Aircraft Propulsion, 2nd Edition

CHAPTER 11 Aircraft Engine Component Matching and Off-Design Analysis

11.1 Introduction

Source: Reproduced with permission from Rolls-Royce plc

How does an existing engine behave in an off-design operating condition? For an existing engine, we know its geometry and design-point parameters, such as compressor pressure ratio or turbine entry temperature. All the parameters that we used in the cycle analysis were in effect “design-point” parameters. For example, take compressor pressure ratio; it may be designed into a number of stages, each with a certain design pressure ratio. Although we cannot change the number of stages after the engine is manufactured, we may operate the compressor at a different shaft speed than its design speed, or, equivalently, we may operate the engine at a different throttle setting than its design. In general, the independent parameters that may be set and create an off-design flight condition for an airbreathing jet engine are

• Altitude of flight, which may include nonstandard ambient pressure and temperature, for example, arctic flight or seasonal effects
• Flight Mach number, from takeoff, climb, cruise, dash, approach, landing
• Throttle setting, that is, turbine entry temperature
• Afterburner setting, for example, -on, -off, or partial setting
• Nozzle area ratio setting, for example, to optimize installed thrust.

We will first consider individual ...