Aspects of Aircraft Design and Control Olivier Cleynen – March 2013 – v1.3.1

Lecture 6 Propulsion

“NNggnniiiaavvrrooooooaaaaaaarrrrooouuummmmm.....” Jacques Darolles

~ foreword ~ ●

●

The present notes serve as a support for in-class work, not the opposite! Refer to the introductory course notes for explanations. These notes are used as a succinct introduction to selected topics. They are purposefully incomplete and must not be used for real-life applications.

Feedback is always appreciated: olivier.cleynen ariadacapo.net

These course documents can be found at: http://aircraft.ariadacapo.net/

This document is published under a Creative Commons license.

Some photos and illustrations have their author and specific license indicated on the bottom of the page. All other content is © 2011-2013 CC by-sa Olivier Cleynen. You are encouraged to copy, modify, and re-use this content under specific conditions: http://creativecommons.org/licenses/by-sa/3.0/

6.1 Generating thrust: Propulsive efficiency

F net

d = m V  dt

Thrust

Thrust = m˙  V out − V in 

Do 328

CC by-sa W:KS-U92

Do 328

GFDL 1.2 Jeff Gilbert

Do 328 Jet

CC by-sa Vincent Edlinger

What power is required to generate 20 kN of thrust?

Do 328

Do 328 Jet

Stationary Do 328 at full thrust:

Air stream velocity: 30 m/s

Stationary Do 328-Jet at full thrust:

Air stream mass flux: 120 kg/s

Stream tubes at cruise conditions:

Propulsion fundamentals ●

The greater the mass flow, the smaller the required power

●

“Always grab as much air as you can!”

●

Large engines are fundamentally more efficient

Propulsive efficiency

E˙ received by aircraft ηP ≡ E˙ spent on air

low for jet fighters, high for airliners, 100% for road vehicles

Propulsive efficiency for a turbojet:

ηP =

P =

F V aircraft m˙ air Δ e K air F V aircraft 1 2 2 m˙ air V out − V in  2

→ these equations need to be adapted for turbofans

6.2 Engine Thermodynamics ~now augmented with fluid mechanics~

6.2.1 Energy accounting within steady flows

Steady Flow Energy Equation

1 2 m˙ u 1 + p1 ν 1 + C 1 + g z 1 + Q˙ + W˙ 2

(

)

1 2 = m ˙ u2 + p2 ν 2 + C 2 + g z 2 2

(

)

Steady Flow Energy Equation

˙ 1→2 = Q˙ 1 → 2 + W 1 2 m˙ Δ u + Δ ( p ν ) + ΔC + g Δz 2

(

)

Steady Flow Energy Equation

q 1 → 2 + w1 → 2 = Δ u + Δ ( p ν ) + Δ e m

q 1 → 2 + w1 → 2 = Δ h + Δ e m

6.2.2 Stagnation properties

Stagnation enthalpy

1 2 h0 ≡ h + C 2

Enthalpy if fluid brought isentropically to zero velocity

Stagnation values ●

Quantifies energy contained in flow, regardless of its speed → changes in cross-section area have no effect on stagnation values

●

●

Stagnation properties cannot be measured (only calculated) “stagnation” = “total” as opposed to real properties (“static”)

SFEE – introducing stagnation enthalpy

˙ 1→2 = m m˙ ( h0 1 ) + Q˙ 1 → 2 + W ˙ ( h0 2 )

q 1 → 2 + w1 → 2 = Δ h0

Stagnation temperature

1 2 cpT0 ≡ cpT + C 2

[for a perfect gas – not water/steam!]

Stagnation pressure and density

1 2 p 0 ≡ p + ρC 2

0 =

[for a perfect gas]

p0 R T0

6.2.3 Jet engine components

Ideal compressors and turbines

T 01 T 02

=

−1  01

  p

p0 2

w 1 2 = h 0 2 − h 0 1 = c p T 0 2−T 0 1  For isentropic (frictionless, infinitely slow) processes

Ideal inlets and nozzles

h01 = h0 2 T 01 = T 02 p01 = p02 No external work done on the fluid Ideal case : frictionless (isentropic) process

Ideal combustion chambers, afterburners and coolers

p01 = p02 q 1 2 = h 0 2 − h 0 1 = c p T 0 2 −T 0 1 

For isentropic (frictionless) flow

© Rolls-Royce

© Rolls-Royce

CC by-sa W:Rios

6.2.4 Jet engine configurations

The gas generator

Useless alone – only produces pressurized air in D

The turbojet

Converting air pressure into air speed

CC by-sa W: Sanjay Acharya

Turboprop

Air expanded to ambient pressure in D

CC by-sa Vincent Edlinger

The turbofan

CC by-sa K. Aainsqatsi

Twin-spool turbofan

GE 90

© Unknown

CFM56 (A320 ; B737)

CC by-sa W:David Monniaux

CC by-sa Vincent Edlinger

6.4.5 Jet engine performance and main parameters

Thermodynamics tells us that: ●

●

●

The thermal efficiency of an engine strongly depends on its maximum temperature The maximum, theoretical thermal efficiency of an engine is very poor (usually 60%) Friction and rapid compression/expansion further reduce this value.

Thermal efficiency

ηth

E˙ received byair ≡ Q˙ engine

Thermal efficiency of a turbojet

ηth

m˙ air Δ e K air = m˙ CC q CC

→ This equation needs to be adapted for turbofans

The compressor [total] pressure ratio, CPR

CPR =

p0 2 p01

Determines temperature before entry in the combustion chamber → a large CPR increases efficiency Similarly, FPR stands for Fan Pressure Ratio

Turbine Entry Temperature (TET) ●

●

TET is the highest temperature in the engine → crucial for efficiency Tremendous efforts spent to cool the turbine, so as to increase TET...

© Rolls-Royce

The bypass ratio

m˙ cold BPR = m ˙ hot

The greater the BPR, the greater the overall mass flow → leads to increased propulsive efficiency → leads to increased engine weight and diameter

6.3 Efficiency and Cost

Overall efficiency

 total =  P  th

Largest for large BPR, large CPR, high TET engines

Weight

Example: power your 747-400 ●

●

GE CF6 4100 kg, 240 kN RR RB-211 4300 kg, 270 kN, -2% SFC

Weight ●

●

●

Higher compression ratios, larger bypass ratios require heavier machines A 1% decrease in specific fuel consumption will be lost if the weight increases by f × 1% Long-range aircraft most likely to benefit from increased efficiency

Cost

Cost ●

●

●

The investment required to purchase a product will [needs] always be compared against merely saving money in the bank Not all fuel savings are worth striving for (especially by airlines in need of liquidities) GE Financial Services is more profitable than GE engines...

6.4 Installation

6.4.1 Ducting

Duct effect on a free flow (engine-less)

6.4.2 Positioning

Engines need to be easily accessible, far away from the wing, far away from the ground, far away from the cabin. And: aircraft layout choices have long-lasting impact

B-737

B-707

CC by-sa W:Mulag

B737-original

CC by-sa W:PhillipC

B-737 original

CC by-sa W:Bryan&altair78

B737 classic

GFDL 1.2 Konstantin von Wedelstaedt

737 classic

CC by-sa Olivier Cleynen

B-737 classic

PD W:arpingstone

B737 NG

CC by-sa Bill Abbott

B-737 NG

CC by-sa F:Andy_Mitchell_UK

737 NG

CC by F:abdallahh

A320

CC-0 Olivier Cleynen

A-320 family

CC by-sa Vincent Edlinger

747-400 #2 pylon

CC by-sa Vincent Edlinger

747 pylons

PD [Olivier Cleynen]

777 pylon (GE version)

CC by-sa Vincent Edlinger

6.4.3 Accessories

GEnX 2B

CC by-sa Olivier Cleynen

GEnX 2B

CC by-sa Olivier Cleynen

Engine accessories within the nacelle ●

Hydraulic systems

●

Pneumatic circuits (in/out)

●

Lubrication

●

Cooling

●

Mechanical control

●

Monitoring systems...

CC by-sa Vincent Edlinger

Project 6 Design of a military turbofan

F-100

PD Shelley Gill/USAF

Project 6 ●

Inlet mass flow is given by aircraft geometry

●

All engine components are off-the-shelf

●

●

Design a turbofan that is able to produce 70kN thrust with afterburning (wet) Should be as efficient as possible when running dry