Special engine ratings
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| Fig. 18-15 Thrust increases with short lift | ratings |
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24. Experience has shown that an engine rating structure can be devised which provides high thrustlevels for short periods of time without reducing engine life. Operation in ground effect and the take- off and landing manoeuvres require maximum thrust for less than 15 seconds so that use of a short lift rating for that time is feasible. Fig. 18-15 shows an example of thrust permissible with a 15 second short lift rating compared to that with a 2.5 minute normal lift rating.
25. At high ambient temperatures, the engine may run into a turbine temperature limit before reaching its maximum r.p.m. and suffer a thrust loss as a result. Restoration of the thrust can be achieved by means of water injection into the combustion chamber (Part 17) which allows operation at a higher turbine gas temperature for a given turbine blade temperature. If desired, water injection can also be used to increase the thrust at low ambient tempera- tures.
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