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Probability methods for engine design
Commercial engine cycle design
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The main goal of engine cycle design is to find a good balance between requirements for engine weight, fuel burn, and flying range. In this study, design range receives the most emphasis. There is a well-defined target for design range, and the best solution is inherently a compromise between engine weight and specific fuel consumption. The system and engine level FoMs tracked include most of the basic measures for evaluating conceptual aircraft at the system level, as well as basic engine FoMs and constraints, as shown in Table 1.
Table 1
Engine and Aircraft Figures of Merit |
Aircraft Performance Measures
- Design range
- Fuel burn for 3000 n mi and 6000 n mi missions
Engine Performance Measures
- Cruise specific fuel consumption at 35,000 ft, Mach 0.85, 9000 lb thrust per engine
- Fan diameter
- Engine weight
- Noise
Other Attributes
- Exhaust gas temperature
- Core flow
- Bypass ratio
- Overall pressure ratio
- Stages in each engine component
- Aircraft drag at Mach 0.85, 35,000 ft
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The Fast Probability Integration (FPI) method of probabilistic design determines values for the noise parameters according to a set of user-defined input distributions. Control parameters are prescribed according to a design of experiments setup. These values are passed to the case execution routine by the FPI-shell script. The shell script then generates the cycle model, calculates aircraft operating empty weight and nacelle drag, and the engine is "flown" to estimate installed performance using a suitable mission analysis model. Finally, response data are sent to FPI for probabilistic analysis, as shown in Figure 1. FPI repeatedly calls this routine to get data to generate a cumulative distribution function (CDF) for the response of interest, such as fuel burn, engine weight, and aircraft range.
Figure 1. Analysis process flowchart.
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