What is the Net thrust?
The difference between the gross thrust and the ram drag is called the net thrust because it is the net force acting on the engine to produce propulsion power. Net thrust delivered by a turbojet engine in flight. Net thrust considers a force of air entering and leaving the engine. Net thrust is the gross thrust of a jet engine minus the drag as a result of the momentum of the incoming air. As aircraft forward speed expands, engine thrust will lessen to a value we call net thrust. The air passing through the engine create an impeding drag constrain alluded to as energy drag. 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), or (MV). If the aircraft is not moving forward, the momentum drag is zero. The maximum static or rated thrust is a gross figure and is also the same as net when there is no forward speed. At the point when the flying machine is flying, energy drag must be subtracted from the gross thrust to give the net thrust.
Figure: Ideal turbojet net thrust
The equation for finding the net thrust of a jet engine:
F(N) = m dot (air) + m dot (j) V(j) – m dot (air) V
F(N) = net thrust
m dot (air) = inlet mass flow
m dot (j) = fuel mass flow
V(j) = final velocity
V = initial velocity
It should be remembered that the net thrust is always the difference between the gross thrust and the ram drag; therefore, it is given by any combination of the various gross thrust and ram drag terms.