Brake Horsepower (BHP)

Brake horsepower (BHP) is the measure of an engine’s power output at the crankshaft, before losses through the transmission, propeller, or other driven components. In piston aircraft engines, BHP is the standard measure of engine power. The term derives from the original method of measuring engine power using a braking device (dynamometer) applied to the output shaft.

BHP and Aircraft Performance

At sea level in standard conditions, a piston engine produces its rated BHP. As altitude increases, air density decreases and the engine produces less power. Normally-aspirated engines lose approximately 3% of power per 1,000 ft of altitude. Turbocharged engines maintain rated BHP up to their critical altitude by using a supercharger to maintain manifold pressure.

BHP vs. Thrust Horsepower

BHP is the power at the crankshaft. Thrust horsepower (THP) is the useful power delivered to propel the aircraft — it accounts for propeller efficiency. A propeller operating at 80% efficiency converts 80% of BHP into THP. This is why propeller efficiency matters significantly for aircraft performance.

Related Terms

See also: Manifold Pressure, Full Throttle Altitude, Tachometer.