Theoretically, it is the difference between the gross and net thrust (Momentum drag = Gross thrust – Net thrust). It is computed as:
Net thrust is a positive aerodynamic force that causes propulsion of an aircraft in air. It is represented as a summation nozzle thrust and momentum of the jet’s motion. It is a resultant propulsion force produced by the jet engine used in providing thrust (speed) to the aircraft. It is the difference between the gross thrust and the momentum drag on the engine of an aircraft (Net thrust = Gross thrust – Maximum drag). It is usually denoted by T and is calculated as follows:
The engine thrust is affected by the flow of air into the engine and is thus a change in the rate of airflow results to a change in the resultant thrust. With net thrust, the increase in the aircrafts speed results to an increase for air fed into the engine, resulting to a higher output. With static thrust, the environmental factors tend to determine the ability of the craft to cause motion. With static thrust, the air speed is kept constant and is not affected by the crafts motion thus differing from the net thrust.
b) Calculate the net thrust of a turbo jet flying at 150 M/S with an air mass flow of 50 Kg/S and a jet stream velocity of 300 M/S with a chocked nozzle. The residual gas pressure in the jet stream is 45 KPa gauge, the nozzle area is 0.2M2.
Net thrust = gross thrust – momentum drag (T = mVj + Aj (Pj - Pam) – mVi) where Pam is the ambient air pressure in question. Form this equation an increase in the ambient air pressure, while holding all other factors constant results to a decrease in the net thrust of the engine, which being the denominator of the SFC formula results to an increase in the resultant value of the specific fuel consumption. As indicated by the new SFC equation
Propulsive efficiency = (2Vi) / (Vi + Vj), where Vi is the speed of the exhaust gases and Vj is the incoming air speed. Increasing the speed of the gas