Re: Contra-rotating propellers

Date:         26 Dec 97 03:28:54 
From:         Joshua Turner <shua@umich.edu>
Organization: University of Michigan ITD News Server
References:   1
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brinkja@alpha.unisa.ac.za wrote:
> I flew on a BAe Jetstream 4100 aircraft the other day, and I noticed that the
> two propellors turned in opposite directions.
>
> It was the first time I saw this on any aircraft.  I presume the opposing
> rotation is to cancel the moment on the body of the aircraft caused by the
> turning engines.  But I suppose then separate sets of spare engines - both
> "clockwise" and "anti-clockwise" have to be kept in store since the engines
> can not be inter-changed.

The American P-38s had props set up this way, and while it might be
theoretically possible to acheive with gearboxes, you are correct, at
least as far the P-38 goes--each engine rotated in the opposite
direction. On the original XP-38, the props both rotated inward. This
caused buffeting problems, so later production -38s had both props
rotating outward. The logistic headaches that this entailed were
apparently considerable. Indeed, the P-322 (-38s built for England) had
this feature eliminated, at the insistence of the British gov't.

At the time (1939 or so), this was fairly uncommon, but I don't know
what the situation is now.

The exlanations I've seen for this was that it was supposed to eliminate
the 'critical engine' that all twin wngine aircraft have. See
Fork-Tailed Devil: The P-38, by Martin Caidin, at 24. Unfortuneately,
the explantion in that book is rather spotty, and not being an engineer,
I can't flesh it out very well.

For a more in-depth discussion of this question, you can search
Deja-News in rec.aviation.military, where there was a substantial
back-and-forth over it within the past couple of weeks.