Definition
The point through which the resultant thrust force of a propeller (or engine) acts on the airframe. On a single-engine airplane, it lies along the propeller's axis of rotation, but the effective center of thrust shifts when the propeller disc is not perpendicular to the relative wind, because the descending blade produces more thrust than the ascending blade.
Plain English
It is the spot where the pulling or pushing force of the propeller is effectively concentrated. When the airplane is flown at a high angle of attack, that spot moves to one side of the propeller, which pulls the nose off-center.
Context Anchor
Seen in discussions of P-factor and other propeller effects that make an airplane tend to yaw, especially during high-power, nose-up flight.
Derivation
Center comes from an older word meaning the middle point of a circle. Thrust means a push. Together, center of thrust means the point where the propeller’s push is treated as being concentrated.
Why Pilots Care
When the nose is raised, the thrust line moves above the center of gravity and creates a yawing moment that must be countered with rudder.
Analogy
If you push a shopping cart from the middle, it goes straight. If you push harder on one side, the cart tries to turn. A shifted center of thrust works the same way with the propeller’s push.
Intuition Check
Do not assume center of thrust means the physical center of the propeller. It means the effective center of the propeller’s push, and that point can move when thrust is uneven.
Example Sentence 1
At a high angle of attack during takeoff, the descending right blade produces more thrust, shifting the center of thrust to the right and yawing the nose left.
Example Sentence 2
The pilot applies right rudder to counteract the yaw created by the offset center of thrust during climb.