thrust an engine produces. Figure 3-70. Turbine inlet temperature (TIT) readings between 2,500 and 3,000 degrees Fahrenheit with com- pression ratios near 32:1 give 50 percent or better thermal efficiencies. Figure 3-71. Compressor and turbine efficiency near 90 per- cent is necessary to reach thermal efficiencies above 20 per- cent.
3-42 Turbine Engines Figure 3-72. Air density decreases as temperature rises, and the lower the density of the air, the less thrust an engine can produce. TEMPERATURE As previously discussed, the air mass flowing through a turbine engine is the working fluid that the engine uses to produce thrust. Furthermore, the more dense the air passing through an engine is, the more thrust the engine can produce. If you recall from your study of aerodynamics, air density is inversely related to temperature. In other words, as outside air temperature (OAT) increases, air density decreases. Anytime the density of the air entering a gas turbine engine decreases, engine thrust also decreases. Conversely, thrust output improves with a reduction in outside air temperature. Engine manufacturers base their thrust calculations for any given engine on a standard temperature of 59 degrees Fahrenheit or 15 degrees Celsius. This provides a reference point to use when calculating thrust and compensating for temperature variations. In the field, all performance calculations must be adjusted for non-standard tem- peratures. [Figure 3-72] To counteract the detrimental effects of hot weather on the amount of thrust an engine can pro- duce, some engines are fitted with a thrust augmentation system. In a water injection thrust augmentation system, water, or a mixture of water and alcohol is injected directly into the compressor inlet or into the combustion chamber. Water injection accomplishes several things; it cools the air-mass and allows more fuel to be burned without exceeding turbine inlet temperature limits, and it maintains the same air pressure in the engine because water molecules are added to the airmass. More information is given on water injection systems in Chapter 6. ALTITUDE As altitude increases, air pressure drops. Air at stan- dard temperature at sea level exerts a pressure of 14.69 pounds per square inch. However, this pres- sure decreases as the altitude increases. Approximately one-half of the air in the atmosphere is below 18,000 feet. Therefore, the pressure at 18,000 feet is about 7.34 psi, or half that at sea level. Above 18,000 feet, air pressure continues to drop, but at a higher rate. At 20,000 feet, standard air pres- sure drops to 6.75 pounds per square inch, and at 30,000 feet it is only 4.36 pounds per square inch. Temperature, like ambient air pressure, also decreases as altitude increases. Normally, decreas- ing temperatures result in an increase in air density, leading some to think that increases in altitude result in higher air density. The opposite, however, is true because the pressure losses experienced when climbing has a greater effect on air density than decreasing temperatures. In other words, the temperature lapse rate is less than the pressure lapse rate. Therefore, as an aircraft climbs, engine
- Spring '17
- Turbofan, Gas turbine, Gas compressor