General Aviation Aircraft Propulsion: Power and Energy ......UNCLASSIFIED UNCLASSIFIED •Environmental: – Greatly reduced aircraft emissions at the point of use – Reduced use

UNCLASSIFIED

UNCLASSIFIED

General Aviation Aircraft Propulsion:Power and Energy Requirements

RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ

• Tim Watkins

• BEng MRAeS MSFTE

• Instructor and Flight Test Engineer

• QinetiQ – Empire Test Pilots’ School

• Boscombe Down

QINETIQ/EMEA/EO/CP191341

UNCLASSIFIED

UNCLASSIFIED

• Benefits of electrifying GA aircraft propulsion

• A review of the underlying physics

• GA Aircraft power requirements

• A brief look at electrifying different GA aircraft types

• Relationship between battery specific energy and range

• Conclusions

RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ2

Contents

UNCLASSIFIED

UNCLASSIFIED

• Environmental:

– Greatly reduced aircraft emissions at the point of use

– Reduced use of fossil fuels

– Reduced noise

• Cost:

– Electric aircraft are forecast to be much cheaper to operate

– Even with increased acquisition cost (due to batteries), whole-life cost will be reduced dramatically

– Large reduction in light aircraft operating costs (e.g. for pilot training)

– Potential to re-invigorate the GA sector

• Opportunities:

– Makes highly distributed propulsion possible

– Makes hybrid propulsion possible

– Key to new designs for emerging urban air mobility and eVTOL sectors


Benefits of electrifying GA aircraft propulsion

UNCLASSIFIED

UNCLASSIFIED

Energy conversion efficiency

Brushless electric motor and controller:

• Conversion efficiency ~ 95% for motor, ~ 90% for controller

• Variable pitch propeller efficiency ~ 85%

• TOTAL ~ 73%

Propulsion Type, Propeller Type Conversion Efficiency Comparison with Electric

Piston engine, fixed pitch 20% 3.65 x worse

Piston engine, constant speed 25% 2.92 x worse

Small turboprop, constant speed 23% 3.17 x worse

Electric motor, variable pitch 73%

4 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ

UNCLASSIFIED

UNCLASSIFIED

• Power output to mass ratio for:

– Power sources, e.g. batteries, fuel cells (not fuel!)

– Power conversion, e.g. engines, fuel cells, motors

• Installed power, max continuous:

~ 0.8 kW/kg for piston engines (AVGAS)

~ 2.5 kW/kg for small turboprops (AVTUR)

~ 5.0 kW/kg for brushless electric motors


Specific Power and Specific Energy

• A measure of the storage capacity of an energy

source compared to its mass

– Approx 0.1kWh to boil 1 Litre of water in a kettle

from 20°C

• Aviation energy storage:

12.14 kWh/kg for AVGAS

11.94 kWh/kg for AVTUR

0.25 kWh/kg for Li-ion Cells

Specific Power (kW/kg) Specific Energy (kWh/kg)

UNCLASSIFIED

UNCLASSIFIED

• Assumptions for a light aircraft power system:

Battery: 0.25 kWh/kg, 90% cycle efficiency, life of 3000 cycles

Battery purchase cost £230 / kWh, Motor and Controller £300 / kW

Electricity price of £0.144 / kWh (typical UK domestic electricity price)

• Over the battery life, an electric aircraft uses:

35% of the energy of an equivalent AVGAS-fuelled aircraft

32% of the energy of an equivalent AVTUR-fuelled aircraft

• The electric aircraft energy cost (£) is :

23% of that for equivalent AVGAS-fuelled aircraft

50% of that for equivalent AVTUR-fuelled aircraft

• Whole life cost (£ up to but not including battery replacement):

53% of equivalent AVGAS-fuelled aircraft

86% of equivalent AVTUR-fuelled aircraft


Estimated cost of electric propulsion system

Strong environmental and

economic arguments for

electrifying AVGAS GA aircraft!

AVGAS-fuelled aircraft

produce only a very small

proportion of aviation CO2

emissions

eVTOL

Super King Air

Pilatus PC-12Pipistrel Alpha Trainer

Piper PA-28-140

Britten Norman BN-2A Islander

Diamond DA-42


UNCLASSIFIED

UNCLASSIFIED

Range of a propeller aircraft

Long range requires minimum drag

• High L/D ratio

• Occurs at minimum drag speed, Vmd

Estimated L/D polar and power polar

Vmd

Max L/D

Pipistrel Panthera


UNCLASSIFIED

UNCLASSIFIED

Electrifying a GA Aircraft – simplified analysis

Assume no change to:

• MTOW

• Passenger and payload capacity

• Total mass of the propulsion system (including energy storage)

Substitute – weight for weight:

• Batteries for fuel tanks (full fuel)

• Electric motor in place of conventional engine, controller in place of fuel system

• Electric motor is smaller and lighter, so make up difference with more batteries!

This does not maximise the range, but enables “like with like” comparison


UNCLASSIFIED

UNCLASSIFIED

Electrification of GA aircraft – 4 examples


© Lilium GmbH

UNCLASSIFIED

UNCLASSIFIED

Pipistrel Alpha

PARAMETER UNITS VALUE

MTOM kg 550

Engine Shaft Power kW 60

Minimum Cruise Power kW 12.2

Minimum Drag Speed kt EAS 66

Max Lift / Drag ratio - 20

Max Range (AVGAS) nm 324

Max Range if electrified nm 84

Range ratio (fuel : electric) - 3.8


UNCLASSIFIED

UNCLASSIFIED

Cassutt Special


MTOM kg 386

Engine Shaft Power kW 75



Max Lift / Drag ratio - 7.5





UNCLASSIFIED

UNCLASSIFIED

Britten-Norman BN2A Islander


MTOM kg 2994

Total Engine Shaft Power kW 388



Max Lift / Drag ratio - 8.2





UNCLASSIFIED

UNCLASSIFIED

Lilium Jet eVTOL (estimated)


MTOM kg 950

Total Motor Shaft Power kW 320

VTOL Power kW 320



Max Lift / Drag ratio - 21

Max Range (battery) nm 160

Max Range (hybrid system) nm 1335

Range ratio (hybrid : battery) - 7.7


© Lilium GmbH

Current Technology

Pipistrel Alpha Trainer

Lilium Jet

Cassutt Special

Britten-Norman Islander


Pipistrel Panthera

4 x Current Technology



Lilium Jet

Cassutt Special


Pipistrel Panthera

16 x Current Technology



Lilium Jet

Cassutt Special


Pipistrel Panthera

UNCLASSIFIED

UNCLASSIFIED

Compared to aviation fuel burning engines, electric motors are:

• Up to 3.65 times more efficient

• 50 – 80% lighter installed weight (same continuous power rating)

Current Li-ion batteries have only 1/48th of the specific energy of fuel:

• Partly compensated by efficiency and light weight of electric motors

• Range reduction factor of 3 – 11 for small GA aircraft (up to 2000kg MTOM)

• Range reduction factor of 9 – 16 for larger GA aircraft (Part 23 Normal Category, up to 5670 kg MTOM)

Battery-only electric propulsion is feasible for small GA aircraft applications:

• Where power requirements are low (small payload, low cruise speed, high L/D ratio)

• Where high power is required but only for a short time (e.g. air racing)

• Where reduced range is not a problem (e.g. basic flight training, short range air mobility)

• eVTOL hover and transition flight phases, and cruise if only very short range is required

Conclusions (1)


UNCLASSIFIED

UNCLASSIFIED

Battery-only electric propulsion is well-suited to:

• Initial pilot training, with significantly reduced cost

• Small GA aircraft (e.g. SSDR, Microlights, CS-VLA)

• Motor gliders, gliders with sustainers

• Air racing, aerobatics (high power, short duration)

• Short-range air-taxis / urban air mobility / eVTOL

To be feasible for larger GA aircraft or longer range, we need:

• Large batteries, as a percentage of airframe empty weight (i.e. reduced payload)

• A step change in battery specific energy, or…

• Hybrid propulsion for the cruise phase (e.g. turboshaft + generator, or fuel cells)

In a hybrid system, batteries are still essential for take-off, climb and go-around

Conclusions (2)


UNCLASSIFIED

UNCLASSIFIEDRAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ20

UNCLASSIFIED

UNCLASSIFIED

• Pipistrel Panthera:– https://commons.wikimedia.org/wiki/File:Pipistrel_Panthera_aircraft.JPG– Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License

• Pipistrel Alpha:– https://commons.wikimedia.org/wiki/File:F-WLAB_Pipistrel_Alpha_Electro_3_(cropped).jpg– Licensed under the Creative Commons Attribution-Share Alike 4.0 International License

• Britten-Normal Islander:– https://commons.wikimedia.org/wiki/File:Anguilla_Air_Services_Britten-Norman_Islander_(VP-

AAC)_at_St_Marteen_(SXM-TNCM).jpg– Licensed under the Creative Commons Attribution-Share Alike 2.0 Generic License

• Cassutt Special:– https://commons.wikimedia.org/wiki/File:Reno_Formula1_Cassutt_8479.jpg– Licensed under the Creative Commons Attribution 3.0 Unported License

• Lilium Jet:– https://lilium.com/newsroom– Images are free to use but should be credited to Lilium and only used in their original form

Photo credits and licensing


https://commons.wikimedia.org/wiki/File:Pipistrel_Panthera_aircraft.JPG

https://commons.wikimedia.org/wiki/File:F-WLAB_Pipistrel_Alpha_Electro_3_(cropped).jpg

https://commons.wikimedia.org/wiki/File:Anguilla_Air_Services_Britten-Norman_Islander_(VP-AAC)_at_St_Marteen_(SXM-TNCM).jpg

https://commons.wikimedia.org/wiki/File:Reno_Formula1_Cassutt_8479.jpg

https://lilium.com/newsroom


GA aircraft analysed for electric propulsion

Aircraft Type MTOM Climb Power Min Cruise

(kg) (kW) Power (kW)

Mini-Max 1650R 318 37 9.6

AMF Chevvron 2-32 382 18 6.3

Cassutt Special (Racer) 386 75 30.6

Pipistrel Alpha 550 60 12.2

Tecnam P2002 600 75 23.2

Tecnam P2008 600 74 23.5

Pipistrel Virus 600 75 15.5

Europa XS 623 74 27.6

Cessna 152 757 82 39.7

Jabiru J430 760 90 30.9

Lilium Jet (eVTOL) 950 320 21.9

Piper PA-28-140 975 112 56.6

Slingsby T-67 M260 1157 194 54.2

Tecnam P2010 1160 130 50.6

Aircraft Type MTOM Climb Power Min Cruise

(kg) (kW) Power (kW)

Tecnam 2006T 1230 150 48.0

Pipistrel Panthera 1315 194 43.4

Cessna 350 1542 230 67.7

Cirrus SR-22 1633 230 65.7

Piper PA-32R 1633 225 87.5

Diamond DA42 1700 250 68.2

Piper PA-46 M350 1969 260 75.3

Piper PA34-220T 2155 328 108.4

Pilatus PC-6 2800 410 125.7

BN-2A Islander 2994 388 168.3

Tecnam 2012 3660 560 213.0

BN Defender 3856 600 214.0

Pilatus PC-12 4740 890 224.0

King Air 250 5670 1250 283.9

Average Pmin / PClimb = 32%

Min Cruise Power is estimated


GA aircraft analysed for electric propulsion

Aircraft Type MTOM Range Ratio Battery Cost

(kg) (Fuel:Electric) (£k) *

Mini-Max 1650R 318 3.2 3.4

AMF Chevvron 2-32 382 5.1 2.5

Cassutt Special (Racer) 386 6.3 8.5

Pipistrel Alpha 550 3.8 5.9

Tecnam P2002 600 9.0 10.5

Tecnam P2008 600 8.5 9.9

Pipistrel Virus 600 7.7 9.5

Europa XS 623 10 10.7

Cessna 152 757 7.3 14.5

Jabiru J430 760 9.2 11.3

Lilium Jet 950 7.7 18.1

Piper PA-28-140 975 9.5 15.2

Slingsby T-67 M260 1157 6.0 19.3

Tecnam P2010 1160 9.5 16.9

Aircraft Type MTOM Range Ratio Battery Cost

(kg) (Fuel:Electric) (£k) *

Tecnam 2006T 1230 8.9 20.8

Pipistrel Panthera 1315 6.6 23.7

Cessna 350 1542 11.4 32.1

Cirrus SR-22 1633 10.2 28.7

Piper PA-32R 1633 10.3 30.8

Diamond DA42 1700 7.0 42.1

Piper PA-46 M350 1969 11.3 37.1

Piper PA34-220T 2155 9.1 41.7

Pilatus PC-6 2800 15.0 32.2

BN-2A Islander 2994 9.3 45.9

Tecnam 2012 3660 10.1 70.8

BN Defender 3856 15.8 61.2

Pilatus PC-12 4740 16.7 83.8

King Air 250 5670 15.2 108.3

* Assumes £230 / kWh

UNCLASSIFIED

UNCLASSIFIED

Energy conversion efficiency

Transformation from input to output

• e.g. from fuel to thrust from the propeller… or from battery to thrust from the propeller

Piston Engine (4-stroke, air cooled):

• Conversion efficiency ~ 30%

• Propeller efficiency ~ 70% (fixed pitch) to 85% (constant speed)

• TOTAL ~ 20% to 25%

Small turboprop engine (e.g. PT-6):

• Conversion efficiency ~ 27%

• Propeller efficiency ~ 85%

• TOTAL ~ 23%


UNCLASSIFIED

UNCLASSIFIED

Breguet Range Equation – fuel burning aircraft

𝑴𝒂𝒙 𝑹𝒂𝒏𝒈𝒆 =𝜼𝒑𝒓𝒐𝒑

𝒈 𝑺𝑭𝑪

𝑳

𝑫 𝒎𝒂𝒙𝒍𝒏

𝑴𝒂𝒔𝒔 𝒂𝒕 𝒕𝒂𝒌𝒆 𝒐𝒇𝒇

𝑴𝒂𝒔𝒔 𝒂𝒕 𝒕𝒂𝒌𝒆𝒐𝒇𝒇 −𝑴𝒂𝒔𝒔 𝒐𝒇 𝒇𝒖𝒆𝒍 𝒃𝒖𝒓𝒏𝒕

ηprop = propeller efficiency

SFC = Specific fuel consumption in kg/s.kW

(L/D)max = maximum lift to drag ratio

g = gravitational constant (9.807 m/s2)

Aircraft that burn fuel get lighter as they do so:

• Induced drag is proportional to weight squared

• Reduced weight = significantly less induced drag

• Allows same drag at higher speed, or same L/D at lower power setting = more range

The bigger the fuel tank, the more important this becomes!25 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ

UNCLASSIFIED

UNCLASSIFIED

Range equation – electric aircraft

𝑴𝒂𝒙 𝑹𝒂𝒏𝒈𝒆 =𝜼 × 𝑽𝒎𝒅 × 𝑬𝒃𝒂𝒕𝒕

𝑷𝒎𝒅=

𝜼 × 𝑬𝒃𝒂𝒕𝒕𝑫𝒎𝒊𝒏

=

𝜼 ×𝑳𝑫 𝒎𝒂𝒙

× 𝑬𝒃𝒂𝒕𝒕

𝑾𝒆𝒊𝒈𝒉𝒕

η = power train efficiency (controller, motor, propeller, losses)

Ebatt = Battery: available stored energy in Joules (J)

Pmd = thrust power at minimum drag ( = Dmin x Vmd)

Dmin = minimum drag

Vmd = Minimum drag speed (True Air Speed)

Battery mass remains constant:

• Therefore drag does not reduce over time

• A small penalty for small aircraft but a major problem for large aircraft


UNCLASSIFIED

UNCLASSIFIED

The QinetiQ Air and Space Division, located primarily at MoD Boscombe Down, are experts in aircraft and related

systems modification, installation, safety, airworthiness, certification, evaluation and flight test. They are approved

to provide these services under EASA Part 21 and the UK MoD Maintenance Approved Organisation Scheme.

The QinetiQ Flight Physics Group are able to provide expertise, consultancy, simulation and flight test capabilities

regarding aircraft handling qualities and performance, including new or modified aircraft that have electrical or

hybrid propulsion systems.

Further information on our aerospace capabilities can be found at https://www.qinetiq.com/What-we-do/Air

Should you require further information regarding this presentation, or wish to make a business enquiry, please

visit the QinetiQ contact page at https://www.qinetiq.com/Contact

The presentation author may be contacted via the MoD Boscombe Down Switchboard on +44 (0)1980 664000


Further Information

https://www.qinetiq.com/What-we-do/Air

https://www.qinetiq.com/Contact

UNCLASSIFIED

UNCLASSIFIEDRAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ29

Documents

General Aviation Aircraft Propulsion: Power and Energy ......UNCLASSIFIED UNCLASSIFIED •Environmental: – Greatly reduced aircraft emissions at the point of use – Reduced use