Momentum theory

Rankine (1865),[1] Alfred George Greenhill (1888) and Robert Edmund Froude (1889).

[2] The rotor is modeled as an infinitely thin disc, inducing a constant velocity along the axis of rotation.

Under certain mathematical premises of the fluid, there can be extracted a mathematical connection between power, radius of the rotor, torque and induced velocity.

For a stationary open rotor with no outer duct, such as a helicopter in hover, the power required to produce a given thrust is: where: A device which converts the translational energy of the fluid into rotational energy of the axis or vice versa is called a Rankine disk actuator.

The real life implementations of such devices include marine and aviation propellers, windmills, helicopter rotors, centrifugal pumps, wind turbines, turbochargers and chemical agitators.