Wednesday, April 12, 2006

Geeking Out

A few days ago Bonnie at frogma noted that "the Tillerman (and his incredibly well-informed commenters) have been geeking out (NO disrespect meant by that, Tillerman, sir! Quite the opposite in fact!) on the physics-of-sails topic".

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.


Anonymous said...

From the brain trust at NASA:
When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

Anonymous said...

Upside Down Flight

If you have ever gone to an airshow, you have seen airplanes briefly fly upside down. People who understand the logic behind Bernoulli lift immediately realize that an upside down wing cannot really produce any Bernoulli lift. They are correct! Watch carefully the next time you see such an upside down aircraft flying. They must depend entirely on Reaction Lift, and therefore they must keep the nose of the airplane noticeably higher than usual, to get the greater angle-of-attack they need.

This sort of demonstration confirms everything we have described here. If ONLY Bernoulli Lift existed, no upside down flight would be possible. If ONLY Reaction Lift existed, then an aircraft could use the same angle-of-attack either shiny side up or upside down. The fact that maybe 1/3 greater angle-of-attack is necessary suggests that around 1/3 of the normal lift is probably provided by Bernoulli Lift (for that speed and altitude) while the other 2/3 is normally provided by Reaction Lift.

the skip said...

Have some links herethe skips blog

Litoralis said...

There is a plane in the middle of this clip that flies on its side...

Post a Comment