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On Wings Wider than a Jumbo Jet...

A propeller-driven near-earth satellite?!

Assembled from material provided by NASA and AeroVironment Inc

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Right now you're looking at one of the most innovative aeroplanes ever. Not only has it recently snatched the record for all-time highest altitude for a non-rocket aircraft, but that bizarre shape hides a machine that will potentially be able to stay aloft for months at a time, using just the sun to provide its energy. And where are the pilots? They're on the ground!

The aircraft is being developed by US technology innovations company AeroVironment Inc, with funding through NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project.

And it's not just about setting records, either. Commercial applications of the Helios flying wing include the replacement of near-earth satellites, with the aircraft able to provide some telecommunications services normally the province of much more expensive satellites.

The Craft

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The Helios is part of an extended research effort into unmanned, low-energy and high altitude aircraft.

The first unmanned solar airplane developed by AeroVironment under the NASA program, the Pathfinder, flew to 71,500 feet in 1997. A modified Pathfinder, known as Pathfinder-Plus, then flew to 82,000 feet, higher than any other propeller-driven aircraft. This record flight was the 39th consecutive successful flight test of the Pathfinder platform.

Building on Pathfinder's success, AeroVironment built a next-generation aircraft with a 206-foot wingspan, called the Centurion, which was test flown in 1998 at Edwards Air Force Base. The wingspan was then further extended to 247 feet, and the aircraft was renamed the Helios Prototype. This wingspan is 41 feet greater than the Centurion, 2? -times that of its solar-powered Pathfinder flying wing - and greater than the wingspan of the Boeing 747 jetliner or a Lockheed C-5 transport aircraft!

The Helios Prototype was then equipped with high efficiency solar cells and underwent high-altitude flight testing in the summer of 2001 in Hawaii, at the US Navy Pacific Missile Range Facility in Kauai. On August 13, 2001 - on its second high altitude flight - Helios flew to 96,863 feet, shattering the world altitude record for both propeller and jet-powered aircraft (the SR-71 spy plane was the previous record holder, having flown to 85,068 feet in July 1976).

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For the record flight, the remotely-piloted wing took off from the US Navy's Pacific Missile Range Facility on the Hawaiian island of Kauai at 8:48am. Flying at about 25 miles an hour (40 km/h), the mission lasted nearly 17 hours, the aircraft landing at 1:43am August 14.

"This is like going to the Olympics and setting a new world record for engineers," said NASA Dryden Flight Research Center solar aircraft project manager John Del Frate. "This achievement did not come easily. Thousands of things had to work right for something like this to come together."

The 100,000-foot altitude flight is one of two major flight milestones set for the craft by NASA, the other being a four-day non-stop endurance demonstration flight above 50,000 feet planned for 2003. The power required to lift a small (100 lb - 45kg) payload to 100,000 ft. also enables the aircraft to carry substantially large (600 lb - 275kg) payloads to altitudes up to 70,000 ft., making this a versatile craft.

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The record altitude was achieved during daylight hours, relying on solar cells on the wing's surface to provide electrical power. Descent after dark was possible as the 14 electric motors were no longer needed to maintain altitude. During descent the propellers acted as generators, providing electrical power to control the aircraft.

Development of a regenerative hydrogen-oxygen energy storage system - which would make the multi-day continuous flight possible - is progressing at AeroVironment. The system uses excess power generated by the solar arrays during the daytime to run an electrolyser that separates water into its component parts, hydrogen and oxygen. These gases are then stored under pressure in specially-designed tanks. At night, the hydrogen and oxygen are recombined in fuel cells, generating electricity to power the Helios's motors.

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During later flights, AeroVironment's flight test team will evaluate new motor-control software which may allow the pitch of the aircraft - the nose-up or nose-down attitude in relation to the horizon - to be controlled entirely by the motors. If successful, production versions of the Helios could eliminate the elevators on the wing's trailing edge now used for pitch control, saving weight and increasing the area of the wing available for installation of solar cells.



Applications

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AeroVironment has formed a new company, SkyTower Inc, to pursue telecommunications applications (such as fixed and mobile broadband internet access) using solar-electric aircraft technology.

The commercial version of Helios will be capable of continuous flight for months at a time at altitudes of 50,000 to 70,000 feet.

According to AeroVironment, acting as a geostationary satellite without the time delay (equivalent to an 18-kilometre tall tower!), a Helios-based network has many potential advantages:

  • Enables the lowest overall system cost.
  • Concentrates capacity over populated areas and provides high 'look' angles resulting in improved coverage over both satellite and terrestrial systems. For example, a single airplane can cover a service area of approximately 40 miles (64km) in diameter with a look angle ranging from 30 to 90 degrees.
  • Can increase bandwidth capacity.
  • Due to the low elevation of Helios versus space satellites, less power is required for transmitting and receiving, smaller/lower cost communications equipment can be used, and/or network performance can be improved.
  • Rapidly deployable to provide immediate target market coverage, and is easily relocated, expanded, maintained and upgraded.
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Unique features of these solar-electric aircraft that are said to make them appealing platforms for telecommunications applications include:

  • Long flight duration of up to six months or more
  • Minimal maintenance costs due to few moving parts (eg each motor has only one moving part)
  • High levels of redundancy (eg aircraft could lose multiple motors and still maintain station and land safely - most failure modes do not require immediate response by ground operator)
  • Highly autonomous controls which enable one ground operator to control multiple aircraft
  • Use of solar energy to minimize fuel costs
  • Tight turn radius which makes platform appear geostationary from ground equipment perspective (ie enables use of stationary user antennas), and enables multiple aircraft to serve same area using same frequency spectrum
  • Flexible flight facility requirements (aircraft can takeoff from even a dirt field and in less distance than the length of its wingspan!)

Specifications

The specifications of a commercially complete Helios would include:

Altitude: 50,000 to 70,000ft
Endurance: 6 Months
Wingspan: 256ft (76.8m)
Length: 12ft (3.6m)
Empty Weight: Approx. 1,600lbs (727kg)
Payload weight & power 220lbs (100kg) & 1,000W
Power: Bi-facial solar cells, max. output approx. 42kW, PEM fuel cell 8kW
Motors: 8 brushless DC electric motors, 1.5kW (2hp) each
Airspeed: Approx. 17-21 mph (15-18 knots) cruise
Primary materials: Carbon fibre and graphite epoxy composite structure, composite structure, Kevlar?, styrofoam leading edge, plastic film covering

Conclusion

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If the technology can be successfully proved in a commercial application, many of the services that are now achievable by satellites (phone, digital radio, TV) may become viable on a smaller scale and for smaller populations.

And even if it doesn't, it's certainly fascinating technology!

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