Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again
From: Rick Willoughby
Date: 8/9/2010, 8:51 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Todd

Not My foil.  Here is the story on the series 07:
http://www.pdas.com/refs/rep763.pdf

Lots of useful stuff in old NACA reports.  Sailing boats are about 70 years behind aircraft development when it comes to these bits.  Structurally only 20 to 30 years although some of the development in high speed sailing boats is narrowing the gap.  They are getting closer to aircraft.

One of the issues with surface exposed rudders is the ventilation on the lifting face.  This is not flow separation as normally defined.  It is a water/air surface interface issue.  At high speed where high negative pressure develop it becomes hopeless.  The big foilers use fences on the leading edges to avoid ventilation of surface piecing foils.  With these high lift sections you really need to fence the top of the lifting face to avoid the problem at high speed.  You may observe this problem at higher speed.

The best example of full fencing is shown in a photo down the bottom of this page:
http://www.macquarie.com.au/speedsailing/design.htm
Again shows the merit of asymmetric foils if you only need to lift one way.  The multiple fences are needed where the foils are going to move relative to the surface either through boat lifting or with  waves.  Another solution is to operate the rudder below a little faired pod or below the hull if you can tolerate the draft issue.  

If the rudders are not fenced then they become deflectors at high speed and the lifting face is useless.  The Cl drops dramatically.  If I ventilate one of my high lift props it is the same feel as breaking a shaft - nothing.

Rick 
On 10/08/2010, at 3:36 AM, tsstproa wrote:

 

Sorry sent before finished.

I see the evolution to a low displacment low draft designed hull would benefit from Your highly defined foil shapes. Send me set $$$

Time for me to build a different leeward hull.

Todd

--- In harryproa@yahoogroups.com.au, "tsstproa" <bitme1234@...> wrote:
>
> The design of the deep V is suppose to do that lift hull to windward when slightly heeled. Taking that into consideration small boards low aspect rounded edges flatish profile low lift low drag.
>
> --- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@> wrote:
> >
> > 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@yahoogroups.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@yahoogroups.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
> > rickwill@
> > 03 9796 2415
> > 0419 104 821
> >
>


Rick Willoughby
03 9796 2415
0419 104 821

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