Two propellers, one engine

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Two propellers, one engine

Postby jba8472 » Wed Apr 16, 2014 5:48 am

So in the film Zeppelin, one of the features of the LZ 36 (and a minor plot point) is the fourth engine, located in the rear gondola. It appears to use some sort of chain drive (or movie magic) to link the motor to two outboard propellers. Here's a link to the best picture I can find: http://www.eccentric-cinema.com/reviews2010/movie_pix_p-z/zeppelin01.jpg. I've found other images suggesting that the system was used on Great War German zeppelins, but nothing that describes how the drive system works or if it was a common feature (or actually existed). It seems to me that if this was possible, it would have been advantageous to use this method over separate engine cars as it would both save weight and increase propulsion (two propellers, one engine). Why did this design not flourish? Or am I missing some design or physics factor limiting the usefulness of this design?
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Re: Two propellers, one engine

Postby Jupiter » Tue Apr 29, 2014 8:40 pm

Of course, there'll be some loss trough the drive train, and the usual possibilities for trouble coming with long chain drives on long, thin and rather non-rigid structures. Also, using multiple props on the same engine doesn't necessarily improve thrust, and varying thrust asymmetrically also requires (assuming fixed-pitch props) some gearbox, differential or slipping parts, which have their own efficiency and weight penalties. Unable to quantify anything here, but my guess is that it's not as straigthforward as you present it here...
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Re: Two propellers, one engine

Postby Mutabilis » Tue May 27, 2014 12:22 am

One big engine is usually more efficient than several smaller engines. That was especially true in the early days of internal combustion engines. That would more than make up for the additional friction and drag from the beltdrives.
Also, there is only so much power a propeller of a given size can handle. The alternative being a larger propeller, which might open yet another can of worms.

Also, belt drives have the advantage of getting engaged and disengaged while the drive shaft is turning. In fact, early clutches were beltdrives where a tigtening wheel controlled the belt tension.
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