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

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,  I don't believe  that the Lazair props were counterrotating.  As I remember them, they were essentially bi-plane props turning together.  I can't remember if there was any stagger between the  blades.  Have never seen any data on their thrust versus the standard props.  The engines that drove the Lazair props were really turning up and the diameter of the props were quite small because of tip speed considerations.  I've often wondered if they were testing for some kind of cascade effect between the blades.

George W

Sorry, you're absolutely right George - thanks for catching that (+1
beer) - I didn't continue the thought.
The 2X2-bladed props were joined at the (0.75"? thick) center.  No
stagger that I remember (basically directly inline).
I think (the Lazair owners?) just happened to already have (a surplus
of) the props and needed something to go with the newer, higher HP,
rotax engines.
I don't remember the arguments/discussion, 2 instances of the same
2-bladed prop, one behind the other, produced more thrust than when
offset 90 degrees at some test point.  (People advocated a  90degree
split for stacking the existing props to emulate a 4 bladed prop
configuration).
My point was that these problems/solutions count as non-trivial and are
likely (iterative/sensitive) to even approach any sort of an optimal
solution  :)
Cheers Again
Cary