In the low-speed range, the specific thrust decreases faster than the airflow increases and causes a decrease in net thrust. The exhaust velocity remains relatively constant thus, the effect of the increase in airspeed results in decreased specific thrust.

As airspeed is increased, ram drag increases rapidly. It is the remainder of specific gross thrust minus specific ram drag. Specific thrust is the net thrust in pounds developed per pound of airflow per second. To explain the airspeed effect, it is necessary to understand first the effect of airspeed on the factors that combine to produce net thrust: specific thrust and engine airflow. The effect of airspeed on the thrust of a gas-turbine engine is shown in Figure 1-85. The net result of increased altitude is a reduction in the thrust output. Although the decreased temperature increases thrust, the effect of decreased density more than offsets the effect of the colder temperature. Since the temperature lapse rate is lower than the pressure lapse rate as altitude is increased, the density is decreased. In this case, an increase in altitude causes a decrease in pressure and temperature. The altitude effect on thrust, as shown in Figure 1-84, can also be discussed as a density and temperature effect. The increase in density causes the mass flow through the engine to increase.

In addition, the thrust output increases since the air at reduced temperature has an increased density. This increase occurs partly because the energy required per pound of airflow to drive the compressor varies directly with the temperature, leaving more energy to develop thrust. Figure 1-83 shows that the thrust output improves rapidly with a reduction in outside air temperature (OAT) at constant altitude, rpm, and airspeed. The next three illustrations show how changing the density by varying altitude, airspeed, and outside air temperature affects the power level of the engine. The power produced by a turbine engine is proportional to the stagnation density at the inlet. Effect of turbine inlet temperature on turbine bucket life. To make the analysis simpler, the combination of these three variables can be represented by a single variable called stagnation density. The three principal variables that affect inlet conditions are the speed of the aircraft, the altitude of the aircraft, and the ambient temperature. Since this is a practical application of a turbine engine, it becomes necessary to analyze the effect of varying inlet conditions on the thrust or power produced. In the previous discussion, it was assumed that the state of the air at the inlet to the compressor remains constant. Figure 1-82 illustrates the effect of turbine inlet temperature on turbine blade life. Since engine life is greatly reduced at high turbine inlet temperatures, the operator should not exceed the exhaust temperatures specified for continuous operation. Rpm is a direct measure of compression ratio therefore, at constant rpm, maximum thermal efficiency can be obtained by maintaining the highest possible exhaust temperature. The effect of compression ratio on thermal efficiency. In actual operation, the turbine engine exhaust temperature varies directly with turbine inlet temperature at a constant compression ratio. The effects that compressor and turbine component efficiencies have on thermal efficiency when turbine and compressor inlet temperatures remain constant are shown in Figure 1-81. Figure 1-80 shows the effect that changing compression ratio (compressor pressure ratio) has on thermal efficiency when compressor inlet temperature and the component efficiencies of the compressor and turbine remain constant. Other factors that affect thermal efficiency are compressor inlet temperature and combustion efficiency. The three most important factors affecting the thermal efficiency are turbine inlet temperature, compression ratio, and the component efficiencies of the compressor and turbine. It is the ratio of net work produced by the engine to the chemical energy supplied in the form of fuel. Thermal efficiency is a prime factor in gas turbine performance.

Ratio of ram pressure thermal pressure