15.2 : ENGINE PERFORMANCE

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THRUST

• Gross Thrust = Total thrust = Momentum Thrust by engine(F=ma)

• Net Thrust = Gross thrust – momentum drag(flying speed) = mv2 – mv1

• Chocked Nozzle Thrust = Pressure thrust(venturi) = P2 – P1(area nozle)

• Resultant Thrust = Net Thrust = Mom thrust + Mom drag + Prss thrust

THRUST DISTRIBUTION

• Distribution of thrust vary in every section of engine.

• It can be calculated base on exit duct area(A), exit presssure(P), exit air velocity(v) and air mass flow(w)

• Thrust Load(sect2) = ((AxP) + WV/g) – thrust load(sect1)

• Value is (+ve) for convergence exit and (-ve) for divergence exit

• Resultance thrust is sum of all trust load in every section.

HORSE POWER

• 1HP = 746watt• Thrust Horse Power(jet engine):

THP = Thrust(Ib) x Aircraft speed(ft/s)550ft/s

• Actual horsepower engine-propeller unit found less compare to jet engine

• Equivalent Shaft Horse Power(turboshaft engine):ESHP = SHP + (jet thrust ÷ 2.5)

SPECIFIC FUEL CONSUMPTION

• Amount of fuel required in a time to produce specific thrust output

• Normallt compare during cruise condition

ENGINE EFFICIENCIES

• Thermal efficiencies : KE ÷ Fuel EnergyRatio of convertion of heat energy to kinetic energy (thrust)

• Propulsive efficiencies :Ratio of convertion of kinetic energy(thurst) to do the work(pushing aircraft)

• Overall efficiency : TE + PE

_________Work done____________Work done + Work wasted in exhaust

RATIO

• By-pass ratio is a ratio mass of air bypasses engine core and air passing through the engine core.

• Engine pressure ratio is ratio between Turbine exit pressure to the engine intake pressure

By-pass air

P(intake)P(exhaust)

EPR = P(intake) P(exhaust)

PERFORMANCE GRAPH

Velocity

PressureTemperature

Peak temp at begining of combustion chamberPeak pressure at the end of compressor sectionPeak velocity at begining of turbine inlet

FACTOR EFFECTING THRUST• Temperature :

Thrust increase with decreasing in temperature (air density increase at low temp)

• Air Pressure :Thrust increase with increasing air pressure(air density increase by pressure)

• Altitude :Thrust decrease with increase in altitude(change rate after 36kft = temp constant)

• Airspeed :Thrust decrease initially then increase back due to ram air effect on jet engine