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?
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).
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.
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,
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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..
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)?
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?
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?
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
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.
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)
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