constant. 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. The three  principal variables that affect inlet conditions are the speed  of the aircraft, the altitude of the aircraft, and the ambient  temperature. To make the analysis simpler, the combination  of these three variables can be represented by a single variable  called stagnation density. The power produced by a turbine engine is proportional to  the stagnation density at the inlet. The next three illustrations  show how changing the density by varying altitude, airspeed,  and outside air temperature affects the power level of the  engine.  Figure 1-84  shows that the thrust output improves  rapidly with a reduction in outside air temperature (OAT)  at constant altitude, rpm, and airspeed. 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. In addition, the thrust  output increases since the air at reduced temperature has an  increased density. The increase in density causes the mass  flow through the engine to increase. The altitude effect on  thrust, as shown in  Figure 1-85 , can also be discussed as  a density and temperature effect. In this case, an increase  in altitude causes a decrease in pressure and temperature. Since the temperature lapse rate is lower than the pressure  lapse rate as altitude is increased, the density is decreased.  Although the decreased temperature increases thrust, the  effect of decreased density more than offsets the effect of  the colder temperature. The net result of increased altitude  is a reduction in the thrust output. The effect of airspeed on the thrust of a gas-turbine engine  is shown in  Figure 1-86 . 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. Specific thrust is the net thrust in pounds  developed per pound of airflow per second. It is the remainder  of specific gross thrust minus specific ram drag. As airspeed  is increased, ram drag increases rapidly. The exhaust velocity  remains relatively constant; thus, the effect of the increase in  airspeed results in decreased specific thrust.  [Figure 1-86]  In  the low-speed range, the specific thrust decreases faster than  the airflow increases and causes a decrease in net thrust. As the  airspeed increases into the higher range, the airflow increases  faster than the specific thrust decreases and causes the net  thrust to increase until sonic velocity is reached. The effect  of the combination on net thrust is illustrated in  Figure 1-87 . Dec STD INC Outside air temperature (OAT) Net thrust 60% 100% Constant altitude, rpm, and airspeed 0 TAS Net thrust Increase 100 50 0 Constant rpm, altitude, and OAT With ram Without ram 0 10 20 30 40 50 Altitude—thousand of ft Thrust—percent sea level rated 100 50 0 Constant airspeed and rpm Figure 1-85. Effect of altitude on thrust output.  Figure 1-86. Effect of airspeed on net thrust. Figure 1-84. Effect of OAT on thrust output.  1-61