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&Simplequer
    y=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


    I'll add to this that if I were to build something of this sort (and I
    plan to) I'd be looking at prime numbers for my blade sets, and the
    secondary set wouldn't rotate at the same speed as the primary.

    There is an old NACA paper on this that is very good.  I think most of
    the really loud applications to this are loud because of the harmonics.
    Reduce or eliminate the harmonic and I think this will be a very good
    idea.

    Noise from propellers with symmetrical sections at zero blade angle, II

    Deming, A F  naca-tn-679 December 1938

    Doug, once you get it built can I just buy it from you so as not to go
    through the hassle? LOL

    Decker, Michael

    Lynn, what kind of gear box are the airboat people using.
    In other words how does it achieve counter rotation?

Paul Lamar

Not a clue here. The coaxial layout and ratio splits suggests a
planetary set. On this site the scrolling photos upper left, the green
and black boat has a counter-rotating set up. There is more than one brand.
Quiet operation is one of the features.

http://www.centurydrivesystemsinc.com/ch3_counter_rotating.htm

Lynn E. Hanover

Thanks Lynn.

    Paul Lamar