Definition
A propeller blade angle set with the chord of the blade nearly parallel to the plane of rotation, so the blades take a very small bite of air per revolution. Flat pitch produces low thrust at low forward speed but allows the engine to turn at high RPM, and on the ground it creates high aerodynamic drag against rotation.
Plain English
The blades are turned so they slice almost edge-on through the air, taking only a thin bite each turn. The engine spins fast, but the propeller doesn't pull the airplane forward very much.
Context Anchor
Seen in discussions of controllable-pitch, constant-speed, and feathering propellers, especially when comparing normal low blade angle with the feathered position used to reduce drag after an engine failure.
Derivation
Flat here means lying nearly level with the direction of rotation — the blade face is close to flat against its circular path, rather than twisted into the oncoming air.
Why Pilots Care
Proper flat-pitch selection prevents engine overspeed while providing the high RPM needed for takeoff thrust and climb performance.
Analogy
It is like twisting a paddle in the water. Held nearly flat, it slices through with less resistance; turned broadside, it pushes much harder against the water.
Intuition Check
Flat pitch does not mean the propeller is physically flat like a board, and it does not mean the airplane is level. It means the propeller blades are set at a low angle so they take a smaller bite of air.
Example Sentence 1
The pilot set the propeller control full forward to flat pitch before applying takeoff power, ensuring maximum RPM for the climb.
Example Sentence 2
After leveling off the pilot increases pitch from flat to reduce RPM and improve cruise efficiency.