Subject: Re: [harryproa] Asymmetric Bi-directional Rudders
From: Dennis Cox
Date: 8/4/2010, 9:29 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Rick,
 
I want to say... I really enjoy the way you let us "see" you think.  I have seen you come up with a speculation (not quite a theory yet) on several occasions and talk about it.  Then you go back and do some analysis to bring it up to a theory with some projections.  You have, on occasion, even changed your mind from when it was at the speculation level.  It says a lot about you.  Many try to sweep under the carpet that they had a turn around.  You're a true scientist.
 
Context - With your extensive background with the human powered speed record machines... 
Some theories you had may pay off with big gains in performance.  But to reach the success levels you have, many things must have come with very small gains.  For the example at hand... given a test Proa in the size you usually have (I know yours are not Proas) and changing from NACA0012 that has to rotate around on every shunt to a symmetric NACA0012-DE and on to one of these NACA07-DE designs. 
 
Question - How would you go about quantifying the results and hopeful gains.  In my old career, I'd specify about a million dollars worth of transducers, strain gauges and computers and the technicians would knock themselves out complying.  I'm at a little loss imaging how I could say, detect and document that one ran at 1 degree less AOA and had 5% less drag.  Especially when wind and water conditions will certainly change more than the test results spread from one test to the next. 
 
Thanks,
Dennis

From: willoughby_rick <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Wed, August 4, 2010 5:26:27 AM
Subject: [harryproa] Asymmetric Bi-directional Rudders

 

I have taken a look at what should be the best section for an asymmetric bidirectional rudder.

It threw up a few surprises. The best I could come up is a double ended version of a NACA 07 series:
http://www.rickwill.bigpondhosting.com/Double_End_NACA07.png

At an Re# of 10E6 and aspect ratio of 4 it has an L/D of 27. It has flow separation on the trailing edge at all angle of operation that reduces at higher Re#. But it is no more than 3% of the chord in the normal operating range. The lowest Cd is 0.004.

By comparison a NACA0012 section at same Re# and AR has a best L/D of 17.7. The in-line Cd is actually higher than the asymmetric section at 0.009. One advantage is that it does not have flow separation until about 7 degrees AoA. It would be designed to work at about 4 degrees for nest L/D.

What surprised me is that the large diameter nose actually has better lift, less flow separation and lower minimum Cd than the pointy nose/tail version. The reason becomes apparent when you look at the pressure profile for the respective sections. Notably the 07 series was designed for a flat pressure profile. The NACA 16 series was a development that factored in air compression for higher speed subsonic flight. However not a factor for water although cavitation could need consideration.

This idea has a lot of upside with the one downside of flow separation and possible resulting vibration. There could be some benefit roughing up the nose/tail to help the flow stay attached - could be tested with a strip of sandpaper glued in place down the edges.

Rick

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