Obviously, the dominant aviation fuel these days is Jet A, and diesels run just fine on it. Several companies have taken the lead in developing diesel engines for aircraft, perhaps the most notable being Austrian manufacturer Diamond Aircraft. Currently, Diamond incorporates German Thielert diesels on its Star and Twin Star models.
Maule Aircraft of Moultrie, Ga., also has adopted a diesel, in this case, a French SMA, to power its line of bush planes. Société de Motorisations Aéronautique (SMA) of France is one of the oldest companies in the business of adapting diesels to aircraft, and it’s now a wholly owned subsidiary of the French jet-engine conglomerate Snecma. SMA began developing an aviation diesel in 1998 and earned FAA certification for its first product in May 2004.
That first product is the SMA SR305-230, a four-stroke, four-cylinder turbodiesel with horsepower potential from 200 to 300. The powerplant has been installed and flight-tested in a number of aircraft, including our case in point, a Maule M-9.
Maule knows a thing or three about burning jet fuel, as the company introduced a 420 shp, Allison 250–powered turboprop M-7 back in the late ’80s. The Georgia company first premiered the new SMA diesel engine at Oshkosh 2003 and has been flying the SR305 on a modified M-7 taildragger for nearly five years, quietly marching toward certification.
SMA is currently pursuing STCs for seven aircraft models, including the Piper Dakota and Seneca, SOCATA Trinidad, Cessna Skyhawk, Maule and others. The company is also developing six-cylinder, 300 to 350 hp models for possible use on big singles and medium twins. (There’s currently an experimental, diesel-powered Duke flying in Europe.)
As one who delivered Maules from Georgia to California for 10 years in the ’70s and ’80s, I can verify that the new diesel model has a very different look. One glance at the cowl is enough to convince you that there’s something different going on ahead of the firewall. In the case of the new Maule M-9-230, the cowling is one of those typical works of art from LoPresti Speed Merchants in Vero Beach, Fla. Curt LoPresti, president of Speed Merchants, reported that the company designed the cowl with attention to improving the drag profile and reducing the engine’s internal operating temperatures.
The SMA engine is air and oil cooled, and the combination of induction and cooling air demands four large inlets. The Hartzell three-blade prop is specifically designed for the diesel engine, intended to provide maximum torque at the constant 2,200 rpm of climb and cruise.
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