Generating thrust to fly an aircraft using the technology behind ionic air purifiers sounds too good to be true - until now.

It has been widely believed that electrohydrodynamic (EHD) thrusters are too inefficient to be used for primary propulsion. But research carried out at the Massachusetts Institute of Technology - and published in the Royal Society's journal of mathematical, physical and engineering sciences - has concluded that EHDs are actually far more efficient than conventional jet engines, and may be of some practical use.

Indeed, Professor Steven Barrett, working with graduate student Kento Masuyama, found that EHDs - often called ionic wind thrusters - are some 50 times as efficient as jet engines.

They can generate 110N/kW of input power, compared to just 2N/kW for the International Aero Engines V2527-A5s that power Airbus A320s.

The pair were measuring thrust per unit power - a measure proportional to the inverse of thrust specific fuel consumption (SFC), which is the conventional metric for comparing efficiency of propulsion systems.

The typical EHD is a pair of electrodes, one much thicker than the other and separated by a large air gap. Applying power to the thin electrode ionizes air molecules, which are accelerated towards the thick electrode and discharged as the so-called ionic wind, generating thrust.

Hobbyists and anti-gravity enthusiasts have done much work with what are often called "lifters", but there has been little peer-reviewed experimentation. The idea was first raised and patented in the UK in 1928, and some work has been done this century by NASA and the US Army.

However, Barrett and Masuyama, based on static thrust to power measurements in their laboratory - and assuming sea level conditions and a forward speed of 70m/s - calculated EHD overall efficiency to be 50%, compared with just 17% for the V2527 at the same forward speed. Moreover, Barrett and Masayama recognised that EHD efficiency increases with speed, and calculated that it would reach some 66% at 140m/s.

Theoretically, at airliner cruise speed of 250m/s and 33,000ft (10km) altitude, the EHD should be 48% efficient - comparable to modern jet engines in cruise.

They concede, however, that while "EHD propulsion may provide practical levels of efficiency", more work is needed to determine whether or not EHD propulsion can produce the thrust density or force per unit area of nozzle exit necessary to be useful.

Much of the efficiency of the EHD comes from its low thrust density. The jet blast from a gas turbine contains lots of kinetic energy, which is wasted. By contrast, the EHD produces low-speed wind over a large surface area. Thrust may be the same, but whether that wide-area but low-speed push can be made into a practical aircraft configuration remains to be seen.

Also, for an EHD thrust increases with the gap between the thin and thick electrodes, and Barrett says for any practical application that gap could be the size of an aircraft.

Electrical requirements are another issue. Even small models need several thousand volts to generate enough thrust to lift at all, so for any real aircraft the demand could be millions of volts, Barrett estimates.

Managing that much voltage even at low current poses practical challenges, including safety issues - as highlighted by the warnings to mechanics contemplating working on 500V hybrid automobile systems.

However, EHDs have no moving parts and are silent, so if they can be connected to a sufficiently lightweight power supply, they could prove attractive for unmanned aircraft, especially in surveillance applications.

Designers of large aircraft, however, probably need not worry any time soon about how to incorporate EHD propulsion.

The apparent practical limits of this very efficient propulsion system will thus be disappointing to anyone concerned about airport environmental quality.

As we know from electronic air purifiers or even lightning storms, ionised air molecules attract dust particles and leave a pleasant smell that is a world away from the soot and kerosene fumes lingering over runways today.

Source: Flight International