Solar Aerofoil and the Future of Flight

Something unique was spotted above the Hawaiian Islands in 1997. Powered by six propellers, it moved slowly and quietly through the sky, staying in flight above the coastal region for hours. The flying machine didn’t look like any other planes or jets. It was just a long flexible wing with no fuselage, cockpit, and (most importantly) a fuel tank. 

The plane, better known as NASA’s Pathfinder, was in the area collecting photos and data of forests and coastal zone ecosystems on Kauai. Its unusual design, as well as the duration of flight, created a lot of attention from onlookers. However, the real fascination for this unmanned flying vehicle had little to do with its aesthetics; it was its mode of solar-powered propulsion.

To be precise, Pathfinder used a culmination of airfoil, propulsion, energy production, storage, and control in what appeared to be a simple design. The long, flexible wing and its design (sometimes referred to as the “solar aerofoil”) had several solar panels laid on top of its flat surface. The panels powered the electric propeller engines.

Pathfinder is a combination of several technologies, as well as an evolution in unmanned flights. The current push for these vehicles is to see how far and long they can fly. Yet, in the not-so-distant future, the planes and its airfoil design may push other boundaries in manned and unmanned flights, as well as find a niche – as Pathfinder has done.

A Brief History

The process leading to the creation of the solar aerofoil stretches as far back as November 4, 1974 when the first solar-powered aircraft took flight. Sunrise I and Sunrise II (flown in 1975) were remote controlled model planes created by Robert J. Boucher of AstroFlight, Inc. The solar panels were mounted on the wings, thus becoming the first solar aerofoil used in flight. Due to its size, it was catapulted into the air at a dry lake at Camp Irwin. It stayed in the air for 20 minutes.

Deemed a success, private companies such as AeroVironment, Inc. soon got involved in this technology. AeroVironment’s founder, Dr. Paul MacCready, was no stranger to alternative powered vehicles. In 1979, his creation, Gossamer Albatross, became the first human-powered plane to cross the English Channel.

He saw that solar power had benefits. He claimed that it can “help business and government recognize and meet their environmental and energy objectives (NASA, 2011).”

MacCready took the body of the Gossamer Albatross II – another human-powered plane – and converted it to solar power. He received help from Astroflight, who supplied him with the electric motor that powered the propeller.

It was later renamed the Gossamer Penguin. The solar panel, however, was not on the wing. It was above, perpendicular to it and facing the sun. The Penguin made its first flight on April 7, 1980 at Shaffer Airport near Bakersfield, California.

Gossamer Penguin may have been the first manned solar-powered plane, but it was the next plane, Solar Challenger, was the first to use the solar aerofoil. This plane was designed by MacCready, sponsored by DuPont, and used AstroFlight, Inc. motors. On July 7, 1981, Solar Challenger, piloted by Stephen Ptacek, flew across the English Channel from Pontoise – Cormeilles Aerodrome in France to Manston Royal Air Force Base in Manston, United Kingdom. The trip was 163 miles and lasted 5 hours and 23 minutes.

Solar Challenger was an incredible leap forward. Not only was a manned flight sustained for so long and over greater distance, the design proved to be ground-breaking.

Although the wingspan was 47 feet (was much smaller than the Penguin’s), it proved to be much more sturdy and viable. Also, it was affixed with solar panels on its surface and wings. The wing’s airfoil design used on Challenger became the basis for future solar aircrafts. It had a flat upper surface that allowed for solar panels to be affixed there.

It wasn’t long before the U.S. Government got involved. AeroVironment and NASA had been working together on the human-powered planes. In fact, the first public showing and flight of the Gossamer Penguin was done at NASA’s Dryden Flight Research Center at Edwards, California.  However, manned flights were not what NASA’s wanted.

NASA’s vision was to have an atmospheric satellite. As a result, unmanned aerial vehicles (UAV) and the altitudes they can reach became the next step in solar-powered planes. In 1981, the U.S. government initiated a classified program to look into creating vehicles that could fly for a long duration at altitudes above 65,000 feet.

Pathfinders and the HALSOL (High-Altitude Solar Energy)

Pathfinder was the first HALSOL vehicle. It had a wing span of 98.4 feet and was originally powered by eight battery powered electric motors (later to be converted to six). In the early 80s, Pathfinder was converted by AeroVironment and Dryden’s team to solar power. Between 1983 and 1995, the unmanned plane was tested and perfected at the Edwards facilities and nearby dry lakes.

September 11, 1995, Pathfinder entered the record books. It flew to an altitude of 50,500 feet, the highest any unmanned plane – let alone a solar-powered plane – had ever flown at that time. 

A year later, it became the property of the U.S. Navy and was shipped to the Pacific Missile Range Facility at Barking Sands, Kauai, Hawaii.  In 1997, Kauai was chosen for further testing of the aircraft due to its high level of sunlight, available airspace and radio frequencies, as well as the island’s diversity of terrestrial and coastal regions. This was when Pathfinder took on the role of ecological surveyor.

While in Hawaii, Pathfinder broke another record. It reached an altitude of 71, 530 feet.

By 1998, the Pathfinder was modified. Now, it had a longer wingspan. It became known as Pathfinder “plus.” And, once again, it flew to new heights on August 6, 1998: 80,201 feet above Kauai. The goal, according to NASA Dryden’s website was to “validate new solar, aerodynamic propulsion and systems.”

These new technologies would be used on Pathfinder’s successor, Centurion. Although using a similar design as the Pathfinder, this new plane had a much longer wingspan of 206 feet. It was designed to be stronger and more capable of carrying numerous payloads up to 600 pounds.

The wing was redesigned to take on higher altitudes, as well as being increased in size. Also, it included 14 motors and four under-wing pods that carried the batteries, flight system components, ballast, and landing gears. 

The Centurion’s first flight happened on November 10, 1998. Two other test flights would occur, mostly to test payload capacity. These initial tests used batteries to power it. In January 1999, the decision was made to expand the wing and test it with solar power. The plane would be renamed Helios Prototype.

Helios Prototype had 41 feet added to its wingspan as well as a fifth landing gear and system pod. It also had incredible successes and failures. On August 14, 2001 it reached an altitude of 96,863 feet, shattering the previous record for sustained horizontal flight by a winged aircraft by 2 miles. It stayed at this altitude for more than 40 minutes.

June 26, 2003 was the last flight for Helios. While being tested for an eventual endurance test, the plane broke up in mid-air about 10 miles off the coast of Kauai.

The Future

The demise of the Helios Prototype wasn’t the end of HALSOL. In 2002, steps were made to turn Pathfinder Plus into a functioning atmospheric satellite. NASA and the Japan Ministry of Telecommunication used it successfully to transmit both HDTV signals and IMT-2000 wireless communications from 65,000 feet.

Also, another vehicle is in the planning stages, and with a new mission in mind. The government agency DARPA (Defense Advanced Research Project Agency) is working on Vulture.  It hopes that it will be able to sustain uninterrupted flying for 5 years.

If artist rendering is correct, the Vulture’s airfoil will include a delta-shaped wing, a second wing, nine propellers, and four pods. In a sense, it looks like a biplane without the fuselage.

The solar aerofoil has been used on manned and unmanned planes. As the technology of the material improves, this system may find wider uses in the future. For now, it’s still in its infancy. In the near future, the military may use it for reconnaissance. And, if possible, it may make it to space.  The future is wide open.


“Dryden Flight Research Center”: NASA:

 “History of Airfoil Development (retrieved 2011)”: Stanford. Edu

Christensen, B (retrieved 2011): “DARPA Vulture Five Year Flying Wing”: