PTT, Dave Un­win

Pilot - - CONTENTS - Dave­un­win

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

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.