Helios Prototype

The NASA Pathfinder, NASA Pathfinder Plus, NASA Centurion and NASA Helios Prototype were an evolutionary series of solar- and fuel-cell

Fuel cell

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

-system-powered unmanned aerial vehicle

Unmanned aerial vehicle

An unmanned aerial vehicle , also known as a unmanned aircraft system , remotely piloted aircraft or unmanned aircraft, is a machine which functions either by the remote control of a navigator or pilot or autonomously, that is, as a self-directing entity...

s. AeroVironment, Inc.

AeroVironment

AeroVironment Inc. is a technology company in Monrovia, California, and Simi Valley, California, that is primarily involved in energy systems, electric vehicle systems, and unmanned aerial vehicles . Paul B. MacCready, Jr., a famous designer of human powered aircraft, founded the company in 1971...

developed the vehicles under NASA's Environmental Research Aircraft and Sensor Technology (ERAST)

ERAST Project

The Environmental Research Aircraft and Sensor Technology, or ERAST program was a NASA program to develop cost-effective, slow-flying unmanned aerial vehicles that can perform long-duration science missions at altitudes above 60,000 feet. The project included a number of different technology...

program. They were built to develop the technologies that would allow long-term, high-altitude aircraft to serve as "atmospheric satellites", to perform atmospheric research tasks as well as serve as communications platforms.

Pathfinder

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

Gossamer Penguin

|-References:...

and Solar Challenger

Solar Challenger

|-See also:-External links:*...

vehicles in the late 1970s and early 1980s, following the pioneering work of Robert Boucher, who built the first solar-powered flying models in 1974. Under ERAST, AeroVironment built four generations of long endurance unmanned aerial vehicles (UAVs), the first of which was the Pathfinder.

Development

In 1983, AeroVironment obtained funding from an unspecified US government agency to secretly investigate a UAV

Unmanned aerial vehicle

concept designated "High Altitude Solar" or HALSOL. The HALSOL prototype first flew in June 1983. Nine HALSOL flights took place in the summer of 1983 at Groom Lake

Area 51

Area 51 is a military base, and a remote detachment of Edwards Air Force Base. It is located in the southern portion of Nevada in the western United States, 83 miles north-northwest of downtown Las Vegas. Situated at its center, on the southern shore of Groom Lake, is a large military airfield...

in Nevada. The flights were conducted using radio control and battery power, as the aircraft had not been fitted with solar cells. HALSOL's aerodynamics were validated, but the investigation led to the conclusion that neither photovoltaic cell nor energy storage technology were mature enough to make the idea practical for the time being, and so HALSOL was put into storage.

In 1993, after ten years in storage, the aircraft was brought back to flight status for a brief mission by the Ballistic Missile Defense Organization

Ballistic Missile Defense Organization

The Ballistic Missile Defense Organization was an agency of the United States Department of Defense that began on 20 May 1974 with the responsibility for all U.S. ballistic missile defense efforts. It evolved from the SAFEGUARD System Organization. The original mission of BMDO was comparable to...

(BMDO). With the addition of small solar arrays, five low-altitude checkout flights were flown under the BMDO program at NASA Dryden in the fall of 1993 and early 1994 on a combination of solar and battery power.

In 1994 the aircraft transferred to the NASA ERAST Program to develop science platform aircraft technology. It was renamed "Pathfinder" because it was "literally the pathfinder for a future fleet of solar-powered aircraft that could stay airborne for weeks or months on scientific sampling and imaging missions". A series of flights were planned to demonstrate that an extremely light and fragile aircraft structure with a very high aspect ratio

Aspect ratio

The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...

(the ratio between the wingspan and the wing chord) can successfully take-off and land from an airport and can be flown to extremely high altitudes (between 50000 feet (15,240 m) and 80000 feet (24,384 m)) propelled by the power of the sun. In addition, the ERAST Project also wanted to determine the feasibility of such a UAV for carrying instruments used in a variety of scientific studies.

On October 21, 1995, the aircraft's fragility was aptly demonstrated when it was severely damaged in a hangar accident, but was subsequently rebuilt.

Aircraft description

Pathfinder was powered by eight electric motors — later reduced to six — which were first powered by batteries. It had a wing span of 98.4 feet (30 m). 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. The solar arrays provide power for the aircraft's electric motors, avionics, communications and other electronic systems. Pathfinder also had a backup battery system that can provide power for between two and five hours to allow limited-duration flight after dark.

Pathfinder flies at an airspeed of only 15 miles per hour (24.1 km/h) to 25 miles per hour (40.2 km/h). Although pitch control is maintained by the use of tiny elevators on the trailing edge of the wing, turns and yaw control are accomplished by slowing down or speeding up the motors on the outboard sections of the wing.

Flight testing and records

Major science activities of Pathfinder missions have included detection of forest nutrient status, forest regrowth after damage caused by Hurricane Iniki

Hurricane Iniki

Hurricane Iniki was the most powerful hurricane to strike the U.S. state of Hawaii in recorded history. Forming on September 5 during the strong El Niño of 1991–1994, Iniki was one of eleven Central Pacific tropical cyclones during the 1992 season. It attained tropical storm status on...

in 1992, sediment/algal concentrations in coastal waters and assessment of coral reef health. Science activities are coordinated by the NASA Ames Research Center and include researchers at the University of Hawaii

University of Hawaii

The University of Hawaii System, formally the University of Hawaii and popularly known as UH, is a public, co-educational college and university system that confers associate, bachelor, master, and doctoral degrees through three university campuses, seven community college campuses, an employment...

and the University of California

University of California

The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...

. Pathfinder flight tested two ERAST-developed scientific instruments, a high spectral resolution Digital Array Scanned Interferometer (DASI) and a high spatial resolution Airborne Real-Time Imaging System (ARTIS), both developed at Ames. These flights were conducted at altitudes between 22000 feet (6,705.6 m) and 49000 feet (14,935.2 m) in 1997.

On September 11, 1995, Pathfinder set an unofficial altitude record for solar powered aircraft of 50000 feet (15,240 m) during a 12 hour flight from NASA Dryden

Dryden Flight Research Center

The Dryden Flight Research Center , located inside Edwards Air Force Base, is an aeronautical research center operated by NASA. On March 26, 1976 it was named in honor of the late Hugh L. Dryden, a prominent aeronautical engineer who at the time of his death in 1965 was NASA's deputy administrator...

. This and subsequent records claimed by NASA for Pathfinder remain unofficial, as they were not validated by the FAI

Fédération Aéronautique Internationale

The Fédération Aéronautique Internationale is the world governing body for air sports and aeronautics and astronautics world records. Its head office is in Lausanne, Switzerland. This includes man-carrying aerospace vehicles from balloons to spacecraft, and unmanned aerial vehicles...

, the internationally-recognized aviation world record sanctioning body. The National Aeronautic Association

National Aeronautic Association

The National Aeronautic Association of the United States is a non-profit 501 organization and a member of the Fédération Aéronautique Internationale , the international standard setting and record-keeping body for aeronautics and astronautics. NAA is the official record-keeper for United States...

presented the NASA-industry ERAST team with an award for one of the "10 Most Memorable Record Flights" of 1995.

After further modifications, the aircraft was moved to the U.S. Navy's Pacific Missile Range Facility

Pacific Missile Range Facility

The Pacific Missile Range Facility, Barking Sands is a U.S. Naval facility and airport located five nautical miles northwest of the central business district of Kekaha, in Kauai County, Hawaii, United States....

(PMRF) on the Hawaiian island of Kauai

Kauai

Kauai or Kauai, known as Tauai in the ancient Kaua'i dialect, is geologically the oldest of the main Hawaiian Islands. With an area of , it is the fourth largest of the main islands in the Hawaiian archipelago, and the 21st largest island in the United States. Known also as the "Garden Isle",...

. On one of seven flights there in the spring and summer of 1997, Pathfinder raised the altitude record for solar-powered aircraft — as well as propeller-driven aircraft — to 71530 feet (21,802.3 m) on July 7, 1997. 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

During 1998, the Pathfinder was modified into the longer-winged Pathfinder-Plus configuration. It used four of the five sections from the original Pathfinder wing, but substituted a new 44 feet (13.4 m) long center wing section that incorporated a high-altitude airfoil designed for the follow-on Centurion/Helios. The new section was twice as long as the original, and increased the overall wingspan of the craft from 98.4 feet (30 m) to 121 feet (36.9 m). The new center section was topped by more-efficient silicon solar cells developed by SunPower Corporation

SunPower

SunPower Corporation designs and manufactures high-efficiency crystalline silicon photovoltaic cells, roof tiles and solar panels based on a silicon all-back-contact solar cell invented at Stanford University. SunPower Corporation is publicly traded on the NASDAQ as SPWRA and SPWRB...

of Sunnyvale, California

Sunnyvale, California

Sunnyvale is a city in Santa Clara County, California, United States. It is one of the major cities that make up the Silicon Valley located in the San Francisco Bay Area...

, which could convert almost 19 percent of the solar energy they receive to useful electrical energy to power the craft's motors, avionics and communication systems. That compared with about 14 percent efficiency for the older solar arrays that cover most of the surface of the mid- and outer wing panels from the original Pathfinder. Maximum potential power was boosted from about 7,500 watts on Pathfinder to about 12,500 watts on Pathfinder-Plus. The number of electric motors was increased to eight, and the motors used were more powerful units, designed for the follow-on aircraft.

The Pathfinder-Plus development flights flown at PMRF in the summer of 1998 validated power, aerodynamic, and systems technologies for its successor, the Centurion. On August 6, 1998, Pathfinder-Plus proved its design by raising the national altitude record to 80201 feet (24,445.3 m) for solar-powered and propeller-driven aircraft.

Atmospheric satellite tests

In July 2002 Pathfinder-Plus carried commercial communications relay equipment developed by Skytower, Inc., a subsidiary of AeroVironment

AeroVironment

, in a test of using the aircraft as a broadcast platform. Skytower, in partnership with NASA and the Japan Ministry of Telecommunications

Ministry of Internal Affairs and Communications

The ' or Ministry of Internal Affairs and Communications is a cabinet-level ministry in the Government of Japan. The English name Ministry of Public Management, Home Affairs, Posts and Telecommunications was used prior to 2004...

, tested the concept of an "atmospheric satellite" by successfully using the aircraft to transmit both an HDTV

High-definition television

High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...

signal as well as an IMT-2000 wireless communications signal from 65000 feet (19,812 m), giving the aircraft the equivalence of a 12 miles (19.3 km) tall transmitter tower. Because of the aircraft's high lookdown angle, the transmission utilized only one watt of power, or 1/10,000 of the power required by a terrestrial tower to provide the same signal. According to Stuart Hindle, Vice President of Strategy & Business Development for SkyTower, "SkyTower platforms are basically geostationary satellites without the time delay." Further, Hindle said that such platforms flying in the stratosphere, as opposed to actual satellites, can achieve much higher levels of frequency use. "A single SkyTower platform can provide over 1,000 times the fixed broadband local access capacity of a geostationary satellite using the same frequency band, on a bytes per second per square mile basis."

Ray Morgan, president of AeroVironment, has described the concept as, "What we're trying to do is create what we call an 'atmospheric satellite,' which operates and performs many of the functions as a satellite would do in space, but does it very close in, in the atmosphere"

Centurion

Centurion, originally built for the 100000 feet (30,480 m) altitude on solar power milestone specified by the ERAST project, was the third generation aircraft in this series. The ERAST program managers had determined that an aircraft based on the Pathfinder/Pathfinder Plus concept would be the lowest risk approach of achieving the altitude goal.

Initially, a quarter-scale model of the Centurion was test flown at El Mirage Dry Lake

El Mirage Dry Lake

El Mirage Lake is a dry lake bed in the Mojave Desert of San Bernardino County, California. The lake is located about west-northwest of the town of Adelanto and north of Highway 18 in San Bernardino County....

on March 4, 1997. The full-size Centurion's maiden flight took place at Rogers Dry Lake

Rogers Dry Lake

Rogers Dry Lake is an endorheic desert salt pan in the Mojave Desert of Kern County, California. The lake derives its name from the Anglicization from the Spanish name, Rodriguez Dry Lake. It is the central part of Edwards Air Force Base as its hard surface provides a natural extension to the...

on November 10, 1998, and lasted a total of 1 hr and 24 minutes. At the time, it weighed in at 1385 pounds (628.2 kg) (including a 150 pounds (68 kg) steel anvil hanging on its centerline to simulate a payload) for its first flight. The flight was nearly flawless and was followed by a second similar performance on November 19, this time before a crowd of VIPs and Media. It lasted 1 hr and 29 minutes. The third and final flight of the low altitude test series took place on December 3. On this flight the vehicle was loaded down to its maximum gross weight of 1806 pounds (819.2 kg) to test its weight carrying capability. Total flight time on this flight was 30 minutes, as it was shortened because high winds were anticipated by mid-morning. All of these flights took place on battery power and verified the design's handling qualities

Handling qualities

Handling qualities is one of the two principal regimes in the science of flight test . Handling qualities involves the study and evaluation of the stability and control characteristics of an aircraft...

, performance, and structural integrity. Following these three flights, NASA decided to expand the aircraft into the Helios Prototype, with work starting in January, 1999.

Aircraft description

The design of Centurion resulted in an aircraft that looked very much like the Pathfinder, but with a much longer wingspan of 206 feet (62.8 m). Although the Centurion shape resembled the Pathfinder, the structure was designed to be stronger and capable of carrying numerous payloads (up to 600 pounds (272.2 kg)) more efficiently. Its wing incorporated a redesigned high-altitude airfoil and the span was increased to 206 feet (62.8 m). 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.

The Centurion was modified into the Helios Prototype configuration by adding a sixth 41 feet (12.5 m) 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 accommodated more solar arrays to provide adequate power for the sun-powered development flights that followed. The aircraft's maiden flight was on September 8, 1999.

The ERAST program had two goals when developing the Helios Prototype: 1) sustained flight at altitudes near 100000 feet (30,480 m) and 2) endurance of at least 24 hours, including at least 14 of those hours above 50000 feet (15,240 m). To this end, the Helios Prototype could be configured in two different ways. The first, designated HP01, focused on achieving the altitude goals and powered the aircraft with batteries and solar cells. The second configuration, HP03, optimized the aircraft for endurance, and used a combination of solar cells, storage batteries and a modified commercial hydrogen–air fuel cell system for power at night. In this configuration, the number of motors was reduced from 14 to ten.

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.

Aircraft description

The Helios Prototype is an ultra-lightweight flying wing aircraft with a wingspan of 247 feet (75.3 m), longer than the wingspans of the U.S. Air Force C-5

C-5 Galaxy

The Lockheed C-5 Galaxy is a large military transport aircraft built by Lockheed. It provides the United States Air Force with a heavy intercontinental-range strategic airlift capability, one that can carry outsize and oversize cargos, including all air-certifiable cargo. The Galaxy has many...

military transport (222 feet (67.7 m) or the Boeing 747

Boeing 747

The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...

(195 feet (59.4 m) or 215 feet (65.5 m), 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

Carbon fiber

Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

, graphite epoxy, Kevlar

Kevlar

Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

, styrofoam

Styrofoam

Styrofoam is a trademark of The Dow Chemical Company for closed-cell currently made for thermal insulation and craft applications. In 1941, researchers in Dow's Chemical Physics Lab found a way to make foamed polystyrene...

, 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

Nomex

Nomex is a registered trademark for flame resistant meta-aramid material developed in the early 1960s by DuPont and first marketed in 1967.- Properties:...

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 8 feet (2.4 m) wing chord (distance from leading to trailing edge) as its Pathfinder and Centurion predecessors. The 247 feet (75.3 m) wingspan gives the Helios Prototype an aspect ratio

Aspect ratio (wing)

In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth . A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings....

of almost 31 to 1. The wing thickness is the same from tip to tip, 11.5 inches (29.2 cm) inches or 12 percent of the chord, and it has no taper or sweep

Swept wing

A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...

. The outer panels have a built-in 10-degree dihedral to give the aircraft more lateral stability. A slight upward twist

Wing twist

Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing.Often, the purpose of lift redistribution is to ensure that the wing tip is the last part of the wing surface to stall, for example when executing a roll or steep climb; it involves twisting the...

at the tips of the trailing edge helps prevent wing tip

Wing tip

A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...

stalls

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

during the slow landings and turns. The wing area is 1976 sq ft (183.6 m²)., 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 (12.5 m) 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 only flight control surfaces used on the Helios Prototype are 72 trailing-edge

Trailing edge

The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge rejoins. Essential control surfaces are attached here to redirect the air flow and exert a controlling force by changing its momentum...

elevators

Elevator (aircraft)

Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...

that provide pitch control. Spanning the entire wing, they are operated by tiny servomotors

Servomechanism

thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...

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. A major test during the initial flight series was the evaluation of differential motor 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 and begin a descent. Conversely, applying additional power to the motors in the center panels caused Helios to pitch up and begin climbing.

Solar panels, supplied by SunPower were then installed in 2000. The cells featured a rear-contact cell design that placed wire

Wire

A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

s on the underside of the cells, so as not to obstruct the cells' exposure to solar radiation.

Records

On August 13, 2001, the Helios Prototype piloted remotely by Greg Kendall reached an altitude of 96863 feet (29,523.8 m), a world record for sustained horizontal flight by a winged aircraft. The altitude reached was more than 11000 feet (3,352.8 m) — or more than 2 miles (3.2 km) — above the previous altitude record for sustained flight by a winged aircraft. In addition, the aircraft spent more than 40 minutes above 96000 feet (29,260.8 m).

Crash

On June 26, 2003, the Helios Prototype broke up and fell into the Pacific Ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

about ten miles (16 km) west of the Hawaiian Island Kauai

Kauai

during a remotely piloted systems checkout flight in preparation for an endurance test scheduled for the following month.

On the morning of the accident, weather forecasts indicated that conditions were inside the acceptable envelope, although during the preflight go/no-go review, the weather forecaster gave it a "very marginal GO." One of the primary concerns was a pair of wind shear

Wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

zones off the island's coast. After a delayed take off, due to the failure of the winds to shift as predicted, Helios spent more time than expected flying through a zone of low-level turbulence on the lee side of Kauai, because it was climbing more slowly than normal, since it had to contend with cloud shadows and the resultant reduction in solar power.

As the aircraft climbed through 2800 feet (853.4 m), according to the subsequent mishap investigation report, "At about 30 minutes into the flight, the aircraft encountered turbulence and morphed into an unexpected, persistent, high dihedral configuration. As a result of the persistent high dihedral, the aircraft became unstable in a very divergent pitch mode in which the airspeed excursions from the nominal flight speed about doubled every cycle of the oscillation

Phugoid

A phugoid or fugoid is an aircraft motion where the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "uphill" and "downhill." This is one of the basic flight dynamics modes of an aircraft , and a classic example of a positive...

. The aircraft’s design airspeed was subsequently exceeded and the resulting high dynamic pressures caused the wing leading edge secondary structure on the outer wing panels to fail and the solar cells and skin on the upper surface of the wing to rip off. The aircraft impacted the ocean within the confines of the PMRF test range and was destroyed. Most of the vehicle structure was recovered except the hydrogen–air fuel cell pod and two of the ten motors, which sank into the ocean."

The investigation report identified a two-part root cause of the accident:

"Lack of adequate analysis methods led to an inaccurate risk assessment of the effects of configuration changes leading to an inappropriate decision to fly an aircraft configuration highly sensitive to disturbances."
"Configuration changes to the aircraft, driven by programmatic and technological constraints, altered the aircraft from a spanloader to a highly point-loaded mass distribution on the same structure significantly reducing design robustness and margins of safety."

Specifications

Specifications
	Pathfinder	Pathfinder-Plus	Centurion	Helios HP01	Helios HP03
Length ft(m)	12 (3.6)	12 (3.6)	12 (3.6)	12 (3.6)	16.5 (5.0)
Chord ft(m)	8 (2.4)
Wingspan ft(m)	98.4 (29.5)	121 (36.3)	206 (61.8)	247 (75.3)
Aspect ratio	12 to 1	15 to 1	26 to 1	30.9 to 1
Glide ratio	18 to 1	21 to 1	?	?	?
Airspeed kts(km/h)			15–18 (27–33)	16.5–23.5 (30.6–43.5)	?
Max altitude ft(m)	71,530 (21,802)	80,201 (24,445)	n/a	96,863 (29,523)	65,000 (19,812)
Empty Wt lb(kg)	?	?	?	1,322 (600)	?
Max. weight lb(kg)	560 (252)	700 (315)	±1,900 (±862)	2,048 (929)	2,320 (1,052)
Payload lb(kg)	100 (45)	150 (67,5)	100–600 (45–270)	726 (329)	?
Engines	electric, 2 hp (1.5 kW) each
No. of engines	6	8	14	14	10
Solar pwr output (kW)	7.5	12.5	31	?	?
Supplemental power	batteries	batteries	batteries	Li batteries	Li batteries, fuel cell

External links

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