NASA 

National Aeronautics and 

Facts

Space Administration 

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523 

Voice 661-276-3449 

FAX 661-276-3566 

pao@dfrc.nasa.gov FS-2002-08-068 DFRC 

Helios Prototype:

The forerunner of 21st century solar-powered “atmospheric satellites” 

Project Summary 

The Helios Prototype is a remotely piloted flying wing aircraft developed under NASA’s Environmental 

Research Aircraft and Sensor Technology (ERAST) project. The two primary goals of the Helios Prototype 

development are to demonstrate sustained flight at an altitude near 100,000 feet and flying non-stop for at least 

24 hours, including at least 14 hours above 50,000 feet. 

In 2001, the Helios Prototype achieved the first of the two goals by reaching an unofficial world-record 

altitude of 96,863 feet and sustaining flight above 96,000 feet for more than 40 minutes during a test flight near 

Hawaii. The aircraft is undergoing modifications and upgrades to enable it to accomplish the flight endurance 

milestone, presently planned for late summer, 2003. 

The operational and technical ability to reach these two goals is critical for NASA’s ERAST project. 

Through ERAST, many new propulsion, materials, control, instrumentation, and sensor technologies are being 

pioneered which could enable the development of a fleet of high-flying uninhabited aircraft that could conduct a 

wide variety of Earth and atmospheric science missions. Flying autonomously with mission-oriented payloads 

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and instrumentation, these ultra-high flyers could carry out 

storm tracking studies, atmospheric sampling, spectral 

imaging for agricultural and natural resources monitoring, 

pipeline monitoring and also serve as relay platforms for 

telecommunications systems. 

Developed by AeroVironment, Inc., of Monrovia, 

California, with the assistance of NASA’s Dryden Flight 

Research Center, the Helios Prototype is one of several 

remotely piloted aircraft that have been involved in 

NASA’s ERAST project. It is an enlarged version of the 

Centurion flying wing, flown at Dryden in late 1998 to 

verify the handling qualities and performance of a lightweight all-wing aircraft of more than 200-foot wingspan. It 

was renamed the Helios Prototype to reflect its role as a 

forerunner of the eventual Helios production aircraft, which 

will be designed to fly continuously for up to six months at 

a time on science and commercial missions. 

Aircraft Description 

The Helios Prototype is an ultra-lightweight flying 

wing aircraft with a wingspan of 247 feet, longer than the 

wingspans of the U.S. Air Force C-5 military transport (222 

feet) or the Boeing 747 commercial jetliner (195 or 215 

feet, depending on the model), the two largest operational 

aircraft in the United States. The electrically powered 

Helios is constructed mostly of composite materials such as 

carbon fiber, graphite epoxy, Kevlar , styrofoam, and a 

thin, transparent plastic skin. The main tubular wing spar is 

made of carbon fiber. The spar, which is thicker on the top 

and bottom to absorb the constant bending motions that 

occur during flight, is also wrapped with Nomex and 

Kevlar for additional strength. The wing ribs are also made 

of epoxy and carbon fiber. Shaped styrofoam is used for 

the wing’s leading edge and a durable clear plastic film 

covers the entire wing. 

The Helios Prototype shares the same eight-foot 

wing chord (distance from leading to trailing edge) as its 

Pathfinder and Centurion predecessors. The 247-foot 

wingspan gives the Helios Prototype an aspect ratio of 

almost 31 to 1. The wing thickness is the same from tip to 

tip, 11.5 inches or 12 percent of the chord, and it has no 

taper or sweep. The outer panels have a built-in 10-degree 

dihedral (upsweep) to give the aircraft more lateral stability. A slight upward twist at the tips of the trailing edge 

helps prevent wingtip stalls during the slow landings and 

turns. The wing area is 1,976 sq. ft., which gives the craft a 

maximum wing loading of only 0.81 lb./sq. ft. when flying 

at a gross weight of 1,600 lb. 

The all-wing aircraft is assembled in six sections, 

each about 41 feet long. An underwing pod is attached at 

each panel joint to carry the landing gear, the battery power 

system, flight control computers, and data instrumentation. 

The five aerodynamically shaped pods are made mostly of 

the same materials as the wing itself, with the exception of 

the transparent wing covering. Two wheels on each pod 

make up the fixed landing gear—rugged mountain bike 

wheels on the rear and smaller scooter wheels on the front. 

The Helios Prototype is powered by 14 brushless 

direct-current electric motors mounted across the wing’s 

entire span. The motors are rated at 2 hp. (1.5 kW) each, 

and drive lightweight two-blade propellers of 79-inch 

diameter. The propellers are made from advanced composite materials and a laminar-flow design for efficiency at 

high altitudes. 

The cruising speed of Helios ranges from 19 to 27 

mph, with takeoff and landing equating to the average 

speed of a bicycle. 

For the initial flight tests at Dryden in 1999, the 

Helios Prototype was powered by lithium battery packs 

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carried in the underwing pods. More than 62,000 solar cells 

were installed on the entire upper surface of the wing 

during 2000. For eventual long duration missions, the solar 

cells will be assisted by an on-board fuel-cell based energy 

storage system now in development that will power the 

motors and aircraft systems through the night. 

The only flight control surfaces used on the Helios 

Prototype are 72 trailing-edge elevators that provide pitch 

control. Spanning the entire wing, they are operated by tiny 

servomotors 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. 

The Helios Prototype weighs in at only 1,322 

pounds empty. During the 1999 development flights, the 

aircraft carried payloads of up to 626 pounds — a combination of ballast and instrumentation, with the amount on 

each flight determined by the flight objectives. During the 

2001 flights, the Helios Prototype flew at a weight of about 

1,600 pounds, including its flight test instrumentation. The 

ultimate objective of the Helios design is to carry a payload 

of scientific instruments or telecommunications relay 

equipment averaging about 200 pounds to high altitudes for 

missions lasting from several days to several months. 

The Helios Prototype is controlled remotely by a 

pilot on the ground, either from a mobile control van or a 

fixed ground station that is equipped with a full flight 

control station and consoles for systems monitoring. A 

flight termination system, required on remotely piloted 

aircraft flown in military restricted airspace, includes a 

parachute system deployed on command, plus a homing 

beacon to aid in the aircraft’s location. In case of loss of 

control or other contingency, the system is designed to 

bring the aircraft down within the restricted airspace area to 

avoid any potential damage or injuries to fixed assets or 

personnel on the ground. 

Helios Prototype Development Path 

AeroVironment initiated its development of fullscale solar-powered aircraft with the Gossamer Penguin 

and Solar Challenger vehicles in the late 1970’s and early 

1980’s, following the pioneering work of Robert Boucher, 

who built the first solar-powered flying models in 1974. 

The Helios Prototype is the fourth generation of all-wing 

aircraft designed and built by AeroVironment at its Design 

Development Center in Simi Valley, Calif. as technology 

demonstrators for future solar-powered high-altitude 

aircraft platforms for science and commercial missions. 

The first generation was the Pathfinder, developed 

as the HALSOL aircraft in the early 1980’s with a wing 

span of 98.4 feet. Its eight electric motors — later reduced 

to six — were first powered by batteries. Two underwing 

pods contain the landing gear, batteries, instrumentation 

system, and flight control computer. By the time the 

aircraft was adopted into the ERAST project in late 1993, 

solar cells were being added, eventually covering the entire 

upper surface of the wing. Pathfinder set a solar-powered 

altitude record of 50,500 feet at NASA Dryden on Sept. 11, 

1995. 

After further modifications, the aircraft was moved 

to the U.S. Navy’s Pacific Missile Range Facility (PMRF) 

on the Hawaiian island of Kauai. On one of seven flights 

there in the spring of 1997, Pathfinder raised the altitude 

record for solar-powered aircraft — as well as propellerdriven aircraft — to 71,530 feet. During those flights, 

Pathfinder carried two lightweight imaging instruments to 

learn more about the island’s terrestrial and coastal ecosystems, demonstrating the potential of such aircraft as platforms for scientific research. 

Pathfinder Plus was the next step leading to Helios. 

It used four of the five sections from the original Pathfinder 

wing, but substituted a longer center section that increased 

the span to 121 feet, increasing the area available for solar 

cells on the upper wing surface. The number of electric 

motors was increased to eight. The Pathfinder Plus development flights flown at PMRF in the summer of 1998 

validated power, aerodynamic, and systems technologies 

for its successor, the Centurion, which at that time was 

being designed to reach 100,000-foot altitude on solar 

power. On Aug. 6, 1998, Pathfinder Plus proved its design 

by raising the world altitude record to 80,201 feet for solarpowered and propeller-driven aircraft. 

Centurion, originally built for the 100,000-foot 

altitude milestone specified by the ERAST project, was the 

third generation. Its wing incorporated a redesigned highaltitude airfoil and the span was increased to 206 feet. The 

number of motors was increased to 14 and the number of 

underwing pods to carry batteries, flight control system 

components, ballast, and landing gear rose to four. Centurion flew three development test flights on battery power in 

late 1998 at low altitudes at NASA Dryden Flight Research 

Center at Edwards, Calif., verifying the design’s handling 

qualities, performance, and structural integrity. 

The Centurion was then modified into the Helios 

Prototype configuration by adding a sixth 41-foot wing 

section and a fifth landing gear and systems pod, becoming 

the fourth configuration in the series of solar-powered 

flying wing demonstrator aircraft developed by 

AeroVironment under the ERAST project. The larger wing 

on the Helios Prototype accommodates more solar arrays to 

provide adequate power for the sun-powered development 

flights that followed. 

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Using the traditional incremental or stair-step 

approach to flight testing, the Helios Prototype was first 

flown in a series of battery-powered development flights in 

late 1999 to validate the longer wing’s performance and the 

aircraft’s handling qualities. Instrumentation that was used 

for the follow-on solar-powered altitude and endurance 

flights was also checked out and calibrated during the 

initial low-altitude flights at NASA Dryden. 

Solar panels were then installed in 2000 for further development flights flown during the summer of 2001 over the 

Pacific Ocean near Hawaii. On Aug. 13, 2001, the Helios 

Prototype reached an altitude of 96,863 feet, a world record 

for sustained horizontal flight by a winged aircraft. The 

altitude reached was more than 11,000 feet — or more than 

two miles — above the previous altitude record for sustained flight by a winged aircraft. 

Round-the-clock Operation 

Flying the solar-electric Helios Prototype at night 

when no sunlight is available will require a supplemental 

electrical energy source to provide power to operate the 

motors, avionics and experiment payloads. The number of 

ordinary rechargeable batteries of the NiCad or Lithium 

families that would be required to successfully power the 

aircraft through an entire night would be too heavy to meet 

either altitude or endurance goals. Therefore several years 

ago, the Helios Team turned to proton-exchange membrane 

fuel cell technology as the best option to give the Helios 

Prototype full day-and-night flight capability. Protonexchange membrane—also known as polymer electrolyte— 

technology has advanced significantly in recent years due 

to the large interest and investment in alternative energy 

research, primarily by the automotive industry. 

The premise of a fuel cell-based system is that 

oxygen and hydrogen are combined to produce electric 

power, heat and water. As long as these gases are supplied, 

the unit continues to produce power. Not only are these 

systems attractive from an environmental standpoint, but 

since they have very few moving parts, these systems have 

the potential of very high reliability. 

The Helios Team—including NASA, 

AeroVironment and several subcontractors—has tested a 

number of different fuel cells and investigated a variety of 

fuel cell-based power systems, with the goal of developing 

a system which would be significantly lighter than conventional energy storage systems. The Helios Team is currently building a fuel cell-based system which will be 

installed and flown on the Helios in 2003 to achieve a 

NASA milestone of flying for at least 14 hours above 

50,000 feet altitude. This will double the amount of time 

the Helios is able to operate above 50,000 feet altitude. In 

the future, a more sophisticated regenerative energy storage 

system is expected to provide night-time power for Helios 

derivatives on flights lasting from several weeks to several 

months. Developmental versions of both systems are 

undergoing operational testing at AeroVironment laboratories. 

Differential Thrust for Pitch Control 

A major test during the initial flight series was the 

evaluation of differential engine power as a means of pitch 

control. During normal cruise the outer wing panels of 

Helios are arched upward and give the aircraft the shape of 

a shallow crescent when viewed from the front or rear. This 

configuration places the motors on the outer wing panels 

higher than the motors on the center panels. Speeding up 

the outer-panel motors caused the aircraft to pitch down 

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and begin a descent. Conversely, applying additional power 

to the motors in the center panels caused Helios to pitch up 

and begin climbing. 

If AeroVironment engineers feel that the procedure 

provides adequate and safe pitch control for all flight 

operations, the elevator servo motors and all of their 

electrical wiring could be removed from a follow-on 

production Helios design, giving it an additional 25 to 30 

pounds of useful payload on future flights. The wing space 

now being used by the elevators could also be covered with 

solar arrays for additional power. 

Flight Test Venues 

The initial Helios Prototype development flights 

were conducted at the NASA Dryden Flight Research 

Center at Edwards, Calif., where NASA’s ERAST project 

is managed. The subsequent development flights on solar 

power which led to the world altitude record flight in the 

summer of 2001 were flown from the U.S. Navy’s Pacific 

Missile Range Facility on the Hawaiian island of Kaua’i. 

That site was chosen due to its more equatorial latitude, 

minimal air traffic and restricted test range airspace over 

the Pacific Ocean. 

ERAST Project Management 

Overall management of the ERAST aeronautical 

technology project is managed by NASA’s Dryden Flight 

Research Center, Edwards, Calif. 

A parallel effort is the development of lightweight, 

microminiaturized sensors that will be used to carry out the 

environmental research and Earth monitoring. This effort is 

led by NASA’s Ames Research Center, Moffett Field, 

Calif. 

Also contributing to the ERAST program in the 

areas of propulsion, energy storage systems, structures and 

systems analysis are NASA’s Glenn and Langley Research 

Centers. 

NASA is also working closely with the Federal 

Aviation Administration to develop “detect, see and avoid” 

systems; over-the-horizon command and control technologies and operational plans so remotely operated aircraft can 

be safely flown in national airspace. 

The ERAST project supports the Revolutionary 

Vehicles element of NASA’s Aerospace Technology 

Enterprise’ Aeronautics Blueprint by creating environmentally compatible aircraft with revolutionary capabilities for 

unprecedented levels of performance and safety. The 

Helios Prototype development is in line with the goal of 

developing efficient electric propulsion with advanced fuel 

cell technology and highly integrated airframe and propulsion designs to enable quiet, low-emission aircraft. 

Aircraft Specifications 

Wingspan: 247 ft.

Length: 12 ft.

Wing Chord: 8 ft.

Wing Thickness: 11.5 in. (12 percent of chord)

Wing area: 1,976 sq. ft.

Aspect Ratio: 30.9 to 1

Empty Weight: 1,322 lb.

Gross Weight: Up to 2,048 lb., varies depending on power

availability and mission profile.

Payload: Up to 726 lb., including ballast, instrumentation,

experiments and a supplemental electrical energy system,

when developed.

Electrical power: 62,120 bi-facial solar cells covering

upper wing surfaces. Cells are silicon-based, and are about

19 percent efficient in converting solar energy into electrical power. Lithium battery pack backup power to allow

limited operation after dark.

Propulsion: 14 brushless direct-current electric motors,

each rated at 2 hp. (1.5 kW), driving two-blade, widechord, 79-in. diameter laminar-flow propellers designed for

high altitude.

Airspeed: From 19 to 27 mph cruise at low altitudes, up to

170 mph ground speed at extreme altitude.

Altitude: Designed to operate at up to 100,000 ft., typical

endurance mission at 50,000 to 70,000 ft.

Endurance: With solar power, limited to daylight hours

plus up to five hours of flight after dark on storage batteries. When equipped with a supplemental electrical energy

system for nighttime flight, from several days to several

months.

Primary Materials: Carbon fiber composite structure,

Kevlar®, Styrofoam leading edge, transparent plastic film

wing covering.

Kevlar and Nomex are registered trademarks of E.I. Du

Pont De Nemours and Co.

August, 2002 

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