Dennis
The frequency is almost linear with speed. It is just that at lower speed, probably under 15kts, you are not going to get big forces unless the rudders are loose enough to flog.
So if you can get frequency above 500Hz then that should get you up around 20kts before there is an issue.
There is plenty on the net regarding this. This paper is reasonable:
If you get to the speeds where this becomes an issue you will likely be happy irrespective of a few odd vibrations.
Rudders usually have fine trailing edges to avoid these things so it is a matter of being aware of the condition and check the design for potential issues. I expect you will struggle to get the stiffness to avoid it. It could be a case of making it stiffer ends up worse because you push the resonance into your operating range.
Rick
On 07/09/2010, at 9:59 PM, Dennis Cox wrote:
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.
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
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