Power Available
Sources of power
Power available comes from the engine-propeller combination or from a jet. For a piston engine, power falls with altitude because density falls, roughly in proportion to density ratio unless the engine is turbocharged. For a propeller, thrust is power divided by speed, so thrust falls as speed rises at constant power; this is why propeller aircraft have abundant thrust at low speed and lose it rapidly. A jet's thrust is approximately constant with speed, so its power available rises with speed.
Candidates should understand the concept of a constant-speed unit: the propeller blade angle increases with speed to absorb the engine's power at a chosen rpm, and manifold pressure is the practical measure of power delivered.
Effects of altitude, temperature and configuration
High altitude, high temperature and high humidity all reduce power available. A turbocharged engine maintains power to a critical altitude and then declines. Fixed-pitch propellers lose rpm in the climb as speed falls, whereas constant-speed propellers hold rpm and increase blade angle.
Traps and a worked example
Traps: assuming power available is constant with speed for a propeller aircraft; forgetting that thrust equals power divided by speed; and confusing brake power with shaft power at the propeller. Another trap is assuming turbocharging gives unlimited altitude performance.
Rule of thumb: for a propeller at constant power, halving speed doubles available thrust.
Worked example: an engine delivers 100 kW to a propeller at 50 m/s. Ideal thrust is 100 000 / 50 = 2000 N. At 100 m/s the same power gives only 1000 N. If propeller efficiency is 80%, the actual figures are 1600 N and 800 N respectively.