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FangyFin MkII Theory for insomniacs

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Created by fangman 1 month ago, 20 Feb 2017
racerX
NSW, 318 posts
8 Mar 2017 3:42AM
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An elliptical un-swept foil, if the taper ratio is correct should stall across its span at the same time. ie. neither the root, nor the tip, as the relative angle of attack is same across the span, and consequently no drag penalty.

If you just swept the same elliptical foil back, and to make matters worse you made it low aspect, you would need to change the taper otherwise the angle of attack will be greater at the tips and you would get a drag penalty. Though it would stall at a greater average angle of attack as bit of bonus. It makes sense to look for optimum curve or taper for a swept foil.

A wing is swept back to address problems with compressibility, as designs have gotten better less sweep is required i.e. compare the sweep on a 777 versus the 747. You have added rake the fin because of the weed, so I doubt you have much choice on the rake...

I have quite a few aerodynamic references on the topic, as wanted to understand why windsurf sails have a twist in them and other phenomena.

This one is probably a good starting point
www.flightlab.net/Flightlab.net/Download_Course_Notes_files/3_Three-DimensionalA%232BA154.pdf

Hopefully some of this makes sense

fangman
WA, 391 posts
8 Mar 2017 7:06AM
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Thanks heaps for pdf :-)
PS 3:42 in the morning! Do you ever sleep?!

racerX
NSW, 318 posts
8 Mar 2017 2:49PM
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Here is the paper on cavitation

www.marin.nl/upload_mm/8/3/e/1806737901_1999999096_120_e_8.pdf

It shows that cavitation doesn't have much effect on the stalling angle of attack, maybe even delaying it slightly... Though it does reduce the overall lift and increase drag. See fig 4. This would suggest an increased tendency to spin out, when your 'under finned' at speeds where cavitation is a consideration. i.e. cavitation would have an effect when spin out is least likely to occur.

The video you posted on cavitation, where it covers partial pressure and the interaction between ventilation and cavitation, is intriguing. Still thinking about that one...

sailquik
VIC, 3312 posts
8 Mar 2017 3:34PM
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The first paper has some interesting things that may explain some strange behaviour I have seen in some fins. Need to study it more. Thanks RacerX.

The second deals mainly with low speeds and very large Angles of Attack. I need to read it more carefully, but at first skim I am not sure of it's relevance. The AoA we see in windsurfing fins is rarely more than a few degrees (except after a 'spinout'!).

barney831
38 posts
8 Mar 2017 10:26PM
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racerX said..

A wing is swept back to address problems with compressibility, as designs have gotten better less sweep is required i.e. compare the sweep on a 777 versus the 747. You have added rake the fin because of the weed, so I doubt you have much choice on the rake...



Wing sweep is used to minimize compressibility drag rise but introduces 'Dutch Roll' instability problems. The sweep angle is a function of speed. The maximum speed of a fighter can be estimated simply by measuring the sweep angle on a plan view photograph.

The 777 has less sweep because it has a lower design cruise speed than a 747. The 747 was flight tested to Mach 0.99 in a power dive during development.

racerX
NSW, 318 posts
Tuesday , 21 Mar 2017 10:57AM
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For those still awake, not sure if this one has been posted before...

www.foils.org/hysecdes.pdf suggests 'any speed above 18 - 20 kt will have some degree of cavitation present'

It also discusses it as both a high speed phenomenon but also a low speed phenomenon, how it may interact with foils tendency to stall i.e. spin out.

fangman
WA, 391 posts
Tuesday , 21 Mar 2017 9:22AM
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Yes that's a good paper, and the last line is the kicker alright! I just wish he had done it with symmetrical foils. foils.org is a fab site but a terrible time thief - I always get sucked in by all the cool looking boats/ hydrofoils and end up reading about all sorts of foil stuff that isn't relevant to what I am after but interesting all the same

yoyo
WA, 1488 posts
Tuesday , 21 Mar 2017 10:12AM
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Windsurfing is different. Why? See if you can work it out....

racerX
NSW, 318 posts
Tuesday , 21 Mar 2017 9:09PM
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yoyo said..
Windsurfing is different. Why? See if you can work it out....

Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.

fangman
WA, 391 posts
Tuesday , 21 Mar 2017 7:47PM
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racerX said..

yoyo said..
Windsurfing is different. Why? See if you can work it out....


Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.


+1 on that one

yoyo
WA, 1488 posts
Wednesday , 22 Mar 2017 8:52AM
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racerX said..

yoyo said..
Windsurfing is different. Why? See if you can work it out....


Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.


No, no, no and no.
Think first principles. What causes cavitation in first place?

mathew
VIC, 1459 posts
Wednesday , 22 Mar 2017 12:53PM
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yoyo said..

racerX said..


yoyo said..
Windsurfing is different. Why? See if you can work it out....



Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.



No, no, no and no.
Think first principles. What causes cavitation in first place?


Instability in the boundary layer, which causes the water to vapourise into a gas - then implode under compression of the surrounding water.

Windsurfing isn't different. We do however run in a fluid that is highly oxygenated, vs most other mediums - but the same laws of physics apply.

racerX
NSW, 318 posts
Wednesday , 23 Mar 2017 1:06AM
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yoyo said..

No, no, no and no.
Think first principles. What causes cavitation in first place?


Sufficiently low pressure, factors that effect the value of this low pressure are: density of the fluid (e.g. salt water/versus fresh), other nuclei in the fluid (e.g. a solid surface, air bubbles big and small, salt), the partial pressure of other gases present.

What is also interesting is that many other the foil shapes/ideas designed to reduce the effects of compressibility in aerodynamics also appear to be applicable to cavitation. e.g. minimise high pressure gradients across the chord, i.e. spread the pressure changes along the chord.

In any case as far as windsurfing is concerned I reckon we are all just guessing...



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"FangyFin MkII Theory for insomniacs" started by fangman