Thrust

The total thrust is equal to the product of the propelling nozzle exit gas velocity and the mass of air passing through the engine.

The gross thrust is often referred to as momentum thrust and does not take into account any retarding forces such as the drag force induced by air passing through the engine in flight known as momentum drag.

Gross thrust=MV j or WVj/g

W = Weight of air per second

M = mass air flow pet sec

vj =Jet-stream velocity

g =acceleration due to gravity 9.81 m/s2, 32 ft/s2

V = Aircraft speed

Note:

The use of MVi or WV/ g is optional. Remember though that weight is a force expressed in either newtons or lbsf and if used, has to be divided by g to convert it to mass either in kilograms or lbs mass.

EXAMPLE 1

A turbo-jet running in a stationary aircraft in still air conditions has a mass airflow of 100kg/s and a jet-stream velocity of 330m/s

Acceleration = Vj−U/t

= 330−0/1

= 300 ms-2

Gross thrust=Mass Air Flow  X  Acceleration

=100×330

=3300N=33KN

EXAMPLE 2

The turbo-jet is now moving through the air at a velocity M of 200m/ s.

The mass airflow is still 100kg/s and the jet-stream velocity is 330m/s.

Thrust = M( Vj−V)

But M(Vj-V)= MVj−MV

 =(100×330)−(100×200) =13KN

If you look at the two examples above you will be reminded that an engine produces its maximum thrust when the engine is static on the ground at sea level ISA (international standard atmosphere) conditions.

At full throttle, this maximum value is known as the maximum static thrust or rated thrust, it is the maximum possible thrust certified by the engine manufacturers.

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