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FROM HERE TO THE STRATOSPHERE ON SUNSHINE

Coen van Wyk

Introduction

Approximately two years ago, during August 1998 the solar-electric aircraft, Pathfinder-Plus, achieved an altitude of 80201ft and thereby set a world altitude record for propeller-driven aircraft. And then a small group of engineers from NASA’s Dryden Flight Research Center and AeroVironment, Inc., planned to fly a unique unmanned solar-powered aircraft, the Helios Prototype, into a hostile environment, 100000 feet above ground. If they could achieve that goal, then they would not only have set a new world altitude record for propeller-driven aircraft, but their aircraft would also exceed the highest reported altitude of 85068 feet achieved in sustained horizontal flight by jet-powered aircraft, being the altitude reached by a SR-71 aircraft in July, 1976.

The stratosphere

The troposphere extends from ground level up to 9 miles (approximately 50000 ft.). It contains most of the atmosphere's mass and nearly all of the atmosphere's water vapour and dust.

The stratosphere, on the other hand, extends from 9 to 31 miles (approximately 50000 to 166000 ft.)  and contains most of the ozone, which forms a layer that absorbs most of the sun's ultraviolet radiation.

The sky is black at 100000 ft above ground, and the curvature of the earth is clearly discernible. At that altitude the air is so thin, being only 1.4 percent of its density at sea level, that it is incapable of supporting life. It is also incapable of supporting sustained horizontal flight of an aircraft. I should actually say that the air at 100000 ft. was presumed to be incapable of supporting sustained horizontal flight of an aircraft, until NASA proved this premise to be fallacious. The reason why I say so that, as is evident from a NASA status report, that is referred to later in this article, Helios got very close, a few weeks ago, to reaching the 100000 ft altitude goal, and in fact shattered the world altitude record for propeller-driven aircraft and jet-powered aircraft. And, to add insult to injury, it did so by using the power of the sun.

The fact of the matter is that the unique lightweight craft established a new unofficial world altitude record of 96500 ft in sustained horizontal flight during an almost 17-hour flight from the U.S. Navy's Pacific Missile Range Facility on the Hawaiian island of Kauai on Monday, August 13, 2001.

The aircraft's record breaking attempt was originally scheduled for August 11, but engineers working on the project on the Hawaiian island of Kauai decided to postpone their first attempt to fly at between 95000 ft and a 100000 ft. The rescheduling was made to accommodate some preflight equipment checks. The delay could also enhance the possibility of Helios reaching an altitude of 100000 ft because high cirrus clouds in the area on the August 11 may have disappeared the following day, a fact that may have permitting more sunlight to charge the aircraft’s solar cells.

Project information

The Helios Prototype is an enlarged version of the Centurion flying wing, which flew a series of test flights at Dryden in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, and 2.5 times that of its solar-powered Pathfinder flying wing, and longer than either the Boeing 747 jetliner or Lockheed C-5 transport aircraft. It is one of several remotely-piloted aircraft, also known as uninhabited aerial vehicles (UAV's), being developed as technology demonstrators by several small airframe manufacturers under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project.

For quite some time the Helios Prototype flights have been conducted from the U.S. Navy’s Pacific Missile Range Facility on the Hawaiian island of Kauai. The location at 22 degrees north latitude gives the Helios Prototype a sun angle advantage and the protection of a vast test range and restricted airspace over the Pacific Ocean, west of the islands.

Helios is a prototype that is being developed for high-altitude, long-duration earth science imaging and atmospheric sampling missions under the Environmental Research Aircraft and Sensor Technology (ERAST) project at NASA Dryden, located at Edwards, California.

John Del Frate, project manager for solar-powered aircraft at NASA Dryden, earlier commented that setting an altitude record is only one of several goals for this summer’s flight tests. He said, "A 100,000-foot altitude record would be the icing on the cake," and added, "Our primary interest in testing this new aircraft is for taking sophisticated lightweight science instruments to greater heights." It appears that an added bonus for NASA in achieving an altitude of 100000 ft is the fact that flight at 100000 feet would be very similar to flight in the Martian atmosphere. In that regard Del Frate said: "In a way, we are going to school on these flights to learn what the aerodynamics are like in these conditions. AeroVironment's vice-president, Robert Curtin, noted that production versions of the Helios could also serve as long-endurance commercial telecommunications relay platforms, orbiting over major population centres at 55000 to 70000 ft altitudes for months at a time. Disaster recovery agencies might one day be able to move a Helios over the scene of a natural calamity where the normal communications infrastructure had been destroyed.

Development of the Helios Prototype flying wing is being funded and managed under NASA's Environmental Research Aircraft and Sensor Technology project.
Aircraft information and specifications

The Helios Prototype is powered by 14 brushless direct-current electric motors mounted across the wing’s entire span. For the initial flight tests, they are drawing power from lithium batteries carried in the underwing pods and are rated at 2 hp (1.5 kw) each. A 79-in. diameter two-blade propeller on each motor features a wide chord and a laminar-flow design for high-altitude flight.
The cruising speed of Helios ranges from 19 to 25 mph, with takeoff and landing equating to the average speed of a bicycle.
The only flight control surfaces used on the Helios Prototype are 72 trailing-edge elevators which provide pitch control. Spanning the entire wing, these elevators are operated by tiny servo motors linked to the aircraft’s fight control computer. To turn the aircraft in flight, yaw control is applied by applying differential power on the motors, speeding up the motors on one outer wing panel, while slowing down motors on the other outer panel.

Helios prototype post-flight status report

On August 13, 2001 Helios took to the sky and ascended to an altitude of 96500 ft. The following are extracts from the NASA Dryden post-flight status report of August 17, 2001 concerning that record breaking flight.

"With a new world altitude record to their credit, engineers and technicians for AeroVironment, Inc., are preparing to disassemble and pack the solar-powered Helios Prototype flying wing for shipment back to California.
The unique lightweight craft established a new unofficial world's altitude record of 96,500 feet in sustained horizontal flight during an almost 17-hour flight from the U.S. Navy's Pacific Missile Range Facility (PMRF) on the Hawaiian island of Kaua'i on Monday, Aug. 13. Only short-duration rocket-powered planes such as the X-15 of the 1960's have flown higher. The new record is subject to certification by the National Aeronautics Association, the certifying agency for all aviation records in the United States.
John Del Frate, solar aircraft project manager at NASA Dryden Flight Research Center, advised that the aircraft and most of the crew will depart PMRF by next weekend. The Helios Prototype will be placed in storage while development work continues on a regenerative energy storage system designed to allow the aircraft to fly at night on excess power generated by the solar arrays during the day. A flight-worthy prototype system is expected to be ready for installation and flight tests in 2003. Those tests are targeted at achieving a NASA milestone of flying continuously for four days and nights above 50,000 feet.
Designed and built by AeroVironment, Inc., of Monrovia, California, the ultra-lightweight Helios Prototype is believed capable of reaching altitudes in the vicinity of 100,000 feet under ideal weather conditions. However, conditions for last Monday's flight were far from ideal. Del Frate noted that the low clouds which delayed the Helios Prototype's takeoff by almost an hour had a significant negative impact on the maximum altitude achieved, since it shortened the time available for ascent to a little over seven hours. Weather also contributed to a severe turbulence encounter during the first hour of flight, but as it had done on a checkout flight July 14, the Helios rode out the turbulence well. Monday's demonstration flight was performed to validate the Helios' capability as a platform for high-altitude earth monitoring and atmospheric sampling missions."
A final word

When considering the fact that Helios reached an altitude just short of 100000 ft the following should be borne in mind:

Only short-duration rocket-powered aircraft have flown higher.