Mythbusters: Blow your own sail review

[JDługosz]

The result was surprising. Their "explanation" was nonsense. What is the reason that the boat is able to move forward?

To those who haven't seen it, they put a fan and a sail on a model cart or boat, and as expected, it didn't move. They made the sail very small and it moved backwards: makes sense as some air missed the sail. Having no sail at all is simply the limiting case.

So they made the sail much larger, and it moved forward! His thought is that the sail could not absorb all the force so some of it reflected. What does that even mean?

 

[Pengwuino]

Can you find a youtube link of this or something?

Nevermind:

 

[Dadface]

Isn't this simply explained in terms of momentum.The momentum of the moving air in one direction is equal and opposite to that of the boat.Using a large sail reverses most of the air flow.

 

[pyrotix]

But the air flow was generated on the boat.

The fan blows the air forwards. This accelerates the boat backwards.

The sail catches the air and captures its momentum. This force is equal and opposite to the force of the fan.

I don't see how this is possible.

 

[rcgldr]

The propeller accelerates the air from zero speed to some positive speed. The large sail accelerates the air from that positive speed to some amount of negative speed, so there's more momentum change at the sail.

It would be eaiser to visualize this if the propeller output went into a u shaped tube to redirect the propeller output backwards.

 

[pyrotix]

Thanks for the clarification. That makes sense. It's a question of how elastic the collision with the sail is.

 

[rcgldr]

It's a question of how elastic the collision with the sail is.

The other requirement is that the flow reversed by the sail is far enough away from the propeller's input flow that the sail reversed flow doesn't interefere with the propellers input flow. This is part of the reason that the large sail is needed for this to work.

 

[olivermsun]

Anyway, if it works, it would work better if you just turned your fan the other way (so that it acts as a propellor).

 

[sophiecentaur]

The video seems to miss the point and is misleading because what counts is not 'forces and forces cancelling out'. What counts is the direction in which most of the air flows when it's finished interacting with the fan and the sail. It's just Newton III at work, as usual; reaction and action etc..

 

[rcgldr]

Note that thrust reversers on jet engines operate on a similar principle. The thrust is reflected forward by the reversers to produce a braking effect on an aircraft.

 

[olivermsun]

Note that thrust reversers on jet engines operate on a similar principle. The thrust is reflected forward by the reversers to produce a braking effect on an aircraft.

I demand to see this tested on MythBusters! :D

 

[Vespa71]

Force vectors:
The fan gives force backwards to the boat, by suction and blowing. The sail cancels blowing force by stopping the airflow. It then diverts equal amounts of that air sideways (=0), and some air blows backwards.

Net: Backwards blow force from sail battles inlet suction force, but suction force is smaller, as air is pulled from all directions. So the MB explanation "the sail could not absorb all the force so some of it reflected" holds, kind of, regarding force vectors.

 

[sophiecentaur]

Since when has "suck" been a force? Pressure is the only force that a gas can apply to an object. It's pressure difference that counts.

 

[olivermsun]

No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.

 

[Vespa71]

Since when has "suck" been a force? Pressure is the only force that a gas can apply to an object. It's pressure difference that counts.

Kinetic energy of air will explain more of the forces here than pressure-difference. (Jet engine?) Then suction is a force. The fan pulls on stationary mass of air.

 

[Vespa71]

I'm not sure if the fan gives a pressure-difference or simply accelerates air, or both. A pressure difference between the fan and incoming air, and a pressure difference between fan and blowed air, will provide the vectors described. It does not matter how the fan works. The jet-engine example by "rcgldr" also has the same aspect. Suction will accelerate the aircraft, but so much less then the reverse thrust brakes.

 

[sophiecentaur]

No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.

I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.

 

[mstacho]

So just out of curiosity, how would this result have changed if the boat/sail combination were enclosed and there was zero gravity?

For example: Put the fan and the sail in the middle of the space shuttle or something. Let's assume that the thrusters are cancelling out the rotation induced by the fan, and that the fan and sail are "small" compared to the size of the shuttle's interior, if that's important.

Would the shuttle still move, or would the air eventually hit the walls or something and cancel all the momentum?

Maybe if it works we'll have a new form of space engine

 

[pallidin]

I don't see how this is possible.

In a closed system there is no way this is possible.
However, the experiment is actually an open system.

 

[olivermsun]

I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.

If a fan doesn't suck air in one end and blow it out the other, then what does it do in the "correct" terms?

 

[rcgldr]

I'm not sure if the fan gives a pressure-difference or simply accelerates air, or both.

At the macroscopic level both. In the immediate vicinity of the propeller, there's almost no change in speed, just an increase in pressure, called the pressure jump. The reduced pressure before the propeller accelerates air towards the propeller, and the increased pressure after the propeller accelerates air away from the propeller.

In this article, the air flow speed across the 'propeller disk' is V_p:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

So just out of curiosity, how would this result have changed if the boat/sail combination were enclosed and there was zero gravity?

The boat would travel across the inside of the closed system until it hit a wall. There would be circulation of air within the closed system. The center of mass of the closed system would not be affected by any of the internal motions and/or internal accelerations.

 

[Fewmet]

No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.

Suppose I pump all the air out of the room on the other side of the wall I am sitting next to. I drill a hole in the wall ad then cover it with my finger. Does the near-vacuum pull the pad of my finger toward the other room, or does air in the room I am in push it that way?

The former seems like describing the change in a room when I turn out the lamp as the darkness moving into the room rather than as the light leaving it.

 

[pallidin]

Uh... what??

Of course your finger tends to get sucked into that vacuum.
Nothing special here.

 

[Fewmet]

Uh... what??

Of course your finger tends to get sucked into that vacuum.
Nothing special here.

Really? I'd see it as the air in my room pushing it.

Maybe it is just a quirk of my training, but I always look for a force wherever two bodies are in contact. That leads me to think air molecules push on my finger, not that the absence of a body contacting my finger is pulling it.

If I were floating in a vacuum, would there be no net force on me because the vacuum pulls me equally in all directions? Or is there no force on me because the vacuum applies no force.

 

[Vespa71]

I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.

You're right.

Ehem.. Force vectors, 2nd take

Suction force can not exist. We'll name it "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller"
The fan gives force backwards to the boat, by "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller" and blowing. The sail cancels blowing force by stopping the airflow. It then diverts equal amounts of that air sideways (=0), some upwards (doesn't matter) and some air blows backwards.

Net: Backwards blow gives force forward to the boat. This battles "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller", but "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller" is smaller, as air is pulled from all directions. So the MB explanation "the sail could not absorb all the force so some of it reflected" holds, kind of, regarding force vectors. They did not describe "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller", presumably to avoid conflicts with scientists when using popular terms to describe the phenomenon.

 

[sophiecentaur]

You're making me feel a bit bad now.
But it's a bit of a minefield and the more carefully you tread, the less likely you are to come to unreasonable conclusions.

 

[Vespa71]

You're making me feel a bit bad now.
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..

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]

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.

 

[Vespa71]

Would you agree the lower pressure zone results in a pressure that exerts backwards force on the boat?

 

[sophiecentaur]

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?

 

[Vespa71]

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.

People rest.

 

[sophiecentaur]

 

[olivermsun]

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.

 

[sophiecentaur]

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.

 

[turf]

Does the shape of the sail matter? e.g. If the sail was flat and rigid would the result change?

 

[sophiecentaur]

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]

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.

 

[boing!]

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??

 

[Fewmet]

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.

 

[pallidin]

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.

 

[exabrial]

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 canceled 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!