SoLong
Solar-Electric UAV 48-hour Flight
On June 3rd, 2005,
AC Propulsion's SoLong UAV, an unmanned solar-electric airplane, completed
it's first 48 hours aloft, a milestone in sustainable flight. For a full
report on the SoLong, you can click to download the .pdf
or the .doc
file. What follows is the official press release:
AC
Propulsion SoLong UAV Flies for 48 Hours on Sunlight
Two Nights Aloft Opens New Era of Sustainable Flight
Desert Center, California
Friday, June 3, 2005
An airplane that never needs to land might sound impossible,
but it is closer to reality now that AC Propulsion’s SoLong has
completed a two-day flight fueled only by energy from the sun. SoLong
took off at 4:08 PM, Wednesday, June 1 from the sun-baked runway at Desert
Center Airport just east of Eagle Mountain in California’s Colorado
desert. It remained aloft until Friday when it skidded to a stop at 4:24
PM after 48 hours and 16 minutes in the air. During that time it had fully
recharged its batteries during the day and then flown through the night
on battery power. Twice. Nothing, save the flagging energy of its pilots
on the ground, kept the SoLong from flying for another two days, or ten,
or a whole month.
The SoLong is an electric-powered UAV (unmanned aerial vehicle) that collects
solar energy from photo-voltaic arrays laminated into its wings. It uses
energy so efficiently that it can fly all night on energy it gathers from
the sun during the day. Remaining aloft for two nights is the milestone
for sustainable flight. One night is possible just by discharging the
batteries, but two or more nights means that the plane has to fully recoup
and store the energy used at night while flying in the sunlight the following
day. Once that is achieved, the cycle can repeat continually, and keep
the plane airborne indefinitely.
“We flew 24 hours in April”, said Alan Cocconi, SoLong’s
creator and chief pilot, “but we split the night in two, flying
midnight to midnight. That was a warm up for this flight. It showed us
that we were getting enough solar energy during the day but we didn’t
have quite enough battery to takes us through the night. Just last week
I got new Sanyo high-capacity Li Ion cells. That made the difference”
Cocconi is founder, Chairman, and Chief Engineer of AC Propulsion, Inc.,
a San Dimas, CA-based R&D shop that specializes in high-efficiency
electric propulsion.
Efficiency is the key to SoLong’s success. The power system includes
a high efficiency electric motor driven by a patented split-phase power
controller developed by AC Propulsion. The controller gives high power
for takeoff and maintains high efficiency even at the low power levels
used in steady flight. A variable pitch propeller allows tuning for maximum
propulsion efficiency under varying flight conditions. Solar cells that
convert solar energy to electricity with an efficiency of 20% are controlled
by proprietary peak power tracking software that makes best use of the
photovoltaic energy. Much of that energy goes to charge the Li-ion battery
pack that powers the motor and a separate pack for the controls and communications.
With a charge-discharge cycle efficiency over 95%, the Li-ion batteries
do not squander the bounty from the solar cells, and at 220 Wh/kg, the
Sanyo cells pack a lot of energy without much weight. Still, the battery
makes up 44% of the aircraft’s total weight because the air frame
uses efficient structure and composite materials so it is light but still
strong enough to withstand 30 mph winds. The 6 servos that move the control
surfaces use special electronics that were developed for this mission
by AC Propulsion to reduce power consumption and to extend durability.
“Every system and part on the SoLong was designed to minimize weight
and drag, and maximize efficiency” Cocconi stated. He continued,
“of course that is true of just about every airplane, but with the
SoLong the entire mission depends on efficiency. We had to push everything
to the limit.”
That included the pilots who flew SoLong from the 5 ft x 8 ft trailer
that serves as SoLong’s ground station. Led by Cocconi, the team
of crack radio-control and hang glider pilots took turns monitoring flight
conditions from the twenty three channels of telemetry plus GPS navigation
and video downlink data available in the ground station. The pilot’s
job is to find updrafts, avoid downdrafts, and make judicious use of the
battery power to maintain altitude and find “good air” that
will lift the plane. The energy budget requires riding thermals with the
motor off as much as possible during the day. With the motor off, the
entire output from the solar wings goes into the battery. The energy margins
are so thin, and the weather so dauntingly capricious that the pilot must
focus intently, always trying to bank energy, either as battery charge
or altitude, that can be drawn upon to get the plane out of trouble when
the air turns bad.
After 46 hours, with the critical achievement of two nights flown already
in the bag, the air did turn bad. In bright clear skies, SoLong flew into
an invisible but huge mass of downrushing air that seemed inescapable
no matter which direction she flew. Sinking at 5 meters per second even
full power was not sufficient to check the descent, SoLong’s altitude
reserve diminished rapidly. An abortive early landing seemed a possibility
until Cocconi, drawing on 30 years of piloting experience, determination
borne of exhaustion, and perhaps some good luck too, found less treacherous
air at low altitude. The atmospheric disturbance lasted a total of 20
minutes, and the SoLong was restored on a course of energy equilibrium.
The first mission of any flight is to land the plane safely. This is no
less true because the SoLong’s pilots remain on the ground. SoLong
represents 4 years of work by Alan Cocconi to develop light, strong, and
efficient aircraft, control, and propulsion systems. SoLong herself took
him the more than a year to build and test. The funding was his own. The
consequences of pilot error or system failure would be devastating. But
still there is no margin to make the plane stronger than it has to be,
or easier to fly, or to add a few extra batteries. The balance points
between strength and weight, between stability and drag, between energy
and power make a very fine line. On one side of that line are airplanes
that cannot fly through the night. On the other side are airplanes that
fall from the sky. Many efforts, some extremely well-funded, have tried
to find the balance that will keep a solar-powered airplane plane in the
air for two consecutive nights. Until today, not one had found it.
By three o’clock with good air and full batteries, the 48th hour
became a formality to make a numerical milestone – two full days
in the air. The network film crew arrived to record SoLong’s historic
landing and the jubilation, showing through the fatigue, on the pilots’
faces. Steve Bellknap, Jerry Bridgeman, David Fee, “RCDave”
Freund, Chuck Grim and Steve Neu had all ably assisted Alan Cocconi in
piloting the SoLong over its record setting flight. They had helped accomplish
something never done before, and in doing it the first time, they made
it easier to do the next time and then again, and again. Now, it is within
the realm of reality that airplanes flying sustainably on rays from the
sun may become commonplace, may become useful tools in the service of
humans and their environment.
SoLong
Specifications
Wingspan: 4.75m
Wing area: 1.50 m2
Mass: 12.6 kg
Power sources: 120 Sanyo 18650 Li-Ion cells
76 Sunpower: A300 solar cells
Solar panel nom. power: 225 W
Battery mass: 5.50 kg
Max motor power: 800W
Min electrical power for level flight: 95W
Stored energy: 1200Wh
Speed range: 27 to 50 mph
Max. climb rate: 2.5 m/s
Control and telemetry range: 8,000 m
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