The maximum speed of a propeller occurs at its tip, and this speed is equal to the vector resulting from its forward speed and tangential velocity. This speed is the helical velocity at the propeller tip.
Propellers are used by ships, aircraft and other machines to translate rotational power into thrust. Each blade of a propeller is a wing with airfoil sections optimally shaped and aligned into the relative wind along the radius of the blade. Because of the rotation of the propeller, the tip of each propeller blade is moving faster than the inboard section or root. As a result, the optimal alignment of the propeller results in a recognizable twist to the airfoil from the root to the tip.
To calculate the helical tip velocity of a propeller blade, the following equation is used: Vtip = square root (V^2 + T^2), where Vtip is the helical tip velocity, V is the forward speed of the propeller and T is the tangential velocity at the propeller tip. To calculate T, multiply the rotational speed of the propeller by the circumference of revolution at the propeller tip. It is important to ensure consistent units when doing these calculations.
The helical tip velocity of an airplane propeller is often expressed in Mach in order to represent the factor of the local speed of sound. Though some airplane propellers have been designed to travel at a supersonic tip velocity, the vast majority are designed for subsonic tip speed in order to reduce noise and drag resulting from shock waves. It is important for propeller designers to calculate the tip speed in order to optimize the propeller’s performance for its specified flight type.