Definition
The tendency of an aircraft to roll or yaw in reaction to the rotation of its engine and propeller (or turbine spool). In propeller-driven airplanes, torque effect is most noticeable as a left-rolling and left-yawing tendency at high power and low airspeed, and is one of several propeller-related left-turning tendencies pilots must counteract with rudder and aileron. In jet aircraft, torque effect from spool rotation exists but is much smaller and is usually felt only as a slight roll tendency during rapid thrust changes.
Plain English
When the engine spins one way, the airplane wants to twist the other way. In propeller airplanes this shows up as the nose and wing wanting to drop to the left when you add power, so you push right rudder to keep things straight.
Context Anchor
Encountered when comparing the control feel of jet airplanes with propeller airplanes, especially during power changes, takeoff, and climb.
Derivation
From Latin torquere, meaning 'to twist.' The engine twists the propeller one way, so the airframe is twisted the opposite way -- the same idea as Newton's third law, action and reaction.
Why Pilots Care
Uncorrected torque effect can produce unwanted roll, especially during takeoff, go-arounds, or rapid power changes, requiring aileron input to maintain wings level.
Grounding Statement
Picture adding full power in a propeller airplane: the propeller twists one way, and the airplane may try to lean the other way.
Intuition Check
Torque effect does not mean the airplane simply has a powerful engine. It means the airplane reacts to a twisting force, usually by trying to roll opposite the engine or propeller rotation.
Example Sentence 1
On the takeoff roll, the student added full power smoothly and applied right rudder to counter the torque effect.
Example Sentence 2
During the high-power climb, the crew noticed the torque effect required continuous but light lateral corrections.