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?

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 said...

THAT is ****ing awesome.

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

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?

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

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

EDIT.

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

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 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