Thanks for the compliment to my commenters Bonnie. I feel like it's time to stimulate them to do some more geeking out. So here goes...
Back in March I was trying to explain why I've come to suspect lately that the conventional explanation of how wings and sails generate lift is totally wrong. That conventional story, which you will find in many high school physics and sailing books usually goes something like this ...
Because of the curvature of the upper surface of the wing, the air passing over that side has to travel a greater distance than that passing under the wing. Since it has to go farther, it has to go faster in order to reach the trailing edge at the same time as the air flowing past the underside of the wing. Because of the Bernoulli effect the faster flowing air on the upper surface has a lower pressure than the slower moving air on the underside of the wing; and the pressure difference generates the lift.I thought I had delivered the coup de grace to this myth when I pointed out that planes can fly upside down and even linked to a video to prove it. It seemed to me that this destroyed the "planes fly because the upper surface of their wing is curved and longer than the other side" nonsense, once and for all. But it seems that I had underestimated the strength of the human spirit's attachment to myths or perhaps a human being's power of denial to believe the evidence of his own eyes.
Some of the commenters to that post responded to the effect that planes can't actually fly upside down for more than a short time. Well, there's some truth in that argument but not for the reason that wings won't work upside down.
Do a Google search for "sustained inverted flight". Now, I haven't checked all 231,000 hits generated but I didn't see any that say that this feat is impossible. No, they are all about the first time it was done (1913), how to learn how to do it, guys that do it to show off and so on. There are a couple of reasons why you don't see it very often as far as I could tell. Firstly, most planes' oil and fuel systems aren't designed to work upside down, and you really don't want the engine to cough and splutter to a stop when you're upside down 300 ft. off the ground. Secondly, it's darned uncomfortable hanging from your seat harness while you pilot an upside down plane, all the while thinking that if the harness gives way you're going to drop headfirst through the cockpit bubble. (Not to mention that it plays havoc with the champagne and caviar service in the first class cabin.)
If you still don't believe me then I encourage you to shell out $258,000 for an M-26 Airwolf, for example, take some lessons and demonstrate sustained inverted flight. I promise I'll post a photo on this blog if you do.
One of the commenters on that last post suggested that some other force might be responsible for the ability of airplanes to fly upside down. Exactly! The explanation of that force is where we're heading, I hope, but at the present rate of progress we might not get there before 2010.
OK, if you're not convinced by 231,000 Google hits then how about this argument ...
Look at that sentence in the conventional "how planes fly" myth: "Since (the air going past the upper surface of the wing) has to go farther, it has to go faster in order to reach the trailing edge at the same time as the air flowing past the underside of the wing." Why? Why does an air molecule going across the upper surface have to arrive at the trailing edge the same time as one going underneath the wing? What's going to happen if it doesn't? And how does it know how fast to go to stay in synch with its buddy molecule on the other side of the wing?
Let me draw an analogy on a more human scale to explain why I think this "path length" argument is totally bogus. Instead of air molecules think of cars. Imagine Tillerwoman and I are driving from New Jersey to Massachusetts to see Cutest Granddaughter in the World (and Son Number One and Daughter-in-Law of course). We're heading along I-287 in Westchester County (we didn't drop in on Bill and Hillary this time) and alongside of us is a red pickup truck with a bumper sticker saying Men age like wine, women age like milk. (I'm not making this up, honest.) I look across at the driver of the pickup truck just to make sure it's not Bill. It isn't.
So then the two little air molecules represented by our two cars arrive at a decision point. We can drive the first 40 miles or so of our journey through Connecticut on the Merritt Parkway or we can go along the shore on I-95. I take the parkway, he goes via I-95. The two routes meet up again around Milford. The two roads are of slightly different length, the traffic drives at different speeds and the sliver of land between them is shaped like a wing. Can you see where I'm heading with this?
Now is there any law of physics, traffic flow or logic that says the misogynist redneck and I must arrive in Milford at exactly the same time? Of course not. We would be astonished if we did. So why would anyone think that two air molecules traveling on different sides of a wing at different speeds must somehow, by magic, arrive at the trailing edge together?
Still not convinced? Think my analogy won't hold for air molecules? OK, try this argument for size ...
Forget wings. This blog is about sailing, not flying. Even if there is some shred of truth in the argument that " the top of a wing is longer so the air has to go faster and this causes lift", what possible relevance can this have to sails? You know the air flows along both the leeward and windward sides of your sails (on a reach say) because you spend a lot of time concentrating to keep both the windward and leeward telltales streaming back nicely. Now how big is the difference in length between the path along the leeward surface of your sail and the path along the windward surface? Miniscule I would say. In the same ballpark as the thickness of your sailcloth. And this minute difference causes a difference in airspeeds that generates the force that can drive you and your fatass crew and your leadmine of a boat forwards? I don't think so.
OK, "well informed commenters", it's your turn. Feel free to geek out and attack my three carefully crafted arguments. No calculus allowed. Feel free to provide more links to websites that have other erroneous explanations of how sails work. In line with the established tradition of this discussion, credit will be given for irrelevancy and irreverence. Do not write on both sides of the paper at once.