General Aviation

August 2004

 

"Game, set & match - to the diesel"

"Pitting the conventional O-200 against Mark Wilksch’s ingenious WAM-120 diesel engine in a back-to-back air test, using identical Thorp aeroplanes. Philip Whiteman finds a clear winner— and salutes a great engineering achievement"

"Heard the one about the Australian who invented a two-stroke diesel engine, using a Hoover motor as the scavenge blower and, with a help from a couple of his mates, fitted it in secret to an old J-3 Cub?

No? Well here’s the punchline: gathering backers and a small team of engineers around him, that same Australian developed that proof-of-concept motor into a finely-honed production item—the 120 hp Wilksch Airmotive WAM-120—and ended up signing an agreement with one of Britain’s longestestablished diesel engine manufacturers, Lister Petter, to build it in quantity.

Not many people at Teledyne Continental and Lycoming would find the story very funny.

Truth is, the tale of Mark Wilksch and his engine stretches credulity. He has pitched his tiny company, based in a small industrial unit in the outskirts of Buckingham and staffed by a handful of people, against the industrial might of the US aero engine makers and SMA in France. These companies have spent tens of millions in chasing the idea of the light aircraft diesel engine, burning up hundreds of thousands of engineering manhours—and failed substantially, only the SMA project coming to belated fruition. Admittedly, the German Thielert concern has short-cut much of the development grief and expenditure by adapting a Mercedes car engine for its Centurion power unit, but even this firm did it on the back of a highly profitable competitioncar business—and not everybody is convinced that using a car engine in an aeroplane is the right way to go.

Wilksch started out with a clean sheet of paper and cut a lone furrow in pursuing a bespoke, two-stroke unit, rather than adapting any existing design. On the way, he recruited two key engineers: from Cosworth, Martin Long—the man who he says “made all my scribbles into working parts”; and ex-Ruston and Holset diesel engine and turbocharger specialist, Phil Franklin. Even his automotive sourced components—fuel injection pumps, scavenge blowers and the like—have given way progressively to lighter, more efficient items engineered by Wilksch Airmotive.

Could this tiny British concern really have succeeded, when so many others had bogged down or hit the buffers? I travelled up to Finmere to find out. Chosen weapons for the duel were a pair of Thorp T-211s; one fitted with a ‘production standard’ WAM-120 diesel, the other retaining its Continental O-200.

Sniffing out the opposition

I have to say that you could literally sniff out the diesel aeroplane on the flightline. It’s not that its Jet A1 fuel gives off a powerful pong, just the slight odour of something different— perhaps I’ve become too used to the scent of avgas. The Wilksch engine is configured like the old DH Gipsy series, as an inverted, in-line unit (although it has three—rather than four— pots and is water cooled). Visually, the diesel engined Thorp is distinguished by its sloping top cowl, leaving the spinner and fairing over the crankcase standing proud. Below the prop is a gaping intake for the inter-cooler/radiator— shades of Hawker Typhoon or Tempest, if one sticks to the historical comparison. As we shall see, there are major and significant differences in the engine instrumention between the two Thorps—but the rest of the airframe, from the firewall back, is identical.

Flight one is to be in the Continental powered machine, G-BXPF, which has a standard, fixed-pitch propeller and 20 hp less than the Wilksch diesel. To check fuel consumption I had wanted to dip the tanks of both aircraft, but was frustrated by the absence of a dipstick for ’PF. At least Mark had one for the WAM Thorp, which represented the lesser-known quantity and would provide the real evidence for the case to be proved.

PIC for both flights was Mark himself. He made what I took to be theatrical protests at having to cope anew with ‘dangerous’ things like carburettor heat—and supported his case by firmly pulling out the wrong knob when running up the O-200 before take-off (a genuine mistake, he insisted). Mark flew the initial part of the sortie, whilst I operated the watches.

Winding up to full throttle before releasing the brakes gave a ground run of 20 seconds, before we lifted off, skimming the poplars that separate Finmere’s none-too-long concrete runway from the Saturday market area, and climbed out over the electricity lines to the east. I am not the lightest of crew members and the 60-odd litres of fuel we had on board meant we were near maximum gross weight— so the 500 fpm I got from VSI and altimeter/stopwatch alike seemed to be a representative figure.

Keeping pace with us and in close formation on the starboard wing was former RAF Lightning pilot and Wilksch associate Simon Braithwaite, in the WAM-120 Thorp, G-TZII. Our photo session gave me the chance to see closely the diesel engine running at a high power setting—and not producing the slightest exhaust haze (a trick I’ve yet to see matched by any high-performance diesel car). Flying in formation puts a premium on ‘throttle response’ and the fact that Simon could keep such accurate station suggests the Wilksch diesel is not wanting in this respect.

Once we’d finished with picture taking, Mark handed over control for me to follow Simon back to Finmere. This is not a handling report, but I should like to add my voice to the others who have expressed pleasure with the Thorp T-211’s pleasant, sporty handling. Less impressive were the flaps, which we left up for the landing, Mark preferring the far more effective means of steepening the approach by side-slipping (he is a very good pair of hands in a light aeroplane, our Mr Wilksch).

Back on the ground we had to level the diesel Thorp by hand to check the fuel level (settling on its tail is another odd T-211 foible). We’d measured 57 litres prior to start-up and found it difficult to be sure that much fuel had been consumed at all in half an hour’s flying. I’d rather be able to make a true, independent assessment—by brimming the tank, say—but will have to take Mark’s word for it that the WAM-120 burns about 14 to 15 litres per hour at the Thorp’s usual 85 knot cruising speed. This, he says, is 25 per cent better than the O-200 version, which consumes 20 to 21 lph at the same speed. I would not challenge these figures—as a salesman, Mark Wilksch is a very good engineer: straight questions get honest answers, even if is is sometimes to the detriment of his product.

As far as fuel consumption goes, the case for the diesel is beyond doubt. Factor in the price of Jet A1—a third of the price of avgas— and you might reckon that the Wilksch diesel engine consumes fuel at a cost equivalent to just over one gallon of 100LL per hour: Group A flying at sub-microlight cost.

Something very clever

Swapping machines, Mark introduces me to the WAM diesel’s unique instrumentation. The familiar flight instruments are all there, as are a set of automotive-style engine dials, but set in the right-hand side of the panel is an LCD screen—the visible part of something very clever. This is WAM’s CI-Log, a combined instrument/warning display and data logger that not only tells you what the motor is doing, but records all the data, once a second. From his pocket our engineer-magician produces a small plastic case containing a standard 32 Mb MMC card. Slotting into the CI-Log this, Mark tells me, has the capacity to store over 50 hours worth of downloaded engine data— sufficient to cover all the running time between engine oil and filter changes.

In normal operation the CI-Log’s clear screen (I found no problem reading it in bright sunlight) displays both analogue and digital readouts of engine speed—in per cent, as per turbine or even Russian radial standard. Manifold air pressure and the essential Ts and Ps are displayed in a similar manner; this is to say that you get a pair of rather small dials, with numbers displayed alongside, and a set of four ribbon gauges for the minor indications. Hmm, I thought; impressive technical wizardry—but does it keep the pilot properly informed and alerted?

Mark can see what I am thinking: “We were concerned about the size of the display—you’ll see in the Europa that it is mounted in a better place; higher up and close to the pilot’s eye-line. There’s a red LED that flashes when anything goes out of limits—we’re thinking of mounting a repeater on the left side of the Thorp’s panel.”

My other concern is that, should the CI-Log or its power supply fail, all engine instrumentation would be lost in one.

“This is also true for a number of production aircraft with all-electric panels. It doesn’t bother the engine—the controls are purely mechanical, and you could easily continue to fly on the normal power setting.”

We will return to power settings later: for now, having allowed the CI-Log to boot up and self-check, we come to starting the engine. The WAM-120 Thorp has two verniers and one simple plunger as primary engine/propeller controls. These are stacked vertically, the power control (equivalent to the throttle) being at the bottom. Next comes the Engine stop plunger, and above that sits the constantspeed propeller control. The last time I was confronted with this kind of array was in my old publisher’s Cessna Cardinal. Starting the Cessna involved briefly running the electric fuel pump—just long enough to see pressure and not so long that you flooded the engine. In contrast, all you do is wait until the WAM CILog’s Glow caption goes out— and turn the key. Thanks to WAM engineer Phil Franklin’s patented mechanical-pneumatic governor, there’s no need to even to set the power control: once it has fired, the engine will automatically sustain idle speed. It’s easier than starting a diesel car.

Unfortunately, it’s also as noisy as starting a diesel car— worse, in fact, as the idling WAM-120 sounds to the Thorp’s occupants much like an old taxi. Even the panel shimmies in sympathetic vibration, but Mark assures me that the engine will smooth out with revs.

As indeed it does: this time, prop and power controls shoved into the panel, we really do strain against the brakes. I hit the stopwatch button as Mark releases the brakes and barely have time to register real, shove-in-the-back acceleration before we are off the ground, ten seconds later (Mark reckons the diesel Thorp makes 0 to 60 mph in less than seven seconds). After the Continental-powered machine, the climb performance is exhilarating—I make it just over 1,000 fpm, and this is once again for a machine at, or close to, maximum weight. Of course, the diesel Thorp has a VP prop and 20 per cent more steam on tap, but here’s the performance bottom line; the Wilksch diesel engine halves the time to take-off and doubles the rate of climb.

We have no sooner reached a thousand feet or so than Mark wants to show me something else important. Pulling the engine speed back to 75 per cent on the propeller control, he finely adjusts the power setting to 150 Kpa MAP.

“Right,” he says, “from this point on you don’t need to touch the power setting. If you want to climb or descend, just increase or reduce the propeller speed—and the engine will shuttle along the best sfc [engineeringspeak for best fuel efficiency] line. It’s not until you are on final approach that you will want to bring the power back.” This is WAM’s version of ‘single-lever’ engine power control. The beauty of it is that it is had, not through any fly-by-wire trickery—the power lever is essentially a direct, mechanical connection with the diesel injection pump—but by basic engine optimisation.

Another great virtue is that the turbocharger automatically compensates for changing air density: as you climb, it simply spins faster to maintain the operating pressure differential. We only went to 3,500 feet or so, but testing by Wilksch Airmotive has shown that the initial rate of climb is sustained all the way up to 14,000 feet.

At normal cruise, the WAM-120’s engine note reminds me of a big outboard. It doesn’t quite sing like a four-stroke six turning at the same speed, but it sounds good. We’d sampled the O-200 Thorp’s cabin environment sans headsets, and found bellowing at each other was the order of the day. It’s much the same with the WAM-120, Mark even reckoning that his engine is a shade noisier.

Curiously, the diesel Thorp felt loose in the roll axis. The profile area of its re-contoured and extended nose might have been expected to have a destabilising effect in yaw and pitch, but this did not seem to be the case. As we found smoother air at altitude it became less apparent—so perhaps it was an intrinsic, minor handling quirk of this particular airframe that I simply took time to adjust to.

In terms of point-to-point aviating, there is simply nothing about the WAM-120 Thorp to remark upon. At low flight levels, it’s the same, nice little aeroplane—the big difference is in the fuel cost.

Mark did, however, have one last trick he wanted to show me. Having had me climb all the way back to Finmere, he took control overhead the airfield, at 3,500 feet. “Imagine we have this engine in a glider tug,” he said. Given the acceleration along the runway and rate of climb, this took little effort. “The big issue with the Lycoming or Continental is getting down quickly without subjecting the engine to thermal shock-loading. Here we just go to 100 per cent,” he pushes the propeller control in “and come down at Vmca. Phil’s governor will simply shut off fuel flow to the engine.” By now, Mark has the nose pitched down and the VSI needle pegged at its stop— 2,000 fpm down. The ground comes swimming up at alarming speed, while the engine hums on, smooth and unperturbed. By the time we’ve corkscrewed around a 360 turn, we are nicely lined up for the approach and Mark gets us gently on the ground within a couple of minutes of initiating the descent. “You can see how useful a Katana with this engine would be to a small gliding club...”

Properly put together

Back at the hangar, Mark lifts the bonnet to show me the motor. Many readers will have seen first mocked-up and then pre-production versions of the engine at past PFA Rallies and other exhibitions. The finished item looks very handsome and properly put together.

One obvious feature is WAM’s ‘parafocal’ engine mount, which is claimed to limit engine movement under g-loading to simple up-anddown motion. “I am disappointed with the noise level in the Thorp,” Mark concedes “although the Europa is much better than we had hoped—having all those curved, composite panels is a help. The Thorp is one big, lightweight tin box, and the engine mount locates right up by the panel.”

Weight saving measures figure extensively in any technical discussion over the engine. Mark is proud of the automotive-based starter motor assembly: at 6 lb, it is the lightest available (and it can be retrofitted to Lycomings, by the way). The sump is a magnesium alloy casting, as is the rear timing cover of the engine fitted to WAM’s Europa. Installed weight of the WAM-120 is approximately 15 lb up on the 0-200, matching the weight of an 0-235 but being 70 lb heavier than the Rotax generally fitted to Europas. “However, this should be balanced against the significant increase in range, or the possibility of reducing the difference in payload to something like 40 lb through reducing the fuel load to give the same range.

“I think the sort of person who will fit a WAM-120 in a Europa will be looking for range and the benefit in altitude performance—you would be able to fly over any European mountain ranges with a considerable margin of safety.”

The fact that the Wilksch diesel engine has first been fitted to kitplanes is significant. “If you think about it, few people are going to want to fit a diesel engine to something like an old Cessna 152—especially if they need to find something getting on for the total value of the aircraft to cover the re-engining exercise.”

One can appreciate Mark’s point; it’s not quite the same thing but, whilst new diesel cars are taking over the world, how many motorists bother to convert their petrol cars to diesel power—or even shell out for relatively low-cost, fuel-saving LPG conversions?

Aircraft homebuilders are, by contrast, adept at installing their favoured engine in all sorts of machines. Excluding the (expensive) MT propeller, the WAM-120 is priced at £12,000. Wilksch Airmotive provides the cooling system, but the cowl design for any new type would be left to the owner (“although we will help out,” says Mark).

He certainly sees no shortage in demand: “There are 200 Europas under construction, there is a strong interest from Vans RV9 builders and the type certificate owner of the Thorp is looking at low-cost manufacture overseas. We plan to build 50 engines this year, 350 in 2005 and 1,000 per year by 2007.”

Demand will increase as WAM diesels find their way into certificated aircraft and there is plenty of scope for development “The crankshaft is stressed to go up to five cylinders, giving the potential for a family of engines—the next one up being a 160 hp four”

But wouldn’t the five be rather a long engine?

“Well, whatever you put it in, you’re not going to be worried about getting it in Tesco’s car park. Look at the Pilatus Porter—the advantage of dropping a turboprop engine in that airframe was so great that nobody ever worried about the nose becoming a metre or more longer.”

If that still doesn’t sound like a recipe for commercial success, bear in mind Wilksch Airmotive’s astonishingly low overheads. To date, Mark reckons the ten-year development project has cost around £2m. This figure will have eyes popping out in big industry and among his specialist competitors: the amount spent by the Americans in failing to produce a light aviation diesel will probably never be accounted for. The actual development cost of the SMA diesel is probably a French State secret.

My last question was about the finish of the Wilksch engine. “Oh, it’s done like that because my wife said black made her legs look smaller,” Mark grins broadly. “Well, it’s a practical finish, because any oil that you drip on the crankcase from the dipstick doesn’t show up, but the real reason...” He glances at me from under his brow “is that Merlins were black.”

Is this a conceit, or does the little Wilksch diesel really bear comparison with Rolls- Royce’s magnificent aero engine? The famous technical motoring writer L K Setright once claimed that the great WWII V12 was, in truth ‘a triumph of development over design’. Given his severely limited resources, Mark Wilksch had to get his little engine right from the outset—a triumph, you might say, of design over development. That this triumph is a very real one should be beyond doubt: the WAM- 120 is a very, very impressive piece of kit.

I salute an ingenious new aero engine and I take my hat off to a great, visionary engineer and his brilliant team."