Definition
The mechanical power delivered by an engine at its output shaft, before any losses from the propeller or other driven components. In a piston aircraft engine, shaft power is what the crankshaft produces and what turns the propeller; it is typically expressed in horsepower.
Plain English
The amount of power the engine actually puts out at the spinning shaft that drives the propeller, measured before the propeller turns it into thrust.
Context Anchor
Seen in range performance discussions for propeller airplanes, where engine power, propeller performance, and fuel use affect how far the airplane can fly.
Derivation
‘Shaft’ refers to the rotating output shaft (the crankshaft in a piston engine) that the power is delivered through. The term distinguishes power measured at that shaft from power measured elsewhere, such as thrust power produced after the propeller.
Why Pilots Care
It shows how much engine output actually reaches the propeller, which affects fuel use and how far the aircraft can fly.
Intuition Check
Shaft power is not the same as thrust. Shaft power is the turning power delivered to the propeller; thrust is the forward push the propeller creates from that turning power.
Example Sentence 1
The engine was rated at 200 horsepower of shaft power, but only a portion of that became usable thrust after the propeller.
Example Sentence 2
High density altitude reduced available shaft power, so the pilot adjusted the mixture to maintain performance.