Topic of The Day: Factors Affecting Thrust
The factors that affect the thrust of a gas turbine engine include:
Air Density
Airspeed
Ram Effect
Engine RPM
Note that the effects of these factors is not restricted to any particular gas turbine engine and the ability to compensate for these factors is dependent upon the type of engine.
Density– Is the mass of a a substance per unit of its volume.
According to the thrust equation:
If mass of airflow increases- thrust increases
If density of air increases- mass will increase and therefore thrust will increase
Air Temperature
Temperature Increase- thrust will decrease
Temperature decrease- thrust will increase
Thrust may vary as much as 20% from standard rated thrust on a hot or cold day
Air Pressure
As air pressure increases- density increases and thrust increases
As air pressure decreases- density decreases and thrust decreases
Altitude
As an aircraft climbs, temperature and pressure decrease. With an increase in altitude, the rate of thrust decreases because the pressure drop is greater than the thrust increase resulting from temperature decreasing. At approximately 36,000 feet, temperature stabilizes. As a result temperature will no longer offset the density decrease. Altitude cruise level is found at 36,000 feet.
Airspeed
In the thrust equation, the difference between the inlet and exhaust velocities plays a major role in determining thrust available. As the inlet velocity approaches the magnitude of exhaust velocity. thrust is reduced. Therefore, if the mass of air and fuel is held constant, thrust will decrease as airspeed increases.
Ram Effect
The thrust equation consists of two variables: mass and acceleration. As airspeed increases the difference between mass and acceleration causes a decrease in thrust. However, as you increase airspeed more air is being rammed into the inlet, increasing mass and pressure of inlet air. This offsets the decrease in acceleration and results in a neutral affect or slight increase in thrust at subsonic airspeeds. This is due to the compressibility of airflow as velocity increases toward supersonic. As airflow becomes compressible, mass due to ram effect increases at an increasing rate.
Density– Is the mass of a a substance per unit of its volume.
According to the thrust equation:
If mass of airflow increases- thrust increases
If density of air increases- mass will increase and therefore thrust will increase
Air Temperature
Temperature Increase- thrust will decrease
Temperature decrease- thrust will increase
Thrust may vary as much as 20% from standard rated thrust on a hot or cold day
Air Pressure
As air pressure increases- density increases and thrust increases
As air pressure decreases- density decreases and thrust decreases
Altitude
As an aircraft climbs, temperature and pressure decrease. With an increase in altitude, the rate of thrust decreases because the pressure drop is greater than the thrust increase resulting from temperature decreasing. At approximately 36,000 feet, temperature stabilizes. As a result temperature will no longer offset the density decrease. Altitude cruise level is found at 36,000 feet.
Airspeed
In the thrust equation, the difference between the inlet and exhaust velocities plays a major role in determining thrust available. As the inlet velocity approaches the magnitude of exhaust velocity. thrust is reduced. Therefore, if the mass of air and fuel is held constant, thrust will decrease as airspeed increases.
Ram Effect
The thrust equation consists of two variables: mass and acceleration. As airspeed increases the difference between mass and acceleration causes a decrease in thrust. However, as you increase airspeed more air is being rammed into the inlet, increasing mass and pressure of inlet air. This offsets the decrease in acceleration and results in a neutral affect or slight increase in thrust at subsonic airspeeds. This is due to the compressibility of airflow as velocity increases toward supersonic. As airflow becomes compressible, mass due to ram effect increases at an increasing rate.
