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


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