Definition
The difference between the geometric pitch of a propeller (the distance it would advance in one revolution if it were moving through a solid medium) and its effective pitch (the distance it actually advances through the air in one revolution). Slippage represents the loss of efficiency caused by the propeller working in a fluid rather than a solid.
Plain English
A propeller is shaped like a screw. If it were turning through something solid, one full turn would move it forward a fixed distance. But air isn't solid, so each turn actually moves the airplane forward a bit less than that. Slippage is that shortfall.
Context Anchor
Seen in basic propeller discussions when comparing the propeller’s intended forward movement with the airplane’s actual forward movement.
Derivation
From the everyday word 'slip' — to slide without gripping. The propeller blade 'slips' through the air rather than biting into it the way a screw bites into wood. Slippage is the amount of that slipping.
Why Pilots Care
Slippage directly reduces propeller efficiency and therefore the thrust produced for a given RPM and power setting.
Analogy
Think of a wood screw turning into a block of wood — one full turn moves it forward by exactly the thread spacing. Now imagine turning that same screw in soft mud. It still moves forward, but less per turn. Air behaves more like the mud than the wood, and slippage is the difference.
Intuition Check
Slippage does not mean the propeller is loose or physically sliding off the airplane. Here it means the propeller’s actual forward travel through the air is less than its theoretical forward travel.
Example Sentence 1
Some slippage is always present because air is a fluid, so a propeller's effective pitch is always less than its geometric pitch.
Example Sentence 2
Lower slippage at higher speeds improves the propeller's ability to convert engine power into thrust.