Definition
The difference between the geometric pitch of a propeller (the theoretical distance it would advance in one revolution if moving through a solid medium) and its effective pitch (the actual distance it advances through the air in one revolution). Slip is the inefficiency caused by the propeller working in air rather than a solid, and it is normally expressed as a distance or a percentage.
Plain English
A propeller would move forward a certain distance in one turn if the air were solid like a screw going into wood. In real air, it moves forward a bit less than that. The shortfall is called slip.
Context Anchor
Seen when studying propeller pitch, takeoff performance, climb, and propeller efficiency.
Derivation
Slip comes from the Old English slipan, meaning to slide or slip away. It captures the idea that the propeller is sliding through the air rather than gripping it perfectly, so some of the theoretical forward travel slips away.
Why Pilots Care
It reduces propeller efficiency, so pilots must consider it when estimating thrust, range, and fuel consumption.
Analogy
Think of a screw turning into firm wood versus turning into soft mud. In wood, every turn moves it forward a fixed distance. In mud, each turn moves it forward a little less because the medium gives way. Air behaves more like mud than wood, and that shortfall is slip.
Grounding Statement
On takeoff, the propeller may be spinning fast while the airplane is still moving slowly, so more of the propeller’s motion is lost as slip.
Intuition Check
Slip does not mean the propeller is loose, damaged, or failing. Here it means the normal difference between how far the propeller would move in a perfect case and how far the airplane actually moves.
Example Sentence 1
Because of propeller slip, the aircraft advanced about 35 inches per revolution rather than the 50 inches its geometric pitch would suggest.
Example Sentence 2
Understanding propeller slip helps calculate realistic cruise performance and fuel burn on long flights.