INTRODUCTION TO PROPULSION AIRCRAFT DESIGN EXERCISEPropulsion evolution 1. the piston engine...

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INTRODUCTION TO PROPULSION

AIRCRAFT DESIGN EXERCISE

20/09/2019

Remy Princivalle

[email protected]

Koen Hillewaert

[email protected]

Safran name of the activity / Date / Department

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0. INTRODUCTION & HISTORY

1. PROPULSION SYSTEMS

2. POWER GENERATORS

3. PROPULSION SYSTEM PRE-DESIGN

Agenda

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Introduction to propulsion, projet intégré, 1er master Aérospatiale, ULiège2


INTRODUCTION & HISTORY

Introduction to propulsion, projet intégré, 1er master

Aérospatiale, ULiège3


■Propulsion system provides the needed translation force from a dedicated power source> Several energy available (combustible -oil, gas, wood-, electrical, chemical, animal…)

> Several way to consume energy into power (muscle, piston, turbine…)

> Several way to generate the translation force (propeller, nozzle, wing…)

■At the end of this lecture, you should know> The several types of existing propulsion systems (or how to invent a new one?)

> How to choose one or one other

> The key parameters to define the right dimensions of your propulsion system

What is propulsion ?

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Introduction to propulsion, aircraft design exercise, 1er master aérospatiale 2019, ULiège4


■Before humans> Birds and insect include propulsion and lift via the motion of the wings

■Man tried to copy them> Thank you Leonardo, but it was not really efficient…

A bit of history : at the beginning was …

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Energy : nutrition

Power : muscle

Force : wing motion


■During 18th, Montgolfier brothers got an easier idea to fly> Energy is used to heat the air inside the balloon

■But no real way of propulsion included

■Finally, requires a lot of energy to fly

■But no energy dedicated to movement

Getting around propulsion difficulties

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Energy : hope

Power : none

Force : wind


■Mid 19th, first dirigibles appeared> Henri Giffard used steamed powered propeller

■And propeller and planes were associated late 18th / early 19th

> Clément Ader and the “Avion III”

> The Wright brothers and the “Flyer”

■Finally, human found the way to associate propulsion and lift> First helicopter in early 20th

Then propulsion appeared

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Energy : combustible

Power : piston

Force : propeller

Energy : combustible

Power : piston

Force : rotating wing


Propulsion evolution

1. the piston engine

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Steam engine

early 1900

V engines

WW1 & 2

Star fixed & rotating engines

WW1 & 2

Wankel engines

> 1950Challenge was

- Increasing power density

- lowering vibrations due to

engine

Associated with

propeller


How do different piston engine types work?

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In-line pistons => Vibrations

V pistons => 1st step smoothing Star pistons : mostly smoothed

try to imagine the center fixed and pistons turning with propeller

Star pistons but no smoothed

vibrations No axial piston : totally smoothed

But what about leakages ?


Propulsion evolution

2. the gas turbine engine

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Turbojet

Afterburn jet engine

Turbofan

Turboprop

Challenge was

- High speed, high temperature materials

- Component efficiency

Associated with

propeller, fan or single

nozzle

High power

density


The last present and the future for planes : the electric engine

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e-fan

Elektra prop Solar impulse

fierfly

Energy : electricity

Power : electric engine

Force : prop or fan


The forgotten past : the nuclear airplane

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Energy : atomic

Power : nuclear engine

Force : nozzle


And don’t forget space & rockets !

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Solid propulsion

Liquid propulsion

Cryogenic propulsion

Plasma propulsion

(satellites)

Needs high thrust to weight

ratio for vertical acceleration

Associated with nozzles

This document and the information therein are the property of Safran. They must not be copied or communicated to a third party without the prior written authorization of SafranMonth/day/year


PROPULSION SYSTEMS

1



POWER GENERATORS

2


■Widely used from 1W to 100kW

> But have a look at the Tesla engine (~500kW)

> Or the Airbus light aircraft demonstrator

■More and more powerfull in the future for a lighter weight

ELECTRICAL ENGINE

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2kW/kg

90% efficiency


■Operation :> Magnet rotor induced in rotation through changing phase stators

ELECTRICAL ENGINE OPERATION

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■Widely used from 5kW (mower) to 500kW

> Can easily rise up to 800kW … and even more but with heavy mass cost

> Can be turbocharged for higher power density & efficiency

Enables constant power with altitude thanks to adapted compression level

PISTON ENGINE

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0.5kW/kg

~50% efficiency


■Piston engine thermodynamic cycle

■Combustion is close to stoechiometric> ~14 fuel/air ratio

■Air mass flow proportionnal> to rotation speed divided by 2 (4 steps engines)

> to atmosphere density, as constant volume

■=> engine power proportional to speed untill mechanical losses increase at very high speed

■=> engine power mainly proportional to air density> But some constant losses lead to the following approximated equation

PW ~ PW0 . (1.132 rho/rho0 - 0.132)

PW0 = power at sea level

Rho0 = air density at sea level:

PISTON ENGINE OPERATION

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■ Increase of piston air inlet density by a compressor> Increase the piston engine power

> Increase the diesel thermodynamic cycle efficiency ~ OPR

■But needs power to drive the compressor> Directly driven on the engine power shaft => supercharger

Compressor speed proportional to engine rotation speed

End shaft power lower than piston power

> Driven by an independent electric engine => e-charger

Help-yourself compressor speed

> Driven by an independent turbine on the exhaust gas

Compressor speed not linked to engine speed, but not à la carte

Turbine power extracted from piston power when exhausting (1->I cycle step)

TURBO- (or SUPER-) CHARGED piston engine

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■The gas generator in 3 parts engine (Brayton cycle) :> One compressor that increases thermal efficiency

> One constant pressure combustion chamber

By opposition to constant volume combustion of a piston

> One turbine that take a part of the exhaust gas energy to drive the compressor

■The gas generator is associated> To a turbine that drives power to propeller or fan

Can be a free low pressure turbine

Or the same turbine than the compressor one, with extra-power going to the power shaft

> To a nozzle for straight propulsion

THE GAS GENERATOR

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10kW/kg

40% to 50%

efficiency


■The higher pressure combustion enables higher extra-power at

turbine outlet

■This is constant continuous air admission> By opposition to piston, that is 1 step admission per round

> => High power density / small size

GAS GENERATOR ADVANTAGES

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GAS GENERATOR DISADVANTAGES

■Combustion temperature limited by turbine materials capability> 3% to 7% (cooled turbines) fuel/air ratio, ie half to quarter of piston engine

■Starting and Idle is touchy

■High rotation speeds needed> Cutting edge technologies

> Gearbox necessary for driving propeller for example

■High altitude impact> Power proportional to inlet pressure and squared temperature



PROPULSION SYSTEM PRE-DESIGN

3


Turboprop pre-design

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Introduction to propulsion, Projet Intégré 1er master Aérospatiale24

■1. Sizing the propeller

2 formulas for Propeller choice :> J = V0/(N.D) – speed coefficient

V0 = airplane speed (m/s)

N = propeller rotation speed (rev/s)

D = diameter (m)

> Ct = F/(rho.N².D4)

F = propeller traction (N)

Rho = air density (kg/m³)

N = propeller rotation speed (rev/s)

D = diameter (m)

2 specifications :> Thrust

> Airplane speed

at key points, as cruise & take-off



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■ 2. Find some propeller maps

Ct, Efficiency (h) characteristics function of J factor and propeller pitch



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■3. Specification

As example (pure imagination), let’s take> Cruise is 1kN traction at 100m/s (~200knts) @ 4500m, ISA

> Take-off is 5kN traction at 70m/s (~140knts) @ 0m, ISA

■4. Propeller diameter first guess

Choose propeller diameter

>D = V0/(N.J)

=> for a typical 2000RPM propeller, Diameter is here 3/J meters

>Let’s start with a 2m propeller, J=1.5

>Best efficiency is for 35° pitch,and then Ct = 0.06



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■5. Propeller cruise pitch

Get necessary traction coefficient at cruise

for choosen J ratio

>Here, J = 1.5 => D = 2m

>Ct = F/(rho.N².D4) = 0.072

F = 1kN, D = 2m, N = 2000/60 rev/s

Rho = P0/(Rair.T0) = 0.78kg/m³○ Rair = 287.05

○ P0 = 57.7kPa @ 4500m, ISA

○ T0 = 258.5K @4500m, ISA

>Pitch ~36°

■6. Control propeller efficiency

From Ct & J factors,

look for propeller efficiency for the dedicated pitch value

>Here, h ~ hmax

>J = 1.5 was a good choice for cruise operating point



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■7. Propeller take-off pitch

Get necessary traction coefficient at take-off

>J = V/(N.D) = 1.1

V = 70m/s, D = 2m, N = 2000/60 rev/s

>Ct = F/(rho.N².D4) = 0.230

F = 5kN

Rho = P0/(Rair.T0) = 1.23kg/m³○ Rair = 287.05

○ P0 = 101.3kPa @ 0m, ISA

○ T0 = 288.2K @ 0m, ISA

> ! Ct > Ct max (~0.18) !

Speed or diameter is too low



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■8. Adapt take-off rotation speed

A safer traction coefficient would be Ct = 0.15

>N² = F/(rho.Ct.D4) = 1693 rev/s

>=> N ~ 2470 RPM

We can increase take-off propeller speed by 25% vs. cruise

to reach the traction specification

>J factor value is now 0.84, & pitch is 32°

But efficiency is poor (~0.65)

To reach better efficiency at take-off, we need

>To decrease the pitch

>Or to increase the J factor



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■9. Iterate on propeller sizing

To decrease take-off pitch, we need to decrease J factor

> Increase propeller diameter

Proportional to 1/J

> In the same time, it will decrease the needed Ct

Proportionnal to J4

Change cruise J factor from 1.5 to 1.4

(diameter reaches 2.1m)

>Cuise Ct coefficient changes from 0.72 to 0.055

>Take-off J factor moves from 0.84 to 0.78

>And associated Ct coeff. from 0.15 to 0.11

Resulting cruise pitch is 32° with still >.8 efficiency

And take-off pitch is now 25° only

and efficiency reaches 0.75



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■10. Define engine power needed

Look for power coefficient for both cruise and take-off

Read the diagram using known J & pitch values from 9.

>Cruise : J = 1.4 & pitch = 32°

Cp value is ~0.09

>Take-off : J = 0.78 & pitch = 25°

Cp value is ~0.11

Then you get the propeller power thanks to the formula

PW = Cp.rho.N³.D6

Cruise power = 55kW

Take-off power = 206kW



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■



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■12. Last, find the engine !

206kW engine (276 horse power) is needed by the propeller

>Don’t forget to consider ~10% installation losses on the engine

>Looking for 300shp engine

For such power, solution will probably be turbine engine

But some piston engine would still be available

Seems one solution exists



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■13. And very last, find the propeller !

Available manufacturers are depending the size & power

>For such power, solution could be found by MT-Propellers or Hartzell for example

>You can check your pre-design

2700RPM @ 350 HP○ We got 2500 for 300 HP

○ Not so bad

78”” diameter ~1.98m○ We got 2.1m

○ Not so bad, still

Here is a possible solution


Engine installation

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■Turboprops are mainly installed> On the nose for single engines

> Under the wings for twin or 4 engines

INTRODUCTION TO PROPULSION AIRCRAFT DESIGN EXERCISEPropulsion evolution 1. the piston engine...

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