# Factors Affecting Thrust:
Gas turbine engine operates under varying condition that affect the amount of thrust the engine produces. These conditions affect either the momentum of air fuel flows through the jet nozzle.
a) Jet nozzle velocity
During most normal high speed operations, the exhaust nozzle operates in a chocked condition. This means that the gases flowing out of the engine have been accelerated to the speed of sound and they cannot be accelerated any further. The value of C j in the thrust formula remains relatively constant, changing only the temp of the exhaust gas changes the speed of sound. Wherever the nozzle is not chocked, varied atmospheric condition will cause some changes in jet nozzle velocity.
b) Airspeed
When a turbojet or turbo fan engine operating at high power and the aircraft in which it is mounted is not moving the value of Ci in thrust energy is zero and thrust is maximum.
When the aircraft is moving forward the velocity C i of the air entering the engine increases, but at higher power, the exhaust nozzle is chocked and the value C j , do not increase proportionally.
Therefore, as airspeed increases the net thrust decreases.
c) Mass Airflow
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.
Air Density & the effect of Temperature & Pressure
Air 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.
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.
Air Density & the effect of Temperature & Pressure
Air 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.
d) 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.
At the beginning of stratosphere at approximately 36,000 feet temperature stabilizes at -56.50C and remains at this temperature up to around 85000 feet. The pressure continues to fall above the 36,000 feet and the thrust therefore drops off at a faster rate than it does at the lower altitudes. This increased drop off in thrust makes 36,000 feet a chosen altitude for a long range cruise in jet powered aircraft.
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.
At the beginning of stratosphere at approximately 36,000 feet temperature stabilizes at -56.50C and remains at this temperature up to around 85000 feet. The pressure continues to fall above the 36,000 feet and the thrust therefore drops off at a faster rate than it does at the lower altitudes. This increased drop off in thrust makes 36,000 feet a chosen altitude for a long range cruise in jet powered aircraft.
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Table of Contents:
- What Are The Factors Affecting Thrust Of A Gas Turbine Engine?
- Factors Affecting Thrust - Air Density
- Factors Affecting Thrust - Altitude
- Factors Affecting Thrust - Airspeed
- Factors Affecting Thrust - Ram Effect
- Factors Affecting Thrust - Engine RPM