Wings, plasma

Video:
http://www.newscientist.com/videoredirect?bctid=2209598837001

CIKK:
Futuristic planes with glowing wings, super-efficient wind turbines, greener cars… can a coat of ionised gas deliver all this magic?

IT WAS a clear, moonlit night when the two men set up their video cameras alongside Highway 58 in the Mojave desert. A little north of Edwards Air Force Base in California, they were hoping to shoot footage of the top-secret futuristic aircraft being tested there. What happened next amazed them.

At about 1 am a glowing object flew out of the west. It was long and thin, photographer Bill Hartenstein recalls, and surrounded by a glowing layer of gold-and-white light. The object accelerated and disappeared so fast that Hartenstein almost didn't catch it on film: "I was too clumsy due to excitement," he says.

At first they assumed the object – later nicknamed "the Dripper" from the way it shed a trail of glowing blobs – was a meteor, but when none were reported, they began to suspect it was a plane. But who had ever heard of one that glowed?

For clues, they showed their footage to physicists at the California Institute of Technology in Pasadena. From the way the blobs moved, the scientists suggested that the glowing light resembled plasma – a cloud of ions and electrons formed when gas is ionised. Plasma is formed in neon lights, by lightning and on the surface of the sun. Could the air force be testing plasma-coated planes?

Very possibly. In the decades since Hartenstein's mysterious encounter in July 1991, there have been suggestions that military labs are wrapping planes with plasma "cloaks" which absorb radar waves, creating a new kind of stealth bomber. But other, unclassified research now suggests a very different rationale for glowing coats.

It turns out that ionised gas can have a dramatic effect on airflow, and this could bring all kinds of benefits. Plasmas on wind-turbine blades, for instance, could help cut the cost of renewable energy. Fuel-guzzling cars and planes can also benefit from plasmas, offering cheaper travel and cleaner journeys. Ultimately, plasmas allow engineers to think of aerodynamics in a whole different way, and this could have remarkable implications for the future of transport.

The foundations of plasma research were laid half a century ago, when the space race was in full swing. At the time, scientists were intrigued by events that occur when space capsules return to Earth. They discovered that as a capsule hits the upper atmosphere, it generates friction, heating the gas around it so much that the molecules ionise, forming a coating of plasma.

This plasma does not behave in the same way as normal gas. In particular, experiments showed that plasmas reduce drag – for fast-moving objects, at least. Tests at the Ioffe Institute in St Petersburg, Russia, during the 1970s revealed that the drag on a steel sphere fired at hypersonic speed through plasma was a third less than expected. The phenomenon fascinated physicists. However, as ionised gas was also known to absorb radar signals – a problem for space agencies since it hampered their ability to track returning capsules – plasmas began to look as if they could provide the key to high-speed, radar-evading bombers. And with the cold war at its height, that work was classified.

Plasma research finally began to emerge from the shadows in the early 1990s following the dissolution of the Soviet Union. As labs round the world gained access to Russian expertise, there was fresh interest in the benefits plasmas could bring to aviation. While physicists struggled to understand the underlying physics, a team led by John Reece Roth, an engineer at the University of Tennessee in Knoxville, made a breakthrough.

In 1998, they devised a simple plasma generator and found that it could reduce drag on slow-moving objects too. Hopes were raised that the effect might prove practical for airliners, and experiments in the wind tunnel at NASA's Langley Research Center in Hampton, Virginia, soon revealed how such a system might work.

Wings of fire

To minimise friction on a wing, the layer of air nearest the surface should always move smoothly – a phenomenon called laminar flow. However in practice this "boundary" layer of air is easily disrupted and flows away from the surface. This creates turbulence, which can account for up to a third of an aircraft's drag. To try to counter this, Roth and his colleagues devised a pair of electrodes, separated by a thin insulating film and set it into the wing's upper surface near the leading edge (see diagram). When the researchers applied a high voltage, the air between the electrodes ionised, creating a strip of glowing purple plasma along the top of the wing. Wind-tunnel tests revealed that this simple arrangement helped to maintain smooth airflow in conditions where it would usually be wildly unstable.

Since then physicists have worked hard to unravel this effect. Experiments suggest that as the plasma forms, it is repelled by the exposed electrode, creating an "ion wind" that flows away down the wing. Reaching speeds of up to 10 metres per second on a stationary wing, this flow accelerates the air in the boundary layer, which helps to prevent it peeling away from the wing surface.

The effect in testing is remarkable, but can plasmas make any difference to real aircraft? Take the Bell Boeing V-22 Osprey with its two tilting rotors. It is a military transport plane that lifts off like a helicopter and then rotates its engines through 90 degrees so it can fly like a twin-propeller aircraft. Maintaining smooth airflow during this transition has proved tricky. To help, the wings of the craft are covered with small "fins" which suppress turbulence at low speed. But these fins have an unwelcome side effect: they add drag when the Osprey flies at full throttle.

According to Chuan He at the University of Notre Dame in Indiana, plasmas are the answer. His tests show that electrodes can do the same job as the fins but at less than half a millimetre thick, they won't spoil the craft's aerodynamics. According to He's results, plasmas reduce the craft's drag by more than 40 per cent compared with the fins on the conventional design and the US Navy has funded a study to develop a complete control system, with an eye to future flight tests.

Airliners could reap similar benefits. Engineer David Birch at the University of Surrey in Guildford, UK, has calculated that small plasma actuators could reduce frictional drag by 30 per cent, which could translate into a fuel saving of approximately 5 per cent. Considering US airliners alone consume some 40 billion litres of aviation fuel each year, this technology could save $1.5 billion in fuel costs and cut carbon dioxide emissions by 5 million tonnes annually.

Where wings lead, aerofoils of all kinds could follow – including the blades on helicopters, wind turbines and even inside gas turbines. The wind-energy industry is particularly interested in the benefits plasma technology can offer. The rotor blades on wind turbines are precisely shaped to generate maximum "lift" over a broad range of wind speeds. To improve efficiency, some turbines use adjustable-pitch blades which are rotated by motors in the turbine hub to ensure they maximise lift in calm conditions or reduce it during storms. Plasma actuators could do a similar job without the need for this heavy, expensive machinery. Better still, with separate actuators mounted along the blade – each controlled by sensors – the airflow over the entire blade could be optimised in real time. "It is a very promising technology, especially due to the low manufacturing cost and absence of moving parts," says Georgios Pechlivanoglou, technical director of Smart Blade based in Ravensburg, Germany, which is developing plasma control for wind turbines. The electrodes can be made so thin that it might even be possible to add them to blades in the form of sticky tape, he says. "The easy application of these actuators is really exciting for the industry."

What benefit might this bring? According to Thomas Corke, an aerospace engineer at the University of Notre Dame, retrofitting plasma actuators should increase the performance of existing turbines by up to 15 per cent, and help make the cost of electricity generated by wind more competitive. Corke's team will be running field trials later this year in a bid to demonstrate this. "We have two wind turbines operating and one will be fitted with plasma actuators," says Corke. "All the electronics to support this are already installed."

Then there are road vehicles to consider. In one study on cars, a team from the University of Orléans in France showed that plasma actuators fitted along the top of the rear window can smooth the airflow behind the vehicle, cutting drag by as much as 8 per cent. Though offering only a tiny improvement in fuel efficiency, rolled out on national scales it could significantly cut fuel consumption and CO2 emissions.

Smooth airflow

Before we can put plasmas to work, there are some serious problems to smooth out. To start with, the position of the actuators is critical. The French team found that adjusting the electrode placement slightly can increase drag on the car by almost 6 per cent. And to make such systems practical, the benefit it brings must outweigh the energy needed to create the plasma in the first place. Here, engineers have made some progress: generating plasmas by pulsing a high voltage signal can maintain smooth airflow while halving the energy used. Still, warns Birch, controlling these systems in real time will be a challenge.

Since 2009, a consortium of European universities and other organisations has been tackling some of these problems head on. Their Plasmaero project has taken a fresh look at the physics underlying these actuators in the hope of improving their efficiency. But it also had a more ambitious goal: to control the flight of an unmanned aerial vehicle (UAV) using plasmas alone.

While plasmas have been used to increase the lift on a wing – in other words, to act as flaps – the Plasmaero collaboration wants to replace ailerons too, by adding actuators to both wings and operating them on one side or the other to make an aircraft bank. Could the same trick be used on rudder and elevators as well? In December 2012, the researchers got their answer during a successful flight of their 4-metre wingspan drone in Darmstadt, Germany. "The devices worked as expected, in real conditions," says Daniel Caruana, Plasmaero's project co-ordinator. "The UAV was manoeuvred only by plasma." And with a blue glow on its surface, the actuators gave the drone a futuristic appearance, he adds.

Though likely to appear first in UAVs, replacing conventional control surfaces on airliners could bring significant benefits. As well as drag reduction, plasma-based controls could make flights smoother because these actuators work much faster than mechanical controls, allowing an aircraft's autopilot to respond more quickly to sudden turbulence. With no moving parts, they require minimal maintenance. And since about a quarter of air accidents are caused by mechanical failure – with movable surfaces such as flaps and tail fins particularly susceptible – plasma controls could mean more reliable, safer planes.

Don't expect to see glowing airliners in the next few years. The failure of an actuator on a wind turbine would not be serious but the situation could be life threatening in flight. Besides, the risk-averse commercial aviation industry is cautious of new technology. Birch suggests that in the short term, plasma actuators may turn up as some kind of back-up control, to automatically prevent a plane from stalling, for instance.

Even so, this technology is already encouraging engineers to take a fresh look at conventional aerodynamics. There could be considerable benefits to coating the entire skin of a wing with plasma actuators, for instance. According to He, this would not merely reduce drag, it would allow engineers to tailor the airflow across it as they desire, creating what He calls a "virtual wing". Rather than using a precisely designed aerofoil, a virtual wing could use a generic shape but with a plasma coat. That way, it could transform at the flick of a switch – acting as a large, curved high-lift surface during take-off and landing or providing a smoother profile for high-speed cruising. Controlled automatically, such wings could flicker with blue light during flight, constantly optimising their aerodynamic performance.

Jamey Jacob at Oklahoma State University in Stillwater is looking even further ahead. He is investigating whether plasmas could even replace engines. To do this he would have to increase >the energy in the ion wind to generate sufficient thrust to get a craft off the ground. For now, this is not feasible, Jacob admits – plasma jets are not powerful enough. "Advances in the field have moved it in the right direction, though," he says.

Pechlivanoglou, too, thinks glowing coats have a bright future. Conventional forms of flow control use brute force, "but plasma actuators manage something much more elegant", he says. "And their development is only just starting."

This article appeared in print under the headline "Flow with the glow"

Fly Like Lightning
Here's a dramatic way to improve an aircraft's performance – fly it through an exploding tube of lightning. Proposed by Kevin Kremeyer, CEO of PM & AM Research in Tucson, Arizona, the technique aims to reduce drag as well as steer a plane by creating a plasma tunnel in front of it.

His plan is to equip an aircraft with powerful lasers that fire a series of intense, ultra-short pulses ahead. Each pulse creates a narrow channel of plasma. Capacitors on the aircraft then create an electrical discharge which flows along these conducting channels, forcing them to expand explosively, much like lightning.

By creating a region of low-density air for the aircraft to fly through, Kremeyer calculates that an aircraft flying at Mach 5 will experience as much as 90 per cent less drag. At subsonic speeds the effect is more modest, reducing drag by about 10 per cent.

Such a plasma tunnel could bring other benefits. By changing the shape of the tunnel, the aircraft could be steered without the need for conventional control surfaces, he says. Tweaking the tunnel should significantly reduce noise and make the aircraft much more stable, Kremeyer predicts.

The theory is certainly valid, says Doyle Knight, director of the Center for Computational Design at Rutgers, The State University of New Jersey. Studies have shown that a "plasma spike" can save more energy than it takes to create. The difficulty lies in generating a suitable spike. In an earlier project, researchers proposed using a focused microwave beam to generate the plasma, says Knight, but they found that the shape of the filaments was not consistently reproducible. "They could come out like spaghetti rather than in straight lines. But when it worked, it was effective."