Wednesday, March 15, 2006

Pretty Darned Cool

Bonnie from frogma said in a comment that she thought my post about the physics of sailing was "pretty darned cool". Thanks Bonnie. Actually there were three posts in all on the topic at here, here and here. But I haven't finished, so let's return to the subject.

What started me off meandering around the question of how sails work was that I was uneasy with the standard explanation that you read in most sailing books. It usually goes something like...

A sail is like an airplane wing and it generates lift for much the same reason. 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.

There are a couple of reasons why this seems like nonsense to me. Or even it does explain some of the lift generated by sails and wings it is by no means the full story. Here's just one reason...

Check out this video.

If an airplane flies because the upper surface of the wing is curved, then how do you think a plane can fly upside down? Surely if the Bernoulli force is balancing gravity when the plane is flying right side up, then there would be double the force of gravity pulling the plane towards the earth when the plane is upside down?

3 comments:

Adrift at Sea said...

While the video of the plane is quite cool...and I'd imagine rather risky to fly... the plane's wings have flaps, which can drastically change the effects of airflow over the wings. By changing the position of the flaps, moving them upwards relative to the top of the plane, changes the airflow sufficiently that the plane can fly upside down for limited periods of time. Not all planes are capable of doing this as far as I know.

Most sails don't have flaps... so the analogy between a wing and a sail starts to fail at the point the wing start using the attached flaps.

Another good site with "virtual lab" experiments to demonstrate the physics of sails and how the work is found here.

carlyon said...

You'll also find that a plane has a source of thrust other than its wings. When one throws a stone in a direction that is not down, one does not expect it to begin its decent for some considerable time. Of course, as things moving forward fall at the same rate as things merely falling down, regardless of horizontal movement, it can be assumed that the plane it positioned at a slightly upward angle, which it must be, otherwise it would propel itself into the ground, and thats no fun...
The fact that it has a large wingspan generates air resitance in itself, and given that they travel so fast, the plane in the clip only went a relatively short distance. It is possible to fly any plane, without flaps, in an upside down position for these reasons. Try it with a paper aeroplane. It will fall eventually, but it'll give it a good run...

Couple that with aforementioned flaps... and the comparison is dead.

A wing may be like a sail, but a plane is NOT like a boat.

bonnie said...

Almost missed it! A pox on budget season anyhow.

that seed.slb.com site is excellent. I had stuff I needed to do tonight 'cause I've got lots of messing about on boats I want to do this weekend and I just frittered away half an hour...

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