Jet Engine Thrust| Factors Affecting Thrust

Jet Engine

Jet Engine Thrust

Thrust is a reaction force which is resulting of accelerating mass of gass at very high speed. The engine does operate in conjunction with the gas, which is accelerated in the opposite direction as it accelerates toward the back. A force acts on the aeroplane as a result of the engine mass's acceleration.

The amount of force required to move an aeroplane through air that is resisting its passage is known as thrust. Thrust is a reaction force described by Newton's third law of motion. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude in opposite direction to be applied to that system.

Constant thrust is necessary for level flight at constant speed; it must be increased or decreased to ascend or descend the aircraft while maintaining constant speed; and it must be altered to increase or decrease speed while maintaining level flight.

Depending on the type of propulsion used in an aircraft, multiple methods are used to generate thrust.

There would be no need to alter the quantities used in the aforementioned formulae for gross and net thrust at any given throttle level if a turbojet or other gas turbine engine were exclusively run under static conditions in an air-conditioned room at typical daytime temperature. All aircraft engines, however, must function under a variety of airspeed and altitude conditions. The temperature and pressure of the air entering the engine, the amount of airflow through the engine, and the jet velocity at the engine exhaust nozzle will all be significantly impacted by these variable conditions. This implies that as the aircraft's airspeed and/or altitude change, various numbers must be entered into the thrust formulae for each given throttle setting. Many of the changes that will take place will have a direct impact on the engine's thrust output, even though the engine fuel control can correct for some of these variables. In reality, it is uncommon to directly calculate engine thrust using the above calculation. To explain how the changing circumstances at the engine inlet affect engine performance in flight and on the ground, it is helpful to understand how various variables that will be present during typical engine operation affect the thrust equations.

Factors Affecting A Gas Turbine Engine's Thrust

When developing and running a jet engine, the following considerations must be made:

  • Air Density
  • Altitude
  • Airspeed
  • Ram Effect
  • Engine RPM

Effects of density on engine thrust

The mass of a substance per unit of its volume is known as its density. The thrust equation predicts that as airflow mass increases, thrust will also increase. As air density rises, mass will as well, resulting in an increase in thrust. The temperature and pressure of the surrounding air will change when an aeroplane flies under various altitudes and climatic conditions. These elements will modify the air's density when it enters the engine, which will modify thrust.

Effects of Air Temperature on engine thrust

Effects of temperature on thrust

Air molecules tend to disperse as temperature rises. This causes the density to drop, which in turn causes the thrust to drop. A warmer engine operating close to the equator will generate less thrust than a colder engine operating in Alaska. On a hot or cold day, thrust may vary by up to 20% from the usual rated thrust.

Effects of Air Pressure on engine thrust

Effects of Air Pressure on engine thrust

Air molecules tend to migrate closer together when air pressure rises. Due to the rise in density, thrust also rises as a result. For instance, a plane flying through a hurricane's low-pressure eye will generate less thrust than a plane flying at ambient pressures that are more typical.

Effects of Ram air on engine thrust

Effects of Ram air on engine thrust

The outside air is moving past an aeroplane at a faster and faster rate as it descends the runway. The outcome is identical to what would occur if the aeroplane were stationary in a wind tunnel and air were being blasted past it by a fan within the tunnel. Air is "rammed" into the inlet duct due to the movement of the aircraft in relation to the ambient air. Because of the ram effect, increasing airspeed also raises the engine's intake air pressure. Then, the ram effect helps to enhance airflow to the engine, which should improve the thrust the engine produces.

The initial momentum of the air in relation to the engine will likewise rise when the aircraft increases its airspeed. Additionally, because the jet nozzle typically operates in constricted conditions, its velocity is typically fixed by the speed of sound. As a result, as the airplane's speed rises, the momentum change decreases, which causes a loss of push.

Eventually, the added thrust from the ram is enough to offset the lost thrust. Ram will also partially make up for the thrust loss brought on by the lower pressure at high altitude.

Because the ram effect finally results in a considerable overall increase in power when the airspeed is high enough, ram effect is crucial, especially in high-speed aircraft. Ram effect has little effect on engine thrust at the subsonic speeds that nonafterburning engine-powered aircraft typically cruise. Ram effect can play a significant role in determining how much thrust an engine will produce at supersonic speeds.

Effects of Altitude on engine thrust

Effects of Altitude on engine thrust

Thrust is actually affected by height in a way that depends on density. Up until the aeroplane reaches around 36,000 feet, the air's temperature and pressure both decrease as an aircraft ascends through the atmosphere. For a fixed flying Mach number, thrust falls as pressure decreases. On the other hand, the thrust rises as the temperature falls. As a result, the thrust keeps decreasing as altitude rises since the air pressure is descending more quickly than the temperature is, on average. 

Effects of Engine RPM on engine thrust

The engine's rotational speed is one of the most evident elements that influences the thrust output. The amount of push increases as the RPM rises. At low RPM, however, increasing the throttle has very little effect on the amount of thrust. A slight increase in the throttle setting will result in a significant increase in thrust at higher rates of revolution. Fuel consumption is significant for the amount of thrust produced at the lower settings. Gas turbine engines are typically run at or close to their maximum RPM for this reason. 

Humidity Effect

The gas turbine engine is barely impacted by humidity, whereas the reciprocating engine is significantly affected. Increasing humidity will reduce the weight per unit volume because water vapour weighs only five-eighths as much as dry air; as a result, the lower density equals less mass at the same rpm. Because a carburetor is essentially a volume measuring device, it won't notice this drop in air weight and will continue to feed the engine with the same amount of fuel, which will make the fuel-to-air ratio excessively rich and reduce engine power.

In contrast, a jet engine runs with more air than is necessary for burning. Air from the cooling air supply will be used if any air is required for the combustion process. Additionally, the fuel control metres fuel flow as a function of pressures, temperatures, and rpm rather than directly measuring the volume of air.

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