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.
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