Nebbian - there is no misconception whatsoever. Lift is generated by the difference in pressure - created by the shape of an airfoil. You state we have a misconception without stating what that may be. Whether the plane be upside down or otherwise - the movement of the wing through the air still generates "lift" . That lift - that lift counters the weight of the aircraft versus the force of gravity. Without lift it falls like a brick. When the aircraft goes inverted other things need to be slightly adjusted like aircraft attitude to maintain balanced flight but the lift doesnt change. So if anyone has a misconception then perhaps you do on inverted flight.
Also, I wasnt making any comment on Decrepits initial post re. the speed of air particles over either side of the airfoil - but rather airfoil versus windsurf board fin(hydrofoil).

P.S.Did some checking re. symmetrical airfoils - they will fly, but that is dependant of angle of attack - if the symmetric airfoil has a zero angle of attack - no lift - so they are dependant on a degree of positive angle of attack to create lift.
So the question for me is - a symmetric windsurf fin creating any lift component (because if it is, it will be sideways) - or is it merely acting in its role converting energy from the sail into forward movement, as a keel does??
Sorry Decrepit - didnt mean to change the topic of your thread!!

nebbian said...

There's your misconception. Fins always operate at an angle of attack when creating lift. The flow separates off to one side of the leading edge (left or right when looking down at it), not smack bang in the middle of the leading edge radius.
This means that there is a pressure difference, although I find that thinking in terms of pressure differences can be misleading.

I am a simple man and a colorful pic would really help, but is the direction the windsurfer is travelling, actually slightly (a couple of degree) different to the direction the fin is pointing? Is this what you mean by angle of attack? So the flow of water is hitting the fin at slight angle?

Thanks Nebbian I remember not long ago i jumped in and said the same things about the lift and faster airflow bernouli's theory and whatnot. You jumped in and corrected me and now i understand it all and it all makes sense.

I belive you directed me to this link > amasci.com/wing/airfoil.html#parts

very helpful indeed.

Glitch, Re the wing twist comment, if both wings are twisted the same then there is nothing stopping one from stalling before the other because they are symmetrical.
Many other factors determine which wing drops first ( if any ) in a stall, and if your quick on the rudder then life doesnt get very interesting.

Symmetrical wings are used on aerobatic aircraft. They get lift by always having a positive angle of attack to get lift. The same goes for the vertical tail on an aircraft to keep it going in a straight line. Whenever the aircraft yaws (goes sideways) one side will get more lift to naturally get it back on track. The same goes for a sailboard fin.
The reason the leading edge is blunt is that it will allow the air to separate more smoothly (have a look at Trev Newmans pic). Sharp leading edges work on high-speed aircraft where the air would compress on a more blunt leading edge. On a low speed airfoil a sharp leading edge will stall quicker & not get as much lift.
Now the even more boring bit about lift. There are 2 types of pressure, dynamic & static. Dynamic pressure is the pressure you can feel such as the wind. Static is the pressure that surrounds an object, like the atmosphere.
An airfoil will get some lift from dynamic pressure with a positive angle of attack. This is Newton's every action there is an opposite reaction theory. This is also the reason a sailboard will plane over the water as it has a positive angle of attack to the water.
As an object moves through a fluid, it will get a negative static pressure; the faster it travels the lower the pressure. You can feel this when you drive with the window open. The wind will blow in at the back of the window to equalize the air being sucked out at the front of the window. As air flows over the curved part of the wing it speeds up and reduces the static pressure to give lift.

Yes maverick there is nothing to stop a wing dropping first with the twist (wash out)being built into a wing, but the stall will be a lot more sedate, and in a lot of aircraft will just mush along in a straight line, although getting closer to the earth, until it gets forward stick pressure.

TrevNewman said...
Thanks for that. I see all now. I think I was given the bro-science version when I started windsurfing, something like "the fin creates lift like an airplane wing, too big a fin creates too much lift, which is why your board was tail walking, which is why you crashed and that is why your head is bleeding"

No, all that is true ...but without the science.
There are still some continuing unbelievers who think you can have one board with one fin, but having an extra fin choice extends your range greatly

Correct me if I'm wrong but the further out you stand (i.e. wider the board) the bigger the fin you need to provide the resistance or "lift" to stop the board sliding out from underneath you. (that's technically why the board width and not the sail size determines optimal fin length size although you can successfully use a smaller fin with a smaller sail due to the fact you're in stronger winds providing an equal amount of lift).

Standing right over the fin like a freestyle or wave board and the area (size) of fin can be significantly reduced. Hence a fin will slip when going through a lull because there is no forward drive from the sail and the fin has limited or no lift to counteract the lateral loading your legs are providing through the back of the board.

PS - I'm in Neb's corner.

decrepit said...

I think some people don't understand what foil lift refers to.
Foil lift is in a direction at 90deg to the foil, not necessarily upwards.

This extract from the thesis Elmo posted diagrammatically explains Decrepit's comment


this also from the thesis
A symmetrical section can only develop lift if it is oriented with an angle of incidence to the freestream (whereas a cambered section will produce lift even at zero angle of
incidence; with its lift also being a function of increasing angle of incidence). Other lifting surfaces which are designed to work in one direction only normally use a cambered section(fixed and variable).

Mark _australia said...

^^^ ahhhh but are you using a bigger fin because you are standing further out and applying more lateral pressure, or standing further out to counteract the lift of the big fin?

Tomarrrto - tomaayyyto

(Seriously, surely all we need to know is that fins develop lateral lift and once the board rails up you are overfinned and need a smaller one?)

I think both statements are correct and are always acting in equilibrium with each other (until one wins out over the other i.e. over finned or sliding out)

PS - it's tomato too

Macroscien said...

Could we have a fin with adjustable pitch (angle of attack) ?
By comparison to airplane's airfoil if we could set or even change " the angle of attack" probably we could use this "side-ways" lift to go nicely up the wind.
Everytime with sail flips (change tack) fin should flip also
At this moment our sailboards construction reminds advanced Raptor -with working vectoring trust but broken Aileron.

In effect we have that - due to the centre of effort of the sail pushing downwind the hi pressure is on the leeward side of the fin, generating upwind lift. And when you swap tacks, the lift swaps side too

If you had any more upwind lift (by way of some kinda aileron as you show) it would be the same as having too big a fin. The fin would not help you get upwind, but rather it would want to rotate the board around it's longitudinal axis (ie: railing up)

sausage said...

>>>>

This extract from the thesis Elmo posted diagrammatically explains Decrepit's comment


>>>>>>

Good one sausage, but note that the vertical lift vector, in the middle diagram, is from the board not the fin!!!!

Thanks for diagrams, I have got it. You are right. Already whole board, sail and fin works as big rudder combination

Still could be room for improvement because at this moment you need to turn whole board to change this angle of attack with not only fin but also board shape having effect.
small tuning at only fin angle still could be beneficial ...at overall performance...
BTW the world "lift" is a bit confusing in such context and more appropriate should be " lateral resistance" as vehicle is in "lifted" of the wind course line but still dropping down of course..
maybe only lifted relativelyto the wind direction.
by analogy we could not say about parachute that is " lifting" a person, just a slowing down drop descent
to be even more confusing I wish to call windsurfer (any sailboat ) trail "negative descent"

muscles?....

Macroscien said...
Could we have a fin with adjustable pitch (angle of attack) ?
By comparison to airplane's airfoil if we could set or even change " the angle of attack" probably we could use this "side-ways" lift to go nicely up the wind.
Everytime with sail flips (change tack) fin should flip also
At this moment our sailboards construction reminds advanced Raptor -with working vectoring trust but broken Aileron.

Large racing yachts already have trim tabs on the trailing edge of their keels. Possibly on their rudders too. Some smallboat owners have experimented with "tacking centre boards" with some success. Almost became common in the '60s and '70s.

Glitch said...

>>>>>>

Does any one want to try and explain the forces that turn a board when cave gybing?

An attempt I'll have, just to start the ball rolling.

I think most of it's down to increased drag on the depressed rail, especially on straight rockered boards.
Wave boards with more tail rocker, present a curved surface to the water when banked over, this enables a tighter turn.
There's also the angle of the rail outline, the more parallel the rails the worse they turn.

Then there's the resistance to g forces involved, at a very sharp banking angle the rails are working as well as the fin at keeping the back from sliding sideways.
(This is the reason we removed any tuck from the rear rails when surfboards became short enough to generate high g turns, didn't realise till later, it also made them a bit faster)

Also why pulling up on the front strap to bank the board, instead of putting more weight on the rail with the back foot, can result in spinning out at higher speed and sharper turns. The inside rail doesn't get buried enough to provide it's share of lateral resistance.

Ah ha

Finally we are starting to get somewhere with the terminology so I can start getting my head around it.

Lateral resistance to counteract the rig forces(the sailors part of the rig).

Lift is the force working along the axis of the fin lifting the board out of the water.

True lift of the fin still comes down to water resistance overcoming the mass holding the board down.

We don't sail sideways so therefore flow from front to back of fin is uniform because the foil is symmetrical (excluding the load a heavy footed gumby like myself exerts when cavitate the fin).

So coming back to the lift. 100L board, 7.0m sail, compare the lift between a 32cm fin and a 40cm fin without changing anything. The 40cm is always going to want to lift the board out of the water and the 32 at the same speed will rarely have the board out of the water.

Both these fins have similar thickness's, the chord length on the 40 may be slightly longer but that only increases lateral resistance.

What the difference is, is the surface area of the fin which you can see when looking from the front of the fin which is trying to push it's way through the water.

Water at low speeds offers very little resistance to an object moving through it but as the speed increases so does the water resistance.

So the faster we go the greater the water resistance we have in front of the fin till it overcomes the balancing act which we have with a windsurfer and lifts the board right out of the water and starts the board precariously balancing on the fin making us uncomfortable.

To overcome this excess lift, we...
1) Shift the mast foot forward
2) Drop the boom,
3) Carry more weight
4) Put on a smaller fin

1 through 3 are changing the balance of the board to counteract the lift from the fin. Nº 4 is reducing the lift by reducing the area affected by water resistance.

Now start thinking about the angle of attack of the leading edge and the effect on lift and speed

Barn must be on the toilet.