Paul,
    >
    > Say, are there any efficiency merits to mounting counter rotating
    > propellers right after each other?  Say two sets of three blades.  P
    > factor would be canceled of course.
    >
    > Doug in Japan
    >
    > Rumor has it that..... that is true but I don't have any
   comparison data.
    >
    > Is this what you have in mind? Only props instead of ducted fan?
    > If so it is a very good idea as there are no issues with torque
   pulses
    > getting into turbines. Good going Doug.
    >
    >
    > Paul Lamar
    >
    > Counterrotating props, or "contrarotating" as is more technically
   used,
    > was a big idea 50 years ago to allow for more hp with reduced prop
    > diameter.  The Tupelov Bear is the best example along with the Hughes
    > spy plane design, the Mustang, the Spitfire, etc. They all went
   for the
    > contrarotating props at some point, and with the Tupelov, are still
    > being used.
    >
    > However, and I explored this a lot a few years ago with designers
   (Jim
    > Bede was one) that the turbulent flow over the second (aft) prop was
    > intense and destructive not to mention the complex coaxial hub
   needed.
    > Vibration was not just to the prop but also the shaft, the
   engine, the
    > mounts, the airframe, etc.  The noise is out of this world loud as
    > demonstrated by the any rear pusher prop flying in the turbulent
    > fuselage wake of air going thru the prop (Piaggio, LearFan, etc.).
    >
    > When I protested that efficiency was increased by negating the
   'swirl'
    > P-factor, I was told it was "small gain vs. big loss" and better to
    > stick with single prop, more blades if necessary.
    >
    > Just my experience from a few years ago.  There's good articles
   on the
    > contra props on the web.  Try this:
    > http://en.wikipedia.org/wiki/Contra-rotating_propellers
    > Greg
    >
    > Here is a bunch of actual data.
    >

http://aerade.cranfield.ac.uk/results.php?rs=All&sf=KW&st=AND&Simplequery=propellers&page=9
    >
    > This is only one of many pdfs on this site.
    >
    > Paul Lamar


   Seems like if the counter rotation props were a big gainer, aircraft
   like the C130H and the Dehavilland DHC8 400 would have gotten them
   instead of just adding blades to the existing setup.  I realize a new
   prop is probably cheaper than a whole new gearbox, but both of these
   aircraft were significant redesigns from their predecessors.

   Peak prop efficiency comes from accelerating a large mass of air to a
   speed slightly higher than the speed of the aircraft.  Counter
   rotating props may gain efficiency at very high speeds - reference the
   Russian Bear.


   Regards,

   Matt-

Airboat speed reduction units have such a feature. Some with the rear
prop turning faster than the front.
Operating in very dirty air with a bunch of people sitting on bench
seats in front of the props, and you know how dirty people can be. Front
prop has wider blades usually 4 or 6 and the rear prop with higher pitch
slightly thinner blades. Also allows for the big block Chevy to be lower
in the mount, and not torque the boat over on its side with a sudden
throttle application. Always a problem with well dressed Northerners in
the boat. Probably arrived at through trial and error. Like the Wright
Brothers.

Lynn E. Hanover

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