The propeller or rotor converts shaft power into thrust, and its efficiency multiplies the efficiency of everything upstream. A propeller sized for the wrong speed or flight condition can waste more energy than the difference between two motors.
Types
Fixed-pitch propellers: the standard on multirotors and many fixed-wing UAVs. Simple and light, but efficient only near one operating point.
Ground-adjustable pitch: pitch set on the ground for a given mission.
Variable-pitch (constant-speed) propellers: pitch changes in flight to stay efficient from take-off to cruise. Common on larger fixed-wing UAVs and on eVTOL cruise propellers.
Lift rotors: large, slow rotors for vertical flight, often designed for low noise.
Tilt-rotor propellers: blades that must work both in hover and in forward flight.
Key specifications
Diameter and pitch, and the number of blades.
Rated speed and maximum power, matched to the motor or engine.
Static thrust and efficiency curves, ideally measured, at the conditions you will fly.
Mass and moment of inertia, which affect the motor’s response and the vehicle’s control.
Noise: tip speed is the main driver; more blades and lower rpm reduce noise.
Balance and tolerances, which affect vibration and the life of the motor bearings.
Matching the propeller
A larger, slower propeller is generally more efficient in hover, but needs more torque and is heavier. For fixed-wing cruise, the pitch must match the airspeed. Most manufacturers publish recommended motor and propeller combinations with test data; use them as a starting point.
Diameter is the size of the circle swept by the blades. Pitch is the distance the propeller would advance in one turn if it moved like a screw through a solid. Larger diameter gives more thrust per watt in hover; higher pitch suits faster flight.
When is a variable-pitch propeller worth it?
When the aircraft must be efficient at very different speeds, such as take-off and cruise on a fixed-wing UAV or the cruise propeller of an eVTOL. It adds mass, cost and a pitch actuator.
How can propeller noise be reduced?
Mainly by lowering the blade tip speed: a larger, slower propeller, more blades or optimised blade shapes. Low-noise rotors are a priority for eVTOL operations near cities.
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