News, facts, and comments on the coming revolution for piston-engine aircraft.
In 1998, one diesel engine flew on a converted airplane for the first time since 1945. Today, close to 4,000 singles and twins are flying. This is the beginning of a worldwide trend which will eventually allow a rebirth of the piston-engined aircraft, around new specs and new missions.
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News of October 12, 2008
The forgotten secret of the superb German diesel flying boats.
(Dornier 26 - Doc: HistoricAircraft.org)
Between 1930 and 1945, flying boats for transport were fashionable because the world was lacking runways, especially big runways for big planes. A flying boat doesn’t need a runway. So the jetset of the days was travelling in Boeing 314s, Martin 130, Sikorsky S-40, 42 and 44, Liore-Olivier 24 and 26, Short Calcuttas and Empires… Germany built many flying boats for various uses, most of them using the Junkers Jumo 205 diesel 2-stroke opposite pistons, with powers from 600 to 880 HP. Best knowns were the Do-18 twin, and the Blohm & Voss Ha 138 and 139. The most beautiful, and possibly the most beautiful flying boat ever made, was the Do-26 See Adler (Sea Eagle), which could carry 4 passengers plus crew of 4 and mail from Lisbon to New York, or during the war Bathurst to Natal, and back. During the battle of Narvik in early 1940 Do-26s were carrying 18 troops from Germany to northern Norway. The Do-26 weighed 25,000 Lbs empty and 50,000 Lbs at take off, therefore she could carry her own weight: So much for those who think that diesels are heavy… Top speed was 185 kts. Cruise economy speed was 135 kts and range was almost 4,000 NM. And this leads me to disclose the secret of these diesel flying boats.
You certainly agree that the power a plane needs at best economy speed is much less than the power at take off. How much less depends on the design. The most economical plane would be the glider if it could glide for ever, which is why the most economical planes, and therefore the ones with the longest range, are the ones with a high aspect ratio (ratio of wing length to width) giving them a lower induced drag and a most efficient gliding ratio. A classical example was the Convair B36 strategic bomber of the fifties, which had an aspect ratio of 13 and could cruise at 230 kts for 9,000NM without bombs (ferry range). Its six R-4360 engines were delivering 1,000HP at cruise speed against 3,500 at take off.
Therefore, to obtain a long range, basically you need a high aspect ratio, and an optimal cruise speed of some 1.3 to 1.5 times stalling speed clean. And here comes the problem with gasoline engines: at 1/3rd to 1/4th their nominal power, the specific fuel consumption (fuel consumed per HP-hour) becomes lousy because combustion is no more in optimal conditions.
At same fractional power, the diesel is as efficient as at full power. So maintaining cruise speed may require down to 50% of the fuel flow needed with gasoline engines operating at same fractional power, meaning so much more range. The Germans therefore said: the future of air transportation is the diesel flying boat flying at some 1.3 to 1.5 Va with a high aspect ratio. And they demonstrated it. Then, in 1945, the secret was lost. Nobody was interested anymore in such planes when one could buy a surplus DC3 or DC4 for a song, and use all the runways built during WWII, and when the future so obviously was the turbojet.
A cargo diesel motor glider with a very high aspect ratio cruising around 100 kts could very well again be the most effective way to ship goods by air. Eventually, a very big one.
Think about it while looking at the plane here under: Do you recognize it?
posted at 8:36 AM
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Every month: news, facts, and comments on the coming revolution for piston-engines aircrafts between 130 and 400 HP: Retrofitting a diesel engine to run on Jetfuel or Kerosene, reduce Gallons/Hour by some 30%, eliminate ignition systems (magnetos, spark plugs) and their problems, eliminate mixture control, increase TBO to 2,400-3,000 hours, increase performance between 6,000 and 12,500 ft., and drastically reduce Operating Costs.
The letter is intended for piston engines aircraft owners, manufacturers, fleet operators and FBOs, re-manufacturers of engines for these aircrafts, manufacturers of engine components and ancillaries, and all professionals acting in decisions of engine exchange or refitting at TBO, in North and South America, Pacific Rim, African continent, and all parts of the world were Avgas, Mogas, Kerosene and Jetfuel are available.
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