*AE 3310Introduction to Aerospace Vehicle Performance7.Propulsion Systems7.1Thrust and propulsive efficiency7.2Reciprocating enginesShaft brake power and fuel consumption7.3Propeller performanceAvailable power, thrust, efficiency, and advance ratioFixed, variable, and constant speed propellersMach number limitations of propellers7.4Reciprocating engine/propeller performanceShaft brake power, available power, available thrust7.5Turbojet enginesHow does a turbojet engine work?Thrust and fuel consumption models

*AE 3310Introduction to Aerospace Vehicle Performance7.6Turbofan enginesHow does a turbofan engine work?Bypass ratioThrust and fuel consumption models for high BPR engines (subsonic)7.7Turboprop engineHow does a turboprop engine work?Thrust, power, and fuel consumption models7.8Comparison of EnginesOptimal Mach number operational range

*Type of jet (reaction) engine in which some, but not all of the air flowing into the engine is processed through the combustion chamber (engine core - a turbojet), but the rest bypasses the combustion chamber and is accelerated by a ducted fan.

Thrust is produced by accelerating the air flowing into the engine, and ejecting it at higher speed - both by the core engine and the fan.

The fan is driven by the turbine in the engine. Part of the power being generated by the engine core is converted into mechanical energy to drive the fan.What is a Turbofan Engine?

*What is a Turbofan Engine?Graphics source:Anderson: Aircraft Performance and DesignCompressorTurbine(Drives fan andcompressor)BurnerNozzleFanEngine Core

*How Does a Turbofan Engine Work?Graphics source:Archer and Saarlas: An Introduction to Aerospace PropulsionFanFan DriveMixingTurbineZone

*How Does a Turbofan Engine Work?A key parameter that determines the performance of a turbofan engine is the bypass ratio:

Low Bypass Ratio Turbofans: 0 < BPR < ~ 1High Bypass Ratio Turbofans: ~ 1 BPR < ~ 7

30% to 60% of the thrust generated by a turbofan engine is generated by the fan

As the bypass ratio decreases, the engine behaves more like a turbojet

*ThrustMaking assumptions similar to those used in deriving the simplified thrust equation for the turbojet, for example:

We can obtain the simplified thrust equation for the turbofan:

*Thrust - Subsonic, High BPRIn this course we will model the thrust lapse of a high bypass ratio turbofan with Mach number at subsonic speeds with curve fits of the form:

where the values for > 0, A > 0, and n > 0 are determined for a specific engine.

( is the thrust at zero velocity at sea level.)

*Thrust - Subsonic, High BPRSimilarly, we will model the thrust lapse with altitude as,

where the subscript 0 indicates sea level maximum thrust and m > 0 depends on the particular engine.

*Graphics source:Anderson, Aircraft Performance and DesignThrust - Subsonic, High BPRTV=0 is the thrust at zero velocity and sea level.

*Fuel ConsumptionFuel consumption for a turbofan engine is defined in terms of thrust (as compared to power for a reciprocating engine).

ct=thrust specific fuel consumption (consistent units)

=weight of fuel burned per unit thrust per unit time

TSFC = thrust specific fuel consumption

*In this course we will model turbofan fuel consumption as it varies with subsonic M as follows:

ct = B(1.0 + kM)

where B and k are empirically determined quantities for a specific engine.

The sensitivity of ct to altitude at subsonic speeds is weak. Thus, we will model turbofan fuel consumption as it varies with altitude as:

ct is constant with altitudeFuel Consumption - High BPR

*Fuel Consumption - High BPRGraphics source:Anderson, Aircraft Performance and Designct - fuel consumption at sea level and zero velocity

*Propulsive Performance Model SummaryThrust, and Specific Fuel Consumption

Subsonic, High BPR:

Subsonic and Supersonic:

Subsonic, High BPR: ct = B(1.0 + kM)

Subsonic, High BPR: ct constant with altitude

*Advantages and Disadvantages of a TurbofanTurbofan engines available in a wide range of sizes. Maximum thrust to approximately 510,000 N (115,000 lbf).

Turbofans operate with high efficiency in the range of Mach numbers used by commercial transports (M from 0.65 to 0.80). Turbofans can also work well at supersonic speeds (up to M ~1.5), although for high M a turbojet is a better choice.

Thrust to weight ratio of 5 to 6.

Turbofans are simpler than turboprops (no reduction gears and variable pitch mechanism necessary) and can operate at higher Mach numbers.

Mixing of hot and cold gasses in exhaust lowers engine noise (less than turbojet or turboprop for the same level of thrust).Adapted from: Ojha: Flight Performance of Aircraft

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