Definition
The total horsepower developed inside the cylinders of a reciprocating engine, calculated from the average pressure of the burning fuel-air mixture acting on the pistons during the power stroke. It represents the raw power produced by combustion before any of it is lost to internal friction, accessory drives, or mechanical resistance.
Plain English
The full power the engine produces inside its cylinders, before some of that power is used up turning the engine's own moving parts. It is the power generated by the burning fuel, not the power that finally reaches the propeller.
Context Anchor
Seen in aircraft engine theory when comparing the power made inside the engine with the power actually delivered for useful work.
Derivation
‘Indicated’ comes from a device called an engine indicator, which was used to draw a pressure-versus-volume diagram of what was happening inside the cylinder. The horsepower calculated from that diagram became known as indicated horsepower because it was literally read from the indicator's trace.
Why Pilots Care
Knowing indicated horsepower helps pilots and mechanics understand how efficiently an engine converts its internal combustion into usable power at the propeller.
Analogy
Indicated horsepower is like gross pay before deductions. It is the larger starting amount, but the useful amount available afterward is lower.
Intuition Check
Do not read “indicated” as “shown on the cockpit instruments.” Here it means calculated from 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
Mechanics compare indicated horsepower to brake horsepower to find the engine's mechanical efficiency.