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How a wing does not work.
Example of the most common wrong theory:How a wing does not work!
I will first of explain what I consider to be the most commonly used incorrect theory in the teaching of sailing theory:
The diagram below shows the form of a wing:
As you can see, the upper surface is curved and the lower surface is almost flat.
An air parcel travelling along the upper surface of the wing has to cover a longer distance than a comparative air parcel travelling along the lower surface of the wing.
At the leading edge of the wing, the flow of air splits into two air parcels: one which will travels along the upper wing surface, the second travelling along the lower “flat” surface.
Both the air parcel travelling along the upper surface and that travelling along the lower surface arrive at the trailing edge simultaneously. Should this not be the case a vacuum or ‘hole’ in the wind-profile would exist.
The air parcel travelling along the upper surface therefore moves over a longer distance in the same time frame, as that travelling at the lower surface. Hence, the air parcel moving along the top of the wing is travelling faster than that at the lower surface.
Bernoulli’s Law” states, that the faster the air moves (over a wing), the lower the pressure, leading to a partial vacuum above the upper surface of the wing, the normal pressure at the lower surface of the wing then “lifts” the wing into the vacuum above.
This can be demonstrated by the following experiment:
If I blow air over a sheet of paper, the air moves faster above the sheet of paper, this results in lower pressure above the piece of paper (and with enough speed eventually creating the partial vacuum), resulting in an upward movement in the paper.
So, because the air along the upper surface of the wing has to travel a longer path within the same time frame, the wing is sucked up (lift).
A sail works in a similar way. On first consideration it appears that the path along the leeward side of a sail is equal to that of the windward side, but because it is curved, the inner-turn an air parcel makes is shorter than that of the outer-turn, resulting in the same effect i.e. - the sail is ‘sucked’ forward, preventing the potential vacuum at the leeward side.
This explains the fact that the more curve a sail is allowed, the more power it has.
I have heard this explanation several times and had adopted it myself at the beginning of my career as a sailing instructor.
Even following my first fluid dynamics lessons as a student in naval architecture I believed this “tale” to be true and continued to use it together with some other theory.
Eventually however, having given it considerable thought I came to the conclusion that this theory is completely wrong.
I now call this theory the "equal transit time" theory.
I will now endeavour, through differing experiments, to show that the “equal transit time” theory is indeed wrong.
Super wingAccording to the equal transit time theory the wing pictured below should work very efficiently because the path along the upper surface of the wing is very long.
Strangely however, this type of wing is never used.
Airplane upside downMany small aircraft have the ability to fly upside down (not too long however because it may 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 that these plates are much too small to achieve this and that this did not occur at all, I was puzzled and needed to re-orientate.
This does not occur many times.
Simple flat plate also can produce liftA simple flat plate also produces lift if placed at an angle to airflow.
By experimenting with a piece of cardboard moving it through the air, one can see this.
Often the piece of cardboard curves the wrong way but 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, as there is still a difference in the path length along the leeward and windward sides, one would expect to experience a lift force.
Similarly, a loosely sheeted sail gives much less power then a correctly sheeted sail, even though the path length stays the same.
Air does not arrive at the same time at the end of the sail.As I was once smoking a cigarette at the foredeck in light weather I noticed that the smoke moving to leeward and the smoke moving to windward were not reaching the trailing edge of the sail at the same time. Huh?
Taking in the genoa did not make any significant difference to the wind movements.
I then went on to play with the tell-tales in my sail (self-made, of tape from my tape recorder, who likes to eat tape?).
Dependant upon how far the sail was sheeted-in, the tell-tales where at the windward side were doing nothing, at leeward were nicely streaming aft.
This indicated that at the windward side there was virtually no speed and at leeward a much higher speed. One would have expected only a small difference in speeds between leeward and windward, as the path length difference between the sides is marginal.
Unfortunately I did not take any photographs of these phenomena, but have since found some results of wind tunnel smoke tests using such a wing and showing the same effect.
The diagrams clearly indicate that the puffs of smoke at the leading edge are equal, but are shifted upon arrival 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 paperA simple test to show that "if air travels faster it creates a lower pressure" is incorrect, is to blow below a sheet of paper. The sheet will move upward with the fast moving air.
Virtually the same can be shown by first rolling the piece of paper so that it forms an upward curve. If one then blows over the sheet of paper it will move down and not up!
Interesting, isn’t it?
I hope that I have now made clear why I consider the equal transit time theory not to be a good model to simulate what happens in the reality; I consider it to be the wrong theory.
As such I would recommend not using the “equal transit time” theory.
In the chapter about sail I present what I consider 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.”