General Aviation Aircraft Propulsion: Power and Energy Requirements

UNCLASSIFIED

• Tim Watkins • BEng MRAeS MSFTE

• Instructor and Flight Test Engineer • QinetiQ – Empire Test Pilots’ School • Boscombe Down

QINETIQ/EMEA/EO/CP191341 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Contents

• 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

2 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Benefits of electrifying GA aircraft propulsion

• 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

3 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ 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 Specific Power and Specific Energy

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

• Power output to mass ratio for: • A measure of the storage capacity of an energy – Power sources, e.g. batteries, fuel cells (not fuel!) source compared to its mass – Power conversion, e.g. engines, fuel cells, motors – Approx 0.1kWh to boil 1 Litre of water in a kettle from 20°C

• Installed power, max continuous: • Aviation energy storage: ~ 0.8 kW/kg for piston engines (AVGAS) 12.14 kWh/kg for AVGAS ~ 2.5 kW/kg for small turboprops (AVTUR) 11.94 kWh/kg for AVTUR ~ 5.0 kW/kg for brushless electric motors 0.25 kWh/kg for Li-ion Cells

5 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Estimated cost of electric propulsion system

• 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 Strong environmental and economic arguments for electrifying AVGAS GA aircraft! • The electric aircraft energy cost (£) is : 23% of that for equivalent AVGAS-fuelled aircraft 50% of that for equivalent AVTUR-fuelled aircraft AVGAS-fuelled aircraft produce only a very small • Whole life cost (£ up to but not including battery replacement): proportion of aviation CO2 53% of equivalent AVGAS-fuelled aircraft emissions 86% of equivalent AVTUR-fuelled aircraft

6 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED Super King Air

Pipistrel Alpha Trainer Pilatus PC-12

Diamond DA-42

eVTOL Britten Norman BN-2A Islander

Piper PA-28-140

7 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED Range of a propeller aircraft Long range requires minimum drag Max L/D • High L/D ratio • Occurs at minimum drag speed, Vmd

Vmd

Pipistrel Panthera Estimated L/D polar and power polar

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

9 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Electrification of GA aircraft – 4 examples

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

11 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Cassutt Special

PARAMETER UNITS VALUE

MTOM kg 386

Engine Shaft Power kW 75

Minimum Cruise Power kW 30.6

Minimum Drag Speed kt EAS 82

Max Lift / Drag ratio - 7.5

Max Range (AVGAS) nm 391

Max Range if electrified nm 63

Range ratio (fuel : electric) - 6.3

12 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Britten-Norman BN2A Islander

PARAMETER UNITS VALUE

MTOM kg 2994

Total Engine Shaft Power kW 388

Minimum Cruise Power kW 168.3

Minimum Drag Speed kt EAS 78

Max Lift / Drag ratio - 8.2

Max Range (AVGAS) nm 539

Max Range if electrified nm 58

Range ratio (fuel : electric) - 9.3

13 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Lilium Jet eVTOL (estimated)

PARAMETER UNITS VALUE © Lilium GmbH

MTOM kg 950

Total Motor Shaft Power kW 320

VTOL Power kW 320

Minimum Cruise Power kW 22.2

Minimum Drag Speed kt EAS 66

Max Lift / Drag ratio - 21

Max Range (battery) nm 160

Max Range (hybrid system) nm 1335

Range ratio (hybrid : battery) - 7.7

14 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED Current Technology

Britten-Norman Islander Lilium Jet Pipistrel Panthera Cassutt Special

Pipistrel Alpha Trainer

15 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ 4 x Current Technology

Britten-Norman Islander

Pipistrel Panthera Cassutt Special

Pipistrel Alpha Trainer Lilium Jet

16 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ 16 x Current Technology

Britten-Norman Islander

Pipistrel Panthera

Pipistrel Alpha Trainer Cassutt Special

Lilium Jet

17 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED Conclusions (1)

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

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

• 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

21 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED GA aircraft analysed for electric propulsion Average Pmin / PClimb = 32%

Aircraft Type MTOM Climb Power Min Cruise Aircraft Type MTOM Climb Power Min Cruise (kg) (kW) Power (kW) (kg) (kW) Power (kW) Mini-Max 1650R 318 37 9.6 Tecnam 2006T 1230 150 48.0 AMF Chevvron 2-32 382 18 6.3 Pipistrel Panthera 1315 194 43.4 Cassutt Special (Racer) 386 75 30.6 Cessna 350 1542 230 67.7 Pipistrel Alpha 550 60 12.2 Cirrus SR-22 1633 230 65.7 Tecnam P2002 600 75 23.2 Piper PA-32R 1633 225 87.5 Tecnam P2008 600 74 23.5 Diamond DA42 1700 250 68.2 Pipistrel Virus 600 75 15.5 Piper PA-46 M350 1969 260 75.3 Europa XS 623 74 27.6 Piper PA34-220T 2155 328 108.4 Cessna 152 757 82 39.7 Pilatus PC-6 2800 410 125.7 Jabiru J430 760 90 30.9 BN-2A Islander 2994 388 168.3 Lilium Jet (eVTOL) 950 320 21.9 Tecnam 2012 3660 560 213.0 Piper PA-28-140 975 112 56.6 BN Defender 3856 600 214.0 Slingsby T-67 M260 1157 194 54.2 Pilatus PC-12 4740 890 224.0 Tecnam P2010 1160 130 50.6 King Air 250 5670 1250 283.9

22 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ Min Cruise Power is estimated GA aircraft analysed for electric propulsion * Assumes £230 / kWh

Aircraft Type MTOM Range Ratio Battery Cost Aircraft Type MTOM Range Ratio Battery Cost (kg) (Fuel:Electric) (£k) * (kg) (Fuel:Electric) (£k) * Mini-Max 1650R 318 3.2 3.4 Tecnam 2006T 1230 8.9 20.8 AMF Chevvron 2-32 382 5.1 2.5 Pipistrel Panthera 1315 6.6 23.7 Cassutt Special (Racer) 386 6.3 8.5 Cessna 350 1542 11.4 32.1 Pipistrel Alpha 550 3.8 5.9 Cirrus SR-22 1633 10.2 28.7 Tecnam P2002 600 9.0 10.5 Piper PA-32R 1633 10.3 30.8 Tecnam P2008 600 8.5 9.9 Diamond DA42 1700 7.0 42.1 Pipistrel Virus 600 7.7 9.5 Piper PA-46 M350 1969 11.3 37.1 Europa XS 623 10 10.7 Piper PA34-220T 2155 9.1 41.7 Cessna 152 757 7.3 14.5 Pilatus PC-6 2800 15.0 32.2 Jabiru J430 760 9.2 11.3 BN-2A Islander 2994 9.3 45.9 Lilium Jet 950 7.7 18.1 Tecnam 2012 3660 10.1 70.8 Piper PA-28-140 975 9.5 15.2 BN Defender 3856 15.8 61.2 Slingsby T-67 M260 1157 6.0 19.3 Pilatus PC-12 4740 16.7 83.8 Tecnam P2010 1160 9.5 16.9 King Air 250 5670 15.2 108.3

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

24 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Breguet Range Equation – fuel burning aircraft

휼 푳 푴풂풔풔 풂풕 풕풂풌풆 풐풇풇 푴풂풙 푹풂풏품풆 = 풑풓풐풑 풍풏 품 푺푭푪 푫 풎풂풙 푴풂풔풔 풂풕 풕풂풌풆풐풇풇 − 푴풂풔풔 풐풇 풇풖풆풍 풃풖풓풏풕

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

푳 휼 × × 푬 휼 × 푽 × 푬 휼 × 푬 푫 풃풂풕풕 푴풂풙 푹풂풏품풆 = 풎풅 풃풂풕풕 = 풃풂풕풕 = 풎풂풙 푷풎풅 푫풎풊풏 푾풆풊품풉풕

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

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.

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