RBAK - Basic Aviation Knowledge
Propellers, Rotors, Thrust and Torque
Understand blade angle, pitch, thrust, torque and why propeller or rotor condition matters before launch.
Lesson references
- Last reviewed
- 2026-05-18
- Lesson references
- RePL Study Guide pp. 166-172; Part 101 MOS C10 p. 93.
A propeller is a rotating wing
Propellers and rotor blades create thrust by accelerating air. Blade angle, helix angle and pitch describe how the blade meets the air as it rotates.
A damaged, incorrectly installed or mismatched propeller can reduce lift, increase vibration, increase current draw and compromise control.
Three steps of about four and a half seconds each. Step 1, blade section: a two-blade propeller spins on a motor, and its disc is the plane of rotation. A cross-section of one blade is an aerofoil. Its chord line is tilted from the plane of rotation by the blade angle, leading edge first, as the blade moves through the air. Step 2, thrust: the blade turns the air downward, air is drawn into the disc and pushed down below it, and thrust acts upward along the shaft, at right angles to the plane of rotation. Step 3, torque, viewed from above: a quadcopter's diagonal pairs spin in opposite directions, front-left and rear-right clockwise (CW), front-right and rear-left counter-clockwise (CCW). The CW pair pushes the body counter-clockwise and the CCW pair pushes it clockwise. The reactions are equal and opposite, so the torques cancel and the aircraft does not yaw. The still frame shows step 2.
Torque has to be managed
When a motor turns a propeller or rotor one way, an opposite torque reaction acts on the aircraft. Multirotors manage this with opposing rotor directions and flight controller mixing.
That is why propeller position and direction matter. A propeller fitted to the wrong arm or upside down may still spin, but it will not produce the expected thrust.
A 12 second loop showing a quadcopter from above with the front at the top. The propellers spin slowly so their direction can be seen. The front-left and rear-right propellers turn clockwise, and the front-right and rear-left propellers turn counter-clockwise, so each diagonal pair turns the same way. Curved arrows around the body show the opposite torque reaction from each pair, and the two reactions balance. A pre-flight check then ticks each propeller whose leading edge faces its direction of turn, and the aircraft lifts off level. With a wrong propeller fitted on the front-right arm, that propeller still spins counter-clockwise but its blade faces the wrong way. The check marks it with a red cross and no thrust, and the aircraft does not launch.
Pre-flight checks are aerodynamic checks
Checking propellers is not just a mechanical habit. It verifies the aircraft still has the aerodynamic surfaces it needs to generate predictable thrust.
Look for cracks, chips, deformation, loose mounts, wrong prop type and contamination before flight.
Practice Questions
Why is an incorrectly installed multirotor propeller dangerous?
- It may not produce the expected thrust or torque balance.
- It automatically improves GPS hold.
- It only affects the aircraft lights.
- It has no effect if the motor spins.
Answer: It may not produce the expected thrust or torque balance.
Propeller direction, pitch and position are critical to thrust and aircraft control.
Next step after study
Complete your Remote Pilot Licence training
The free study guide is a strong theory foundation. To actually be issued with a RePL, students still complete approved training, practical flying and assessment with a certified provider.