Definition
The total horsepower developed inside the cylinders of a reciprocating engine by the burning fuel-air mixture, calculated from the mean effective pressure on the pistons, the piston area, the stroke length, and the number of power strokes per minute. It represents the theoretical power produced before any losses from friction, accessories, or driving the propeller are subtracted.
Plain English
The raw power produced inside the engine's cylinders by combustion, before any of it is lost to internal friction or used to turn things like the oil pump or generator. It's bigger than the power that actually reaches the propeller.
Context Anchor
Seen in aircraft engine performance, engine testing, and maintenance discussions comparing power made inside the engine with power actually available to turn the propeller.
Derivation
"Indicated" comes from the Latin indicare, meaning "to point out" or "to show." The term originally referred to power values calculated from a device called an engine indicator, which traced the pressure inside the cylinder during each stroke. So indicated horsepower is the power the cylinder pressures show is being produced internally.
Why Pilots Care
Allows comparison with brake horsepower to determine how much power is lost to friction inside the engine.
Intuition Check
“Indicated” does not mean the horsepower shown on a cockpit gauge. Here it means power calculated from measured pressure inside the engine cylinders.
Example Sentence 1
The engine's indicated horsepower was higher than its brake horsepower because some power was lost overcoming internal friction.
Example Sentence 2
A high IHP reading compared with brake horsepower showed the engine was losing power to internal friction.