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 Post subject: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 1:01 am 
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I just saw a video of cavitation .
how can i improve connection from mast to fuselaje and fuselaje to wing?
what would be the best transition?Radius ?
Pedro


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 1:58 am 
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Can you post a link to the video?

I don't think those transitions are very important because of the direction of the flow. The flow is going from front to back, not really up and down.


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 2:03 am 
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https://www.youtube.com/watch?v=K_w3gcvA87I
at 3 minute mark


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 3:05 am 
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Great video! I didn't get a chance to watch the whole thing but looks really interesting.

I didn't realize that the strut intersection would further reduce pressure and start cavitation earlier like that. Most hydrofoils have the front wing mounted below the fuselage, so the low pressure side has the fuselage attached. This is better strucurally, wing pushing on fuselage instead of bolts in tension, but maybe better for cavitation if the wing was on top and the high pressure attached to the fuselage. This would leave the low pressure side open.

I never understood how super cavitating propellers work. I still probably don't :D but at least have somewhat of an idea now from this video. Has anyone built an axe blade shaped, super cavitating kite hydrofoil?


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 4:26 pm 
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cool video.
the best part is that it was probably made in the 50s.....long before "videos" existed.

those old guys were pretty clever back then.

regarding cavitation.
it happens, but i bet you'd need to go a whole lot faster (>30kts) before it becomes your particular problem.

things you may need to improve upon first:
ventilation from the surface.
drag coefficient of the strut and wings
handling (stability and control)
mechanical stregth and stiffness
the list goes on and its all important .
the best we can do is try to keep track of the big picture and build a compromise to gets you where you're going.

-bill


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sat Jun 28, 2014 6:55 pm 
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GOOD ONE !! :thumb:
Had to take a dump ; had to hold it till the end . :( :lol: :D

In regards to your question :

Yea , best way to reduce interference drag, is to just get Rid of the intersection ; why
do Ya think they invented J-foils ?

Second best way to reduce interference drag, would be to estimate the AOA
on your main foil , and make a symmetrical cross-section that interfaces with
the strut or body, that flies at (O) AOA to flow, and tapers into your wing-profile.

And the third best way, would be to use a wing profile at the body/strut interface, that
can fly at same AOA as main wing, but with Less drag for a given AOA.
Now the example given in #2, won't be producing negative lift at start.

LOOK at how the Humpback Whale does it :


Bille


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 1:05 am 
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Guys thanks for taking the time to answer my question!
i guess my question is not relevant to the speed i will probably travel.
the speed on the video is not revealed.
My impression was that this was the first place where cavitation started.
when routing my wings i did the easiiest fastest way to do the profile without change of bit.
but if taken the proper time and chage of bit the transition can be made smooth without Sharp angles!
i guess i have to make my wings and strut connections with more atention!
just wanted to know if it made a big difference!
:thumb:


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 3:26 am 
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windfreak74 wrote:
Guys thanks for taking the time to answer my question!
i guess my question is not relevant to the speed i will probably travel.
the speed on the video is not revealed.
My impression was that this was the first place where cavitation started.
when routing my wings i did the easiiest fastest way to do the profile without change of bit.
but if taken the proper time and chage of bit the transition can be made smooth without Sharp angles!
i guess i have to make my wings and strut connections with more atention!
just wanted to know if it made a big difference!
:thumb:


the speed sailers have pretty universally found that foil cavitation takes place at approx 50 kts.


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 6:20 am 
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At 15 knots cavitation seem trivia compared to ventilation. The air get suck down from the rear of the mast to the fuselage.


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 4:32 pm 
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taken that ventilation is most likely phenomena to occur.
what methods in construction or design can prevent or diminish this effect?
Does the fence method aplies?
slightly curveD to the front or the back?
Does it make a difference?
ive got everything i need to start a new foil but im taking my time to figure out how i will build!


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