Contra-rotating propellers

Date:         21 Dec 97 02:32:42 
Organization: University of South Africa
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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.

Are there any other reasons for having this?

Which other aircraft also use this kind of arrangement?

Johan Brink