Definition
The difference between the geometric pitch of a propeller and its effective pitch. Geometric pitch is the distance the propeller would advance in one revolution if it were moving through a solid medium. Effective pitch is the distance it actually advances through the air in one revolution. Slip is the shortfall between the two, expressed as a distance or percentage.
Plain English
A propeller is shaped like a screw. In a solid material, one full turn would move it forward a fixed distance. In air, it doesn't move forward quite that far because air is fluid and gives way. Slip is how much shorter the actual forward travel is compared to the theoretical travel.
Context Anchor
Seen in propeller theory, propeller performance, and maintenance discussions about pitch, thrust, and efficiency.
Derivation
From the everyday sense of 'slip' meaning to slide or lose grip. The propeller blade 'slips' through the air rather than biting into it perfectly the way a screw bites into wood.
Why Pilots Care
Slip directly affects propeller efficiency, cruise speed, and fuel consumption.
Analogy
Think of stirring a spoon through water versus screwing it into wood. The wood holds the spoon's threads exactly; the water lets it slide a little. That sliding is slip.
Intuition Check
Do not confuse this with an aircraft slip during flight. Here, slip means the propeller’s shortfall between ideal forward travel and actual forward travel.
Example Sentence 1
Some slip is unavoidable because the propeller is moving through air, not a solid material.
Example Sentence 2
Mechanics compare measured slip to the published value after a propeller overhaul.