Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again
From: Rick Willoughby
Date: 8/8/2010, 10:19 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Dennis

Most of the data I use for foils comes out of JavaFoil.  I have one Volume of the Selig data on foils but the range is too limited to use practically.  There are a few sites with single polars but you need a range of Re# to be useful for design.  I have compared the actual test data from Selig and others with results from JavaFoil and they essentially agree.  Javafoil gives reasonably reliable results.  Its advantage over Xfoil is that it handles multiple sections simultaneously so it is useful for getting sail data.  There are some comparisons that show Xfoil is more technically accurate but for most analysis JavaFoil is fine.  If you look through the various analysis pages it gives insight into what is going on and with some knowledge you can pick up if there is an anomaly.  

For a constant section in wood you probably run into and AR limit around 4.  With higher strength material you could go higher.  Usually on boats it is draft that becomes a constraint to increasing AR.  With thin sections and 360 degree rudders you have to consider the load when the rudder is perpendicular to flow as this will be higher than any steering force it can produce.   I like the 16 series because it has a lot of meat as well as the favourable L/D and almost constant pressure profile.

If you go to varying section then aircaft wings are a good example of what can be achieved.  Gliders use AR of 40 or so on the wing.  

There are diminishing returns to AR though.

I guess if you are on to rudders you have the hulls done!

Rick
On 09/08/2010, at 11:47 AM, Dennis Cox wrote:

 

Rick,
 
Friday, I took my first look at JavaFoil.  Did not get too far.  Are all these analyzes and results concerning the symmetric NACA07310 (Lift, Drag, Moment, and aspect ratio being done in JavaFoil?  BTW, do you have a L/D vs AR curve for it (say 2:1 to 10:1)?  I note you stopped at 4:1.  Is it diminishing returns or did you decide that this was the realistic best that could be made structurally out of wood?  If one went to fiberglass or even carbon, higher aspect ratios could be used... but would it be significant fluid dynamic advantage?
 
Dennis

From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Sun, August 8, 2010 9:30:48 PM
Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again

 

Todd

On you last statement regarding the lifted tack.  That is what I am talking about with the trim adjustment.  The rudders are set up to give you more lift then you need for some conditions.  They are dragging the hull sideways into the wind.  So you have the rudders fighting the hull, creating unnecessary drag.  Inevitably it will be faster to go where you point rather than ending up higher than you point.  

Rick  
On 09/08/2010, at 11:06 AM, tsstproa wrote:

 

Knowing you need aoa for best flow on different types of foil shapes. The foil that needs the least amount of aoa for best flow is again a compromise to over come the drag you describe as opposing force from each board if in deed this is the case.

The whole design goal of the linked boards hung on center chord was just that zero aoa for best possible flow, speed, along with a neutral helm. How controllable the set could be and what if any negatives there where. Everyone I talked to said the rudders would be over balanced and the set up would be uncontrollable.

So far in most all my shunts leeway is not even an issue. I'm am almost always on a lifted tack after a shunt.

I think its about refining the compromises. Not replicating the all ready known.

Todd

--- In harryproa@yahoogrou ps.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Todd
> With your AR around 2 they will have a best lift to drag just under
> 10. If you were to increase the AR to 4 then the best L/D is just
> under 15.
>
> To get the full benefit from the asymmetry you need to be able to
> trim them with respect to each other. You could set them up for
> close hauled but then they will have more lift AND drag than needed
> off the wind.
>
> They have similar performance to a NACA0010 section. The minimum
> drag is about 20% lower than a NACA0010.
>
> Rick
> On 07/08/2010, at 4:25 AM, tsstproa wrote:
>
> > RG14 mirrored...
> >
> > --- In harryproa@yahoogrou ps.com.au, "willoughby_ rick"
> > <rickwill@> wrote:
> > >
> > > I have posted the co-ordinates for the best L/D rudder section I
> > can come up with for AR of 4 and Re# 10E6:
> > > http://www.rickwill .bigpondhosting. com/NACA_ 07-310.txt
> > >
> > > It is worthwhile noting that if the AR is not limited
> > structurally or by draft then the best L/D section will get
> > thicker. So if someone wants to play and can handle higher AR then
> > I can go through the exercise.
> > >
> > > The best L/D for this section occurs between 0 and 0.5 degrees AoA.
> > >
> > > It is VERY important to realise that they need to be able to have
> > relative angle trimming. If the boat goes best on the wind with the
> > combined trim greater than 1 degree into the windward side then the
> > blade area is likely too small to get the best L/D. Typically these
> > rudders will have the combined area similar to a centreboard and
> > rudder of other boats with the same sail area. With the Harry
> > configuration there should be a tendency to round-up with no
> > rudders until the ww hull unloads but these rudders are not going
> > to have much feel so there may be no indication of weather helm. So
> > it may require actually observing the angle of trim between the
> > rudders.
> > >
> > > The aim should be to trim the rudders to eliminate leeway.
> > Current aside, the boat should go where it is pointed. So the idea
> > would be to set a course and then slowly trim the rudders relative
> > to each other while maintaining the course to get the best speed.
> > Once this has been done a couple of times the best settings will
> > become intuitive for any particular conditions.
> > >
> > > These rudders will not have a cavitation issue for any reasonable
> > speed expectation - Cp less than 0.5 in best working region. If
> > thicker section for higher AR then cavitation would need to be
> > checked and the vortex shedding will be more significant.
> > >
> > > The vortex shedding should not be an issue on this section either
> > as the separation is only on the last 1 to 2%.
> > >
> > > Rick
> > >
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>


Rick Willoughby
03 9796 2415
0419 104 821




Rick Willoughby
03 9796 2415
0419 104 821

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