For those of us who never felt comfortable with the grade school explanation of how wings and sails worked (air moves faster across curved top surface):
Coanda effect (Also look him up, really smart guy - developed first "jet" aircraft in 1910)
http://en.wikipedia.org/wiki/Coanda_Effect
which led to an article by one of the original designers of the Mac
http://www.jefraskin.com/forjef/jefweb-compiled/published/coanda_effect.html
while you are at his site, some fun stuff on sails:
http://www.jefraskin.com/forjef2/jefweb-compiled/unpublished/hard_sails.html
Now, inevitably, there will be someone who will quote some cherished physics book-
this is the link for you:
http://www.aa.washington.edu/faculty/eberhardt/lift.htm
In order to explain why the air goes faster over the top of the wing, many have resorted to the geometric argument that the distance the air must travel is directly related to its speed. The usual claim is that when the air separates at the leading edge, the part that goes over the top must converge at the trailing edge with the part that goes under the bottom. This is the so-called "principle of equal transit times".
One might ask if the numbers calculated by the Popular Description really work. Let us look at an example. Take the case of a Cessna 172, which is popular, high-winged, four-seat airplane. The wings must lift 2300 lb (1045 kg) at its maximum flying weight. The path length for the air over the top of the wing is only about 1.5% greater than under the wing. Using the Popular Description of lift, the wing would develop only about 2% of the needed lift at 65 mph (104 km/h), which is "slow flight" for this airplane. In fact, the calculations say that the minimum speed for this wing to develop sufficient lift is over 400 mph (640 km/h). If one works the problem the other way and asks what the difference in path length would have to be for the Popular Description to account for lift in slow flight, the answer would be 50%. The thickness of the wing would be almost the same as the chord length.
Enjoy.
PS To anticipate some replies: Modern theory still satisfies Bernoulli but the simple explanation so many of us grew up on really didn't tell the whole story.
OK. I am not qualified (or do I desire) to get into a physics & math discussion about this but....
Just before the Cessna 172 takes off it lowers its flaps which increases the wing thickness and path length. Not sure if it is 50% but it is a great deal larger than 1.5% that they describe above.
As the wind speed increases the wing thickness does not have to be as great to support the same weight so the flaps come up.
I do not see how the Coanda theory accounts for wind speed across the wing surface.
In any event, I now have a new excuse for why I am going so slow on the water.
Patrick
Which also increases the deflection of the wind flow.
Let's put this in a sailing analogy. The Cessna is doing the equivalent of letting off the outhaul and increasing the camber of the sail. Lift is increased but so is drag. The sail hasn't "thickened" but its shape has changed.
The Cessna's wings aren't any thicker - only longer and more curved - in a flaps down configuration. The path is longer (because the flaps are extended) but proportionately so (the path on the bottom is longer, too). The camber - curvature - has increased. What has materially changed is the extent the airflow is directed downward.
Coanda refers to the propensity for a fluid to stay attached to a curved object - which is only part of the explanation. But it was the starting point for me in discovering the other parts of the explanation.
If you accept the conventional "Bernouli" explanation, it would be hard to envision how letting off the outhaul increases the sail's power. The difference in distance between the windward and leeward sides is the same and almost infinitesmal.
from The Soft Sell on Hard Sails:
Lift is what makes sailboats go (excepting when they are running directly downwind).
Does everybody agree with this? (many do not)
agree, which is why we can sail at 1.5 to 2 times windspeed. I don't know enough to say what defines lift but you need to have airflow attached to both sides of the sail (again, our friend Coanda) which is why sailboats use telltales to observe airflow and trim the sails. Going directly downwind, there is no attached flow to the backside of the sail - again, which is why for more efficient hulls like windsurfers, cats, hi-performance sailboats to "tack" (jibe) downwind. Even though the distance sailed is further the VMG is better.
It's also what makes fins work.