Plane & Pilot
Monday, August 1, 2005

"Star Wars!"

The Force behind the Diamond DA42 Austrian invasion

Okay, perhaps it’s true other countries outdo the USA when it comes to manufacturing automobiles, computers and TV sets, but there has never been any serious competition with America’s general aviation airplanes. Companies such as Piper, Cessna, Beech, Mooney, Maule, Cirrus, Lancair, American Champion, American General, Commander and Grumman-American have accounted for the vast majority of light aircraft sales in the last half-century. " />

Unlike the Seneca and Seminole that employ counter-rotating propellers, the DA42 turns both engines left, so the left mill is critical. In other words, you’ll generate more of a rolling moment if the left engine fails than if the right quits. Fortunately, an electrohydraulic auto-feather system will sense any failure and allow the prop to move to the low-drag feathered position with a minimum of fuss. It’s interesting to note that the Twin Star’s dirty stall is 56 knots, only 12 knots below Vmc. Certainly, you could fly the airplane into an asymmetric power roll, but you’d have to be asleep not to notice it coming.

In keeping with the universal principle of TINSTAAFL (There Is No Such Thing As A Free Lunch), the diesel makes some trade-offs in exchange for its remarkable economy. Partially for that reason, Diamond is currently flight-testing a Lycoming version of the Twin Star that may be fitted with all-metal Hartzell propellers rather than the standard airplane’s composite MTs. With a pair of 180 hp IO-360s mounted on the wings, the Americanized Diamond twin will sport an extra 90 hp over the TAE airplane, and with the same 3,748-pound gross weight (1,700 kilograms in Euro-speak), that should translate to a true rocket ship in climb. Standard empty weight is listed as 2,400 pounds on both Twin Stars, but in the real world, the diesel may weigh in slightly heavier. A gross weight increase is also being investigated by Diamond.

Similarly, I’d bet cruise in the Lycoming-powered Twin Star at typical normally aspirated altitudes will be at least 185 knots. This airplane may be more attractive to private owners because of higher speeds and better climb, neither especially important attributes for a trainer. With the benefit of turbocharging, however, the Thielert model should surpass the Lycoming Twin Star in both climb and cruise above 12,000 feet.

Since announcing the Twin Star three years ago, certification and production timetables have been pushed back several times, and the latest schedule doesn’t call for North American deliveries until the Q1 of 2006. Delays in the Twin Star program have been partially a function of certification problems in Europe on the Garmin G1000 that comes standard on the DA42, as well as waiting for the establishment of suitable levels of technical service capability for the diesel engines in the U.S.

Another significant delay was related to the Thielert engine’s liquid cooling system. Diesels generate massive amounts of heat, and despite the use of intercoolers and liquid cooling, Diamond had to go back to the drawing board to come up with adequate cooling for high-altitude operation. Engine heat has been a consistent problem on the diesel, despite the use of four cooling radiators.

If anyone can solve such problems, however, Diamond can. The company has more experience than practically anyone with aircraft diesel engines. Diamond already has delivered some 150 diesel-powered Stars in Europe, and it has learned quite a bit about the Thielert Centurions in actual service. Time between replacement (TBR) on the Thielerts is currently 1,000 hours, but Diamond is now hard at work to upgrade that to 2,400 hours. Rather than overhaul the engines, you’ll replace everything from the firewall forward for about $25,000 a side.


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