NET THRUST

As aircraft forward speed increases, engine thrust will decrease to a value we call net thrust.

I guess you knew this was coming! There is always a penalty to pay, like tax on your salary.

The air passing through the engine creates a retarding drag force that we shall refer to as momentum drag (MV).

It may be calculated quite easily by multiplying the mass of air (M) passing through the engine per second by the aircraft’s forward speed(V).

But, you will say, if the aircraft is not moving forward the momentum drag will be zero! Well done, you would be correct in that assumption.

The maximum static, or rated, thrust is a gross figure and is also the same as net when there is no forward speed.

When the aircraft is flying, the story is different, however, momentum drag becomes a consideration and has to be subtracted from the gross, or momentum, thrust to give the net thrust value.

Net thrust= Gross  thrust(MVj)-Momentum Drag(mv)

Remember our example of the turbo-jet in flight? Let’s have another look at it.

EXAMPLE 1

A turbo-jet engine flying at 200 m/s has a mass airflow of 100 kg/s. The Jet stream velocity is 330m/s.

 

Net thrust= MVj-MV

=(100 X 330)-(100 X 200)

=33000-20000

=13KN

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