Propulsion Pt2

8/8/2019 Propulsion Pt2

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Engine Selection as Part of theAircraft Design Process

Right Propulsion System for Right Job

Low bypass turbofan

Solid fuel rocket

Hybrid rocket motor using

nitrous oxide & HPTB rubber

2 GE J85 turbojets

Rotax 4-Stroke w/ 2 Blade Prop2 HBR Turbofans



Engine Operational Environments

• Engine selection is an important part of the aircraft

design process - engine requirements are largely

dictated by the aircraft mission profile and

aerodynamic & structural requirements (L/D & GTOW)

StructuralLimit

TemperatureLimit

LiftLimit

Mach Number

A l t i t u d e

RamjetScramjet

Turbofan/turbojet

Turboprop

Reciprocating

Rockets

VTOL

Engine Development

• Metrics: power (hp) and power loading (hp/lb)



Propulsion

• When selecting a propulsion system for a aircraft, ingeneral, determine your desired thrust generation

method

» Propeller

» Jet (turbojet/rocket)

» Other (ornithopter)

• then determine how much power you will need and

which power generation fits best

» Gasoline engine (recip.)

» Gas turbine

» Other chemical (rocket)

» Electric

Propulsion Selection

• Important Criteria

» Thrust required (determined by drag)

» Power required (determined by drag and velocity)

» Weight of engine

» Type of propulsion (propeller, turbine, other)

» Efficiency

» Fuel consumption (SFC or TSFC)

» Range

o Combine with fuel consumption to determine amount of fuel required

» Power loading (hp/W)

• Note that on the thrust and power curves, the maximum velocity

is the same (U max for power and thrust are identical)

• However, the velocity for minimum thrust and minimum power

are different



How Much Fuel?

• Assume characteristics of JP-8, used for almost all jet aircraft

» Density: 6.7 lb/gallon (0.8 kg/l) or 42 lb/ft 3 (1 gallon = 0.134 ft 3)

slightly heavier than gasoline (6.0 lb/gallon)

» h pr : 18,400 BTU/lbm (42,800 kJ/kg)

• Weight of fuel is determined by the mission range and profile, and

TSFC - this must be determined iteratively since

» The TSFC changes with throttle setting for different portions of the

mission

» The drag changes as fuel is burned and weight of the aircraft drops

• However, you can use the endurance eq’n to estimate (dr’vd later)

where Wi is the initial weight (w/ fuel) and Wf is the final weight

(w/o fuel) - note that L/D is usually not constant as W drops!

• Find E, then volume of fuel from fuel flow rate (SFC or TSFC)

E =

1

ct

L

D ln

W i

W f

for jets

E ≈η

c

L

D

1

V ln

W i

W f

for recips

Fuel Volume Examples



Real Engine Performance

• While idealized, these plots reveal the trends seen inreal engines - note that engine thrust, power, and

TSFC are functions of temperature and density,

hence altitude

Number and Placement

• Number and placement of

engines is dependent

available thrust from

individual engines, required

thrust, and other aircraft

requirements (such as

maintenance, stealth,

V/STOL, FOD, FAA, etc.)

• Start with available engine

specs and iterate around

your plane design



Evolution of Engine Placement - General Aviation

• Propeller aircraft; simple» 1 engine: nose

» 2 engines: wing

» Rare exceptions (pods, tail)

Evolution of Engine Placement - Transports

• Dictated by cost and efficiency

» 4 variations that eventually converge to a single configuration



Engine Placement - Transports

• Rear-Mounted Engines

» Advantageso Aerodynamically clean, particularly wings

o Small asymmetric moments

o Reduced cabin noise levels

» Disadvantages

o Heavier wing structure for lost bending relief

o c.g. further aft, resulting in larger tail to balance moments (also limits taildesign)

o Long slender forebody tendency for deep stall

o Engines closer to passengers in crashes

o More difficult to reach for maintenance

• Wing Mounted Engines

» Advantages

o Can be placed for optimal bending relief on wing, area ruling and flutter

o Easier to maintain and replaceo Closer to c.g., thus easier to balance and install different engine sizes

» Disadvantages

o Higher drag, both nacelle and interference

o Higher noise, both for passengers and surroundings

o Large range of intake angle of attack results in inlet inefficiencies

o Engines closer to ground (FOD, clearance for smaller aircraft)

Still used for commuterand business jets

Now only design option

for larger commercial jets

• Flow field before and behind wing showing AoA at

intakes of both underwing podded engine and rear-

mounted engine configurations



• Flow field characteristics of underwing installation

• Overwing noise shielding

Commuter & Business Jets



FOD

• Foreign Object Debris» Can occur in flight, but particularly important for engines on TO-L

» Engines close to the tarmac act like a vacuum and suck up objects

on the ground

» Solution: keep engine or engine inlet away from runway

» Some designs vary inlet shape to avoid FOD (e.g. Mig-29)

Engine Location Summary Table

Criterion Wing-Mounted Fuselage-Mounted

Ground clearance Poor, particularly on small aircraft

Good

Cabin noise Fair Good

Fatigue Flaps and wings Fuselage

Crash safety Good Possible problems

Propulsive efficiency Good OK if well positioned

Longitudinal stability(pitch)

Good - delays tip stall Poor - short tail arm and tip stall

Asymmetric thrust Poor - large moment arm

Good

Weight Good - wing bending and torsion relief

Poor - heavy tail,heavy fuselage frame

Maintenance Good Fair

Wing aerodynamics Possible flap and LE interference

Very good

Fuel line location Good Passes through cabin

Anti-icing bleed Good Passes through cabin

FOD Poor Good



Supersonic Transports

• External wing boxes are used to take advantage of theshock at the inlet to increase the inlet compression ratio

Evolution of Engine Placement - Fighters

• Dictated by performance (supersonic speed and stealth)

» 2+ variations that converge to a single configuration (w/ exceptions)

Inlet placement key



• Internal engine placement results in efficientaerodynamics and signature masking, making it the

most obvious choice for fighter engine placement

Saab Viggen JSF (VTOL version)

Special Cases - UAVs



Asymmetry

Rutan Boomerang

VTOL/VSTOL Design Choices

• Which have been tested? Put in service?

Propulsion Pt2

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Transcript of Propulsion Pt2