UNCLASSIFIED General Aviation Aircraft Propulsion: Power and Energy Requirements • 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 © Lilium GmbH 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 18 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Conclusions (2) 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 19 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED 20 RAeS Light Aircraft Design Conference | 18 Nov 2019 | © QinetiQ UNCLASSIFIED UNCLASSIFIED Photo credits and licensing • 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