Page 755 - PPL-engelsk 2025

Aircraft general knowledge

8.11.1 How the propeller works

The aircraft propeller converts the engine's brake horsepower (BHK) into thrust

horsepower (THP).

A propeller is subject to large and often opposing forces.

- Centrifugal force tries to throw the blades away from the hub.

- Thrust force tries to bend the tips forward.

- Drag tries to bend the blades backward in relation to the direction of rotation

- Centrifugal twist tries to bend the blade to smaller blade angles
- Aerodynamic twist tries to give the blades a larger blade angle.

The blade angle is greatest at the hub of
the propeller and decreases outwards.

- Low speed – small blade angle

- High speed – large blade angle

Each propeller is designed to deliver the greatest thrust when it rotates at its

best "designed" speed.

8.11.1.1 Climb propeller and cruise propeller

In practice, this gives us two different types of propellers for each aircraft. A

propeller for the pilot who wants good take-off performance, and a propeller for

the pilot who wants good cruising speed. In everyday language, a climbing
propeller or a cruising propeller.

8.11.1.2 Pitch / blade angle

There are propellers that can be operated in 2 different positions via a lever. Fine

pitch (small blade angle) for take-off and coarse pitch for cruising.

A special type of propeller can maintain a fixed speed and vary the blade angle

depending on the engine performance (constant speed propeller).

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 755