Introduction to propulsion systemsIntegrated project: AIAA aircraft design competition1er master Aerospace 2019

Koen HillewaertKoen.hillewaert@uliege.be

Remy Princivalleremy.princivalle@safrangroup.com

Propulsion● Weight → lift

● Lift → drag

● Drag → thrust T

● Propulsive power

Propulsion: action = reaction● Rocket engines: ejection of high speed gas jet

● Acceleration of a stream tube

– Lift based / mechanical acceleration : propeller

– Thermal acceleration : turbojet

– Combination : turbofan

Rocket engine● Nozzle converts heat into speed

● Starting from high pressure oxydant & combustible

Thermodynamics – Brayton cycle● Thermodynamic cycle between Pt1 and Pt2

– Compression 1 → 2 : Wc

– Combustion 2 → 3 : Q

– Expansion 3-4 : Wt

● Energy balance

– Useful work : W= Wt - Wc

– Energy input : Q

– Thermal efficiency = W/Q

Turbojet - M ~ 1 … 3● Gas turbine: hot gas jet

– Recup/RAM inlet velocity : 1’ - 1

– Compressor : 1 - 2

– Combustion : 2 - 3

– Turbine -> compressor : 3 - 4

– Exhaust jet by adjustable nozzle : 4 – 5

● (Thrust vectoring)

Turbojet/fan – airbreathing jet M ~ 1 … 3● Gas turbine: hot gas jet

– Ram inlet velocity : 1’ - 1

– Compressor : 1 - 2

– Combustion : 2 - 3

– Turbine -> compressor : 3 - 4

– Afterburning : 4 – 4’

– Exhaust by adjustable nozzle : 4’ – 5

● (Thrust vectoring)

Turbofan – M = 0 … 1

● Bypass flow mb: acceleration by fan

● Core flow mc : power + hot gas jet

– Fan + compressor : 1 - 2

– Combustion : 2 - 3

– Turbine → fan & compressor 3 - 4

– Exhaust jet : 4 – 5

● Bypass ratio: BPR = mb/(mb+mc)

RAM/Scram: airbreathing M > 3● Cycle without machines

– Convert kinetic energy va in pressure Pt by “RAM” effect : 1 - 2

– Combustion : 2-3

● RAMJET : subsonic combustion

● Supersonic Combustion RAMJET

– Expand in nozzle to form jet : 3 – 4

Combined turbojet & ramjet

Propulsion approach 1 : Propeller● Mechanical acceleration of external flow

Propeller global operation (Rankine-Froude)

● Stream tube through propeller

● Propeller = actuator disk

● No losses up/down - Bernoulli

Propeller global operation (Rankine-Froude)

● Induction factor a

● Thrust computed two ways

– stream tube control volume

– pressure difference

Propeller global operation (Rankine-Froude)

● Thrust

● Power

● Propulsive efficiency

Lift and drag forces on an airfoil

wβ

wβ

L

L

D

D

Propeller: shape and operating parameters● Geometry

– Tip radius Rt

– (Radial distribution of) blade profile

– Radial distribution of chord C(r) / Solidity σ(r) = N C(r)/2 r) = N C(r) = N C(r)/2 r)/2

● Operating parameters

– Stagger or pitch angle ξ

– Rotation speed Ω → local blade speed u = Ω r

– Advance ratio J = va/ut = va/Ω rt

ξ

u

v

Propeller forces (Blade Element Method)● Induction factor a → v(1+a)

● Relative velocity w, flow angle β

● Lift/drag forces L & D

● Compute thrust T and tangential force U

● Recompute induction factor a from T

ξ

u

v(1+a)

w

L

DT

v

β

U

Propeller performance parameters● Thrust coefficient

● Power coefficient

Propeller: force balance

ξ

u

v(1+a)

w

L

DT

β

C

Propeller: variation of advance ratio

ξ

u

v(1+a)

w

L

DT

β

C

Propeller: pitch control

ξ

u

v(1+a)

w

L

DT

β

C

Propeller: use of pitch control

Choosing the propulsion system● From cruise speed

– Propeller : 0.1 < Ma < 0.7

– High BPR turbofan : 0.7 < Ma < 1

– Low BPR turbofan / turbojet : Ma > 1

– RAMJET : Ma > 2

– SCRAMJET : Ma > 5

The type of propulsion system

■How to choose between propeller, turbofan, turbojet or even ramjet?> From your cruise speed !> 0.1 < Mn < 0.7 => propeller> 0.7 < Mn < 1.0 => turbofan

The higher cruise speed, the lower bypass ratio> Mn > 1.0 => turbojet

> Mn > 2 = RAMjet or SCRAMjet> RAMjet uses aircraft speed recovery for compression

P = Ps0 + ½ rho V²

needs to be associated with jet engine for reaching the right speed> SCRAMjet is RAMjet with still supersonic combustion

Classification

■ Propeller> The best efficiency> Big size> Limited cruise speed

■ Open rotor> Very efficient> High cruise speed> Big size> Very noisy> Complexity

■Turbofan> Good efficiency> High cruise speed> Mid size> Low noise> Shroud additionnal weight

■Turbojet / nozzle> Small size & weight> Simplicity> The highest speeds> Poor efficiency