Monday, August 1, 2005
The Force behind the Diamond DA42 Austrian invasion
Certainly, the Twin Star’s most innovative feature is its pair of Thielert Centurion 1.7, water-cooled, turbo-diesel engines, rated for 135 hp each. Diesels have been used for decades on cars and trucks and have proven remarkably reliable and efficient. The Thielert Aircraft Engines used on the Twin Star are essentially adapted versions of powerplants employed on Mercedes diesels. Diamond mounts composite, three-blade, constant-speed, MT propellers as standard.
Perhaps the primary deficit of diesels for aircraft use is the weight of the engine. Diesels are inherently heavier than piston powerplants, as much as 20% more, and weight is critical on airplanes. In exchange for that deficiency, however, diesels offer rugged reliability and remarkable fuel efficiency, burning cheaper diesel or jet fuel.
In fact, however, it’s unlikely many pilots will choose to burn diesel, even if they can find it. Diesel fuel is fairly dense and weighs in at more than 7.1 pounds per gallon, compared to 6.7 pounds for jet fuel and 6.0 pounds for avgas. In the U.S., it’s likely all DA42s will operate on Jet-A.
A typical specific fuel consumption for a general-aviation piston engine is .42 to .45 pounds per hp per hour. In contrast, diesels regularly achieve sfcs of .35 pounds per hp per hour or better, especially when operated at low power settings. This 20% advantage is partially offset by the fact that jet fuel is 10% heavier than avgas.
(Even so, there’s little question that diesels are several orders of magnitude more efficient than piston products. A Diamond factory pilot, returning to Austria after the 2004 EAA AirVenture, flew 1,900 nm nonstop across the Atlantic from St. Johns, Newfoundland, to Porto, Portugal. He set power at 42% for the crossing and burned only 72 gallons of jet fuel. Average consumption was a mere 2.9 gallons per engine per hour, or 5.8 gph total. Average groundspeed worked out to 152 knots with the help of 20-knot tailwinds.)
Certainly, flying behind turbo-diesels will demand some attitude adjustments. For one thing, starts are very different from piston practices, read that as easier. Power is controlled by a single lever. Turn on both engine control units (ECUs), wait for the glow plug annunciator lights to extinguish, then twist the key. FADEC (Full Authority Digital Engine Control) does the rest, optimizing timing for all phases of flight, startup, climb and cruise. The runup is similarly uncomplicated. Press the ECU test buttons, and the system goes through its own self-diagnostics. After that, you’re ready to fly.
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