ACA0204 Gas turbine and reciprocating engine types

Thrust specific fuel consumption (TSFC)

amount of fuel required to produce one pound of thrust

propulsive efficiency

determined by the efficient conversion of KE to propulsive force (by its propelling mechanism)

turbojet consists of

inlet section + GTE + exhaust section

turbojet derives it thrust by

highly accelerating a small mass of air through the engine and out the exhaust section

focuses on the a in T= ma

PCL determines thrust available

turbojet propulsive force

dependent upon the amount of fuel added to the air mass; “more air needs more fuel”

Turbojet Advantages

1. lightest specific weight (weight per pound of thrust produced)

2. higher and faster than any other engine

3. best high-end performance turbine engine

Turbojet Disadvantages

1. low propulsive efficiency @ low forward speeds

   1. TJ wastes more energy @ low A/s

2. high TSFC @ low altitude and low airspeeds

3. long takeoff roll required

turbofan definition

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

combines best qualities of turbo jet and turbo prop

duct-encolsed fans are driven by

• normal turbine: fan connected to the front of the compressor of gas generator

• free turbine / power turbine: no physical connection to the gas generator

bypassed air definition

airflow that goes around the gas generator

bypass ratio

amount air bypass gas generator : amount air through gas generator

higher bypass ratio =

more fuel-efficient engine and lower TSFC

Turbofan Thrust Percentages

30-60 total thrust from ducted air

40-70 from gas generator exhaust

free or power turbine

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

turbofan advantages

compared to turbojet

1. higher thrust @ low airspeeds

2. lower TSFC

   1. converts more fuel energy into pressure energy

3. shorter takeoff distance / heavier

4. considerable noise reduction

turbofan disadvantages

compared to turbojet

1. higher specific weight

2. larger frontal area (drag)

3. inefficient @ higher altitudes

for turbfans and turboprops, larger fan = ? and increased altitude = ?

larger fan = lower V due to increased drag

increased altitude = lower efficiency due to density decrease

propulsive efficiency and turboprops, fans, and jets

TPs: highest for subsonic, decreases in supersonic

TF: between TP and TJ subsonic, remains climbing in supersonic

TJ: lowest for all, but always climbing

turboprop

couples a gas generator with a propeller which is driven by the turbine section

combines adv of gas gen + prop eff of propeller

turboprop consists of

reduction gear assembly

torquemeter assembly

gas generator

turboprop thrust production %

T = ma —> small a for very large m

90% from propeller

10% from the exhaust gases of the gas generator

propeller main components

blades: installed into the hub

hub assembly: attached to propeller shaft

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

reduction gear box (RGB)

• prevents propeller blades from reaching supersonic speeds

• converts high rpm and low torque of the gas generator to low rpm, high torque

• located: between the propeller assembly and the gas generator

• is a one-speed transmission

• acts as a mounting pad for accessories

Torquemeter assembly purpose and location

to connect the RGB to the gas generator and to transmit and measure the power output from the gas generator to the RGB; located between RGB and Gen

torquemeter assembly major components and what they do

torque shaft/inner shaft - carries the load from the prop and produces measure twist

reference/outer shaft - provides reference to the torque shaft and is not twisted

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 turbine

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

turboprop has these 2 types of ranges of operation

Alpha and Beta

Alpha range

• flight range

• PCL can be positioned from flight idle to full power

• the PCL sends signals to the FCU for fuel flow.

• the FCU works 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

turboprop advantages

1. develops very high thrust at low airspeeds

2. excellent take-off, slow speed, low altitude characteristics

3. superior for lifting heavy loads off short and medium length runways

4. lowest TSFC of any gas turbine

turboprop disadvantages

1. heavier and more complicated

2. limited speed

turboshaft engine uses a ____ to drive the ____ of a helicopter

free/power turbine

rotor

two sections of a turboshaft engine

gas generator

free/power turbine

free/power turbine (PT) definition

mechanically independent from the gas generator; driven by the GTE exhaust gases

propulsive energy of turboshaft engine’s exhaust gases

negligible; it all gets converted to shaft horsepower

reciprocating engines key points

• propulsive energy: comes from propeller driven by crankshaft

• propeller can be directly connected to crankshaft (low powered eng) or RGB

• utilizes governor to control the blade angle and prop speed