Definition
Two related but distinct measures of an engine's output. Torque is the twisting force the engine produces at the crankshaft, measured in foot-pounds (ft-lb) or newton-meters (Nm). Horsepower is the rate at which that work is done, calculated from torque multiplied by rotational speed (rpm) and divided by a constant. In a turboprop or piston engine driving a propeller, torque indicates how hard the engine is twisting the prop shaft, while horsepower indicates how much total work the engine is producing per unit of time.
Plain English
Torque is how hard the engine twists the propeller shaft. Horsepower is how much work the engine is doing overall, which depends on both that twisting force and how fast the shaft is spinning. They are related but they are not the same thing.
Context Anchor
Seen in fixed-shaft engine discussions, power-setting procedures, engine instruments, and operating limits.
Derivation
Torque comes from the Latin torquere, meaning 'to twist'—the same root as 'torsion.' Horsepower was coined by James Watt in the 1700s to compare steam engine output to the work a draft horse could do. Both names describe the idea directly: one is twisting force, the other is work measured against a horse's effort.
Why Pilots Care
Correct interpretation prevents over-torque or under-power conditions that affect performance and engine longevity.
Analogy
Think of turning a wrench. Torque is how hard you twist the wrench; horsepower is how much useful work you can keep doing with that twisting force over time.
Intuition Check
Do not treat torque as the same thing as engine speed. Torque is twisting force; horsepower is power produced over time, usually depending on both twisting force and how fast the engine is turning.
Example Sentence 1
On this turboprop, the pilot sets power by reference to the torque gauge, since torque is the limiting factor on takeoff.
Example Sentence 2
At constant RPM in a fixed-shaft engine, an increase in torque produces a direct increase in horsepower.