Factors affecting Thrust-Effect of pressure, velocity,

temperature changes of air entering the compressor

ByS.R.Shyam Sundar

Thrust Equation

• Net thrust

• where• ma (1+f) ue= momentum thrust• (Pe-Pa) = Pressure thrust• ma = mass flow rate• f = fuel ratio• ue = exhaust velocity• u = flight speed• Pe = Pressure at exit• Pa = Atmospheric pressure

Factors affecting the thrustAir velocity (forward speed)Engine RPMMass air flowFuel flow rateTemperature Altitude & DensityBleed air and power extraction

Air speed

Engine benefits from a ram effect. At high speeds (greater than M=0.4) air becomes compressible

producing an area of relatively high pressure in the intake.Drag also increases with speed but the ram effect dominates.

Engine RPM

Very little thrust is developed at low rpm as compared with the high thrust at high rpm

Amount of air pumped by compressor is the function of its rpm

Mass air flow The most significant variable in the thrust equation is mass air flow.Many factors affect the mass airflow, the most important being air

temperature, pressure, because these factors determine the density of the air entering the engine & ram effect.

Fuel flow rate Fuel flow rate determines energy transferred to the flow.Burning of more fuel results in increased thrust.

          Air Density & the effect of Temperature & PressureAir density has a profound effect on the thrust produced. The volume of the

air flowing through the engine is relatively fixed for any particular rpm by the size and geometry of the inlet duct system. But since the thrust is determined by mass, not the volume of air, any increases in its density increases the mass and thus the thrust.

As the temperature of the air increases its density decreases. Therefore the thrust produced by the engine decreases.

As the air pressure increases, its density increases, causing thrust produced by the engine to increase.

Density ratio= K (P/T)

Altitude effect• Altitude has a double effect on thrust. As the altitude increases, the air

becomes colder and denser, up to the beginning of stratosphere. This causes the thrust to increase. • But at the same time, the increase in altitude causes decrease in pressure,

thus a decrease in density and corresponding decrease in thrust. Since the loss of thrust caused by decreasing pressure is greater than the increase caused by decreasing temperature.• Thus the thrust decreases as the aircraft ascends.

Bleed air and power extraction

Engines equipped with the cabin pressurization , secondary flows need the bled air from compressor

This results in low pressure in the burner and lower mass flow at the nozzle and thus reduction in the thrust

Effect of pressureAir enters the compressor at a pressure less than the ambient This can be over come by the ram effect as the airplane speed

increasesHence there will be a considerable increase in total pressure in the

successive stages of compression.Ratio of fuel flow, exit area of the nozzle determines the pressure ratioThe density increases with increase in pressure.

Effect of temperature

Air enters the compressor at the sea level temperature of 150CDue to compression temperature gradually climbs to a point

determined by the number of compressor stages.Increase in temperature decreases the density of the intake air hence

the power needed to compress the increases resulting in the thrust decrease.