Definition
The portion of an engine's total developed horsepower that is consumed internally to overcome mechanical friction between moving parts — pistons sliding in cylinders, bearings turning on shafts, gears meshing, and accessories being driven. Friction horsepower is power produced by the engine but not delivered to the propeller; it is the difference between indicated horsepower (the power developed inside the cylinders by combustion) and brake horsepower (the usable power available at the output shaft).
Plain English
The amount of an engine's power that gets used up just keeping its own internal parts moving against each other. It never reaches the propeller — it is the cost of the engine running itself.
Context Anchor
Seen in aircraft engine performance, maintenance, and powerplant discussions, especially when comparing power made inside the cylinders with power actually available from the engine.
Derivation
From Latin frictio meaning 'a rubbing,' combined with horsepower — the standard unit for measuring engine output. The name describes exactly what it measures: the power lost to rubbing inside the engine.
Why Pilots Care
High friction horsepower reduces usable power and signals possible engine wear or lubrication problems that affect reliability and performance.
Intuition Check
Friction horsepower does not mean horsepower created by friction. It means horsepower lost because the engine must overcome friction and resistance inside itself.
Example Sentence 1
Some of the power produced inside the cylinders is lost as friction horsepower before it ever reaches the propeller.
Example Sentence 2
Knowing the friction horsepower helped the mechanic judge whether the engine was delivering its expected brake horsepower at the propeller.