Maules M-9 Aero Diesel

Who says diesels are stinky, noisy and vibey? This one is smoother than a Lyc but at a princely price premium.

Anyone whos been around airplanes for, say, 10 minutes, has probably seen the famous picture of B.D. Maule demonstrating the STOL performance of his airplane by launching one out of a hangar like a mortar round out of a tube. He only did it a couple of times but the photo has been so widely circulated-there’s even a Sam Lyons print-that youd think hangar takeoffs are a regular occurrence around Moultrie, Georgia, where Maules are made.

They arent. But the factory pilots don’t think twice about using the parking ramp in front of the plant as a runway, as Ray Maule recently did in demonstrating the companys just completed SMA-diesel powered M-9. Despite the fact that the airframe and engine had only 25 hours on it, Ray Maule was confident enough in the diesels performance to take off-and land-from the plant ramp, as he has done with Maules gas-powered aircraft for years.

Judging by the diesels performance, our guess is that the confidence isn’t misplaced. The SMA diesel turns out to be surprisingly quiet and smooth and at first blush, it seems to be a natural match for Maules utilitarian, no-nonsense airframes.

During our demonstration flight with Ray Maule, we were impressed with the engines simplicity and flyability but less enthralled with the stiff cost premium the engine will command. The yet-to-be-answered question is whether the diesels numbers and reliability over the equivalent gasoline engine will attract enough buyers to make Maules efforts worth the trouble. Our impression is that Maule believes it has a great deal riding on the success of the diesel but it will have to demonstrate that there’s a good reason to buy this engine over the Lycomings traditionally used in Maules.

Export Market
When we first spoke to Maule about its SMA diesel project a year ago, it had just entered into an agreement to install the SR305 four-cylinder diesel into one its airframes. Maule is a small but steady player in the GA field so we were surprised that the company front-burnered the project with enough enthusiasm to have it ready for EAA AirVenture 2003. But there it was, having been flown from Moultrie to Oshkosh on what was essentially its shakedown cruise.

Maule is an old line traditional manufacturer of airplanes, adhering to a tried and true-if somewhat dated-combination of rag-and-tube fuselages and riveted metal wings. It hardly seems like the kind of place that would be interested in bleeding-edge engine technology, let alone something as potentially chancy as aero diesel engines.

But the canny side of Maule is that it has remained in continuous production since 1961 while other aircraft companies have blossomed and faded or drifted in and out of bankruptcy. The fact that Maule has endured suggests that it knows its market and has a reliable eye for the durable idea.

Ray Maule told us the companys interest in diesels began nearly a decade ago when Maule was an Oshkosh display neighbor of Michael Zoche, an eccentric German who had wowed homebuilders with his exquisitely crafted radial diesel engines. Maule saw potential and tried to convince Zoche to do what SMA finally did: deliver a production-ready engine that Maule could install in one of its airframes. (Zoche never did and has been sidelined by the likes of SMA and Thielert, another European maker of diesels.)

Maule believes the diesel option expands the companys export market, which is potentially large. Because they do what no other new aircraft do-namely impressive STOL performance, good payload and simple, straightforward maintenance requirements-Maules are in service all over the world but not in large numbers. (Total Maule production since day one is just over 2000 aircraft, at an annual production rate of 60 to 70 aircraft.)

In current production aircraft, Maule is an all-Lycoming company, offering some 15 models powered by O-320, O-360 or O-540 series engines. It also offers two models powered by an Allison 250-B17-C turbine, one a taildragger and one a tri-gear. The addition of the diesel adds yet another choice to an already bewildering number of powerplant/model options for a small company. Maule says it has little interest in the FADEC systems already certified or under development, including Lycomings EPiC engine control.

M-9 Diesel
As currently construed, the Maule diesel is a new model, designated the M-9-230, with the latter number signifying horsepower. Its based on the M-7-235 airframe, what the company calls 235s.

Like the M-7, its a four-place taildragger with two full-size seats up front and a tight bench seat in the back. Maule calls this a five-seat airplane but if thats true, the three people occupying the benchseat had better be both small and friendly. If Maule adheres to its usual options package, a larger cargo door can be fitted in place of the small rear passenger door and the rear seat can be removed to open up a large cargo area.

The M-9 follows Maules standard construction, which is Ceconite over a 4130 Chromolly for the fuselage and vertical fin, with a stainless steel firewall and a fiberglass cowling. The wings are traditional riveted-rib construction with built-in fuel tanks, not wet wings. The rudder and elevator are fabric covered, the ailerons are riveted aluminum.

Brent Maule, the companys marketing director, told us the M-9 is all but identical to the M-7 but has a beefed up wing to accommodate higher gross weight, a heavier tailwheel and an additional crossover tube in the fuselage. The airplane we flew represents a prototype and may vary from the final production model. The M-9 will sell for about $200,000, a $55,000 price premium over the equivalent gasoline model selling for about $145,000.

Gross weight of the M-9 will be 2800 pounds, up from 2500 pounds for the M-7. Part of that will be offset to accommodate the diesel, which is more than 100 pounds heavier than the O-540 Lycoming. The higher gross will also accommodate additional fuel, up to 85 gallons feeding from two mains and two aux tanks. The fuel system is Cessna style, gravity feed from the mains and aux tanks through a left/right/both off fuel valve.

What will the thing carry? The empty weight hasnt been nailed down yet but given the heavier engine, we would estimate an empty weight of about 1800 pounds for a useful load of close to 1000 pounds, which is a bit more than the gasoline-powered M-7, according to Maule specs. Fill the tanks with Jet-A-at 6.6 pounds per gallon-and there’s payload for two standard people and all the baggage theyd want to carry.

The diesel Maule theoretically has greater loading flexibility over a gasoline-powered equivalent thanks to the engines better fuel specifics. You could offload some Jet-A and apply the weight savings toward carrying more payload and still be able to fly the same distance as the gasoline-powered airplane, a diesel selling point. If the diesel performance claims pan out, the range/endurance advantage is not trivial.

Ray Maule told us he flightplans a gasoline-powered M-7-235 at about 137 knots (157 MPH) and 14 GPH. The diesel will do the same speed, according to Maule, on just under 11 GPH. So for the equivalent weight of fuel, the diesel will fly 17 percent further.

And theoretically, thanks to the diesels turbocharging, you can go a little faster on the equivalent fuel, if youre willing to climb and suck oxygen. Theoretically, all this is true. Practically, we cant tell if the airplane will deliver on that promise or not because the Maule we flew was equipped with neither a fuel flow gauge nor a power indicator of any kind other than an incorrectly calibrated manifold pressure gauge.

As we reported in the August, 2002 issue of Aviation Consumer, the SMA SR305 is one of two leading aero diesels, the other being the Thielert Centurion engine, formerly the TAE 125. SMA came out of the ground first with its engine but we believe Thielert has more engines flying, given the success of the diesel-powered Diamond DA40 TDI.

The SR305 is a four-cylinder, turbocharged diesel with direct injection and air/oil cooling for the cylinders and heads. It has a 15-to-1 compression ratio and relies entirely on compression ignition; no mags. It produces a maximum of 230 HP for five minutes and 200 HP continuously after that to an altitude of 11,000 feet, at which point the engines FADEC, as a means of improving durability, reduces power as altitude increases above the limit.

In the Maule M-9, the engine is set-up to maintain full power at 2200 RPM until reaching 5000 feet, at which point the FADEC reduces output to 200 HP. If the pilot decides to remain below 5000 feet, the engine will churn out 230 HP until the throttle is reduced and that has to be done manually by the wristwatch.

Maule declined to provide us much detail on the engine installation although we did inspect it carefully and, with Ray Maules permission, photographed it. However, the engineering department intervened and asked that we not publish the photographs. (In fact, in one of the weirder turnarounds weve seen, Maule confiscated our photos.)

The engine lives inside a large fiberglass cowl designed by LoPresti Speed Merchants. It looks conventional on the top but has an enormous fish-mouth opening on the bottom cowl for induction and cooling air. A wedge-shaped splitter routs air to oil and fuel coolers and a turbo intercooler. Inside the cowling, there’s plenty of room between the cowl and cylinder heads and lots of space behind the engine for service access.

The rear of the engine is dominated by the fuel injection pump and by the plumbing and radiators for oil cooling. On balance, we would say the installation is noticeably less cluttered than a gasoline engine, which it should be, since there are no magnetos. The M-9 has a three-blade Hartzell Kevlar prop but it can also swing a three-blade MT composite.

On the other side of the firewall, there’s a throttle and two other controls: a pull out lever for setting the FADEC from electronic to manual control and an emergency shutoff to choke off the engines air supply to shut it down. Neither of these should be used in normal operations. This is a true single-lever set-up.

For instrumentation, the aircraft has MAP, CHT, TIT, prop RPM and oil temperature and pressure. There’s also an engine-specific annunciator panel for general fault analysis .

Flying It
If SMA set out to build an easier-to-operate engine, it would appear that it has achieved that goal. Because the airplane is new with few hours, Ray Maule took us through the starting checklist. But there’s not much to it. Once the master is on, the throttle is cycled for free movement, the fuel valve is checked and the annunciator panel is reviewed for any fault lights. In cold weather, a blue lamp on the fault panel indicates that the glow plugs are on. These were hardly needed on 90-degree Georgia day.

With a conventional key switch engaged, the engine has a high cranking speed-it has a Sky-Tech starter-and it started instantly, after no more than a blade or two. It settled immediately into a smooth 800 RPM idle. From the outset, our impression is that its smoother than the Lycoming O-540. Diesels have a vigorous power pulse that produces torsional vibes that are difficult to isolate but in the Maule, these don’t seem to telegraph through the airframe.

The idle noise level is unremarkable but the exhaust note is…different. Its more of steady low rumble than the lopping trot of Lyc; more Honda, less Harley. Oh, the smell. Very little. With the cabin windows open, we noted a barely detectable oily exhaust smell. In flight, there’s no odor at all.

Lacking magnetos, the SR305 obviously doesnt require a conventional run-up. After a brief warm-up period, Ray Maule taxied to the end of the factory ramp and turned the airplane into a weak breeze. What run-up is required consists of throttling up to what amounts to a high percentage of boost pressure and checking the instruments and fault panel for anomalies. Boost for this engine is nothing short of record setting; typical cruise boost is 77 inches while takeoff requires about 86 inches. The airplane we flew wasnt equipped with a calibrated MAP gauge so Ray Maule estimated boost performance during the pre-takeoff check. With no prop, no mixture, no carb heat and no boost pump, there’s nothing else to do but push the throttle forward and go.

We thought both the engine and airplane acceleration were brisk. The engine comes right up to its 2200 RPM limit and is kept there by a mechanical governor independent of the FADEC; below the governor range, the FADEC controls RPM. There’s a noticeable change in the exhaust note but not much; from inside the cockpit, the engine is impressively quiet and has a low vibe level. Our initial climb out was at 100 MPH and 1100 FPM at a weight we estimate to be about 150 pounds below gross. Through 5000 feet, where the automatic reduction from 230 HP to 200 HP occurs, the airplane maintained between 900 and 1000 FPM. CHTs were in the 320-degree F range and remained there throughout the flight, except when we idled the engine in flight, when they went nearly off-scale low.

Operationally, the throttle doesnt have to be touched again if youre climbing above 5000 feet. Above that, the FADEC reduces power gracefully. When we reduced the throttle a bit, the airplane merely slowed down. Presumably, it burns less fuel when that happens but without instrumentation, we don’t know how much less. In level cruise flight at 7500 feet, we observed a true airspeed of 137 knots at 200 HP, which is 87 percent power.

Ray Maule told us that this duplicates the performance of the M-7-235 at the same altitude but the M-7 burns 14 GPH of gasoline, the M-9 about 11 GPH of Jet-A. Because Maule doesnt publish power or performance charts for its aircraft, we cant compare the two directly based on stated claims. Generally, however, a normally aspirated engine cant maintain 87 percent power at 7500 feet so if the gasoline-powered M-7 is truly making the same speed, its doing it with less horsepower but on more fuel, from both the fuel volume and fuel cost standpoint.

This begs the question: if the diesel is making more power at that altitude, why isn’t the airplane going faster? Or will it go faster if its taken higher? We wonder if there’s a slug of cooling drag due to that massive intake in the cowl. SMAs Thierry Saint Loup, the companys technical rep in the U.S., says thats possible. Future development by Maule or others may wiggle out the cooling drag and eke out more speed. For now, we would say any speed gain is hiding. It may emerge later.

We also tried some maneuvering to test throttle response and acceleration. Ray Maule cautioned us not to ham hand throttle but then conceded that the FADEC actually controls throttle response. In any case, we noted there’s a definite power step when the throttle is advanced. The engine powers up evenly at first, then suddenly accelerates more quickly. SMAs Saint Loup told us this was probably due to set-up anomalies in the Maule; throttle response should be smooth, with no spikes.

The larger question is what kind of fuel economy is the SR305 really delivering. Not to be paranoid, but we sometimes think there’s a conspiracy afoot to keep us in the dark. We take it on faith that the SR305 produces 200 HP above 5000 feet in the Maule.

But the airplane had no fuel flow instrumentation so we don’t know what its burning. Ray Maule told us the M-9 burned about 10.9 GPH block-to-block on its roundtrip to Oshkosh at altitudes where full throttle should have been producing 200 HP.

If thats correct, it yields a brake specific fuel consumption of .36 pounds per horsepower/hour, which is both believable and impressive, although perhaps not so impressive to beat out a gasoline engine at .40 to .43 BSFC.

In our view, Maule is the perfect company to pioneer aero diesels in the U.S. Its accustomed to delivering aircraft with many engine options and it isn’t wedded to any particular engine type. The company is prepared to deliver anything the customer wants, from a Lycoming four-banger to a Allison turbine, and now a diesel.

Further, the SR305 nicely fits the Maule niche, which is owner-flown pleasure flying to distant, outback destinations plus working missions hauling people and cargo on wheels and floats. The M-9s loading flexibility and endurance are plusses for these jobs, although we still pine to see actual proven fuel consumption data.

What Maule now has to prove is that the $55,000 price premium for this engine will yield longer range, lower operating costs and performance equal to or better than gasoline engines. Further, the engine needs to demonstrate that its tough enough for a 2000- to 3000-hour TBO without eating an owners wallet in the process. (Initial TBO is set at 2000 hours.)

We cant predict the outcome of the campaign to TBO. But based on flying the M-9, we think the flyability question has been answered. The engine is smooth, quiet and simple to operate and appears to generate all the power the M-9 needs. Were glad to see Maule pursuing it and absent any durability or maintenance doubts, we wont be surprised if they find a niche market for the M-9.

Also With This Article
Click here to view “Checklist.”
Click here to view “One Lever, Mechanical Back-Up.”
Click here to view “Maules LSA: Recyled-M-4.”
Click here to view “Life Cycle Cost Comparison.”