>> 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

<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
 >
 >Hi Doug;
 >
 >I recall seeing improved efficiency numbers for CR propfan/unducted fan
 >research back in the 80's? These may just be better design points.
 >Some of the propfan/related cruise missiles reports were available for
 >free years ago on the NASA servers.
 >IIRC, the Martin-Baker-5? prototype had CR props - It kept shedding the
 >tips of the second prop leading to the design choice of starting with a
 >shorter 2nd prop before it decided to on its own.
 >Some other aircraft such as the lazair found that mounting 2, 2-bladed
 >props right behind each other generated more thrust (?speed) than
 >90degress apart.
 >The general rule of thumb i have seen is to put a prop 1 chord length
 >behind the lifting surface to reduce turbulence. Never seen a number
 >for props. It will probably require fluid dynamics/FEA modelling and/or
 >trial and error.
 >I would be more tempted to look at one of Paul Lipps multi-bladed props.
 >
 >Cheers
 >Cary
 >

Cary , Guys,

What did I start here:) Ask a simple question and look what happens!!!
Now you even got me interested in what was a mind exercise as to what
happens if you bolt two rotary engines together to a transmission with
both engines plugs up. Answer: One of them is pointing backwards and
it will turn in the opposite way. This neccesstates either coaxial
contra props or if the power is brought to a single shaft one more gear
reversal for one of the engines.

But yea I am interested in the concept. Not worth it for a 13B however.
Yes I plan to meet Mr. Lipps some day. First I want to get my cowl as
tight as possible and then figure out the best prop to use.

Good stuff these posts. Thanks for the info.

Doug in Japan

Doug,
 Using two engines facing fore and aft has been done many times. One of
the most interesting was the Cozy built by brothers in Venezuela. They
used two Suzuki Swift engines and built a carrier for the prop shaft
between the engines driven by multiple v belts. The prop shafts were
built co-axially with counter rotation was done without trouble by
running the engines in the same direction but facing opposite
directions. They didn't report any trouble with the props. They flew it
to Oshkosh at least twice.
Bill Jepson

Bill,

Yes their plane gave me the original idea. I think they were featured in
EAA's Sport Aviation a few years back.  Concerning two 20Bs, my concern
would be the amount of power (500hp) needed to be transferred from each
of them and the side forces on the e-shaft.  The best way would be with
gears  of course.  Just a mental exercise however.  Time to get back to
making parts:)


Doug