lift, mass is the density of the air.
** There are some experimental systems, but in general these are not available to most pilots.  In addition, this discussion does not consider stabilizers or other systems that may change the surface area slightly, as these are not a significant factor in helicopter flight.


Principles of Helicopter Flight, 2nd Edition, pg. 18

Other Helicopter Aerodynamic Principles

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