Definition
The speed of the spiraling tube of air pushed rearward by a propeller. Slipstream velocity increases with engine power and decreases as the airplane's forward speed increases.
Plain English
How fast the corkscrew of air coming off the propeller is moving backward over the airplane.
Context Anchor
Seen in takeoff, climb, and slow-flight discussions where propeller airflow affects lift, cooling, and control response.
Derivation
Slipstream comes from 'slip' (to slide past) plus 'stream' (a flow). It's the stream of air that slides past the airplane after being accelerated by the propeller. Knowing this helps because the term is literally describing a flow of air sliding rearward along the fuselage.
Why Pilots Care
Higher slipstream velocity increases airflow over the rudder and elevator at low forward speeds, improving control response and contributing to yaw when power is applied abruptly.
Grounding Statement
When the airplane is rolling slowly with high power set, the propeller may still be pushing a strong stream of air back over the airplane.
Intuition Check
Do not assume slipstream velocity is the same as airspeed. Airspeed is the airplane’s speed through the air; slipstream velocity is the speed of the propeller-driven airflow moving over parts of the airplane.
Example Sentence 1
At full power on the takeoff roll, slipstream velocity is high while forward speed is still low, which is why right rudder is needed to keep the nose straight.
Example Sentence 2
As the airplane accelerated in the climb, the relative effect of slipstream velocity on the tail diminished.