The Johnson Indoor Track at MIT probably won't go down in history in the same way as Kitty Hawk has, but it was the scene of a first in powered flight. A team of researchers has managed to build the first aircraft powered by an ionic wind, a propulsion system that requires no moving parts. While the flight took place using a small drone, the researchers' calculations suggest that the efficiency of the design would double simply by building a larger craft.
Ionic wind
In conventional aircraft, air is pushed around by moving parts, either propellers or the turbines within jet engines. But we've known for a while that it's also possible to use electrical fields to push air around.
The challenge is that air is largely made of uncharged molecules that don't respond to electric fields. But at sufficiently high voltages, it's possible to ionize the nitrogen and oxygen that make up our atmosphere, just as lightning does all the time. The electrons that are liberated speed away, collide with other molecules, and ionize some of them as well. If this takes place in an electric field, all those ions will start moving to the appropriate electrode. In the process, they'll collide with neutral molecules and push them along. The resulting bulk movement of atmospheric molecules is called an ionic wind.
Calculations done decades ago, however, suggested that it wasn't possible to generate a practical amount of thrust using an ionic wind. Given advances in batteries, electronics, and materials, however, a team from MIT decided the time may have come to revisit the issue.
Doing so requires navigating a large series of trade-offs. For example, the lower the electric field strength of an ionic wind drive, the more thrust you get for a given power. Of course, if you drop the field strength enough, nothing will get ionized in the first place. Since the thrust per unit area is small, a more extensive thruster system makes sense—other than the fact that it will add to the drag and slow the craft down.
Still, after playing around with different thruster designs, the researchers found that it should be possible to generate sufficient thrust to get something airborne: "This level of performance suggested that steady-level flight of a fixed-wing unmanned aircraft might be feasible but at the limit of what is technologically possible using current materials and power electronics technology."
Finding a balance
The design they chose has a thin wire as its leading edge, where nitrogen and oxygen get ionized. Trailing behind that is a thin airfoil covered by the second electrode. This can both provide a little additional lift and allow the generation of an electric field that accelerates the ionized molecules from the wire to the foil.
But this design had to be integrated with the battery and electronics that make it work, as well as the wing and body that turned the whole thing into an aircraft. Some of those ingredients weren't even available until the team set to designing them.
"Weight constraints necessitated the design and construction of both a custom battery stack and a custom high-voltage power converter," the researchers write, "which stepped up the battery voltage to 40 kilovolts." To handle the aircraft's body, they fed a computer algorithm with a list of their constraints and had it optimize these to allow for stable flight with a limit on the potential wingspan.
The resulting hardware included a five-meter wing with a thin body containing the battery and electronics suspended below it before trailing off to a tail. On either side of the body, hanging off the wing, was a series of the wire/airfoil ionizers (two rows from front to back, both in a column of four for a total of eight). The whole thing weighed just under 2.5kg.
Given a bungee-cord-based launch catapult, the craft could fly about 10 meters when powered off. Fire up the ionic wind, and it could cover 60 meters and would frequently gain altitude while powered on. Measurements showed the thrusters collectively generated five newtons for each kilowatt of power, which is actually similar to the output of jet engines. But because of many inefficiencies in the system, the overall efficiency was only about 2.5 percent—well below that of conventional aircraft.
Still, the researchers have a huge list of potential improvements. The current design was limited by the decision to keep it flying inside the track at MIT. Allowing larger wings and a higher speed could get that efficiency up to five percent without any changes to the underlying technology. They also plan to explore things like different ways of generating ions, electrode designs that reduce drag and/or are integrated into the wing, and better power conversion electronics, all of which could boost things further.
While the paper concludes with talk of silent urban drones and environmental monitoring craft that could stay aloft indefinitely, there's a chance that the technology will end up being little more than a curiosity. Still, given that it was enabled by developments that had nothing to do with flight, it'll be worth seeing what happens when people actually focus on developing this tech further.
Nature, 2018. DOI: 10.1038/s41586-018-0707-9 (About DOIs).
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