Troll Physics: Blow Your Own Sail

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Troll Physics: Blow Your Own Sail

Postby xcthulhu on May 12th, 2011, 1:17 pm 

I love Mythbusters:



(it's 4m50s in)

Unbelievable.
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Re: Troll Physics: Blow Your Own Sail

Postby BioWizard on May 12th, 2011, 3:02 pm 

I don't think this qualifies as blowing-your-own-sail movement, but rather using your sail to reverse the direction of airflow. In this case the sail is acting as a reflector which moves the vehicle forward by redirecting the air flowing towards it, not by acting as a mere sail (i.e not by creating a reaction force due to the push of wind).

It would be easy to make the distinction and test my hypothesis. If they used a hard flat sail, the blow your own sail cart won't move, because that's a lot like standing inside a boat and pushing against one of the edges. That won't move the boat. The reason their sail car moved was because the bulge in the sail caused a region of increased air pressure (hence the bulge) which pushed air outward towards the edges of the sail. And since the direction of the air going from the center to the edges points backwards, that's what created the forward motion.

A hard flat sail will still cause the car to move if wind was blowing against it, but it won't move the car by the air blown by an on-board engine. This myth should've been busted.
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 2:21 pm 

BioWizard wrote:I don't think this qualifies as blowing-your-own-sail movement, but rather using your sail as a reverse the direction of airflow. In this case the sail is acting as a reflector which moves the vehicle forward by redirecting the air flowing towards it, not by acting as a mere sail (i.e not by creating a reaction force due to the push of wind).

It would be easy to make the distinction and test my hypothesis. If they used a hard flat sail, the blow your own sail cart won't move, because that's a lot like standing inside a boat and pushing against one of the edges. That won't move the boat. The reason their sail car moved was because the bulge in the sail caused a region of increased air pressure (hence the bulge) which pushed air outward towards the edges of the sail. And since the direction of the air going from the center to the edges points backwards, that's what created the forward motion.

A hard flat sail will still cause the car to move if wind was blowing against it, but it won't move the car by the air blown by an on-board engine. This myth should've been busted.


This myth has some unfinished edges. It's somewhat similar to Feynmann's reverse water sprinkler puzzler in that it's so hard to set up experimentally, especially using compressible air. But, in theory, it ought to be possible to make the boat move forward (or even backward if you permit me some ductwork fairings); I say this only because I think I see a power cord running away from the fan. Without that cord, I think I would have to jettison mass somehow to make this happen (Don't ask how I jettison air mass; I am not that smart).
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Re: Troll Physics: Blow Your Own Sail

Postby xcthulhu on May 15th, 2011, 3:07 pm 

CanadysPeak wrote:This myth has some unfinished edges. It's somewhat similar to Feynmann's reverse water sprinkler puzzler in that it's so hard to set up experimentally, especially using compressible air. But, in theory, it ought to be possible to make the boat move forward (or even backward if you permit me some ductwork fairings); I say this only because I think I see a power cord running away from the fan. Without that cord, I think I would have to jettison mass somehow to make this happen (Don't ask how I jettison air mass; I am not that smart).


Watch the video. They observed all three behaviors. Generally speaking:
(1) Under most circumstances they observed the "Newtonian" behavior, like Biowizard predicts
(2) When the sail was small, they saw the "Going Backward" behavior
(3) When the fan/jet was really powerful, they saw the "Cartoon" behavior

They also did the experiment on different scales. The larger scales (using a jet-powered skate-bored and finally an airboat) they did not have cords.
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 3:41 pm 

xcthulhu wrote:
CanadysPeak wrote:This myth has some unfinished edges. It's somewhat similar to Feynmann's reverse water sprinkler puzzler in that it's so hard to set up experimentally, especially using compressible air. But, in theory, it ought to be possible to make the boat move forward (or even backward if you permit me some ductwork fairings); I say this only because I think I see a power cord running away from the fan. Without that cord, I think I would have to jettison mass somehow to make this happen (Don't ask how I jettison air mass; I am not that smart).


Watch the video. They observed all three behaviors. Generally speaking:
(1) Under most circumstances they observed the "Newtonian" behavior, like Biowizard predicts
(2) When the sail was small, they saw the "Going Backward" behavior
(3) When the fan/jet was really powerful, they saw the "Cartoon" behavior

They also did the experiment on different scales. The larger scales (using a jet-powered skate-bored and finally an airboat) they did not have cords.



Yes, but I want to do it without "cheating". A small sail oesn't count since i can obviously shrink the sail to the point where i have a reversed airboat.
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Re: Troll Physics: Blow Your Own Sail

Postby Natural ChemE on May 15th, 2011, 4:06 pm 

The behavior's pretty easily explained by the conservation of momentum. As BioWizard pointed out, the ship can move forward when the net effect of the fan and sail is to reverse air motion.

The confusing part here is probably that we often think of sails as merely stopping airflow as opposed to reversing it. But, for higher airspeeds and sails of certain shapes (such as a concave sail) and sufficient size, the sail actually reverses airflow as opposed to merely stopping it.

This said, if you're on an ocean with a sail and a fan, just use the darn fan; it'll be more efficient. Raise the sail above the fan's air current's flow to catch atmospheric airflow if it's in the right direction in the inertial frame that the fan'll get you to at steady state (which will be less than half the time if everything's random; much less than half the time if the fan's relatively strong).
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 4:13 pm 

Natural ChemE wrote:The behavior's pretty easily explained by the conservation of momentum. As BioWizard pointed out, the ship can move forward when the net effect of the fan and sail is to reverse air motion.

The confusing part here is probably that we often think of sails as merely stopping airflow as opposed to reversing it. But, for higher airspeeds and sails of certain shapes (such as a concave sail) and sufficient size, the sail actually reverses airflow as opposed to merely stopping it.

This said, if you're on an ocean with a sail and a fan, just use the darn fan; it'll be more efficient. Raise the sail above the fan's air current's flow to catch atmospheric airflow if it's in the right direction in the inertial frame that the fan'll get you to at steady state (which will be less than half the time if everything's random; much less than half the time if the fan's relatively strong).


I should think a concave sail would be a problem. Wouldn't that give a fairly inelastic collision?
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Re: Troll Physics: Blow Your Own Sail

Postby Fuqin on May 15th, 2011, 4:20 pm 

Anyone ever sit near an engine on landing , they don’t stopem with wheel breaks uknow.
That stoping power u feel is a tiny cone shaped sail at the back of each engine + wing brakes , then your run way can be about 2K and under
Last edited by Fuqin on May 15th, 2011, 4:34 pm, edited 1 time in total.
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Re: Troll Physics: Blow Your Own Sail

Postby Natural ChemE on May 15th, 2011, 4:29 pm 

CanadysPeak,

I'm not entirely sure what you mean by an inelastic collision in the context of a fluid since it's about bulk flow phenomena. Overall, air flowing directly into bowl-like structure will tend to exit with its forward/backward velocity component being in the direction opposite to which the air entered the bowl at.

For an extreme example, we can think of the jet engine with a hard bowl sail cupped over the jet's output. Overall, the jet tends to take in air that's relatively at rest and the bowl expels it in a backwards direction. The balance of momentum is gained by the ship.
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 4:31 pm 

Fuqin wrote:Anyone ever sit near an engine on landing , they don’t stopem with wheel breaks uknow


Yeah, but, as best I remember, they just use negative pitch on the propeller blades. How can we use that here?
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 4:37 pm 

Natural ChemE wrote:CanadysPeak,

I'm not entirely sure what you mean by an inelastic collision in the context of a fluid since it's about bulk flow phenomena. Overall, air flowing directly into bowl-like structure will tend to exit with its forward/backward velocity component being in the direction opposite to which the air entered the bowl at.

For an extreme example, we can think of the jet engine with a hard bowl sail cupped over the jet's output. Overall, the jet tends to take in air that's relatively at rest and the bowl expels it in a backwards direction. The balance of momentum is gained by the ship.


I think you need a delta of 2 mv when the air strikes the sail (or something close). If you blow into a concave sail, you get a lot of turbulent mixing. It seems intuitively like you might get, say, 1.2 mv delta plus a temperature rise. Although anything above 1 mv delta should give forward motion, I suspect there's enough losses in a sailboat that it just sits there below, oh, I dunno know, maybe 1.6 mv delta.

My CFD program license went south for want of a renewal fee, so I can't do much modelling, and am only guessing.
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Re: Troll Physics: Blow Your Own Sail

Postby Fuqin on May 15th, 2011, 4:42 pm 

propeller blades ????? no there is a foil that expands at the end of a jet , it’s the breaks! it’s a thrust deflector , but in the physics of things it’s on board , relative I would have thought , but I may be wrong
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Re: Troll Physics: Blow Your Own Sail

Postby CanadysPeak on May 15th, 2011, 4:43 pm 

Fuqin wrote:propeller blades ????? no there is a foil that expands at the end of a jet , it’s the breaks! it’s a thrust deflector , but in the physics of things it’s on board , relative I would have thought , but I may be wrong


Ah, yes, excuse me. I was focused on a fan. You are right about jets.
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Re: Troll Physics: Blow Your Own Sail

Postby BioWizard on May 15th, 2011, 7:28 pm 

NCE got exactly what I was saying.
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Re: Troll Physics: Blow Your Own Sail

Postby pulikuttan on June 8th, 2011, 9:17 pm 

I think instead of thinking about this using the three laws of motion. It is better if you use control volumes. The sail acts as a reflector. The air molecules lose a lot of energy at the sail. That is why you need a lot of energy in the air to begin with (therefore a jet/massive fan). and the concave sail forces the air to have an overall backward flow. If you drew a control volume around the boat, there would a net flow of air backwards.
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Re: Troll Physics: Blow Your Own Sail

Postby edy420 on September 10th, 2011, 2:56 am 

Wow cool, this reminds me of a theory I came up with when I was 5...
1. I can lift a piece of wood and throw it in the air.
2. I can tie a string to that wood and still lift and throw the wood.
Hypotheses: stand on the wood and throw myself in the air.
It didn't work when I tried it but I concluded that I needed bigger muscles and forgot about that experiment.
Now that Iv'e seen this video, its time to reconsider my experiment, so I'm off to the gym *hyuk hyuk*

From what I can tell, the sail is pushing air in the reverse direction, as if to act as another fan that's less powerful.
But that's basically what the guys in mythbusters and the above posts pointed out :P
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Re: Troll Physics: Blow Your Own Sail

Postby wolfhnd on September 10th, 2011, 4:12 am 

The air doesn't just move backward but offers resistance to the blades which pulls in the opposite direction. As the velocity of the air increases the resistance decreases as the air in front of the blades starts to move at almost the same velocity as the air rushing back. Also friction loses at the blades and sail are important with large slow moving blades being the least efficient. Large slow moving blades being the most efficent minus the sails reversed trust. To simulate blowing they should have used compressed air.
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