07/05/2018 – Science & Technology / Aviation / Lightning
A bolt from the blue
Lightning strikes have proved a pervasive challenge for the aviation industry, not to mention a worry to passengers. Now, scientists at MIT – funded by Boeing – may have hit on a novel solution.
Aviation experts estimate that every commercial aeroplane in the world is struck by lightning at least once per year. Around 90 per cent of such strikes are likely triggered by the aircraft itself: In thunderstorm environments, a plane’s electrically conductive exterior can act as a lightning rod, sparking a strike that could potentially damage the plane’s outer structures and compromise its onboard electronics.
To avoid such bolts from the blue, flights are typically rerouted around stormy regions of the sky. Now however, MIT engineers are proposing a new way to reduce a plane’s lightning risk, with an onboard system that would protect a plane by electrically charging it. The proposal may seem counterintuitive, but the team found that if a plane were charged to just the right level, its likelihood of being struck by lighting would be significantly reduced.
The idea stems from the fact that, when a plane flies through an ambient electric field, its external electrical state – normally in balance – shifts. As an external electric field polarises the aircraft, one end of the plane becomes more positively charged, while the other end swings towards a more negative charge. As the plane becomes increasingly polarised, it can set off a highly conductive flow of plasma, called a ‘positive leader’ – the preceding stage to a lightning strike.
In such a precarious scenario, the researchers at MIT propose temporarily charging a plane to a negative level to dampen the more highly charged positive end, thus preventing that end from reaching a critical level and initiating a lightning strike.
The researchers have shown through modelling that such a method would work, at least conceptually, and have reported their results in the American Institute of Aeronautics and Astronautics Journal.
Creating an “invisible” aircraft
The team, which includes Emeritus Professor Manuel Martinez-Sanchez and Assistant Professor Carmen Guerra-Garcia, envisions outfitting a plane with an automated control system consisting of sensors and actuators fitted with small power supplies. The sensors would monitor the surrounding electric field for signs of possible leader formation, in response to which the actuators would emit a current to charge the aircraft in the appropriate direction. Surprisingly, the researchers say such charging would only require power levels lower than that for a standard light-bulb.
“We’re trying to make the aircraft as invisible to lightning as possible,” says co-author Jaime Peraire, head of MIT’s Department of Aeronautics and Astronautics. “Aside from this technological solution, we’re working on modelling 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.”
Beware hidden charges
To be clear, while unsettling to many, lightning poses very little danger to passengers inside an aircraft, as a plane’s cabin is well insulated against any external electrical activity. In most cases, passengers may only see a bright flash or hear a loud bang. Nevertheless, an aircraft that has been hit by lightning often requires follow-up inspections and safety checks that may delay its next flight. And if there is physical damage to the plane, it may be taken out of service – something that airlines would obviously rather avoid.
Furthermore, it seems that newer aircraft made partly from non-metallic composite structures like carbon fibre may be more vulnerable to lightning-related damage, when compared with their older, all-metal counterparts, as charge may accumulate on the poorly-conducting composite panels of newer models.
“Modern aircraft are about 50-per-cent composites, which changes the picture very significantly,” says Assistant Professor Carmen Guerra-Garcia. “Lightning-related damage is very different, and repairs are much more costly for composite versus metallic aircraft. This is why research on lightning strikes is flourishing now.”
Yet temporarily charging a plane to effectively dampen down the risk of a lightening strike could prove tricky when time is of the essence. “Numerically, one can see that if you could implement this charge strategy, you would have a significant reduction in the incidents of lightning strikes,” resumes Martinez-Sanchez. “There’s a big ‘if’: Can you implement it? And that’s where we’re working now.”
Graduate student Theodore Mouratidis is performing preliminary experiments
in MIT’s Wright Brothers Wind Tunnel, testing the feasibility of charging on a simple, metallic sphere. The researchers also hope to carry out experiments in more realistic environments – for instance, by flying drones through a thunderstorm.
Yet to make the charging system practical, Martinez-Sanchez says researchers will have to work to speed up its response time. Based on their modelling, he and his colleagues have found that such a system could charge and protect a plane within fractions of a second – yet even that would not be fast enough to protect against some forms of triggered lightning.
“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,” he advises. “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.”