I don’t know about you, but I can’t imagine a world without avgas. Within a few years, I may need to stretch my imagination. The reality is that avgas may not be with us for more than about another decade (if that long).
Mobil has already backed out of the avgas business, and several other companies have significantly reduced production and would love to find an excuse to cease refining avgas altogether. 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 going away is inconceivable.
Still, volume is profit in the energy business. Avgas has always been one of the lowest-volume fuels on the market. If there’s an existing STC and you can retard the timing on your airplane to use premium car gas, great. If not, you may be faced with some tough choices in a few years. Sell your airplane for parts, or what?
The pending scarcity of avgas is one strong motivation for the emergence of diesel engines in general aviation. “Discovering” diesel power for airplanes in the 21st century is perhaps a little surprising, as the diesel engine dates back to the 19th century. Diesel is one of the world’s oldest internal combustion concepts, dating back to Rudolf Diesel’s 1892 patent in Germany.
A quartet of 1,200 hp, Daimler-Benz, V-16 diesels powered the Hindenburg, but there’s been little development in diesel engines for fixed-wing aircraft until the last decade. Diesels are simple, durable powerplants that can burn practically any combustible liquid.
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.
From the firewall aft, there are few differences between the standard avgas Maule M-7 and the new Diesel M-9. Adapting the diesel does demand different instrumentation and controls and larger fuel filler openings atop the wings to accommodate the larger jet fuel nozzles. The panel includes changeouts of several gauges: manifold pressure, tachometer, oil pressure and temp, cylinder head temps, turbine inlet temp and fuel level (graduated in pounds rather than gallons).
The SMA diesel utilizes an engine control unit with a full FADEC system to maximize performance and efficiency, so in essence, the only operational engine control is the throttle. The ECU automatically maintains rpm at 2,200.
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. Ground operation is conventional Maule M-7, with no surprises, but slightly better over-the-nose visibility by reason of the sloping cowling.
The sensations of takeoff aren’t that different from what you’d experience in a standard Maule, provided you ignore the manifold pressure gauge. Takeoff power is about 90 inches of manifold pressure (yes, we said 90 inches, and no, don’t expect to see any Maules running the pylons at Reno), purely a function of the diesel’s turbocharger.
Climb is similar to that of a standard Maule at sea level, but the diesel airplane gradually pulls away from the avgas model as the ground falls away, owing to the airplane’s turbocharger. Above 5,000 feet, the diesel version will easily leave the avgas Maule behind.
The SMA’s engine control unit automatically manages 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 almost 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 engines.
Using the Maule M-7’s standard O-540 or IO-540 as the logical examples, avgas SFC works out to about 0.48 pounds/hp/hour at full power, 0.44 pounds at 65%. Converted to diesel power, the same airplane scores more like 0.35 pounds/hp/hour at all power settings. In more familiar terms, that means the SMA-powered Maule will cruise along at 75% power on about 8.5 gph, whereas the stock airplane will burn 11 to 12 gph.
The drag profiles of the modified and stock airplanes are similar, so with 85 gallons in the tanks, the diesel Maule scores a kidney-stretching nine hours’ range plus reserve, compared to only 5.5 hours in the stock airplane. At max cruise, that works out to nearly a 1,000 nm range for the Diesel Maule.
Cruise numbers between the two engine types are similar at lower altitudes, but the SMA-powered airplane’s turbodiesel really begins to shine at higher altitudes where the turbo begins to provide some advantages. Critical altitude (the maximum height at which the turbo can supply sea level air) is 10,000 feet. Up at 12,500 feet, the diesel manages 138 knots compared to more like 130 knots in a standard Maule. The 12,500-foot altitude is currently the limit on the SMA Diesel Maule, but the company is hoping to receive approval for a step up to a max altitude of 20,000 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 retains a few 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.
Additional fuel weight also reduces the diesel model’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—just as well, as it turns out. Diesel fuel is fairly dense (7.2 pounds/gallon), not a consideration for an 18-wheeler cruising the interstate, but a definite concern for weight-sensitive airplanes.
Eighty-five gallons of diesel weighs in at 612 pounds versus 510 pounds for 85 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 suffers a slight weight disadvantage (at 6.7 pounds/gallon, 85 gallons of Jet A weighs 570 pounds).
In other words, the total weight penalty between a heavier engine and heavier fuel in a Maule converted to use jet fuel is about 125 pounds. Fortunately, the Maule hardly notices the difference, and performance benefits more than offset the reduction in payload.
Maule hopes to certify the M-9-230 diesel at 2,800 pounds gross on wheels, and the airplane on display at this year’s Oshkosh AirVenture had a redesigned wing root section and tougher main landing gear to accommodate the higher weight. When the M-9-230 is certified some time next year, Maule hopes to offer a greater than 1,000-pound useful load for a 430-pound payload after full fuel. Like most previous Maules, the M-9-230 is a master of utility, with four doors to feed five seats or a quick-change option that allows pulling the aft seats to open up a large cargo area.
The target price for the new Maule M-9-230 diesel is $250,000, and it’s a safe bet there will be a number of takers. Many bush operators who fly a variety of aircraft ranging from Helios and Twin Otters to Maules will probably be delighted to stock their outlying strips with a single type of fuel. Similarly, there are also a number of places in the world where avgas may be simply unavailable. It’s already happening in certain parts of the South Pacific.
As one of the least known general aviation manufacturers, Maule continues to turn out tough, little, go-anywhere taildraggers and nosedraggers designed to endure in places where other airplanes would fear to roll a tread. The M-9-230 adds use of jet fuel to the Maule’s many other talents.
SPECS: Maule M-9-230 Diesel