How a wing does not work.

Example of the most common wrong theory:

Look, this is the way a wing looks like:

As you can see, the top surface is curved and the bottom surface is almost flat.
Travelling along the top surface of the wing, an air parcel has to move over a longer distance compared to travelling along the bottom surface.

At the leading edge of the wing, the air will be split into two parcels: one which will travel along the upper wing surface and one that will travel along the flat bottom surface.
Both the air parcel travelling along the upper surface and the parcel travelling along the bottom surface side will arrive at the trailing edge at the same time. This has to be the case, otherwise there would be some strange vacuum or ‘hole’ in the wind-profile.

The air parcel travelling along the upper surface thus moves over a longer distance than the parcel moving along the bottom surface, in the same time. Therefore, the parcel moving along the top of the wing will go faster.

Bernoulli Laws says that the faster the air moves, the lower the pressure will be. I will demonstrate this physical law by means of the following experiment:

If I blow air over this sheet of paper, the air will move faster above the sheet of paper, which will result in a lower pressure above the piece of paper and the paper will move up.

So, because the air has to travel a longer path in the same time the wing will be sucked up.

A sail works in a similar way. At first sight, it looks like the path along the leeward side of a sail equal to the windward side, but because it is curved the inner turn an air parcel makes is shorter than the outer turn, resulting in the same effect: the sail is ‘sucked’ forward, preventing the potential vacuum at the leeward side.

This explains the fact that the more curve a sail gets, the more power it has..

I have heard this tale several times, and have even told it myself in my start as a sailing instructor.
Even when I had my first fluid dynamics lessons as a student in naval architecture I believed this tale, and used it together with some other theory.
Eventually I figured out that this theory is completely wrong.
I now call this theory the "equal transit time" theory.

Some small experiments to show that the equal transit time is wrong:

Super wing

According the equal transit time theory the wing pictured below should work very well, because the path along the upper side is very long.
But "strangely" enough this type of wing is never used.

Airplane upside down

Many small airplanes can fly upside down (not too long because it might result in speed loss for many small planes) I used to think that this was achieved by changing the curvature of the wing by moving the plates at the trailing edge of the wing.
When I noticed this plates are much too small to achieve this, And one was not doing this at all I was puzzled.

This does not occur many times.

Simple flat plate also can produce lift

A simple flat plate also produces lift if placed under an angle with the flow.
One can see this themselves by moving a piece of cardbord through the air.
Often this piece of cardboard curves the wrong way, and still produces lift.

A fully battened sail produces no lift if the sheet is without tension.

If one does not sheet a fully battened sail it does not produce lift.
But, there is still a difference in the path length along leeward and windward side, so a liftforce is expected.
Similar is that a loosely sheeted sail is giving much less power then a correct sheeted sail, but the path length will stay the same.

Air does not arrive at the same time at the end of the sail.

At one moment I was smoking a cigarette at the foredeck in light weather and noticed the smoke was absolutely not reaching the trailing edge at the same time.
Rolling in the genoa did not make a lot of difference.

Then I went on playing with some small wires in my sail (self-made telltales of tape from my tape recorder, who likes to eat tape).
Depending on how much the sail was sheeted in the telltales where at windward side doing nothing, and at the leeward side nicely streaming aft.
This indicates that at the windward side there was virtually no speed, and at the leeward side a much higher speed.
One would expect only a small difference between leeward and windward since the path length difference is little.
Unfortunately I did not take pictures, but I have found some pictures of this wing in a windtunnel showing the same effect.
It clearly shows the puffs of smoke equal at the leading edge, but shifted at the trailing edge.

If one calculates the difference in path length the lift is much less as in reality.

Somewhere I saw this type of picture showing a wing and the corresponding lift Coefficient.

When I measured the pathlength difference at an angle of 0, calculated the speed difference, and calculated the lift with this I found the calculated lift 5 to 50 times less as stated.(depending on how I measured the distance) Strange, isn't it?

Blowing below a sheet of paper

A simple test to show that "if the air is faster it creates a lower pressure" is not correct is to blow below a sheet of paper.
The sheet will move up, so towards the fast moving air.

Virtually the same can be shown by first rolling the piece of paper so that it gets a curve directed up when you blow over it.
If one then blows over the sheet of paper, it will move up.

Interesting, is nt it?

I hope I made clear that the equal transit time theory is not a good model to simulate the reality, and therefore called "wrong" by me.
Please do not use the equal transit time theory.

In the chapter about sail I present a better theory, which is to my opinion also simpler:

“A sail deflects the wind, a force is needed to deflect the wind, and this force is called the sail force.”

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