AC Propulsion Achieves 24-hour Endurance
with a Solar Electric UAV
El Mirage Dry Lake, California
Friday, April 22, 2005
Earth Day
From out of the moonlit pre-dawn sky over California’s
high desert, an unmanned aircraft silently returned to the earth after
spending more than 24 hours aloft. AC Propulsion’s SoLong aerial
vehicle had just flown longer than any electric-powered aircraft had
flown before. During the flight the SoLong had made no noise, consumed
no fossil fuels, emitted no pollution, and contributed nothing to global
warming. The flight gave an Earth Day present to our planet and set
a milestone in the development of aircraft that can sustain flight autonomously
for days on end.
The SoLong took off just after midnight, at 12:34 AM on Thursday April
21 and climbed to altitude powered by its high efficiency electric motor.
Leveling off, the 15.6 foot (4.75 m) wingspan aircraft flew above the
desert responding to commands from pilots in the ground station via
2.4 GHz video and data downlink. Once at altitude, the 25.3 pound (11.5
kg) aircraft, optimized for low-power flight drew the minimum current
from its Li Ion battery as the motor cut back to less than 15% of its
takeoff power sustaining an air speed of 25mph. A patented split-phase
power controller developed by AC Propulsion maintained high efficiency
even at low power, and the variable pitch propeller was trimmed to give
maximum propulsion efficiency. Even the 6 servos that move the control
surfaces used electronics developed by AC Propulsion for this mission
to save a few watts. Efficiency was the crux of the matter because only
the energy stored in the battery was available to fly the airplane,
and it had to last until dawn.
With sunrise at 6:10 AM came the first rays direct from the sun. As
they fell on the 76 high-efficiency solar cells molded into the skin
of SoLong’s wings, electric current began to flow augmenting the
battery current. As the sun rose higher and its rays became stronger,
the flow of solar electricity increased until the current from the Li
Ion batteries diminished to zero and then reversed to become a charging
current replenishing the energy that had been withdrawn in the hours
from takeoff and dawn. Peak power tracking software allowed the best
use of the photovoltaic energy and by mid-afternoon the batteries were
full, ready to keep SoLong flying long into the night.
SoLong used solar energy in another way too. The sun’s warmth
gave rise to thermals, currents of warm air rising from the desert floor.
The lightweight materials, efficient structure, and low-drag aerodynamics
designed into SoLong allowed it to ride these thermals to maximum effect,
sustaining and even gaining altitude without using any motor power at
all. With the motor off, the propeller folded back against the fuselage
to minimize drag, and the SoLong glided on rising air. GPS-based autopilot
software gave the ground-based pilots the ability to seek and find thermals
and gain altitude throughout the day. With altitude and battery charge,
the SoLong was well stocked with the two currencies it would need to
stay airborne as day turned back to night.
After sunset at 7:25 PM, SoLong was again dependent on the energy stored
in its 30-volt battery. Thermal lift continued but gradually diminished
as the sky darkened and the air grew still. The motor was turned on
again and it drew power in amounts just sufficient to sustain altitude.
23 channels of telemetry plus GPS navigation data available to the ground
station were analyzed to assess system conditions, plan flying patterns,
and monitor energy consumption during flight.
All of the systems worked properly, the aircraft flew steadfastly, and
the energy budget proved more than adequate. By the 23rd hour, the battery
had discharged less than half of its 800 watt-hour energy capacity,
the sky was clear and calm, and within the 5ft x 8 ft ground station
parked below on the dry lakebed, confidence and anticipation grew. At
12:15 AM, SoLong’s motor was turned off and the aircraft began
a gradual descent circling over the ground station until its onboard
video camera picked up the 10 lights marking the landing zone laid out
next to the ground station. After a few more circles overhead, the waypoint
navigation was turned off and Alan Cocconi, SoLongs’s inventor,
designer, builder and chief test pilot took control, set up the approach,
and flew SoLong to a perfect landing. The time was 12:45 AM, Friday
morning, Earth Day. The plane was safe on the ground, the 24-hour 11-minute
flight was complete, and the goal of sustainable flight had been brought
closer.
Even with the help of autopilot and thermal tracking, flying the SoLong
efficiently demanded full concentration from a pilot on the ground.
A crew of expert and dedicated volunteer pilots including Dave Carr,
“RC” Dave Freund, Jeff Keesaman, and Steve Neu assisted
Alan Cocconi in piloting the SoLong over its record setting flight.
Sustainable flight, sending a plane into the air to fly for days using
only solar energy, is the goal of AC Propulsion’s SoLong program.
To achieve it means developing and applying technology that can capture
every nugget of available energy and use that energy with utmost efficiency.
Sustainable flight will not replace conventional fueled flight for most
aviation, but it does have a growing list of potential applications
including communications relay, and atmospheric, forest, and agricultural
monitoring. Perhaps as important, sustainable flight might serve as
the model for what we need as a planet flying around our sun.
