Definition
A mechanical device used to measure the brake horsepower output of an engine by applying a controlled friction load to the engine's output shaft and measuring the resulting torque on a calibrated arm and scale.
Plain English
A test rig that squeezes a brake onto a spinning engine shaft to measure how much turning power the engine is actually producing.
Context Anchor
Seen in aircraft engine maintenance and powerplant testing, especially when discussing how engine power can be measured on a test stand.
Derivation
Named after Gaspard de Prony, a French engineer who developed the device in 1821. The 'brake' part of the name refers to the friction brake that loads the engine — and it's also the origin of the term 'brake horsepower' (bhp) used to describe usable shaft power.
Why Pilots Care
Understanding the prony brake explains why engine power is rated in 'brake horsepower' — it's the power measured at the shaft after mechanical losses, which is what actually turns the propeller.
Grounding Statement
Picture an engine running on a test stand while a controlled brake resists its turning force so the mechanic can measure the engine’s power.
Intuition Check
A Prony brake is not an aircraft wheel brake. Here, “brake” means a test device that creates resistance so engine power can be measured.
Example Sentence 1
The engine was mounted on a test stand and connected to a prony brake to verify its rated horsepower output.
Example Sentence 2
The Prony brake reading verified that the overhauled engine met its rated power before reinstallation.