Chapter OutlineTrading Thrust and EfficiencyHow Thrust is ProducedHow Thrust is ProducedPropulsive EfficiencyPropulsive EfficiencyPropulsive EfficiencyPropulsive Efficiency and EnginesReciprocating Engine/PropellerPower Generated by EngineSpecific Fuel ConsumptionVariations of Power and SFC with Velocity and AltitudeVariations of Power and SFC with Velocity and AltitudeSupercharged EnginesThe PropellerAdvance Ratio, JAdvance Ratio, JPropeller Efficiency and JShape of Propeller Efficiency CurveEvolution of PropellersFeatheringRate of Climb and PropellersThe Turbojet EngineTurbojet EngineGeneration of ThrustCalculation of ThrustTurbojet Thrust EquationExample of a TurbojetSFC for a TurbojetThrust varying Velocity and AltitudeTSFC varying Velocity and AltitudeSupersonic VariationsSupersonic Variation CommentsTurbofan EngineBypass RatioTypical High Bypass TurbofanVariation of Thrust with Velocity and AltitudeVariation of Thrust with Velocity and AltitudeTSFC varying with Velocity and AltitudeLow Bypass Ratio variationsTurbopropCharacteristics of a TurbopropTurboprop EquationsTurboprop EquationsVariations of Power with Velocity and Altitude for TurbopropVariations of SFC with Velocity and Altitude for TurbopropAfterburnersPerformance of AfterburnerAfterburnerTransforming Fuel ConsumptionsFuel InjectionInstrumentationStators in a CompressorThrust ReversersThrust ReversersThrust ReversersReferences for Chapter 3Chapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyChapter OutlineThrust vs. Efficiency - basic conceptsReciprocating Engines with PropHow it worksPower/SFC varying with Velocity/AltitudeTurbojet EngineHow it worksThrust/SFC varying with Velocity/AltitudeTurbofan EngineHow it worksThrust/SFC varying with Velocity/AltitudeTurbopropHow it worksThrust/SFC varying with Velocity/AltitudeChapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyTrading Thrust and EfficiencyPropeller/reciprocating engineTurbojetRocket EngineThrust EfficiencyLow HighHigherSubstantialLowerPoorIn general, more thrust = less efficiencyThis tradeoff helps explain why there are different propulsive devices in use today.Chapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyHow Thrust is ProducedPropulsiveDeviceVjV∞PropulsiveDeviceTVjV∞TGeneric propulsive device (jet engine,propeller, etc) whose function is to produce thrust, T, acting towards theleftRegardless of type of device, the thrustexerted on the device is the net resultantof pressure and shear distributions, atpoints where air contacts device (internaland external)Air experiences equal and oppositereaction (Newton’s 3rd Law) to thrustChapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyHow Thrust is ProducedVjV∞TAir is accelerated to velocity Vj,called the jet velocityNewton’s 2nd Law: the force on an object is equal to the time rate of change ofmomentum of that objectT = m (Vj-V )∞time rate of change of momentumm Vjmomentum per unit timeentering stream tubem V∞momentum per unit timeexiting stream tubeTHRUST EQUATIONfor generic thrust device(Note: we are neglecting the pressure force on the stream tube for this simplified analysis)Chapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyPropulsive EfficiencyVjV∞TInstead of considering air moving throughdevice, picture device moving throughroom of stationary air.Before device enters the room, air is stationary and has no kinetic energyAfter device leaves room, air is moving at velocity Vj- V and has kinetic energy of∞( Vj- V )2∞wasted energy-source of inefficiencyRecall: power = force x velocityPA= T V∞Since power is energy per unit time, the power wasted in the air jet behind the device:m ( Vj- V )2∞1212this is the useful power availableChapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyPropulsive EfficiencyTotal power generated by device = TV + m ( Vj-V )2∞12∞Propulsive efficiencyηp =useful power availabletotal power generatedSubstituting in previous expressions,ηp =T V∞m ( Vj-V )2∞12T V∞+T = m (Vj-V )∞Recall:and substitute to get...Chapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyPropulsive Efficiencyηp =21 + Vj/ V∞Max efficiency occurs when Vj= V(ηp= 1) but then T=0 (ultimate efficiency but no propulsive force) ∞ηp =m(Vj-V ) V∞∞m ( Vj-V )2∞12m(Vj-V ) V∞+∞m(Vj-V ) V∞∞Divide by:ηp =12(Vj- V )/ V∞∞1 +1=12(1 + Vj/V )∞1T = m (Vj-V )∞Chapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyPropeller provides large m but small Vj-V∞so has high efficiencyPropulsive Efficiency and EnginesGas turbine jet engines gives a smaller mass of air a larger increasein velocity, but at a lesser efficiency.Q: so why don’t we use propellers on faster aircraft?Turbofans try to combine the thrust generating capabilities of the jet engine with the efficiency of a propeller. Similarly, the turboproptries to achieve the same. A: as speed increases, the tip speed increases. At high enough speeds, shock waves will form. This increases drag, which increases the torque on the reciprocating engine, which reduces the rotational speed (rpm) of the engine, which reduces power obtrained from the engine, which reduces thrust. Also, shock waves on the propeller airfoils increase drag, reducting thrust.speedlowhighChapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyReciprocating Engine/PropellerFour Stroke Otto CycleVertical movement of the piston is translated to rotary motion of the crankshaftChapter 3- Some Propulsion CharacteristicsAE 3310 PerformanceDr. Danielle SobanGeorgia Institute of TechnologyPower Generated by EngineDisplacement: size of the engine. Volume of piston sweep from top dead center to bottom dead center is called displacement of the cylinder. Total displacement for the engine is cylinder displacement multiplied by number of cylinders. The larger the displacement, d, the more power output by the engine.Power strokes: the number of times the piston moves through its four-stroke cycle per unit time.