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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 6:30 pm 
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Ventilation can occur :
1 the trailing edge is not sharp, and tapered.
2. Riding too high.
3. Riding too fast, not a problem for a slower foil


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 6:42 pm 
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TPink wrote:
...

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


Absolutely correct ; guess i got tricked by the topic title :
(best way to reduce interference drag?) ;
so that was what i was referring to with my last post , and it all occurs at sub cavitational speeds.



Bille


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Sun Jun 29, 2014 11:41 pm 
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Location: rhode island
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:


hi pedro.
The foil in the video is attached directly to the bottom of the strut/mast , which is also foil shaped.
the fact that both sections are jointly reducing the pressure in that region is why cavitaion first apears there.

Most of the current kite hydrofoils we've seen ( including yours) have sufficient separation between the strut and front wing that the pressure drop is not additive or combined to cavitate prematurely.

Regardless, sharp corners at any joint , whether they are internal or external tend to increase drag
The use of fillets where ever possible help reduce the formation of vorticity in corners, reduce the total exposed wetted area of the joint ( and its skin friction) and help stabilize the joint structurally.

Every foil shape has a well defined pressure distribution that is easily calculated ( or looked up in any of a number of reference sites). if the pressure distribution lowers the local pressure to the point of cavitating, you can choose a foil profile with less lift coeff and increase area ( and/or aspect ratio) to compensate.

always more.
but just some thoiughts.
-bill

(edits for spelling)


Last edited by zfennell on Mon Jun 30, 2014 3:25 pm, edited 1 time in total.

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 Post subject: Re: best way to reduce interference drag?
PostPosted: Mon Jun 30, 2014 12:37 am 
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Location: Chile/Brasil
hello Bill,
thanks for pointing out the problem and the solution.
i didnt picture the whole 3d structure!
your insight always very helpfull! :thumb:


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Mon Jun 30, 2014 3:21 pm 
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Location: rhode island
its the blueberries.

i'm thinking much clearer now that they are in season here.
:)


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Mon Jun 30, 2014 9:02 pm 
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hello Bill,
keep eating them!!
we produce tons but dont eat very much..... probably should eat more!
,maybe the reynolds numbers and all the formulas will start making more sense! :D :thumb:


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Tue Jul 01, 2014 9:00 am 
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Funny, I was watching this video the other day and noticed very quickly, cavitation. Weather it's because the foil is close to the surface, lower pressure has caused it like Bille said! but it is very noticeable. I tried to see where it formed from, I took some screen shots and it looks like it came from a wing tip.....
You can see it at 3.29

Cheers

https://vimeo.com/88175084


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 Post subject: Re: best way to reduce interference drag?
PostPosted: Tue Jul 01, 2014 12:53 pm 
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Edge19 wrote:
.... I tried to see where it formed from, I took some screen shots and it looks like it came from a wing tip.....
You can see it at 3.29

Cheers

...


Yea , took several tries ; but got it .
Good Find ; kinda would like to know WHY , on that one also !! :thumb:

Could it be at the wing/body junction ? Look at the surface-tension of the water
just above the Right wing-tip ; doesn't look disturbed ??

Bille


Attachments:
cavitation.bmp.jpg
cavitation.bmp.jpg [ 72.89 KIB | Viewed 246 times ]
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 Post subject: Re: best way to reduce interference drag?
PostPosted: Tue Jul 01, 2014 1:11 pm 
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zfennell wrote:
...

Regardless, sharp corners at any joint , whether they are internal or external tend to increase drag
The use of fillets where ever possible help reduce the formation of vorticity in corners, reduce the total exposed wetted area of the joint ( and its skin friction) and help stabilize the joint structurally.

...

always more.
but just some thoiughts.
-bill

(edits for spelling)


No Way do i wanna even Try to argue the your point about fillets Vs sharp corners
with you ; already know your smarter and more knowledgeable than me on the subject.

Can Ya enplane why all the 50:1 + glide ratio sailplanes , have sharp corners at
there wing junctions ?

Bille

from : http://www.robertsohana.com/


Attachments:
ye-newtoy.jpg
ye-newtoy.jpg [ 112.1 KIB | Viewed 238 times ]
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 Post subject: Re: best way to reduce interference drag?
PostPosted: Tue Jul 01, 2014 2:20 pm 
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Location: Denmark
Hi Bille

Well, they are actually not sharp.

Many are, because of practical reasons, and some also because of the big and high drag fuselage because it has to hold a person or two, and have a canopy that opens up, so compared is the gain very small.

But if you look closely at your picture, you will see it has some tapering, especially behind the junction.

Can be seen on many gliders, take a look at the classic Discus, here the duo version:

Image



Or look at this interesting article http://soaringcafe.com/2011/01/design-of-a-competition-sailplane/ where you can see how the Concordia junction is tapered.


Also on high performance low drag RC gliders, these junctions/fillets are used.

8) PF


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