Re: Contra-rotating propellers

Date:         21 Dec 97 17:01:33 
From: (Larry Stone)
Organization: Maybe
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In article <airliners.1997.2991@ohare.Chicago.COM>, 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.
>Are there any other reasons for having this?

I can only comment from the view of light twins. Both the Beech Duchess
and the Piper Seminole (both 4 seat airplanes and both of which I've
flown) have counter-rotating engines. Any propeller driven aircraft has 4
factors that, when all (or the only) engine is turning in the conventional
direction, will cause the aircraft to want to turn left during take-offs
requiring a lot of right rudder to be used. These are torque (which is
what I think you're referring to above), the propeller slip-stream, and at
rotation, gyroscopic precession and the unequal angle of attack of the
upwards and downwards propeller blades. Having counter-rotating engines
neatly equalizes all these forces so that no extra rudder forces are

OTOH, they do make your check-rides more difficult since failure of the
most-critical engine is required (the one that will most adversely effect
perfromance). With counter-rotating engines, they're equally critical so
you can't aniticipate which one the examiner will fail. :-(

-- Larry Stone ---
   Schaumburg, IL, USA
   I work for United Airlines but never, never speak for them