If Charles Lindbergh had equipped the Spirit of St. Louis with a Packard Dr-980 instead of the Wright Whirlwind, he might not have had to stuff a gas tank in the nose in lieu of a windshield and he could have enjoyed the view while crossing the Atlantic. And that would have made ditching easier, too, since it's probably a 50/50 shot that the Packardthe first aircraft diesel of notewould have run long enough to get across the pond.
So it is with aerodiesels and sometimes I think not much has changed in the intervening 80 years. For an article in Aviation Consumer on the state of the diesel market, I picked up a copy of Robert Meyer's excellent The First Airplane Diesel Engine, part of the Smithsonian's Annals of Flight Series. I'd seen pictures of the Packard diesel in a Stinson Detroiter and always assumed it was a one-off experimental. It wasn't.
The Dr-980 was a full-up industrial project funded by the equivalent of more than $8 million in today's dollars. A 600-person factory was set up to build it. Think of it as the 1928 version of the Austro project. But it failed. There were several reasons for this. Packard had trouble keeping the cylinders on the thing, its quirky four-cycle, single-valve design tended to fill the cabin with stinky fumes and it was 30 percent more expensive than its competition, the Wright Whirlwind.
But like modern diesels, it had terrific fuel specifics--.0.40 BSFC compared to the Whirlwind's awful 0.60. I now see why Lindbergh had to carry all that gas. Actually, the Packard represented a much greater leap in efficiency against the Whirlwind than modern diesels do against their gasoline counterparts. A Continental IO-550 running lean of peak can operate in the .39 to .40 range, while today's aerodiesels typically run in the mid-.3s. A Packard-powered Bellanca CH-300 held the unrefueled time-aloft record of more than 84 hours until Dick Rutan broke it with the Voyager 1986.
What finally did the Packard in was higher-performance gasoline engines made possible by 87-octane gas. Pilots were more interested in speed than economy and range and I have to wonder if that's still true. Or will the imminent loss of 100LL drive enough buyers toward aerodiesels to make these engines real players? Austro, Continental, Centurion and SMA hope so.
But the ghosts of the Packard failure may still inhabit the diesel market. When you reduce the numbers on diesels, they sometimes turn out to be heavier, more expensive and less power dense than gasoline engines. In fact, to some extent, things have gone a little backward. The Packard matched the Whirlwind's weight and power density, but no modern diesels do that against their gasoline competition because gas engines have gotten lighter and more powerful. An O-470 can claim 1.78 pounds/horsepower; the Austro a portly 2.42worse than the Packard. But that extra weight may do what the Packard couldn't: deliver long-term durability. With diesel, you can have light or you can have durable. Getting both is the designer's balancing act. SMA and DeltaHawk may have it figured out.
What's different now is assured fuel supply and that may be enough to tip it, even if diesels still struggle with weight. In looking at fuel production history, I was surprised to see that peak production for avgas occurred more than 50 years ago, in 1958. It's been downhill ever since. That was the year the military finally got enough jet aircraft and had retired enough piston airplanes to tip the balance toward kerosene.
So if ever there were overwhelmingly favorable conditions for aircraft diesels to succeed, we're lookin' at 'em.