We’ve often heard the expression “that won’t fly,” but an MIT-based experimental model literally contradicts that statement for one type of flight propulsion. Like a science-fiction story come true, what’s claimed to be the first-ever heavier-than-air craft with no moving parts has flown using only ion-based propulsion (formally called electroaerodynamics and commonly known as sonic wind).
While ions have been used for demonstrations of small toy-like devices hovering over a desktop while still tethered to a power cable, this aircraft—with a wingspan of about 5 meters and weighing 2.5 kg—made 10 successive nine-second-long indoor flights of about 60 meters distance at a height of about a half meter, while cruising at about 4.8 m/sec (10 mph). The silent aircraft (Fig. 1) could be a precursor of silent drones and engines with a barely detectable infrared signature.
1. The team prepares the ion-wind aircraft for flight at the MIT’s duPont Athletic Center (the largest indoor space they could find) where the plane flew 60 meters, the maximum distance within the gym. (Source: Steven Barrett/MIT)
The project was led by Steven Barrett, associate professor of aeronautics and astronautics at MIT, and head of their Electric Aircraft Initiative. The principle of the ion propulsion, which has been discussed since the 1930s, uses the “wind” that’s created when current is passed between a thin and a thick electrode as its thrust source (Fig. 2).
2. An electric field (not shown) is applied to the region surrounding a fine emitter wire (shown in cross-section) that induces electron cascades in which free electrons collide with air molecules and thus free up more electrons (a). This process produces charged air molecules (corona discharge) in the vicinity of the emitter (a corona discharge), which then collide with neutral air molecules, thus developing an ionic wind (black arrows). The aircraft uses a series of emitters and collectors, and the flow of charged air molecules occurs mainly along the directions (red arrows) joining emitters and collectors; the ionic wind is accelerated (black arrows) predominantly in these regions (b). (Source: Nature/Springer)
The arrangement strips away negatively charged electrons from the surrounding air molecules, and the ionized molecules are attracted to the negatively charged electrodes at the back of the aircraft. As this cloud of ions flows toward the negatively charged wires, each ion collides millions of times with other air molecules, resulting in thrust that pushes the aircraft. (Note that a variation of ion propulsion has also been proposed for spacecraft, but it uses the sun’s stream of particles rather than air-derived ions.)
To evaluate the almost countless possible design arrangements, the team used geometric programming to find the optimum set of design variables that would also minimize the aircraft’s wingspan, weight, power needs, and cost. For this aircraft, the thin wires of the positively charged electrodes (+20 kV) were strung along and beneath the front end of the plane’s wing, while similarly arranged thicker wires (actually, in the form of very thin foil) were run along the back end of the plane’s wing as the negative electrodes (−20 kV) (Fig. 3). The fuselage of the plane contains an assembly of lithium-polymer (Li-Po) batteries providing between 160 and 250 V dc input to the high-voltage supply.
3. The actual construction of the emitter wires, collector foil, and aircraft wings is an implementation of the theoretical concept. (Source: Nature/Springer)
The 600-W power supply (Fig. 4) was designed by Professor David Perreault’s Power Electronics Research Group in the Research Laboratory of Electronics. It uses a switching regulator followed by a 1:15 step-up transformer, and then a six-stage full-wave Cockcroft-Walton voltage multiplier. This voltage-multiplier topology is used extensively in high-voltage step-up applications, including Tesla coils and research laboratories. However, implementing it within the aircraft weight restrictions and basic safety concerns was a challenge.
4. The overview of the aircraft design and its high-voltage schematic can only hint at the simultaneous simplicity and complexity of the design. (Source: Nature/Springer)
Previous analysis showed that the power/weight ratio and overall 1% energy-to-propulsion efficiency of ion-wind approach was so low that such an aircraft could never fly. However, Prof. Barrett redid the analysis and was convinced that the previous conclusions were incorrect (like the bumblebee, which shouldn’t be able to fly, according to simple theory).
“I did some back-of-the-envelope calculations and found that, yes, it might become a viable propulsion system,” said Barrett (and who also admitted that he was inspired somewhat by the silent gliding of the shuttlecraft in Star Trek). “And it turned out it needed many years of work to get from that to a first test flight.” The published paper notes that “the conventionally accepted limitations in thrust-to-power ratio and thrust density, which were previously thought to make electroaerodynamics unfeasible as a method of aeroplane propulsion, are surmountable.”
Of course, no such project is ever “done” and the team is working the energy-to-propulsion efficiency of this design (about 2.6%) to produce more ionic wind with less voltage. Among the areas to improve, researchers want to increase the design’s thrust density (the amount of thrust generated per unit area). Further, the plane requires a large area of electrodes, and Barrett would like to design an aircraft with no visible propulsion system or separate controls surfaces (rudders and elevators).
Details of the project, including the power supply, are in a paper published in Nature, “Flight of an aeroplane with solid-state propulsion” with videos and additional discussions at the references cited below (and the references with the published paper are also informative). This research was supported in part by MIT Lincoln Laboratory Autonomous Systems Line, the Professor Amar G. Bose Research Grant, and the Singapore-MIT Alliance for Research and Technology (SMART). The work was also funded through the Charles Stark Draper and Leonardo career development chairs at MIT.
References (for MIT project and for Cockcroft-Walton voltage multipliers):
Nature, “Flying with ionic wind”
Nature, “First flight of ion-drive aircraft”
MIT Electric Aircraft Initiative, “MIT engineers fly first-ever plane with no moving parts in propulsion system”
Georgia State University Hyperphysics, “Cockroft-Walton Accelerators”
Wikipedia, “Cockcroft–Walton generator”
Wenzel Associates, TechLib, “Cockcroft-Walton voltage multipliers”