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