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Forums > Windsurfing General

One for the hydrodynamicists

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Created by ikw777 > 9 months ago, 7 Mar 2011
ikw777
QLD, 2995 posts
7 Mar 2011 10:12PM
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Ok, I've always wondered about this. How does a symmetrical foil like a fin or centreboard generate useful lift? I can understand how a sail works, and a aeroplane's wing, but a symmetrical foil would seem to be have balanced forces on either side.

Can anyone set me straight? How does that symmetrical foil pull me up to windward?

jonesmb
QLD, 75 posts
7 Mar 2011 10:20PM
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To generate lift it has to move through the water at an Angle-of-Attack. Once set at an angle of attack the flow is not symmetric relative to the fin and differential pressure are produced. High angle of attack more non-symmetric more lift ... until it stalls (spin out).

decrepit
WA, 12351 posts
7 Mar 2011 8:40PM
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Yep, Jones has it right.

Sail pressure wants to push the board sideways, as soon as this happens you have an angle of attack and the fin generates enough lift to stop most of the sideways slide. The board never quite points where it's going, (unless you're running an asymmetric fin).
The more angle of attack, the more lift and the more drag.

barn
WA, 2960 posts
7 Mar 2011 9:44PM
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An asymmetric foil will give lift with zero angle of attack, as the medium must travel further over bla bla bla bla...

That's the high pressure BS, but you can also break these problems down in other ways,

...


Give the symmetric fin an angle of attack, the fin throws water in downwind, Newtons 2nd law give you lift, upwind.. easy.

Or a plane wing throws air downwards, and for every action there is an equal and opp.. bla bla bla.. the wing generates lift, upwards... easy

Used to have to teach this in windsurf lessons, the second approach always seemed to work better... Lesson 2 was to then get a fin and take it in the pool and spin around in circles making wooshing sounds, you'll be amazed how much lift they make and any speed..

I learnt all this when Dad took me to the Aus War Memorial and I couldn't understand why the Lancaster Bomber's wings were symmetrical..



Also, asymmetric foils work just fine upside down, I had an outrageously asymmetric fin for ages, no difference on either tack..

ozpricey
WA, 333 posts
7 Mar 2011 11:01PM
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barn said...






THAT is ****ing awesome.

No way i'd have my bodyweight behind that bar..

ikw777
QLD, 2995 posts
8 Mar 2011 7:26AM
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So foils are shaped to try to retain flow at an angle of attack. The lift is created by the foil's angle of attack as it moves through water (or air)?

barn
WA, 2960 posts
8 Mar 2011 6:50AM
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Well they have a nice shape so there is minimal drag. The lift is generated by pushing water in one direction (Action) and the water pushes back in the other direction (reaction).

You can also get lift off the rails if you arrange them to push water downwind.. much harder to explain that with the high and low pressure BS..

Now why does the water push back?? Why is frozen water so hard and slippery? How do magnets work?

oldie
VIC, 356 posts
8 Mar 2011 12:37PM
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Let us consider a non-planing board, cos that is a common situation for a sailboard.
The fin does little at slow speeds. (Needs to be infinitely large at zero speed?).
Engaging the upwind rail get back to the launch area is very inefficient as it sinks and drags along in the water.
Engaging the downwind rail works well if the board has V from front to back as one just tips onto that side.
So why don't low wind shortboards have a cut off nose to allow the best rail shape for this situation?

MavericK040
WA, 583 posts
8 Mar 2011 12:10PM
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barn said...


Or a plane wing throws air downwards, and for every action there is an equal and opp.. bla bla bla.. the wing generates lift, upwards... easy


I learnt all this when Dad took me to the Aus War Memorial and I couldn't understand why the Lancaster Bomber's wings were symmetrical..



Also, asymmetric foils work just fine upside down, I had an outrageously asymmetric fin for ages, no difference on either tack..




Sorry mate im going to have to step in here and make a small correction.

An aircraft's wing does not throw air downward if this were the case it wouldn't matter what the top of the wing was shaped like only the bottom and this is definitely not the case. it creates a low pressure area on top of the wing due the faster flowing air having a lower pressure ( bernouli's theory and all that) and the high pressure air on the bottom of the wing pushes it up towards the lower pressure area bringing the wing structure with it.

Im Pretty sure a lancaster bomber does not have symmetrical wing profile , while the wings are cambered somewhat on the bottom surface, they will not be fully symmetrical. the reason it is cambered on the bottom is that the designers needed a high lift capacity wing profile for the bomber plane and a flat bottom profile does not provide the best lift characteristics.

Also, the Sukhoi SU-31 in the second image DOES have a fully symmetrical airfoil and does not make any lift at zero angles of attack because of this. therefore it only makes lift on the airfoil surface ( either the top or the bottom) which has a positive angle of attack. Generally this type of aircraft or stuntplane, rely heavily on the engine to simply drag them through the air, the airfoil is less critical than on something somewhat less powerful.
and as a general rule of thumb most airfoils are most efficient at 4? AOA ( angle of attack) and stall around 16? AOA.

If you care to read this wiki page most of this is explained. ignore the maths at the end
en.wikipedia.org/wiki/Aerofoil

i know this from 2 years of aeronautics at school and im now a student pilot. we covered this all in my first lesson a few months ago... i also like planes

Mark _australia
WA, 22723 posts
8 Mar 2011 1:26PM
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MavericK040 said...




snip



Also, the Sukhoi SU-31 in the second image DOES have a fully symmetrical airfoil and does not make any lift at zero angles of attack because of this. therefore it only makes lift on the airfoil surface ( either the top or the bottom) which has a positive angle of attack. Generally this type of aircraft or stuntplane, rely heavily on the engine to simply drag them through the air, .............


Maverick, you are full of it.

The Sukhoi's propellor is not moving, as you can clearly see in the pic.
If the people are blurred and the propellor is NOT blurred are you saying the people are moving faster than the propellor?

This is one of Reflex Films' ones with the plane being dangled from a fishing rod type contraption


MavericK040
WA, 583 posts
8 Mar 2011 1:37PM
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of course!!
you can even see the strings dangling down towards the people on the ground gee the least the coulda done is cut them the right length. talk about un professional.


I think i will go and create nuclear fusion now

nebbian
WA, 6277 posts
8 Mar 2011 2:23PM
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MavericK040 said...


Sorry mate im going to have to step in here and make a small correction.

An aircraft's wing does not throw air downward if this were the case it wouldn't matter what the top of the wing was shaped like only the bottom and this is definitely not the case. it creates a low pressure area on top of the wing due the faster flowing air having a lower pressure ( bernouli's theory and all that) and the high pressure air on the bottom of the wing pushes it up towards the lower pressure area bringing the wing structure with it.



Sorry again, I'm going to have to make a small correction:

amasci.com/wing/airfoil.html

It's a common misconception, although it's worrying that it's being taught at pilot's school...

In fact the side of the wing that has lower pressure (the top of the wing in normal flight) has more of an effect on the airflow than the side that's under high pressure. This is how the coanda effect works.

Wings DO throw air downwards, it's called "downwash", and the mass per unit time of air that is deflected downwards is exactly equal to the lift. This is why it's windy if you stand underneath a helicopter.

MavericK040
WA, 583 posts
8 Mar 2011 2:39PM
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EDIT.

Seems to make sense, i will show my instructors and CFI.

barn
WA, 2960 posts
8 Mar 2011 4:57PM
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MavericK040 said...





Sorry mate im going to have to step in here and make a small correction.

An aircraft's wing does not throw air downward if this were the case it wouldn't matter what the top of the wing was shaped like only the bottom and this is definitely not the case. it creates a low pressure area on top of the wing due the faster flowing air having a lower pressure ( bernouli's theory and all that) and the high pressure air on the bottom of the wing pushes it up towards the lower pressure area bringing the wing structure with it.

i know this from 2 years of aeronautics at school and im now a student pilot. we covered this all in my first lesson a few months ago... i also like planes



That's fine, but no correction was necessary!. Like I said, there are many ways to look at this problem.

Wings throw air downwards, its simple physics. Helicopters also throw air downwards.. Gyros throw air downwards.. Propellers throw air backwards..

Think about this, if you attached to a plane, an Asymmetrical wing on an axle at the centre of lift, the asymmetrical wing would also not create any lift as the wing would sheet itself out (tilt forward).. Do you not agree? So all wings have an angle of attack, as they all throw air downwards..

An Asymmetrical wing can be bolted on a plane with zero 'angle', according to the line that divides the top and bottom half, but this is an arbitrary line drawn by the engineers. The wing still has an angle of attack.



The lift can be directly related to the rate of down wash momentum.

F= d/dt (mv)

F= force in Newtons, d/dt of mv describes the rate down wash is produced.

So if you have a one tonne plane, using this formula you can work out how much air the plane throws downwards every second. The planes needs to throw more air downwards per second if its moving slowly, and as it speeds up the down wash can be reduced.

Its not rocket science, oh wait, it is rocket science!!!

Chris 249
NSW, 3419 posts
8 Mar 2011 8:11PM
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There's a lot of great stuff about aerodynamics on the Boat Design Forum. Look for posts by Tom Speer (a Boeing aerodynamicist) and Mark Drela - a MIT professor of aerodynamics who gets huge respect from many and, as the holder of the world human powered flight and boatspeed records, is also very much into low-speed aerofoils. Both were involved in the design of the wing mast that won the last America's Cup.

I've emailed Tom a bit and he tends to use the Newtonian/angle of attack model but points out that there are lots of different ways to describe what's happening.

Like Barn, I find the "wings push air down so wing goes up" Newtonian model pretty easy to relate to and therefore good when teaching people.

barn
WA, 2960 posts
8 Mar 2011 5:29PM
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I've taken the liberty to draw a wing on its plane on a plane, (plane, as in the 'lying flat' sense of the word and plane as in the 'flying thing')..

Also drew my amazing 'wing on an axle' concept.. Not sure if its gunna take off! (pun intended)




Edit: moving the axle (in green) forward past the centre of lift would work better! (obviously we know this by sailing with only front hand on the boom)

DrJ
ACT, 481 posts
8 Mar 2011 9:17PM
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I just checked my fins.... None of them are made of foil? They seem to be a kind of plastic stuff... Maybe that's why I don't seem to be as good as everyone else?

Where can I get one of these 'foil' fins



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Forums > Windsurfing General


"One for the hydrodynamicists" started by ikw777