Definition
The theoretical distance a propeller would advance forward in one full revolution if it were moving through a solid medium with no slippage. It is determined by the blade angle and the diameter of the propeller, and is calculated rather than measured in flight.
Plain English
How far a propeller would move forward in one turn if nothing was holding it back. It's a theoretical number based on the blade's angle, not what actually happens in the air.
Context Anchor
Seen in propeller maintenance, propeller inspection, and discussions of fixed-pitch and adjustable-pitch propellers.
Derivation
From Greek 'geometria' (measurement of the earth), here meaning 'calculated from physical dimensions.' Pitch comes from the same root as 'pitched' — set at an angle. So 'geometric pitch' literally means the advance distance worked out from the blade's geometry alone.
Why Pilots Care
It lets mechanics and pilots compare ideal performance against actual effective pitch to judge efficiency and slip.
Analogy
Think of a screw turning into wood. If the wood were perfectly solid and the screw didn't strip, one full turn would advance the screw exactly the distance set by its thread angle. That ideal distance is the screw's 'geometric pitch.' Air, of course, is not wood — so the propeller never quite gets there.
Intuition Check
Geometric pitch is not the airplane’s nose attitude, and it is not the actual distance the airplane moves forward in one propeller turn. It is the theoretical distance based on the propeller’s blade shape, before allowing for slip in the air.
Example Sentence 1
The propeller's geometric pitch was 60 inches, but in cruise the aircraft only advanced about 50 inches per revolution due to slip.
Example Sentence 2
A large difference between geometric pitch and effective pitch shows high slip in the current conditions.