SoLong Solar-Electric UAV 48 hour flight

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http://www.acpropulsion.com/whats_new.htm
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 gather enough energy
during the day to keep flying all night. 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 down rushing 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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