Plane & Pilot
Saturday, April 1, 2006

Diesel Skylane


A viable alternative to avgas has arrived


Diesel SkylaneIf you fly a typical general-aviation airplane, you probably can’t imagine a world without avgas. I fly a Mooney with a four-cylinder, 200 hp Lycoming, and there’s currently no alternative engine available. For me and for thousands of other aircraft owners, the thought of avgas becoming obsolete is simply inconceivable.

 

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As is typical of diesels, engine start requires the pilot to activate the glow plugs, one per cylinder, and initiate the start when the annunciators indicate the plugs are up to temp. In fact, on a typical Southern California day, you may not even need to activate the glow plug, as the engine will fire by simply engaging the starter. Similarly, hot starts aren’t cause for concern in a diesel. The engine starts pretty much the same, hot or cold.

By the time you reach the run-up area, there’s little to do but fly. The sensations of takeoff aren’t that different from those in a standard Skylane, provided you ignore the manifold pressure gauge. Takeoff power is about 90 inches of manifold pressure, a function of the diesel’s turbocharger. Climb is similar to a standard Skylane at sea level, but the diesel airplane gradually pulls away from the avgas model as the ground falls away.

The SMA’s ECU automatically controls rpm and mixture, so the only adjustment necessary for cruise is a reduction of manifold pressure, and that’s where the diesel’s advantage becomes most obvious. The near-universal availability of jet fuel is one benefit of the diesel engine, but another is a specific fuel consumption (sfc) that’s only about two-thirds that of the best piston mills.

Using the Skylane’s O-470U as the logical example, sfc works out to about 0.48 pounds per horsepower per hour at full power or 0.44 pounds at 65%. Converted to diesel power, the same airplane scores around 0.35 pounds per horsepower per hour at all power settings. In more familiar terms, that means the SMA Skylane will cruise along at 65% on about 7 gph, whereas the stock airplane will burn 11 gph. The drag profiles of the modified and stock airplanes are similar, so with 88 gallons in the tanks, the Diesel Skylane scores a kidney-stretching 11-hour range plus reserve, compared to only four hours in the stock airplane. At max cruise, that works out to more than a thousand miles.

Cruise numbers between the two engine types are similar at lower altitudes, but the SMA-powered airplane’s turbo diesel really begins to shine at high altitudes, where the turbocharger begins to pay for itself. At 10,000 feet, the standard and diesel airplanes are fairly neck-and-neck at 138 knots, but up at 12,500 feet, the diesel manages 140 knots compared to only 124 knots in a standard Skylane. The 12,500-foot altitude is currently the limit on the SMA Diesel Skylane, but the company is hoping to receive approval to step up to a max altitude of 18,000 feet in the near future.

While the vast majority of the numbers for the SMA Diesel are in the positive column, the standard piston airplane does retain several payload advantages. Diesel engines dispense with such extravagances as carburetors, spark plugs and magnetos, but they nevertheless wind up slightly heavier than standard avgas engines. Comparing the four-cylinder, SMA SR305-230 to the stock, six-cylinder Continental O-470U, the diesel installation winds up about 65 pounds heavier.

Additional fuel weight also reduces the diesel’s payload. Diesel engines were theoretically developed to burn diesel fuel (big surprise), but most SMA or Thielert-powered airplanes will probably never do so, which, as it turns out, is just as well. Diesel fuel is fairly dense (7.2 pounds per gallon), not a problem for an 18-wheeler cruising the interstate, but a definite concern for weight-sensitive airplanes.

Eighty-eight gallons of diesel weighs in at 634 pounds versus 528 pounds for 88 gallons of avgas. Considering that diesel is almost never available at airports, aircraft fitted with diesel engines obviously will use Jet A practically all the time, though even that has a slight weight disadvantage (at 6.7 pounds per gallon, 88 gallons of Jet A weighs 590 pounds). If jet fuel isn’t available, a diesel engine can burn avgas, though the opposite isn’t the case.





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