General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
PlainName:
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As you can see, blowing air across the pipe sucks air out of the bag, giving the exact same effect as the chap in your video.
How does he, and you, know that he is not recreating this situation?
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Yes, I can see what you see, and here is the explanation.
Firstly, there is no such thing as "suction". Therefore, the pipe cannot suck air out of the bag. When we talk about suction this is just loose and imprecise terminology.
Gases (like air) consist of particles that are all pushing each other apart and trying to spread out. In the absence of overriding gravitational or accelerating fields, gases will spread out to evenly fill any container. The gas pushing against the walls of the container manifests as pressure, which can be measured. There is no such thing as "suction" because gases are pushing apart and trying to spread out, not pulling together. To have "suction" requires a "pull", which gases do not have.
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I cut the quote short to save space - I did read it :)
First, to be absolutely clear, that vacuum cleaner pipe is blowing air. I first used an airline and swarf clearing nozzle, but it was very directional and I didn't want to have the effect of the nozzle (the venturi) intruding so switched to the relatively large and slow output.
Now, I accept that the terms being used are incorrect, in that 'suction' is perhaps confusing in a literal sense. However, from my point of view there is high pressure across the top of that pipe - if a sheet of something was placed there, at right angles to the flow, it would be pushed away from the hose outlet. Now, you might say that this is merely air flowing, but if it is flowing then it is being pushed against something. Either the sheet of whatever or just ambient air - there is a restriction against which the flowing air is being pushed, and that must surely be increasing the pressure. A fan is taking air from one side and pushing it to the other despite there already being air there, so the pressure must increase.
The way I understood there being an area of low pressure, which the air in the pipe will flow to fill, is because the air moving across the end of the pipe drags molecules of air at the pipe entrance along with the flow. Kind of like water eroding the sides of a stream. So technically there is low pressure there, but it is caused by the existence of the pipe - if the pipe weren't there then there wouldn't be a created low pressure.
So I am suggesting that the chap is measuring an artifact caused by the act of measuring. Instead of the pipe, what if he put, say, a flap of paper in the airstream. Would that bend toward the blades? I doubt it. But if he placed the paper edge-on, would it be pulled into the stream? It likely would (if it could retain it's shape) but only because the surface of the paper is causing the low pressure that pulls it in.
iMo:
The propeller's blades act as airplane's wings as they have got a so called "airfoil" cross-section.
The blades with a proper angle of attack create a forward lift force because the air pressure above the upper surface of the blade ("suction surface") is lower than the pressure below the lower surface ("pressure surface") of the blade.
The pressure difference is "local" to the blade's surfaces actually (all happens close to the blade's surfaces).
The fact you may see a huge air stream flowing into the intake part of the propeller is because the Earth's atmosphere is "pushing" the surrounding air masses into the place with the local lower pressure (at the blade's suction surface). On the outtake side the local high pressure (at the blade's pressure surface) "pushes" the Earth's air masses to the places with lower pressure.
The propeller's lift force is proportional to the square of blade's velocity (ie. its leading edge rotational speed) and the blade's area (plus other params). The propeller also stores energy as it works as a flywheel.
https://en.wikipedia.org/wiki/Airfoil
PlainName:
--- Quote ---The fact you may see a huge air stream flowing into the intake part of the propeller
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Ah-ha! I think I've spotted the problem...
The prop on the bench is blowing towards the camera. That is, the opposite of if the bench were an airplane. Perhaps I am wrong (I checked the angle of the blades and rotational direction, but may have got confused), if half of us assume that and the rest assume the reverse then we'll never find an acceptable explanation :)
electrodacus:
--- Quote from: Brumby on August 31, 2021, 12:02:16 pm ---
--- Quote from: IanB on August 31, 2021, 05:52:46 am ---The interesting thing about the treadmill experiment is that they missed an opportunity to show something more. They let the model vehicle fly forwards and drive off the end of the treadmill. But if they had simply anchored it in place with some thread, they could have shown it pulling against the thread indefinitely while the treadmill was running.
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Now THAT is a brilliant idea. Put a tension measurement device on that thread and you could generate data of tension vs speed with alternative gearing and propeller parameters.
You could chart the results and get a very good visual understanding of the behaviour.
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Look at my model that has the spiral spring to understand why this is not a proof of anything. The anchored vehicle will be nothing more than a treadmill powered fan. As soon as you keep the vehicle in place you are fully charging the energy storage so if you keep the vehicle in place you can do that indefinitely since the stored energy is not being used (is better to see that in my model).
electrodacus:
--- Quote from: cbutlera on August 31, 2021, 09:55:20 am ---Now imagine if Blackbird plus driver weighs 200 kg, and is travelling at 11 m/s, with a 10 m/s tailwind, both with respect to the ground, so Blackbird's speed in the frame of reference of the air is 1 m/s. With respect to the ground, Blackbird has a kinetic energy of 12,100 joules (mv2/2). Suppose that the wheels now extract 1,000 joules of this energy, reducing the kinetic energy (with respect to the ground) to 11,100 joules and therefore the ground speed to 10.54 m/s. With respect to the air, Blackbird now has a speed of 0.54 m/s and a therefore a kinetic energy of 29 joules. Suppose that the propeller, after losses, manages to add 700 joules to this kinetic energy, increasing it to 729 joules. The speed with respect to the air will now be 2.7 m/s, so the speed with respect to the ground will be 12.7 m/s. This is greater than the starting figure of 11 m/s. So Blackbird can indeed accelerate beyond wind speed without breaking any laws of physics or requiring energy storage.
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Maybe it is a good idea to solve a problem.
So 200kg vehicle traveling at 11m/s with 10m/s tailwind.
Kinetic energy of the vehicle is indeed 12100Ws and that is referenced to ground.
So you say we extract 1000 joules (1000Ws) from vehicle wheel reducing vehicle kinetic energy and obviously speed.
Now say total efficiency from wheel to propeller is 70% that will mean you put back just 700 joules (700Ws) in to vehicle meaning the vehicle now has lower kinetic energy and speed than we started with.
You can think about the air as a parallel lane that is like a treadmill surface and that travels at 10m/s so vehicle can decide to use the 1000Ws extracted from the wheel on the ground and either push against the ground with another wheel at 90% efficiency is 900Ws or use another less efficient wheel 70% efficient 700Ws and push against the parallel lane than itself travels at 10m/s relative to the ground.
So since vehicle has now 10.54m/s that parallel lane look from vehicle perspective as traveling backwards (opposite to vehicle direction of travel) at 0.54m/s
If you decide to go with the 90% efficient wheel on the same road / ground the vehicle travels you recover 90% of the lost kinetic energy so now vehicle is at 12000Ws a bit less than we started.
If you decide to use the 70% efficient wheel t push against the parallel lane witch is like a treadmill moving at 10m/s so relative to vehicle that will look like opposite to travel direction at 0.54m/s
Keep in mind that not the entire vehicle will move on that lane just one of the traction wheels will be touching that lane and vehicle will need to be pushed against that lane but on the ground road.
So your mistake is that you decided the entire vehicle moved in to air same as the entire vehicle will have moved on to that other lane. I think with that treadmill lane you can see the consequences of such action (vehicle changing lanes instead of just using one wheel to push against that lane).
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