SOARING silently above the barren flats of NASA鈥檚 Dryden Flight Center in California, Pathfinder is an extraordinary sight. No rudder, no tail, not even a fuselage. This pilotless aircraft is simply a flying wing 30 metres across and 2.5 metres wide. 鈥淔rom a distance it almost looks like a hawk soaring gracefully in the thermals,鈥� says Nicholas Colella of Lawrence Livermore National Laboratory in California, who originally headed the programme that administered the Pathfinder project. Unlike the supersonic jets that roar above these testing grounds, Pathfinder carries no fuel. Instead, the elec tric motors that drive its eight propellers draw their power from the solar panels covering its wings (see Diagram).
Any solar-powered aircraft is impressive, but Pathfinder is the prototype for an even more ambitious craft, called Helios. Pathfinder can only fly when the Sun is in the sky. Helios will store enough energy during the day to keep it aloft at night, and so might never have to land. Engineers working on the project call it 鈥渢he eternal airplane鈥�.
So far, Pathfinder has been tested only at low altitude. Next week, researchers hope to test Pathfinder鈥檚 flight worthiness after recent modifications. If all goes to plan, in July, it will climb to 20 kilometres, where winds are relatively light. This is the altitude at which eternal aircraft will cruise on flights that could easily circumnavigate the globe. If all goes well, Helios could be flying within a year.
Advertisement
Eternal aircraft are more than a flight of fancy. AeroVironment of Simi Valley in California, the company that built pathfinder, says that these planes might take on many of the tasks that can now only be carried out by satellites, and at a fraction of the cost. For example, they might survey the ground beneath them, taking pictures at higher resolution than satellites ever could because they will be much closer. Unlike the equipment on board a satellite, instruments carried on an aircraft can be replaced if they wear out or become out of date. And this equipment is likely to be cheaper, since it does not have to withstand the rigours of space, where instruments are bombarded by radiation and there is no airflow to cool electronic circuits.
In addition, an aircraft can circle above an area of interest, unlike a satellite, which passes fleetingly overhead as it orbits the Earth. 鈥淐ompared to a satellite, you can practically park this airplane in the sky,鈥� says Martin Cowley, a designer at AeroVironment. All this for as little as one-tenth of the cost of operating and launching a satellite.
Pathfinder brings together several strands of AeroVironment鈥檚 expertise. The company built Gossamer Albatross, the human-powered plane that was flown across the English Channel in 1979. It also built the SunRaycer solar-powered vehicle for General Motors, which in 1987 won the first Solar Challenge race across Australia. AeroVironment later built a highly efficient electric motor, which General Motors uses in its prototype electric car.
To build Pathfinder, however, AeroVironment鈥檚 engineers had to tackle a host of new problems. The Sun is a relatively weak source of energy, so the solar-powered craft must be an extremely efficient flyer. A typical small aircraft requires about 100 kilowatts to fly, a power level that is way beyond the reach of even the best solar cells that could be fitted. The Sun delivers a mere 1 kilo watt to each square metre, and solar cells convert only a small fraction of that energy into electricity.
To minimise the demand for power, Pathfinder has to be extremely light. 鈥淭he constant battle is keeping the weight down,鈥� says Greg Kendall an aeronautical engineer at AeroVironment. Most aircraft are built with aluminium, which weighs about 2700 kilograms per cubic metre. Pathfinder鈥檚 wings are formed from ribs made of expanded polystyrene, which weighs only 16 kilograms per cubic metre. The result is a craft that weighs a mere 180 kilograms.
Lightweight strength
The obsession with weight is the reason for Pathfinder鈥檚 unorthodox shape. A traditional design with a fuselage and wings would place heavy stresses on certain parts of the plane, which would need to be built of stronger, heavier components to cope. In the flying wing design used for Pathfinder, weight is so evenly distributed that if the plane were cut into segments each piece could fly by itself. Most of this weight is supported by a carbon fibre spar that runs the length of the wing. 鈥淭his material has the best strength-to-weight ratio of any material we could use,鈥� says Kendall. It is more than twice as strong as aluminium for a given weight. The polystyrene ribs protrude from this spar, and the entire structure is covered, like a tightly wrapped sandwich, with transparent polymer sheeting.
The lightness of the structure means that Pathfinder鈥檚 wings carry a load of only 2.5 kilograms per square metre. By comparison, a jumbo jet鈥檚 wings support 1000 kilograms per square metre, and even the force on a falcon鈥檚 wings works out at about 25 kilograms per square metre. The load on pathfinder鈥檚 wings is so light that the Mylar film needs to be only one-hundredth of a millimetre thick to withstand the forces of flight. The plane鈥檚 structure is flexible, so it reacts to turbulence and gusts of winds in the same way that an inflatable mattress floats over ocean waves, says Colella. During flight it can withstand forces of up to 5 g.
But it is a different matter when the plane is on the ground. 鈥淎ll of the structures are so fragile that you could crush them in your hand,鈥� says Kendall. 鈥淚n fact, it鈥檚 hard to move parts of the airplane from one place to the other without breaking them.鈥� One of the engineers鈥� biggest problems is preventing 鈥渉angar rash鈥� 鈥� the nicks, bumps and tears that afflict the plane when it is on the ground. According to James Daley, a mechanical engineer at AeroVironment, Pathfinder鈥檚 design takes account of the fact that the craft must sometimes cope with bigger forces on the ground than in the air. To protect the plane from accidental knocks it is kept suspended from the ceiling of its hangar.
Pathfinder gets its power from solar panels covering a surface area of roughly 45 square metres, containing more than 8000 cells each made of single crystals of silicon. The cells are laminated between a layer of silicone adhesive five-hundredths of a millimetre thick and a clear polymer coating a hundredth of a millimetre thick, which forms a smooth surface. The adhesive fills in the gaps between cells, eliminating bumps that create drag. In addition, the coating holds the cells together so that they will continue to conduct a current even if they become cracked as the wing flexes.
Records shattered
鈥淚t鈥檚 exciting to have that kind of damage tolerance,鈥� says Daley, whose job is to ensure the plane is durable. Reliability is a crucial factor in the project, as the eternal plane may eventually fly missions lasting three months or more 鈥� throughout a polar summer for example. The world record for the longest powered flight is currently 11 days, but if solar-powered planes perform to plan that record looks likely to be broken easily.
The solar array has to provide as much electrical power as possible, but here light weight is also essential. Silicon solar cells produce up to five times more power per unit weight than they did a decade ago, and the price has dropped by a factor of nearly 10. 鈥淓fficiency is going up, and thickness is going down,鈥� says Daley. But each 0.6-volt cell still captures only 14.5 per cent of the energy that falls on it. 鈥淭here are cells that are in the 20 per cent range,鈥� says Daley, 鈥渂ut we sacrifice this extra efficiency for a better power-to-weight ratio.鈥� In addition, both sides of a cell can be used to generate electricity. Sunlight reflected from the ground and clouds below the plane boosts the efficiency to about 17 per cent. In total, the solar array produces over 7 kilowatts to drive the plane.
Designing a propulsion system to use this power efficiently was another problem. Typical electric motors run most efficiently at speeds of about 3000 revolutions per minute, but Pathfinder鈥檚 propellers have to run at a maximum of about half that speed at high altitude and as slowly as 300 rpm at low altitude, where the atmosphere is denser. The usual way to maintain an efficient engine speed is to use gears and a variable-pitch propeller. But these systems contain many moving parts. As well as being heavy they would be likely to wear out quickly at high altitude, where the temperature is low and lubricants are less effective.
Electronic fix
The alternative AeroVironment鈥檚 engineers choice was to use fixed-pitch propellers and control the torque and speed of each motor electronically. This left the problem of how to maintain engine efficiency. Ten years ago, AeroVironment鈥檚 propulsion systems converted only 4 per cent of the solar energy they collected into thrust; the latest system can convert 7 per cent. According to Wally Rippel, an electrical engineer who worked on the motors for the SunRaycer and the General Motors electric vehicle, the circuits that control the motor are now more efficient at lower speeds. 鈥淭he electronics acts like a variable-speed transmission.鈥� Engineers have also developed a secret blend of rare-earth metals for the permanent magnets inside each motor, which also improve efficiency.
The result is a motor with only three moving parts: 鈥渢wo bearings and an armature 鈥� that鈥檚 it,鈥� says Colella. 鈥淭hese motors are the technical hallmark of the airplane. When you turn them on and bring them to full thrust, you can hardly hear them.鈥�
The two computers that control the plane are housed in separate pods beneath the wing. 鈥淭he computers have become smaller and much more powerful,鈥� says Kendall. As well as controlling the speed of the craft with the 26 elevator sections in the wings, they steer it by adjusting the speed of some of the motors, and control its altitude by altering the speed of them all together. For safety, the plane can fly with half the elevators and motors out of action. The pods also contain Global Positioning System satellite navigation equipment, gyroscopes that measure the rate of pitch, roll and yaw, and a radio to keep the plane in touch with the ground.
For most of each mission, Pathfinder will be controlled by the on-board computers alone. They will be programmed with information such as the stall speeds, how changing the speed of individual propellers turns the plane, and the rate at which it can turn safely. But with an unpiloted craft, the manually controlled test flights that are needed to gain this information pose a problem. Last year, AeroVironment engineers controlled the low-altitude test flights from a pair of seats mounted on top of a van, which raced to keep the plane in view. The results of these tests have been used to program the autopilot.
While Pathfinder can convert sunlight into electricity, it has no way to store it 鈥� apart from the limited potential energy it can acquire when it gains altitude. When the Sun goes down, Pathfinder has to go down too. But Helios, its planned round-the-clock successor, is already on the drawing board. With a 60-metre wingspan, it will collect twice as much sunlight as Pathfinder. In the search for the best way to store this energy, engineers at Livermore looked at supercapacitors, fuel cells, chemical batteries and even flywheels. The clear winner, they found, was the fuel cell 鈥� a device that produces electricity by converting hydrogen and oxygen into water, and stores it by reversing the reaction. With an energy density of 300 watt-hours per kilogram, the modern fuel cell can store ten times the energy of lead-acid batteries of the same weight and about twice the amount of the most advanced chemical batteries.
Polymer bladders
Helios鈥檚 fuel cells will be unique. 鈥淢ost existing fuel cell systems have been installed in submarines or cars, where weight isn鈥檛 an issue,鈥� says Bob Curtain, the project manager at AeroVironment. 鈥淲hat hasn鈥檛 been built yet is a system that is very light.鈥� Conventional storage tanks for the gases would have weighed 40 kilograms. To save much of this weight, the gases will be stored under pressure in metal-coated polymer bladders, stowed inside the hollow main wing spar.
The fuel cells are 鈥渁 technology that didn鈥檛 exist ten years ago鈥�, says Bill Parks, a senior engineer at AeroVironment who is working on the fuel cells. The hydrogen and oxygen will react through a proton-exchange membrane, a thin polymer with a catalytic coating on each side. The membrane keeps the gas molecules apart but allows through the much smaller hydrogen ions, produced when the molecules react with the catalyst. The electrons formed in this reaction flow from the membrane around an external circuit, which powers the plane. Conventional fuel cells require two separate systems: one to produce electricity from hydrogen and oxygen and another to reverse the process. AeroVironment鈥檚 system does both jobs in a single cell.
The disadvantage of using fuel cells is that they convert only half of their stored energy into electricity. The rest is lost as heat. This means that Helios has to store two-thirds of the energy it collects during the day, leaving only one-third for it to fly on. But this is a price worth paying for a lighter system, says Parks.
Putting together the designs for the solar powered planes is a major task. 鈥淭he most鈥� difficult challenge,鈥� says Curtain, 鈥渋s making all the different systems work together as one. 鈥淐urtain鈥檚 team plan to test this next week, when Pathfinder again takes to the skies. A 16-hour flight is planned for July, when the plane will attempt to reach an altitude of more than 20 kilometres.
The project鈥檚 success, however, depends not only on AeroVironment鈥檚 engineering feats, but also on funding. About $2.5 million has been spent so far on Pathfinder, and about $25 million more will be needed to complete the project. Until 1994 it was funded by the Ballistic Missile Defense Organization, the successor to the Star Wars project. It hoped to use Helios for military purposes: to carry and launch missiles, for example. But recently these funds have dried up as defence budgets dwindled.
Now the plane鈥檚 civilian potential has attracted funds from NASA. An eternal aircraft could, for instance, track hurricanes as they are formed in the tropics, map ocean currents, regulate marine traffic, track oil spills, monitor crops and natural resources, serve as a communications relay, and collect high-altitude air samples for research into global warming. Best of all, the craft would be nonpolluting and cost as little as $2 million each.
鈥淚f Henry Ford got his hands on them, they could be even cheaper,鈥� says Cowley. He believes there are dozens of applications no one has thought of yet. 鈥淲ho knows what people will come up with?鈥�