PTT, Dave Unwin
Is ion drive technology the future of propulsion or just fantasy?
The news about an ion-drive for aircraft caused me to ponder just how far the design and development of the small GA engine has actually come over the last 115 years.
The motor of the Wright Flyer was a four-cylinder petrol-fuelled engine. It weighed about 77 kilograms but only produced twelve horse power, giving it (by modern standards) a miserable power-to-weight ratio. In fact, the Wrights soon realised that−away from the astonishingly reliable wind and favourable density altitude found at Kitty Hawk−their Flyers needed assistance from a catapult to launch! Nevertheless, their little engine worked, and it’s probably fair to say that, of the hundreds of thousands of engines built for small, propeller-driven GA types, the majority had four cylinders and were fuelled by petrol.
The rotary engine had reached its zenith by the 1920s, and while large aircraft were powered by a bewildering array of motors before the invention of the turbine engine, most small aircraft were powered by permutations of air-cooled flat-fours, sixes and eights. Indeed, and quite remarkably, the Continental O-200−which is still in production today−can trace its direct lineage all the way back to the A-40 of 1931. The A-40 was a direct-drive petrol-fuelled air-cooled flat-four fitted with dual magnetos and an up-draught carburettor, and the O-200 is a directdrive petrol-fuelled air-cooled flat-four fitted with dual magnetos and an updraught carburettor!
In the last 35 years, however, there have been some developments. First to shake things up were Austrian company Rotax. Having built many successful two-stroke aero-engines, the ‘Nineseries’ transformed GA with its good power-to-weight ratio and low fuel consumption. However, although such innovative design features as dry sump lubrication, ram-air cooled cylinders with liquid-cooled heads, relatively small swept volume, and a reduction gearbox make these engines very different from Lycomings and Continentals, they are still petrol-fuelled flat-fours with spark-ignition.
The diesel engine, on the other hand, uses compression-ignition and, on paper at least, the diesel engine has always looked quite promising. It is a fact that large two-stroke marine diesels have the lowest specific fuel consumption of any prime mover, and even small diesel engines burn fewer kilos of fuel per horsepower/hour than a petrol engine. As a bonus, jet fuel is also substantially cheaper.
Of course, sticking a diesel engine in an aeroplane is not a new idea. The first diesel-fuelled aero-engine flew as far back as 1928, and several other aero-diesels were also designed around this time, However, they all shared one characteristic: poor power-to-weight ratios. This generally made them unsuitable for anything but long-range aircraft (where the reduced fuel load made up for greater engine weight) and airships, although even here they were found wanting! The new breed of turbocharged aero-diesel is a far more satisfactory proposition. The Diamond DA40, 42 and 62 are all powered by diesels, and an option for a new Piper Archer is the Continental Motors CD-155, which can also be fitted under an STC to older PA-28S and Cessna 172s.
Of course, we’re all waiting patiently for a viable electric aircraft, but although satisfactory motors have existed for years, two factors that combine to greatly restrict development are energy density and recharge time. Unfortunately, two kilograms of the very latest Lithium-ion Polymer batteries simply do not contain anywhere near the energy contained in 0.719kg (one litre) of avgas. Similarly, enough fuel to propel a modern two-seat light aircraft several hundred miles can be uplifted in about five minutes, whereas even the best batteries would probably need several hours. There are several electric aircraft coming onto the market, but a truly practical electric aircraft with real, general-purpose usability doesn’t yet exist.
One solution that makes the best use of both electricity and fossil fuel is the hybrid. As mentioned in last month’s ‘ Pilot Notes’, Diamond Aircraft and Siemens AG recently flew a jointly developed multiengine hybrid electric aircraft based on a DA40. The hybrid powertrain comprises a diesel generator located in the nose of the aircraft, with two independent drive systems consisting of a propeller electric motor, battery and inverter.
The most recent−and possibly most exciting−development in aero-engine technology is the ion-drive. Recent advances in ‘ionic wind technology’, a concept which was first investigated in the 1920s and again in the ’50s, are starting to come to fruition. At MIT (the Massachusetts Institute of Technology) they recently flew the first ever ‘solid state’ flying machine. The prototype− named, somewhat uninspiringly
Version Two− has no moving parts in its propulsion system. Instead, wires at the wing’s leading edge carry 600 watts at 40,000 volts, enough to induce ‘electron cascades’ which charge air molecules near the wire. These charged molecules flow along the electrical field towards a second wire at the trailing edge, hitting and imparting energy to neutral air molecules on the way. These stream out, producing thrust and proving that heavier-than-air flight is possible without jets or propellers.
It’s still early days, but Version Two has already flown over sixty metres and ironically has two things in common with the Wright Flyers. Just as with the Flyers, people with little or no imagination have already dismissed the idea as unworkable, and Version Two requires a catapult launch!
Advances in ionic wind technology are starting to come to fruition
Pilot’s Flight Test Editor operates a Jodel D.9 from a farm strip and has logged stick-time on everything from ultralights to fast jets