Thrust Available

Engines are air breathing engines. We use very simple calculations for engine performance with speed and altitude for level flight operations. Thrust and Power available from an engine are specified by the manufacturer. Thrust and power required are what you calculate using the drag coefficient. In level flight they must be equal.

Thrust and Power available with altitude:

Thrust and Power delivered by the engine are controlled by throttle setting. Maximum throttle setting is 1.

Power delivered is reduced by propeller efficiency.

P_av = T_av*V -- (1)

The following is easy to remember:

• Available thrust of a jet engine is constant with speed at the same altitude,
• Available power of a jet engine varies linearly with velocity (see equation 1)
• Available thrust of a jet engine reduces with altitude - square root of the density ratio
• Available power of a propeller engine is constant with speed at any altitude
• Available thrust of a piston engine varies inversely with speed (see equation 1)
• Available power of a piston engine reduces with altitude - square root of the density ratio

These are described by the following illustrations

Note: Thrust/power available is different from thrust/power required