Definition
Thrust horsepower is the actual power delivered by the propeller to move the aircraft through the air. It equals the engine's brake horsepower minus the losses caused by propeller inefficiency, and is therefore always less than the power produced by the engine itself.
Plain English
It's the useful pulling or pushing power the propeller actually puts into the air, after losses. The engine makes a certain amount of power, but the propeller can never convert all of it — what's left after those losses is the thrust horsepower.
Context Anchor
Seen when studying how a propeller turns engine power into usable force that moves the airplane forward.
Derivation
Thrust' is the forward force produced by the propeller. 'Horsepower' is a unit of power originally based on the work a horse could do. Together, the term describes how much of the engine's power is actually converted into useful forward force.
Why Pilots Care
Knowing thrust horsepower helps pilots understand why not all engine power reaches the airframe and how propeller condition directly affects climb and cruise performance.
Grounding Statement
During takeoff, the airplane accelerates because part of the engine’s power leaves the propeller as forward pull; that useful part is thrust horsepower.
Intuition Check
Do not read THP as the total horsepower the engine can make. THP means the useful horsepower that has come through the propeller as forward thrust.
Example Sentence 1
Even though the engine produces 180 horsepower, the thrust horsepower delivered by the propeller is noticeably lower due to propeller inefficiency.
Example Sentence 2
A damaged propeller blade reduces thrust horsepower and therefore lengthens the takeoff roll.