Momentum drag is the negative thrust (force) opposing the jet thrust or motion through air, it is also referred to as the ram drag. It is a product of the incoming air (flow rate) and the velocity via which it is fed to the jet engine. It is normally denoted as Ti or To. Mathematically, it is the difference between the gross and net thrust (Momentum drag = Gross thrust – Net thrust) (Farokhi 290). It is computed as (Gunston 187):
Momentum drag is the opposition the aircraft experiences as it surges through the air. to the engine, it may be referred to as the ‘friction’ experienced by the engine as it propels the aircraft forwards.
Net thrust is the resultant aerodynamic force that causes propulsion of an aircraft in air, which is represented by the summation nozzle thrust and momentum of motion. It is a result of the propulsion system, engine, overcoming momentum drag. It is thus the difference between the gross thrust and the momentum drag on the engine of an aircraft (Net thrust = Gross thrust – Maximum drag) (El-Sayed 125). It is usually denoted by T and is calculated as follows:
The aircraft thrust is influenced by the airflow into the engine. A change in the rate of airflow triggers a resultant thrust alteration. In the case of net thrust, the increase in the aircrafts speed results to an increase for air fed into the engine, resulting to a higher output (Cumpsty 193). With static thrust, the environmental factors tend to determine the ability of the craft to engage in 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.
It is evident from the derived