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In this section, we deal with one of the forces acting on an aircraft, namely, the thrust produced by the aircraft's engine. In the first part of this section we will look at propellers and their efficiency. In the second part of this section, we will provide the formula for the thrust of a jet engine.
Propellers are used to drive many lightweight aircraft and
were the principal means of propulsion for military aircraft
until the advent of the jet engine. As such, it is
important to know how propellers work and how efficient they
are. The propeller efficiency can never reach the ideal
efficiency of 100 %. This is because in the development of the
propeller efficiency several concepts are ignored,
1. The friction drag of the blades.
2. The kinetic energy of the rotation of the slipstream.
3. The fact that the thrust is not uniformly distributed over the blades.
The maximun propeller efficiency is about 90 %. This is due to
the combined effects of drag from the nacelle and wings upon the
propeller. This combined effect drops propeller efficiency to
about 87 %. From there the thrust horsepower provided by the
= thrust (lb)
= velocity (ft/s)
= engine brake horsepower
550 = conversion factor from ft-lbs to horsepower
= propeller efficiency
The thrust equation for a turbojet can be derived from the
general form of Newton's second law (i.e., force equals the time
rate of change of momentum),
The figure below shows the inlet and exhaust flows of the
turbojet. The negative thrust due to bringing the freestream air
almost to rest just ahead of the engine is called momentum drag
or ram drag. The resulting thrust is given by following equation,
Schematic of a turbojet engine.
= is weight flow rate of the air passing through the engine.
= jet stream velocity
= static pressure across propelling nozzle
= atmospheric pressure
= propelling nozzle area
= aircraft speed
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Updated: March 12, 2004