I had promised to give some background on the ABC Dragonfly engine (posted above a ways) and I particularly like the description in "Plane Speaking; A personal view of aviation history" by Bill Gunston, noted aircraft engine historian and author. His Chapter 8, "January 1918 - Cancel the Others …" is only two pages long but certainly tells the story well. Here is a summary in my words, with quotes from the chapter.
The chief designer of ABC Motors had designed a 7-cylinder radial engine, the ABC Wasp, of 170-200 horsepower whose dry weight was only 290 pounds. The Dragonfly was to be an upsized 9-cylinder radial engine of 340 horsepower, with a weight of 600 pounds. In January 1918, the British Air Board decided to make the Dragonfly the "virtually standard engine for all future fighters and bombers." Orders were placed with 13 companies to produce 9,700 engines. One design, the Nieuport Nighthawk, was going to be the new standard fighter, powered by a Dragonfly engine.
Unfortunately, the Dragonfly turned out heavier, 656 pounds, and with less power, only 295 horsepower at its rated 1650 rpm. But that wasn't all, as the engine overheated badly, and after a few hours in the air, it basically shook itself apart! The short answer was that the engine was designed, without realizing it, to "run at the critical torsional-vibration frequency of the crankshaft." After over 1,300 of the engines had been produced, it was realized that the whole design would have to be redesigned completely. Mr. Gunston ends his chapter with "The one good result was it spurred Major B.C. Carter at Farnborough to undertake valuable research into torsional vibration."
As a side note, the Air Board had earlier ordered another engine, the Sunbeam Arab V-8 of 200 horsepower, which "proved to suffer from a weak crankcase, faulty cylinder attachment, and severe vibration, which took many months to cure." At this time, several hundred SE.5a fighters were "waiting for engines" and "British engine supplies were in a state of crisis." If the war had continued past November 1918, the British would have been almost without reliable, modern aircraft engines.
Hopefully, Dan K's response above will make more sense to those without prior knowledge of the ABC Dragonfly engine. By the way, Bill Gunston's "Plane Speaking" is a great read for any aviation buff. Since we seem to be stuck home for a bit longer, heck I may just reread it myself
Randy
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