Friday, June 1, 2007
Diamond Twin Star: 21st Century Multi
Diamond Aircraft, the world’s third-largest manufacturer of GA, fixed-wing aircraft, is betting that the diesel-powered Twin Star will be the multi trainer of the future
Considering the source, the choice of a diesel powerplant was only logical. While German engineer Rudolf Diesel’s late-19th-century engines are among the world’s oldest form of mechanical propulsion, and have been employed sparingly on military airplanes and dirigibles since the 1920s, they’re a relatively new phenomenon in general aviation. SMA of France and Thielert of Germany have been the pioneers in aircraft diesel development for the little guy.
In fact, the Thielert 1.7 Centurion turbo-diesel is based on an automotive engine design by DaimlerChrysler. It’s extremely similar to a mill used by Mercedes in one of its diesel automobiles, though geared down in the aviation application from 3,900 engine rpm to 2,300 prop rpm. Diamond Aircraft currently offers two Thielert, diesel-powered airplanes: the dedicated diesel Twin Star and the single-engine Star with your choice of a conventional avgas or diesel mill. The Twin Star was originally slated for Thielerts, with Lycomings as an option, but the diesel version has been so successful, Diamond dropped plans for the avgas model.
So why would anyone want a diesel-powered airplane in the first place? One reason is the aforementioned efficiency. Most avgas engines score a specific fuel consumption (SFC) of about 0.44 lbs./hp/hr. The Thielert 1.7 Centurion manages an SFC of more like 0.36 lbs./hp/hr., 20% better. Obviously, diesels can legally burn road diesel, but you’re not liable to find that at most airports, so the alternative is Jet A1, still usually less expensive than avgas and, perhaps equally important, almost universally available at all but the smallest strips.
Avgas is rapidly disappearing at many international destinations. Back in the early ’90s, I delivered a primo Cessna 421C to Subic Bay, Philippines, for a hospital management company. Two years later, the company called and said avgas was becoming so scarce in much of the Far East, they’d been forced to replace the Golden Eagle with a King Air C90. I picked up the 421 in Manila and returned it to the United States by way of Guam, Majuro and Honolulu. (Coincidentally, I had to have avgas shipped in to Majuro, as the airport no longer offers anything except Jet A1.)
The reduced hourly cost of operating a diesel is one reason the type has been so eagerly embraced overseas where avgas has long cost $5 per gallon or more. Jet fuel often sells for as much as $1 per gallon less. The bottom line is a dramatic savings for operators of airplanes that burn jet fuel.
If there’s any bad news, it may be that the American FAA has dictated that the current Mercedes-built Thielert 1.7 Centurion engines must be replaced after only 1,000 hours. On the plus side, the all-Thielert-manufactured 2.0 engines that will supplant the 1.7 will be rated for 2,400 hours or 12 years, whichever comes first. Robert Stewart, of Diamond dealer U.S. Aero in Long Beach, Calif., says the new engines will likely sell for $24,500 in today’s dollars, but that’s for a firewall-forward new powerplant, and it’s prorated to allow for the 1,000 hours already flown. In other words, the owner will pay more like $12,000 per engine. Multiply that by two, and it’s about what you’d pay for a single factory overhaul on an IO-360 Lycoming.
If you learned to fly twins in a comparative antique, such as a Travel Air or Apache, as I did, the Twin Star will come as a revelation. These days, Diamond’s glass is more than half full, and those who allege there’s been no innovation in general aviation need only take a ride in a Twin Star to understand the error of their position.
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Labels: Piston Twins