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HELIOS On August 13th 2001 over Hawaii, the AeroVironment Helios Prototype powered flying wing reached a height of 96,863 feet, thereby setting a new altitude record for winged aircraft. At first glance, this is a wonderful achievement. But that is only the beginning of an even more stunning set of achievements planned for this amazing aircraft, including its first commercial test flights this month. So, what is the Helios Prototype and just what is the story of this most remarkable and unique aircraft? T he Helios Prototype is a remotely-piloted solarpowered flying wing developed to demonstrate the capability of achieving two significant milestones for NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) project. Firstly, reaching and sustaining flight at an altitude near 100,000 feet and secondly, flying non-stop for at least 24 hours including at least 14 hours above 50,000 feet. In 2001, Helios achieved the first of these goals by reaching an unofficial world-record altitude for a non-rocket powered aircraft of 96,863 feet and sustaining flight above 96,000 feet for more than 40 minutes during a test flight near Hawaii. The Helios Prototype is an enlarged version of the Centurion flying wing, flown at Dryden, California in late 1998 to verify the handling qualities and performance of a lightweight all-wing aircraft of more than 60-metre wingspan. It was renamed the Helios Prototype to clearly identify it as a forerunner of the eventual Helios production 8  Silicon Chip aircraft, which will be designed to fly continuously for up to six months at a time on scientific and commercial missions. Developed by AeroVironment Inc, of Monrovia, California, the Helios Prototype has what is probably the most interesting pedigree in aviation history. In 1959 the British industrialist Henry Kremer announced a competition with a prize of $95,000 for the first man-powered aircraft to successfully demonstrate sustained, manoeuvrable human-powered flight. Dr Paul MacCready and Dr Peter Lissamen designed the “Gossamer Condor”, constructed of thin aluminium tubes and Mylar film, supported with stainless steel wire. On August 23, 1977, championship bicyclist and hang-glider enthusiast Bryan Allen flew the Condor for 7 minutes, 2.7 seconds, over a closed figure-8 course to win the coveted $95,000 Kremer Prize. Gossamer Albatross In 1979, MacCready’s Gossamer Albatross, with the same 32kg weight and 29-metre wing span as the Condor, www.siliconchip.com.au the solar powered plane by Bob Young Helios Altitude, 13 August 2001 100,000 90,000 GPS Altitude (feet) 80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 Fig.1: record flight altitude/versus time chart. crossed the English Channel in turbulent winds in three hours. Cyclist Bryan Allen, who pedaled the Gossamer Condor, also provided the human power for the Albatross. For MacCready and the other manpower enthusiasts, it was a tough battle. To illustrate just how tough, consider the following. A hang-glider requires 1.5hp to sustain level flight whereas a man can only generate about 0.30.5hp. MacCready believed that a big, efficient, super-light wing was the answer and set about to prove it. While the knockers stood around with their hands in their pockets, betting it could not be done, MacCready simply went about his business putting his muscle where his mouth is, quietly betting that it could be done. MacCready won! And he won in more ways than one. As a result of the public exposure from the Gossamer Condor and Gossamer Albatross, Dr MacCready’s company AeroVironment, dedicated to environmentally friendly technologies, embarked on a remarkable series of projects, some of which are shown in Fig.2. While seeking ways of storing energy on board a human-powered aircraft – by means of a battery charged by www.siliconchip.com.au 0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 Hawaii Standard Time (Hours) the pilot’s pedaling – MacCready’s team gained insights into making efficient use of very limited battery power. Back on terra firma, he has made his mark as well. He guided the team that developed the GM Sunraycer, a solar-powered car that won a 3000km race across Australia. MacCready’s team, with GM support and help, then developed the Impact demonstrator electric vehicle, which in 1991 stimulated California’s zero-emissions mandate. The Impact became the currently available EV1. MacCready traces his company’s success in this field in no small part to the experience his team gained while running after his fragile flying machines. This is a stunning story about a remarkable man and it all began because a friend defaulted on a $100,000 loan that Paul MacCready had guaranteed and he needed that $95,000 Kremer prize to pay it back. Following Solar Challenger and making use of the expertise gained on human-powered aircraft, MacCready’s team developed the unmanned and solar-powered Pathfinder, the first of the high-flying solar UAVs. In July 1997, Pathfinder set a new altitude record for propeller-driven June 2002  9 Fig.2: Paul MacCready’s Aeronvironment Inc is also responsible for many other environmentally-based projects and is not confined to aircraft by any means. They’re into electric vehicles and renewable energy – and even power-assisted pushbikes! Our apologies for the quality of this graphic . . . planes by reaching 21.8 kilometres (71,500 feet). Pathfinder-Plus followed and pushed the propeller-driven altitude record to 82,000 feet. Pathfinder was followed by the 62-metre span Centurion which was flown in 1998. The Centurion’s wingspan was then extended to 75 metres and the aircraft was renamed the Helios Prototype. The Helios Prototype is only one of many remotely piloted aircraft that have been involved in NASA’s ERAST project (see Fig.3). The Helios Prototype was designed as a solar-powered propeller-driven aircraft, although the first series of test and evaluation flights in the summer of 1999 used batteries to power its 14 electric motors. High efficiency solar panels were installed in 2000 for further development flights, which were flown during the summer of 2001 over the Pacific Ocean near Hawaii. At the limits Flight at the absolute ceiling for any aircraft is a precarious business. As the air thins, propellers lose efficiency, thrust drops off and the wings struggle to maintain the required lift. Approaching absolute ceiling, the rate of climb falls away and once it falls below 100 feet/minute the aircraft has reached what the military people define as the “service ceiling”. At “absolute ceiling”, the rate of climb has fallen to zero and the maximum speed and the stalling speed have finally converged, so there is only one speed at which the aircraft can fly. At this point, nasty things can happen to an inattentive pilot. An interesting sidelight here is that flight at 100,000 feet roughly approximates atmospheric conditions on Mars, 10  Silicon Chip which means the Helios Prototype is providing valuable data for the proposed Martian Aircraft. NASA’s ERAST project is aimed at the development of aeronautical technologies that are expected to produce a new generation of remotely piloted or autonomous aircraft for a variety of upper-atmospheric science missions. The ERAST project aims at revolutionising the way in which aircraft are designed and built. Flying at slow speeds for long periods of time at altitudes of up to 100,000 feet, post-ERAST vehicles may be used to gather, identify and monitor environmental data. Other applications may include assessing global climate changes, studying Earth resources, assisting in disaster recovery situations or serving as telecommunications platforms, all at a fraction of the cost of placing satellites into space. Here one wonders at the practical problems to be encountered with sustained operations at altitudes in excess of 60,000 feet. Ultraviolet radiation strips plastic of its plasticiser and the film becomes brittle and easy to snap. Add to this the extreme cold at those altitudes, exacerbating the brittleness, and suddenly the job of keeping the airframe intact for six months becomes an awesome task. Still, does anyone doubt that it will be done? A parallel effort to developing the aircraft is the development of the lightweight, micro-miniaturised sensors that will be used to carry out the environmental research and Earth monitoring. Also contributing to the ERAST program in the areas of propulsion, energy storage systems, structures, systems analysis and sensor technology are NASA’s Glenn, Langley and Ames Research Centers. NASA is also working closely with the Federal Aviation Administration to develop “dewww.siliconchip.com.au Coming or going? Actually, it’s going: Helios Prototype taking off from the US Pacific Missile Range Facility, Kauai, Hawaii, at the start of its record-breaking flight: 8.48AM, August 18 2001. The first commercial test flights of Helios (with communications technology and remote imaging payloads) are actually planned for this month (June 2002). tect, see and avoid” systems which are over-the-horizon command and control technologies and operational plans so that remotely-controlled aircraft can be safely flown in national airspace. All of this is part of the rapidly developing unmanned aerial vehicle movement that long-term readers of SILICON CHIP have been kept well informed about over the past 10 years. As a result of the successful Helios Prototype flights, Aerovironment have established a subsidiary company, SkyTower Inc, to commercialise Helios. Here Helios is envisioned as merely one component in a complex communications network known as SkyTower. As part of the SkyTower network, Helios is to be used as a virtual geo-sta- tionary satellite, circling for periods of up to six months. According to AeroVironment, Helios, acting as a geo-stationary satellite but without the time delay (equivalent to an 18km high tower), has many advantages: • Low overall system cost. • Concentrates capacity over populated areas and provides high look angles, resulting in improved coverage compared to satellite and terrestrial systems. For example, a single aeroplane can cover a service area of approximately 64km in diameter with a look angle from 30-90°. d • Can increase bandwidth capacity. • Due to the lower elevation of Helios compared with space satellites, less power is required for transmitting Fig.3: other aircraft associated with NASA’s ERAST project. In the main pic are the Proteus (Sealed Composites), the Perseus (Aurora), the Centurion (AeroVironment) and the Altus II (General Atomics). Inset above are the Pathfinder Plus (AeroVironment) and Altair (General Atomics). www.siliconchip.com.au June 2002  11 Fig.4: Cruising above 60,000 feet, well out of reach of commercial air traffic and weather disturbances, Helios, as part of the proposed SkyTower network, will serve as an information gathering and communications relay station. and receiving, smaller/lower cost communications equipment can be used and/or network performance can be improved. • Rapidly deployable to provide immediate target coverage and easily relocated, maintained and upgraded. Aircraft Description The Helios Prototype is an ultra-lightweight flying wing aircraft with a wingspan of 75 metres. This is longer than the wingspans of the US Air Force C-5 military transport (68m) or the Boeing 747 jetliner (65m). The electrically powered Helios is constructed mostly of composite materials such as carbon fibre, graphite epoxy, Kevlar, styrofoam and a thin, transparent plastic skin. There are 14 1.5kW electric motors on the aircraft. During the dark descent on the record-breaking flight, these became generators to power the aircraft electrics. 12  Silicon Chip The main tubular wing spar is made of carbon fibre. The spar is thicker on the top and bottom to absorb the constant bending motions that occur during flight and is also wrapped with Nomex and Kevlar for additional strength. The wing ribs are also made of epoxy and carbon fibre. Shaped styrofoam is used for the wing’s leading edge and a durable clear plastic film covers the entire wing. The Helios Prototype uses the same wing plan-form as its predecessors, Pathfinder and Centurion. With a wingspan of 75.3m and a chord of 2.43m, (distance from leading to trailing edge) the Helios Prototype has an aspect ratio of almost 31:1. The wing thickness is the same from tip to tip, 292mm or 12% of the chord, and it has no taper or sweep. The outer panels have a built-in 10 ° dihedral (upsweep) to give the aircraft more lateral (roll) stability. A slight upward twist of the tips at the trailing edge (washout) helps prevent wingtip stalls during the slow landings and turns. The wing area is 183 square metres, giving the aircraft a maximum wing loading of 4kg/m2 when flying at a gross weight of 750kg. This is an extremely low wing loading when one considers that the typical R/C model flies with a wing loading of 7-9kg per square metre and full size aircraft may push the wing loading up into the hundreds of kilograms per square metre. However, this low wing loading is absolutely essential in the ultra-thin air at 100,000 feet. The flying wing aircraft is assembled in six sections, each 12.5 metres 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 constructed mostly of the same materials as the wing itself, with the www.siliconchip.com.au exception of the transparent wing covering. The fixed landing gear is contained in the underwing pods and consists of rugged mountain bike wheels on the rear and smaller scooter wheels on the front; the lineage from Gossamer Condor is unmistakable. Power is provided by 14 brushless DC electric motors mounted across the wing’s entire span. The motors are each rated at 1.5kW and drive lightweight two-blade, wide-blade propellers two metres in diameter. The propellers are made from advanced composite materials and feature a laminar-flow design for maximum efficiency at high altitudes. For the first flight tests carried out at Dryden in 1999, the Helios Prototype was powered by lithium battery packs carried in the underwing pods. Eventually, more than 62,000 solar cells were installed on the entire upper surface of the wing during the year 2000. The final design stage for long-duration missions calls for the solar cells to not only power the electric motors but also to charge an on-board fuel-cell based energy storage system. This system now in development will power the motors and avionics through the night. The cruising speed of Helios ranges from 19-27mph at sea level to 170mph ground speed at extreme altitudes, with takeoff and landing speeds not quoted. However these are presumably around the 10-12mph mark. Here one wonders about the practical problems encountered when operating an aircraft with such low airspeeds. Ground speed can be very quickly eroded and assume negative values (in other words, flying backwards relative to the ground) in any sort of headwind. Some of the small Fitting just some of those 62,120 high-efficiency bi-facial PVCs (solar cells). They account for about $US10 million of the Helios Prototype’s $US15million price tag. Helios Prototype Specifications Wingspan: ��������������75.3 metres. Length: �������������������3.6 metres. Wing Chord: �����������2.4 metres. Wing Thickness: �����292mm (12% of chord). Wing area: �������������185 square metres. Aspect Ratio: ���������30.9:1 Empty Weight: ��������600kg. Gross Weight: ��������Up to 928kg; varies depending on power availability and mission profile. Payload: �����������������Up to 330kg, including ballast, instrumentation, experiments and a supplemental electrical energy system, when developed. Electrical power: ����62120 bi-facial solar cells covering upper wing surfaces. Cells are silicon-based and are about 19% efficient in converting solar energy into electrical power. Lithium battery backup to allow limited operation after dark. Propulsion: �������������14 brushless DC electric motors, each rated at 2 HP (1.5kW), driving two-blade, wide-chord, 2-metre diameter laminar-flow propellers designed for high altitude. Airspeed: ����������������19-27 mph cruise at low altitudes, up to 170 mph ground speed at extreme altitude. Altitude: ������������������Designed to operate at up to 100,000 feet, typical endurance mission at 50,000 to 70,000 feet. 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 night-time flight, from days to several months. Primary Materials: ��Carbon fibre composite structure, Kevlar, styrofoam leading edge, transparent plastic film wing covering. www.siliconchip.com.au June 2002  13 electric-powered UAVs used in operation Desert Storm suffered badly due to their low speed envelope and proved unusable in windy conditions. Yaw (turning) control is effected by applying differential power on the motors – speeding up the motors on one outer wing panel while slowing down motors on the corresponding inner panel. Pitch control is currently via 72 small trailing-edge elevators operated by 72 small servos. Spanning the entire wing, they are operated by the aircraft’s fight control computer. There is no mention of roll control in any of the literature provided. An alternative method of pitch control is currently under investigation using the dihedral and inflight wing flex to provide some differential in height between the inboard and outboard motors. As the outboard motors are higher than the inboard motors, increasing the power on the outboard and decreasing the power on the inboard will result in a nose-down pitch angle. Conversely, increasing the inboard power and decreasing outboard power will result in pitch up or climb. If successful, using this system will allow the removal of about 15kg of servos and control equipment, a valuable saving in such a lightweight structure. Also, the wing space now being used by the elevators could also be covered with solar arrays for additional power. The ultimate objective of the Helios design is to carry a payload of scientific instruments or telecommunications relay equipment averaging about 90kg to high altitudes for missions lasting from several days to several months. Empty, the Helios Prototype weighs in at only 600kg. Payloads vary depending upon the type of mission to be flown. During the 1999 development flights, the aircraft carried payloads of up to 280kg – 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 725kg, including its flight test instrumentation. The Helios Prototype follows the normal UAV control pattern, being controlled remotely by a pilot on the ground, either from a mobile control van or a fixed ground station equipped with a full flight control station and consoles for systems monitoring. As required on all remotely piloted aircraft flown in military restricted airspace, a flight termination system is provided. This 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, this system is designed to bring the aircraft down within the restricted airspace area to avoid any potential damage or injuries to personnel on the ground. Round-the-clock operation A supplemental electrical energy source will be required to provide power to operate the motors, avionics and experiment payloads when flying the solar-electric Helios Prototype at night or when no sunlight is available. Two versions are currently under development, one regenerative, one non-regenerative. AeroVironment is developing an intermediate fuel cellbased system without regenerative capability that will enable the Helios Prototype to achieve flight over a full 14  Silicon Chip diurnal cycle (ie, day and night) by the NASA milestone deadline of September, 2003. Fuel cells using proton-exchange membranes will combine hydrogen carried in pressurised tanks with oxygen from the atmosphere, producing electricity to power the aircraft at night. Although the goal is at least 24 hours, project officials hope to demonstrate that Helios can stay aloft for several days. The more ambitious regenerative system, based on hydrogen-oxygen fuel cell and electrolyser concepts, is a long-term goal. Briefly, the system would employ water as the primary component, with an electrolyser using excess electricity to break water into hydrogen and oxygen during the daytime, with the gases released being stored under pressure. At night, the process would be reversed, with a fuel cell recombining the two gases into water, with electricity produced as a byproduct. Depending upon funding availability and the overcoming of a variety of technical problems, development of the fully regenerative system would allow for a long-endurance demonstration mission of at least four days, some time in the future. Perhaps this eventually will allow Helios to fly for weeks or months on end. However, even the prototype Helios can achieve extended flight times by judicious use of the on-board storage batteries and solar cell banks. Taking off early in the morning uses all the daylight hours to provide the propulsion for climb to altitude. Descent and return home requires significantly less power (avionics and control only) and can then be carried out in darkness using the internal batteries, augmented by the regenerative power produced in the now freewheeling motors. Referring to the record breaking altitude/versus time chart in Fig.1, we see take off from the US Navy’s Pacific Missile Range Facility on the Hawaiian island of Kauai at 8:48 AM on August 13th and landing some 17 hours later at approximately 1:43 AM the following morning, August 14, several hours into darkness. So there you have it, truly a most interesting story. Perhaps the last word belongs to Dr MacCready’s company citing some of the potential advantages for this impressive aeroplane: * Long flight duration – of up to 6 months or more. * Minimal maintenance costs due to few moving parts (each motor has only one moving part). High levels of redundancy (the aircraft could lose several * motors and still maintain station and land safely – most failure modes do not require immediate response by the ground station operator). Highly autonomous controls which enables one ground * operator to control multiple aircraft. * Use of solar energy to minimise fuel costs. * Tight turn radius which makes the platform appear geostationary from the ground equipment perspective (ie, enables the use of stationary user antennas) and enables multiple aircraft to serve the same area using the same frequency spectrum. Flexible flight facility requirements (the aircraft can even * take off from a dirt field and in less distance than the length of its wingspan). SC Acknowledgments: Thanks to Alan Brown of NASA and the people at AeroVironment. www.siliconchip.com.au