Thunderstorms are something air pilots prefer to avoid, but nonetheless it is estimated that every commercial aircraft is struck by lightning once a year. While this statistic isn't as alarming as it seems, engineers from MIT, under the sponsorship of Boeing, are looking to bring the numbers down by electrically charging airplanes to make them less attractive to lightning.

When large commercial aircraft encounter lightning strikes (which are often triggered by the plane itself) it may result in an alarming flash and bang, but very little damage usually occurs. In fact, the last airplane crash in the United States due to lightning occurred in 1967 when a strike caused a catastrophic fuel explosion.

The reason lightning strikes, while dramatic, cause so little damage is because a conventional aircraft with an aluminum alloy hull is basically a flying Faraday cage. That is, a hollow metal cylinder that's impervious to static electrical fields.

When an airplane is in the vicinity of an electrical storm, its hull develops an electrical charge, which polarizes it. One end of the plane develops a negative charge and the other a positive charge. When this charge becomes strong enough, it generates positive leaders. That is, a highly conductive flow of plasma, which is the preceding stage to a lightning strike. These leaders can close the circuit between electrically charged clouds and the ground, and a bolt of lighting passes through the plane carrying up to 300,000 amps at a billion volts.

Normally, this doesn't cause any damage due to the Faraday effect, but an airplane hull isn't uniform and there are all sorts of antennae, wires and other structures that lightning can induce an electrical current in. Interior panels can also be charged, which can then arc and damage sensitive electrical equipment. In addition, many planes today incorporate composite materials, which are less conductive than aluminum.

For decades, engineers have protected planes against lightning by installing shielding, grounds, surge suppressors, and other measures. This is especially the case in regard to the fuel tanks. In addition, aircraft made out of composites have a fine metallic mesh incorporated in them to act as a Faraday cage.

However, even though modern airplanes can survive a lightning strike with relatively little damage, such an incident can mean repairs that are especially costly for composites or, at the very least, a careful inspection of every inch of the aircraft to make sure it's still airworthy. This causes delays and costs money in an industry that already has very thin profit margins, so anything that can avoid such situations is welcome.

The MIT idea is to avoid lightning strikes by putting a temporary negative electric charge on a plane as a way of dampening any build up of a positive charge. This prevents the overall charge from reaching a critical level and starting a lightning strike. This would be done though an automated system of sensors and actuators fed by small power supplies. The sensors would detect any build up of an electric charge and the actuators would send out a counter charge not much stronger than that needed to light a light bulb.

The MIT researchers developed a mathematical model of an aircraft-triggered lightning strike that shows how the leaders appear at opposite points on the plane and how these evolve into a lightning strike. They found that by charging the airplane, the ambient electrical field had to be 50 percent stronger to produce a leader – greatly reducing the chance of a strike.

"We're trying to make the aircraft as invisible to lightning as possible," says Jaime Peraire, head of MIT's Department of Aeronautics and Astronautics. "Aside from this technological solution, we are working on modeling the physics behind the process. This is a field where there was little understanding, and this is really an attempt at creating some understanding of aircraft-triggered lightning strikes, from the ground up."

The team is currently looking into how feasible the idea is in practice. Graduate student Theodore Mouratidis is conducting tests on a metal sphere a in MIT's Wright Brothers Wind Tunnel, and the hope is to soon go on to more realistic experiments by flying drones through actual thunderstorms. One particular obstacle is how to make the system respond fast enough in storm conditions.

"The scenario we can take care of is flying into an area where there are storm clouds, and the storm clouds produce an intensification of the electric field in the atmosphere," says Emeritus Professor Manuel Martinez-Sanchez. "That can be sensed and measured on board, and we can claim that for such relatively slow-developing events, you can charge a plane and adapt in real time. That is quite feasible."

The research was published in the American Institute of Aeronautics and Astronautics Journal.

Source: MIT