Rick,
I gather you read BoatDesign brief on MLM... 
... and my lapse in understanding of the trailing edge. It just out of reach over my head... I'm fine with Abbot level of wing theory... much beyond 2D and I'm on shaky ground. Maybe you can recommend a good book on the subject of what you've just described.
... and my lapse in understanding of the trailing edge. It just out of reach over my head... I'm fine with Abbot level of wing theory... much beyond 2D and I'm on shaky ground. Maybe you can recommend a good book on the subject of what you've just described. But since you've given me a driving force at 500 Hz... I know what to do with that! I finally did a post cure on the one rudder I have filament wound. I plan on doing some simple non destructive tests and so I can back calculate actual stiffnesses of the as-built condition. My pre- (post curing) deflections were not encouraging. I don't have documentation on my fiberglass tow and I'm finding my fiberglass is probably not continuous fiber... with the resulting reduction in stiffness and strength. We shall see. With those revised numbers I should be able to finish an FEA model and run some modal analyses. It shouldn't be too hard to avoid the 500 Hz. I'm used to this type of design.
Thanks for the 500 Hz driving force... that I had no clue about how to determine.
Dennis
From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Mon, September 6, 2010 11:41:13 PM
Subject: Re: [harryproa] MLM Build Update
Dennis
You may recall that I pointed out JavaFoil was showing a small degree of flow separation on the rounded trailing edge of the bi-directional rudder. The vortices shed at a frequency closely related to speed and are influenced by the geometry of the section shape.
At 20kts I estimate a Strouhal frequency around 500Hz. The vortices actually alter the lift characteristics of the foil so their tiny influence is amplified to rapidly changing moments and lift force. It is a bit like having a rapidly oscillating flap on the back of the rudder. Oscillating foils like the tails on sea creatures take advantage of the vortex shedding to improve efficiency.
To get to the point, if you have torsional or bending modes in your rudders that will tune to the shedding they will amplify forces even more. In fact you can get "lock-in" of the vortex generation over about 20% speed range because of the structure interacting hydrodynamically.
For interest sake it would be worthwhile looking at the natural frequencies. If there is an easy way to ensure they are well above 500Hz then that should be achieved. If not then get a rough idea of the speed where flutter might begin so you can be alert to it. I expect your torsional frequency will be low if I understand what you have in mind. This is bad for flutter if it starts because you will shed full strength vortices down the full depth of the blade.
All this might be getting a bit precious because the separation only occurred over a very short length at the normal operating AoA. They should not have much energy. It would be no worse than say a 6mm thick edge on a standard rudder other than this would be a larger rudder than you are making.
Rick
On 07/09/2010, at 9:13 AM, Dennis Cox wrote:
Hey Rick,Seeing it in person, I think I went too far on the windward flat bottom. It will be a ball breaker. Strangely enough, I could turn it over force it closer together and it would come out pretty much like you're first sketch. I'll argue with myself some more and maybe run some Godzilla on an upside down one. But I like the 5" depth of the windward with the big bottom.The rudders will be basically in cassettes. I haven't drawn them, but basically the fillament wound pictures show the full height (24" below and15" above the hull bottom). The blank will be cut back to have a shaft diameter of 1.3". The cassette will be wedge shaped and gasketed. Can't let it suck any air down next to the shaft.Dennis
From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Mon, September 6, 2010 6:24:35 PM
Subject: Re: [harryproa] MLM Build Update
Dennis
Looks like the drawings so far. It is getting crowded in your basement.I am interested in the stiffening you plan for the hulls and the mounting of the rudders in the lw hull.I prefer outward flare on the hulls but if you get away without burying your bows I might rethink that.The ww hull is going to be bumpy - I know you are looking forward to this but it means it will get a hammering so needs ample stiffening.Rick
On 07/09/2010, at 4:38 AM, Dennis Cox wrote:
Cost $109.44Hours 61Leeward hull cut, glued up.Windward hull cut, glued up.Diagonal beams cut, formed up.See pictures in My Little Mule photo album.
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby
03 9796 2415
0419 104 821
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