Feasibility Analysis:
Electric Aircraft Using Regenerative
Soaring and Solar Power
Abstract: Electric Aircraft using no Fuel. Propeller powered electric aircraft takes off on batteries and
actively searches for updrafts. After encountering an updraft the aircraft switches of the propulsion
electric motor and soars. The propeller works as a turbine and the electric motor works as a generator,
producing electric energy to recharge the batteries. Energy gain is improved using solar power. The
proposed aircraft can stay aloft for long periods of time and land with full batteries.
Table of Contents
1 Feasibility Analysis...............................................................................................................................2
1.1 The Aircraft...................................................................................................................................2
1.2 Energy Balance..............................................................................................................................4
1.2.1 Self Launch and Climb to 300m............................................................................................6
1.2.2 Cruise for 20 minutes in search for updrafts..........................................................................6
1.2.3 Recharge batteries to full capacity ........................................................................................7
1.2.4 Cruise till 50% of the battery capacity remains.....................................................................7
1.2.5 Recharge batteries to full capacity and land..........................................................................8
2 Conclusions...........................................................................................................................................9
3 Acknowledgments...............................................................................................................................10
4 Sources:...............................................................................................................................................11
1
1 Feasibility Analysis
1.1 The Aircraft
We have chosen SWIFT rigid wing hang glider produced by Belgium based firm Aériane as an
candidate aircraft [2]. It was designed by Bright Star Gliders in collaboration with engineers at
Stanford University The SWIFT is a high performance sailplane, designed to combine some of the
convenience of hang gliders
with the soaring performance
of sailplanes. It takes off and
lands like a hang glider, yet
maintains
exceptional
performance at high speeds,
achieving a lift-to-drag ratio
of about 25:1. Although it is
a fully-cantilevered rigid
wing with aerodynamic
controls and flaps, it weighs
only 48 kg and is easily
transported on the top of a
car. It is sold in many
countries. Aériane produces
also engine kit (with Illustration 1: SWIFT rigid wing hang glider
combustion engine) which is
easy to adapt to a standard SWIFT frame (with steerable front wheel, disk brake, wheeled tiplets for
taxing). The optional equipment of SWIFT includes rocket parachute and car roof transportation
container. Except of the pilot and his equipment the payload of the aircraft will consist of lithiumpolymer battery pack (5.6kWh [1]), brushless electric motor (13.5kW [1]), solar array, electric
equipment (instruments, radio
transmission system etc.) and
other
equipment
used
for
adaptation of the aircraft. Table 1
summarizes the specifications of
the SWIFT with the proposed
adaptations.
The
propulsion
system is of the Electraflyer brand
[1], Illustration 3. It has been used
for an aircraft of the same weight.
The motor and the propeller will
enable to climb at estimated rate
of 1ms-1. It will be necessary to
develop a new propeller optimized
for regenerative soaring. The
commercially available propellers
Illustration 2: SWIFT with engine kit
are
optimized
to
provide
maximum trust. The propeller for our purpose should be optimized to work also as a turbine with high
efficiency which among other requires symmetrical blades sections [4]. Optional features of the
propeller include adjustable pitch and collapsibility.
2
Illustration 3: Electraflyer trike, Electric Aircraft Corporation
Glide ratio (best, at 75km/h) [2]
24:1
Minimum sink rate (at 45km/h) [2]
0.65ms-1
Never exceed speed (VNE) [2]
120 km/h
Climb rate (estimated)
1ms-1
Wing area [2]
12.5m2
Maximum load [2]
+5.3g/-2.65g
tested +7.95g/-3.98g
Weight empty [2]
48kg
Payload total:
74kg
battery pack (5.6kWh [1])
el. motor (13.5kW [1])
solar array 1
equipment and accesories
pilot
35kg
12kg
10kg
15kg
80kg
Gross weight
200kg
Table 1: Proposed aircraft specifications.
1 With installation aids and accessories
3
1.2 Energy Balance
The purpose of this section is to show that it is possible to build an electric aircraft capable of self
launch, long daytime flight and landing with a full battery. The proposed aircraft is described in the
previous section. The scenario investigated consists of 5 phases:
1. Self launch and climb to 300m,
2. 20 minutes cruise in search for updrafts,
3. Recharge batteries to full capacity
4. Cruise till 50% of the battery capacity remains.
5. Recharge batteries to full capacity and land
The following calculation requires to estimate certain values. The estimated values were carefully
calculated, consulted (see Acknowledgments) and adjusted to slightly under or over estimate the actual
values against the benefit of the aircraft. Also, only the energy balance considering the electric energy
is shown here. It is ignored that the plane can use the potential energy gained by climbing in the
updrafts during regeneration for travel by gliding. But please note that traditional sailplanes utilize
only this form of energy to fly over large distances (Free out-and-return distance record: 2 247.6 km,
[6]) and to stay aloft for long periods (~15 hours).
The proposed battery pack lasts for 1-1.5 hours of Electraflyer trike's powered flight. The
aerodynamics of the proposed aircraft is better than Electraflyer trike by the same weight. Therefore it
is assumed that the battery pack will last for at least 1.5 hours of flight. The proposed propeller works
with 85% estimated efficiency ([4], efficiency of airborne turbine is defined differently than efficiency
of a ground based turbine and Betz limit does not apply here). The brushless electro motor used in this
example is 90% efficient as a generator [2].When the propeller works in a turbine mode it creates drag
which leads to increased sink rate. If we assume the sailplane to be in level flight in steady air it is
loosing potential energy at certain rate corresponding to the sink rate at given speed. If additionally
electric energy is to be generated the sailplane has to loose potential energy at higher rate. Assuming
the potential energy is converted into kinetic energy and then to electric energy with the overall
efficiency corresponding to the efficiency of the propeller-turbine times the efficiency of the generator
the increase of the sink rate of the aircraft can be calculated if the gross weight of the aircraft is known.
We assume that increasing the sink rate by 0.75ms-1 will not seriously impair the flight performance of
the aircraft. Therefore we estimate to obtain 1126W on the output of the generator during regenerative
soaring. The battery charger works with 80% estimated efficiency (charging efficiency of the batteries
is 99.9%). The charger works only when the power balance is positive. We estimate that 80% of the
wing area can be used to install solar cells. The efficiency of solar cells suitable for airplanes varies
between ~6% to ~20% [5]. Let us assume installing solar cells with 15% efficiency providing
maximum output 150W per m2. The solar cells are estimated to provide on average 70% of their
maximum output during cruising (the aircraft flies sometimes under the shadows of clouds and the
wing is not always in the optimal position relative to the sun). During regeneration are the solar cells
estimated to provide on average 40% of their maximum output (the aircraft is moving in banked turns
and the wing is only part of the time exposed to the sun). The electric equipment of the aircraft
(instruments, radio etc.) is estimated to consume on average 10W. Table 2. summarizes the values and
estimates used in this example.
4
Battery pack [1]
5.6kWh (20.2MJ)
Average powered flight time [1]
1.5 hour
Electric motor [1]
13.5kW, 90% efficient
Average energy consumption rate – motor
3.7kW
Equipment input
10W
Propeller efficiency (turbine mode)
85%
Electric motor efficiency (generator mode)
90%
Charger efficiency
80%
Generator output in regen [4]
1126W
Solar power per m2 max [5]
150W
Solar cells surface
10m2
Solar array output - max
1500W
Solar array output - cruising
1050W
Solar array output - regen
600W
Sink rate total in regen [2]
1.4ms-1
Table 2: Values and estimates used for calculation.
5
1.2.1 Self Launch and Climb to 300m
300m is the lowest altitude suitable for search for thermal updrafts [4]. The electro motor is running at
full power during the start only [2]. The climb is done at reduced power (high discharge rate more
rapidly depletes the battery capacity). During climbing electric energy is used to sustain flight
(compensate the drag) and also converted to potential energy of the aircraft The only source of energy
now is the solar array . Table 3 summarizes the energy balance for this phase. The values are rounded.
Altitude gained = 300m
Balance:
Time spent = 5 minutes
Power (Watt)
Solar array
Energy (Joule)
1050
315000
0
0
-10
-3000
Motor (sustain flight)
-3733
-1120000
Motor (to potential energy)
-1962
-588600
Total
-4656
-1396600
Turbine (Regen)
Equipment
Battery capacity spent:
7%
Battery capacity spent total:
7%
Table 3: Self launch and climb to 300m, energy balance.
1.2.2 Cruise for 20 minutes in search for updrafts
It is likely that the aircraft will encounter updraft in 20 minutes after start. As it maintains the same
altitude the motor spends energy only to sustain flight level flight. Table 4 summarizes the energy
balance for this phase. The values are rounded.
Altitude gained = 0m
Balance:
Time spent = 20 minutes
Power (Watt)
Solar array
Turbine (Regen)
Equipment
Motor (sustain flight)
Motor (to potential energy)
Total
Energy (Joule)
1050
1260000
0
0
-10
-12000
-3733
-4480000
0
0
-2693
-3232000
Battery capacity spent:
16%
Battery capacity spent total:
23%
Table 4: Cruise for 20 minutes in search for updrafts, energy balance.
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1.2.3 Recharge batteries to full capacity
The aircraft has found an updraft (thermal, upwind slope lift) and it is regenerating. The propeller
works as a turbine. The aircraft will continue to recharge until the batteries are full. The efficiency of
the charger has to be taken into account in this phase. If the updraft is strong enough (1.4ms-1 is the
minimum sink rate in this phase) the aircraft gains altitude. Then it can glide in the direction of the
mission objective and continue recharging.
Altitude gained = Ignored
Balance:
Time spent = 56 minutes
Power (Watt)
Solar array
Energy (Joule)
600
2023346
1126
3796126
-10
-33722
0
0
NA
NA
Total
Recharge (80% eff.)
1716
1373
5785750
4628600
Battery capacity spent:
-23%
Turbine (Regen)
Equipment
Motor (sustain flight)
Motor (to potential energy)
Battery capacity spent total:
0%
Table 5: Recharge batteries to full capacity, energy balance.
1.2.4 Cruise till 50% of the battery capacity remains
The aircraft is cruising freely and accomplishing the mission objectives.
Altitude gained = 0m
Balance:
Time spent = 62 minutes
Power (Watt)
Solar array
Turbine (Regen)
Equipment
Motor (sustain flight)
Motor (to potential energy)
Total
Energy (Joule)
1050
3929703
0
0
-10
-37426
-3733
-13972277
0
0
--2693
-10080000
Battery capacity spent:
50%
Battery capacity spent total:
50%
Table 6: Cruise till 50% of the battery capacity remains, energy balance.
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1.2.5 Recharge batteries to full capacity and land
The aircraft is recharging and continues to accomplish the mission objectives. The turbine generated
power is higher during the final descent what can shorten the recharge time before landing but the
turbine power is considered to be constant during the entire final phase of the investigated scenario for
simplicity.
Altitude gained = NA
Balance:
Time spent = 122 minutes
Power (Watt)
Solar array
Energy (Joule)
600
4406371
1126
8267068
-10
-73440
0
0
NA
NA
Total
Recharge (80% eff.)
1716
1373
12600000
10080000
Battery capacity spent:
-50%
Turbine (Regen)
Equipment
Motor (sustain flight)
Motor (to potential energy)
Battery capacity spent total:
100%
8
2 Conclusions
Following this analysis it has been shown that it is possible to build an aircraft powered with solar
power and regenerative soaring from an existing aircraft so that it would be capable to self launch, fly
and land with full battery. The total flight time in the investigated scenario was 4 hours and 26
minutes. In this configuration the ratio between free cruising and regen is 1:2 which means that the
aircraft would need to regenerate for 2 minutes for every minute of powered cruising to have the
batteries fully charged by landing. It should be considered that the regeneration time may be used to
accomplish mission objectives (e.g. the aircraft can fly in general direction of the destination and regen
at the same time). The analysis shows that using regenerative soaring in combination with solar power
shortens the recharge time significantly.
To maximize the efficiency of the aircraft the following should be considered:
•
solar cells should cover maximum of the suitable surfaces and have the highest possible
efficiency
•
power input of the equipment should be minimized
•
drag of the aircraft should be minimized
•
large and slow (more efficient) propellers should be used (possibly installed as ducted fans)
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3 Acknowledgments
We would like to thank to the following people who contributed to the project proposal by providing
advice and consultation:
Phil Barnes is Principal Engineer at Northrop Grumman Corporation. He has
a Master’s Degree in Aerospace Engineering from Cal Poly Pomona and a
Bachelor’s Degree in Mechanical Engineering from the University of Arizona.
He has 25-years of experience in the performance analysis and computer
modeling of aerospace vehicles and subsystems at Northrop Grumman. Phil
has authored technical papers on aerodynamics, gears, and flight mechanics.
Phil Barnes is the author of the paper “Flight Without Fuel - Regenerative
Soaring Feasibility Study” [4]
Randall Fishman is the president of Electric Aircraft Corporation. He has
won numerous awards and accolades for his work on electric flight and
already has built an electric-powered ultralight and a single-seat motorglider.
In April 2007 the Electric Aircraft Corporation began offering complete
electric ultralights and engine kits under the ElectraFlyer brand name, to
convert existing ultralight aircraft to electric power, in what is the first
commercial offering of an electric aircraft.
Jukka Tervamaki graduated from the Helsinki University of Technology in
1963 specialized in Aeronautical Engineering. Experimenting, creating,
designing and building has been everyday work as well as a hobby for him for
four decades. He designed and build several rotary wing aircrafts (autogyros),
a motor glider and cooperated on development of a fixed wing tow plane. He
has logged total 2200 flight hours of which 150 hours in autogyros. He is
aviation writer for several Finnish and foreign aviation magazines.
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4 Sources:
[1] Electric Aircraft Corporation, http://www.electraflyer.com/, email correspondence with Randall
Fishman, president of Electric Aircraft Corporation
[2] Aériane SWIFT rigid wing hang glider http://www.aeriane.com
[3] Northwing, Stratus, http://www.northwing.com/
[4] Philip Barnes - Pelican Aero Group, Flight Without Fuel - Regenerative Soaring Feasibility Study,
presented at General Aviation Technology Conference & Exhibition, August 2006, Wichita, KS, USA,
Session: Propulsion Dynamics and Advanced Engine Concepts,
http://www.sae.org/technical/papers/2006-01-2422, email correspondence with Philip Barnes.
[5] NASA Dryden Fact Sheet - Pathfinder Solar-Powered Aircraft
http://www.nasa.gov/centers/dryden/news/FactSheets/FS-034-DFRC.html;
[6] The Worlds Air Sports Federation http://records.fai.org/, as of Oct. 26 2009.
[7] J. Wharington, I. Herszberg (1998), 'Control of High Endurance unmanned air vehicle', Proc. 21st
Congress of the International Council of the Aeronautical Sciences, Rodney S. Thomson & Murray L.
Scott, eds., AIAA Electronic Publication - CD ROM, ISBN:1-56347-287-2 98-1141, (ICAS '98, 1318 Sept. '98 - RMIT Fishermen's Bend, Melb. Vic.)
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