How it this possible? Even if the gif is fake, the Mythbusters did it and with a large sail it really moves forward. What is the explanation?
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asked Sep 15, 2014 at 0:01
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$\begingroup$ Related and possible duplicate: physics.stackexchange.com/q/35852 $\endgroup$– Alfred CentauriCommented Sep 15, 2014 at 0:20
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9$\begingroup$ it's no different, whatsoever, that simply blowing air out the back. it's exactly how jet engines work! for example ... by blowing air out the back. (the fact that the air is "turned around" by the umbrella or a pipe is irrelevant.) the particular gif is fake because the fan's way too small. $\endgroup$– FattieCommented Sep 15, 2014 at 6:03
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5$\begingroup$ Something that may be causing problems for you here @Olcayto. the sail in question has utterly no connection, at all, to how a sail is normally used. there is zero relationship. no connection whatsoever. the fan/sail device is just a device for sending air backwards. it's that simple. note that if there was a fan plus a large pipe, and the pipe happened to bend around 90 or 180 or 540 degrees, and that device sent air backwards .. you wouldn't be asking the question right? the "fan/sail" is just a (silly) device to send air backwards. $\endgroup$– FattieCommented Sep 15, 2014 at 9:20
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$\begingroup$ But it's still cool, in a fun way. Even more mystifycational is the validated DDWFTW (dead-downwind faster than windspeed) $\endgroup$– Carl WitthoftCommented Sep 15, 2014 at 11:47
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2$\begingroup$ If you look at the umbrella it's flapping towards the rider in question. As someone in an answer posted it looks like it's actually a backpack leaf blower on his back. $\endgroup$– Wayne WernerCommented Sep 16, 2014 at 19:27
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6 Answers
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The concept of blowing your own sail really does have to do with conservation of momentum. In that very episode of Mythbusters you speak of, the sail was removed, the fan was spun around and the ship/boat was propelled forward much faster than with the fan facing into the sail (i.e. figure (1) is much faster than figure (2)).
The reason is really quite simple and can be explained with throwing a ball off a boat.
Suppose you are on a boat carrying a ball with total mass $m_{ball}+m_{boat}$ where $m_{boat}$ also takes into consideration your mass. Now if you throw the ball off the boat at velocity $v_{ball}$ then you and the boat will have momentum $m_{boat}v_{boat}=-m_{ball}v_{ball}$. This is analogous to the figure (1).
Now consider the case of figure (2). In this case, I throw the ball at the sail, it bounces off the sail and into the water behind me. Because the process is inelastic, the ball now leaves the boat with $v'_{ball} < v_{ball}$. Therefore my momentum is now $m_{boat}v_{boat} = -m_{ball}v'_{ball}$.
Now just replace the ball with air molecules and the analogy is complete. Therefore it will always be more efficient to spin the fan around and blow the fan in the opposite direction while forgetting the sail.
Hope this helps!
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$\begingroup$ Seems to me that if this gent had spars that angled forward and curved backwards, the whole process would be rather efficient. As it stands, why would you not just mount the fan the other way around and gain vision? $\endgroup$– GusdorCommented Sep 15, 2014 at 8:35
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3$\begingroup$ Gusdor -- if you're referring to the second photo, it was simply a fun experiment. of course, there is utterly no reason to ever do that if you have a big powerful fan. you'd just throw away the sail, and point the fan the way you want to go!! no mystery. $\endgroup$– FattieCommented Sep 15, 2014 at 9:11
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1$\begingroup$ @stonemetal I don't believe so as the mythbusters did a small scale design in the show that did not have any "turning force". $\endgroup$– XcheckrCommented Sep 15, 2014 at 18:09
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2$\begingroup$ Re: "unless the fan is really strong". Backpack leaf blowers such as the type shown are available with 50CC gas engines, which are capable of driving motor bikes at around 35MPH. There is some inefficiency in the use of a fan vs. driving the wheels, but it looks plausible. $\endgroup$– nobodyCommented Sep 15, 2014 at 20:32
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1$\begingroup$ @Brilliand And I bet he gets better visibility by not having an umbrella in front of him, but you can't have everything, eh? $\endgroup$– XcheckrCommented Sep 16, 2014 at 15:30
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Of course it's possible. The air is blown forwards and bounces off the umbrella. The net flow is to the back. Then its a question of conservation of momentum...
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3$\begingroup$ But the machine blowing air pushes the skate backwards. Why do they not cancel each other out? $\endgroup$ Commented Sep 15, 2014 at 0:20
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2$\begingroup$ Even though the wind machine is aimed forwards, the air is effectively blown backwards because of the shape of the umbrella. $\endgroup$ Commented Sep 15, 2014 at 0:25
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1$\begingroup$ In the system, there are two forces. One backwards from machine one forwards to umbrella. I assume they are not equal to each other. Why not? Why airs moving backwards should be significant $\endgroup$ Commented Sep 15, 2014 at 0:30
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3$\begingroup$ @Olcayto Because upon reversing direction, the air lost all its forward momentum (pushing the vehicle forward exactly enough to balance out the backward thrust it imparted to the vehicle upon getting its initial push), and then it got more momentum going backward thus pushing the vehicle forward. The change in airspeed is greater than the original speed. $\endgroup$– user10851Commented Sep 15, 2014 at 0:45
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2$\begingroup$ Because there is something approaching twice the delta-v (change in velocity). Like Chris said. $\endgroup$ Commented Sep 15, 2014 at 1:22
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As others have said, the skater would move faster if he simply pointed the leafblower behind him, rather than bouncing it off the umbrella.
However, there is a real use for this technique. Jet engines normally suck in air from all directions and blow it out of the back in order to move forward. However they are also capable of reverse thrust if fitted with a device to redirect the air towards the front. This is used for braking after landing.
http://en.wikipedia.org/wiki/Thrust_reversal
Of course this is much less efficient than normal forward thrust. Often, only air from the large fan on the front of the engine is passed through the reverse thrusters. The hot exhaust gas which drives that fan still goes out of the rear (for obvious reasons of material temperature.) This makes the efficiency even worse. Still, as it is only used for a few seconds on landing, this does not matter. And it saves a lot of wear on the wheel brakes.
Next time you are on a plane, listen for the brief but strong boost in engine power that occurs immediately after landing. That is the thrust reversers being applied.
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$\begingroup$ They don't always use thrust reversers so you won't always hear it. $\endgroup$ Commented Sep 15, 2014 at 21:55
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1$\begingroup$ @LorenPechtel true, not always, but in most cases they do as it saves a lot of wear on the wheel brakes, which are tiny compared to the aircraft and tend to get very hot. I fly a lot between London and Madrid and I am very accustomed to hearing this sound on (I believe, all) the various models of aircraft that fly that route. I know the original A380 design didn't have thrust reversers and they were included as an afterthought. Of course, that plane flies much longer routes and therefore does less taking off and landing. $\endgroup$ Commented Sep 16, 2014 at 10:18
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The question is whether this is a form of "pulling yourself up by your own bootstraps". Clearly the driving force here is the leafblower the skater is wearing, which takes ambient air with zero average momentum and sends it out a vent with large average momentum. One would expect, absent some deviousness having to do with turbulence, that the skater would accelerate faster if he ditched the umbrella and pointed the leafblower's exhaust to the rear.
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$\begingroup$ The Mythbusters took this on in an episode and got fairly efficient propulsion by blowing toward the sail from the rear of their craft---but only if the sail had a much larger area than the fan and billowed. Think thrust-reversers. $\endgroup$ Commented Sep 15, 2014 at 0:25
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$\begingroup$ I've actually tried this (or been with some people who were doing this in the carpark outside my office building): without measurement, the sail on the front of the skateboard definitely seemed to develop more thrust than if the fan was pointed straight backwards. The fan in this case was a very high speed electric motor "developed" for the purpose and it made a narrower, but much greater velocity air jet than a leaf blower. You could also hear the engine speed drop more when pointed towards the sail, showing that it was pumping appreciably more air than when pointed away and thus was ... $\endgroup$ Commented Sep 15, 2014 at 2:02
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$\begingroup$ ... slowing down under the load of more pumped air. The effects would seem complicated and probably not unlike the design of a rocket nozzle. The motivation for the experiment was that I think someone had seen the mythbusters or something like the OPs video and someone else in the group postulated exactly what you say in your last sentence. I can't recall the motor's power exactly as the measurement was fiddly - we didn't have a power meter but we had only an ammeter and oscilloscope (the latter to estimate power factor) - one could see the power factor rising when the fan was pointed ... $\endgroup$ Commented Sep 15, 2014 at 2:05
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$\begingroup$ ...towards the sail and I seem to recall the device sucked several (maybe 5) amps out of a 240V, three phase supply (the fan dragged a slack power cord across the carpark) with a pretty high (by inspection of the oscilloscope, anyway) power factor, so we're talking several kilowatts power - rather bigger than a leaf blower. $\endgroup$ Commented Sep 15, 2014 at 2:10
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Remember the sail isn't catching all the moving air from the fan, it's reflecting wind back towards the fan which pushes the boat forward
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Of course:
