Gas turbine and reciprocating engine types

Thrust specific fuel consumption (TSFC)

amount of fuel required to produce one pound of thrust

Turbojet characteristics

adv: lightest specific weight, higher and faster than any other engine

dis: low propulsive efficiency at low forward speeds, relatively high TSFC at low altitude and low airspeeds, long takeoff roll required

turbojet exhaust gases splits

75 to compressor and accessories

25 to thrust

turbofan

combines thrust of the exhaust gases from the gas generator with additional thrust that is generated by utilizing a duct-enclosed fan which accelerates a large mass of air around the gas generator

bypassed or ducted air and the power split of the air

airflow that goes around the gas generator

30-60 total thrust for bypassed air

40-70 from exhaust gases

free or power turbine

turbine aft of the gas generator turbines and is not connected to the gas generator, may drive the fan

what increases efficiency of a turbofan over a turbojet

converting more of the fuel energy into pressure energy rather than the kinetic (dynamic) energy of a high-velocity exhaust gas stream. lower TSFC

bypass ratio and what do most fuel-efficient engines have

amount of air that bypasses the gas generator in comparison with the amount of air that passes through the gas generator

higher bypass ratio

turbofan characteristics

adv: higher thrust at low airspeeds, lower TSFC, shorter takeoff distance, considerable noise reduction

dis: higher specific weight, larger frontal area, inefficient at higher altitudes

turboprop engine

couples a gas generator with a reduction gear box and propeller driven by the turbine section with the propeller providing most of the thrust

heat energy splits for a turboprop

75 for compressor and accessories

25 for thrust but 10% coms from the exhaust gases

how much thrust comes from the propeller

90%

propeller components

blades: installed into the hub

hub: attached to propeller shaft

pitch change/dome assembly: mechanism that changes the blade angle of the propeller

reduction gear box (RGB)

located between the propeller assembly and the gas generator and is a one-speed transmission

prevents propeller blades from reaching supersonic speeds, converts high rpm and low torque of the gas generator to low rpm, high torque

Torquemeter assembly

torque/inner shaft carries load from propeller and produces measured twist. reference/ outer shaft provides references to the torque shaft

propeller assembly, RGB, and torquemeter assembly may be connected to the gas generator in two possible configurations

1: attached to the front of the compressor drive shaft

2: attached to the free/power turbines

the turboprop accelerates a ______ with its propeller to a ___ speed

very large mass of air

moderate

during flight operations the propeller assembly maintains the propeller at a constant ___ % RPM

100%

two turboprop engine ranges of operation

Alpha and Beta

Alpha range

PCL can be positioned from flight idle to full power. the PCL sends signals to the FCU for fuel flow. the FCU wors in conjunction with the prop governor to ensure constant propeller RPM by adjusting the blade angle

Beta Range

only used during ground operations. PCL can be positioned from flight idle to max reverse. PCL is mechanically connected to the pitch change assembly as well as the FCU to allow the pilot direct control of blade angle

Characteristics of turboprop

adv: develops very high thrust at low airspeeds, excellent take-off, slow speed, low altitude characteristics, superior for lifting heavy loads off short and medium length runways

dis: heavier and more complicated aircraft, limited speed

turboshaft engine

the shaft turns more than just the propeller such as a rotor for a helicopter

two sections of a turboshaft engine

gas generator

free/power turbine sections

free/power turbine (PT)

mechanically independent from the gas generator

the propulsive energy from the exhaust in a turboshaft is

negligible

basic components of a reciprocating engine

crankcase, cylinders, pistons, connecting rods, valves, valve-operating mechanism, and the crankshaft