Mythbusters: Blow your own sail review

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sophiecentaur

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You're making me feel a bit bad now. :smile:
But it's a bit of a minefield and the more carefully you tread, the less likely you are to come to unreasonable conclusions.
 
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You're making me feel a bit bad now. :smile:
But it's a bit of a minefield and the more carefully you tread, the less likely you are to come to unreasonable conclusions.
I appreciate your remark, it was inspiring. And I hope people working on more substantial matters are more educated than I am. Nobody found out anything interesting by focusing on treading carefully..:smile:

Suction is a factor on a vacuum-cleaner, on a jet's reverse thrust, and on this boat. That I'm sure of, allthough the defining of the force is difficult.
 

sophiecentaur

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More people have made cockups by not treading carefully than the reverse. In all worthwhile endeavours it's 99% perspiration and 1% inspiration. Don't be mislead by the popular dramatisation of Science History. All the Greats had their feet well on the ground when making their big breakthroughs. Sloppy doesn't make valid advances.
"suck" is a shorthand that we all use but a competent jet engine designer knows exactly what she means when she uses it in an explanation. This is more than your average arm waver.
 
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Would you agree the lower pressure zone results in a pressure that exerts backwards force on the boat?
 

sophiecentaur

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Oh yes. That's fair enough - as far as it goes. But you could imagine a fan that takes in a lot of air at the side and that has a very low area facing backwards. The sum of the forces in a backwards direction could be vanishingly small - i.e. the sum of momentum changes of all molecules entering the fan could be as small as you like. The effect of low pressure at the back could be finite but not appreciable. You could even draw air in from the front of the boat and still propel it backwards.
The sum of momentum changes given to the expelled molecules, however, would be as high as you wanted by suitable choice of the nozzle / sail arrangement. This would be what produces the driving force.

In any case, I have no objection to speaking in terms of pressure difference. It explains all of this very well on a macroscopic level. My problem is with the actual word Suck, which implies a pulling force (adhesion / cohesion) - which you don't get with gases. . So why use it?
 
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I used the term "suction force" as I did not know better. My faulted presumption was that we could define 1 bar to be 0-pressure, and that applying work to reduce this pressure would then give an average, directed negative (pulling) force on the boat, from the accelerating air's increasing momentum. Under a system-pressure of 1bar, air bodys wants to stay in a certain temperature-dependent average vicinity of each other, allthough they are not adhesive or cohesive.

I recognize this perception was wrong.

Your recognition of the (previously wrongly named) force vector sum's existence is appreciated.

What fired me up was that nobody seemed to care if this force was joined with thrust-force, even though they have different directions finally.

I'll tread carefully:smile:.

People rest.
 

sophiecentaur

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:smile:
 

olivermsun

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On any case, I have no objection to speaking in terms of pressure difference. It explains all of this very well on a macroscopic level. My problem is with the actual word Suck, which implies a pulling force (adhesion / cohesion) - which you don't get with gases. . So why use it?
Well, we have continuity in fluid mechanics, so the various points of a flow are connected -- we imagine streamlines even though we don't actually believe there are little strings pulling on the bits of fluid.
 
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sophiecentaur

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Well, we have continuity in fluid mechanics, so the various points of a flow are connected --
Yes, because there is positive pressure keeping things together in a gas - which is what we're discussing. But I don't recall any mention of 'suck' in the fluid mechanics I have read about.

Look, I have no problem with sloppy speak between the cognoscenti but people on this forum very often need help with understanding and with resolving misconceptions. This is not helped by using terms which are blatantly not valid and which can be avoided with just a little care.
 
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Does the shape of the sail matter? e.g. If the sail was flat and rigid would the result change?
 

sophiecentaur

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Oh yes, definitely. The main effect would, I'm sure, be the direction in which the air spills off the edge of the sail. For a flat, rigid sail (a board?) the faster moving air would mainly emerge at right angles to the line of the craft, with equal forces in each lateral direction - no resultant force forward or backward.

But you can't afford to be too 'intuitive' when it comes to aerodynamics.
 

nrqed

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I cringed when I heard them say that Newton's laws predict that the boat would not move. I wish they had asked a physicist!! It's obvious that if the air recoils, the boat will move. And then they concluded something to the effect that Newton's laws had been proven wrong or something to that effect. :eek:
 
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At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??
 
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At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??
Welcome to Physics Forums, boing!

This scenario also came up in https://www.physicsforums.com/showthread.php?t=510723". I offered an answer in terms of conservation of momentum in post #20.
 
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At first, I had no explanation...the fan should be pushing backward with an equal force to the one that air is hitting the sail, but then I thought about Bernoulli. Isn't that the answer here? Put the fan back far enough from the sail and there will be more air particles hitting the sail than were originally moved by the fan. Bernoulli's principle FTW??
If you "close the system" you will find that your scenerio does not work.
If you "open the system" the environmental air simply will not do as you suggest.
 
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I too was baffled by this, but I think I have an explanation not covered anywhere in this thread yet... I can also explain why they didn't get a result until they cranked the propeller to 11 and a had massive sail!

First, all of newtons laws are in effect. Technically the force by the boat would be cancelled if all energy is caught by there sail. Simple as that, there should be 0 net force on the system! But....

I think this is explained by the shape of their Sail. Air currents hitting the center of the sail have four escape routes: left/right, or up/down. Here's the thing, the sail is _not_ flat, it's parabolic!

The left/right escape routes are on the same plane (depth) as the entry point (the center of the sail). In other terms, the left/right escape routes are at the apex of of the parabola. Any air exiting left/right does so perpendicular to entrance of the air current, leaving 0 net force on the boat.

However, the top/bottom escape routes are _rearward_ of the apex. This means any air currents escaping top/bottom would _not_ be escaping exactly perpendicular! They would be angled slightly aft (backwards probably 15degrees), giving a net force forward!

The vertically oriented air currents are likely to be very minor, as the left/right escape routes offer much less resistance. So cranking the fan to 11 finally produces enough force to get the affect.

The oversized sail (Much LARGER than the incoming stream) allows the the vertical oriented air currents to escape around the incoming stream. A small sail would simply not allow the currents to escape around the incoming airstream, cancelling those vectors.

Two experiments would confirm this...

First, create a perfectly flat sail. Air must be able to escape equally in all directions with no pressure differences in any vector.

Second, create a massive "Angel Food Cake Pan" shaped sail and a very very VERY narrow central airstream. The shape of the "Angel Food Cake Pan" would reverse the incoming vector backwards moving the boat forward.

This was really bothering me, but finally I think I have a reasonable explanation!!!
 

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