Thrust

The Role of Thrust in Flight

Thrust is the forward force produced by an aircraft’s propulsion system. In steady, level flight, thrust must equal drag for the aircraft to maintain constant speed. Thrust acts parallel to the engine's axis of rotation, typically along the longitudinal axis of the aircraft.

Thrust and Aircraft Moments

The position of the engine relative to the centre of gravity (CG) affects the pitching moment. If thrust acts above or below the CG, or if the thrust line is inclined, it can create nose-up or nose-down moments. For example, a propeller mounted ahead of the CG with an upward-inclined thrust line produces a nose-up moment, which can destabilise the aircraft.

Thrust on Propeller Blades

The aerodynamic force on a propeller blade can be resolved into two components: thrust (parallel to the rotational axis) and torque (perpendicular to the axis, in the plane of rotation). Thrust moves the aircraft forward, while torque must be balanced by the engine to prevent unwanted rotation.

Thrust and Performance

Increasing thrust can improve performance, but this is often offset by added weight from larger engines or supporting structures. The most efficient way to improve performance is to reduce drag, thereby requiring less thrust to maintain speed.

Thrust and Elevator Deflection

When thrust changes, especially if the engine is not aligned with the CG, elevator input may be needed to maintain pitch equilibrium. The amount and direction of elevator deflection required depend on the engine’s position relative to the CG and the direction of the thrust change.