General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: Alex Eisenhut on September 18, 2021, 03:00:30 pm ---Jesus Christ! STILL can't spell BRAKING correctly? |O
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Not quite sure what that fictional character has to do with me not spelling braking correctly but I get your point and I try to improve. I did not made the English grammar rules where two words with completely different meaning have the same pronunciation but different spelling. And the limited spell checker is not able to help me here.
--- Quote from: Alex Eisenhut on September 18, 2021, 03:00:30 pm ---Look, the spinning propeller creates a virtual sail behind the vehicle. It pushes against the wind, if you sum the velocities you end up with 0, but since it's blowing backwards, it adds to the ground velocity of the vehicle.
You can view this virtual sail as storing the initial energy of the vehicle being blown from rest to its cruising velocity.
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If you sum up the wind speed with the artificial one created by the propeller you do not end up with zero but with the sum of the two. Both the natural wind and the one created by the propeller push against vehicle in the same direction so they will add up and since air is compressible energy is being stored in there.
You can see this energy storage like a spring with wind pushing from one end and the propeller created wind from the other end compressing this spring. This is a very simplified view as there is another spring on the other side of the propeller that is extended.
A sail also has stored energy behind when wind speed is higher than sail speed but that stored energy is only large enough to push the vehicle up to wind speed in ideal case.
So a stationary sail will have the max stored energy behind is like a compressed spring due to high pressure (higher than ambient) at the back of the sail and similarly there is a low pressure on the front of the sail and this pressure differential represent stored energy.
Equivalent will be a propeller that is at wind speed (ideal case) and here the stored energy is equivalent with the pressure differential created by the artificial wind that is dependent on propeller pitch and rotational speed.
A propeller that is not connected to wheel will still accelerate the vehicle but for a very short time and this is what you are thinking when you say there is insignificant amount of energy stored in pressure differential (it is not it just gets wasted in this way). But if you connect the propeller to the wheel then the pressure differential stored energy can be maintained for longer as part of energy is used to maintain that artificial wind so much more of the pressure differential stored energy can be used by the vehicle.
Kleinstein:
The flyheel effect of the propeller can not be used to accelerate the vehicle, unless they have a variable gear box that they would switch. The speed is linkt to the wheels and thus the speed to ground. So like heavy wheels the flywheel part acts similar to additional mass of the vehicle, making it harder to accelerate. From that argument there is also no real way to osciallate energy between the pressure field around the vehicle and the propeller RPM.
Keep in mind that the time scale for the pressure field is the speed of sound divided by the dimensions - so this is less than 1/10 s. Without a container, compressed air is not really effective storrage. So if at all you may get a very short time effect, hard to see on a normal video. So the air pressure around the vehicle can not be the magical energy storrage to drive the vehicle.
It is the stationary pressure field that builds up that drives the vehicle. The wind and the power from the wheels transferred to the prop keep this pressure field around the vehicle alife. Usually one uses a simpler pricture of the working the prop and calls it the prop driving the vehilcle.
electrodacus:
--- Quote from: Brumby on September 18, 2021, 01:53:08 pm ---is completely invalid. You presume your presentation models the Blackbird mechanism - which it clearly does not.
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Obviously it is an analogy and not exactly the same thing but the idea is the same.
The vehicle in my video shows the belt as an energy storage device allowing vehicle to accelerate from left to right and even exceed moving paper speed for short periods.
Blackbird stores energy in pressure differential (equivalent of the rubber band) and can exceed wind speed both direct down wind and against wind direction.
Is just that pressure differential energy storage is much larger and vehicle friction much lower so that time scales are different.
If you will record blackbird for say maybe half an hour and speed up the video you will see the same sort movement as in my wheel only model.
So blackbird average speed over that longer duration will be way lower than wind speed but there will be peak periods where blackbird exceeded wind speed even significantly.
So the experiment done by blackbird team was just to short to show the entire story and based on that experimental result and lack of physics knowledge wrong conclusions where extracted from that.
ejeffrey:
--- Quote from: electrodacus on September 18, 2021, 07:05:25 am ---
You also need to consider what the example represents.
Moving paper is the energy source for the vehicle and is the analog of win in a wind powered vehicle. So in my video you see the equivalent of a vehicle moving in the opposite direction to wind direction at around half the wind speed.
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Sure, it was your demo proposal, not mine. You claimed this was a demo that would satisfy you and that it was impossible for the cart to go the opposite direction from the applied force. It clearly is possible as I have demonstrated and as I have explained both in terms of forces and torques as well as power transfer.
You can alter it to go the other way if you want, or faster than the paper in either direction with a suitable choice of gear ratio. The traction requirement becomes larger so you will probable need to add some weight to the cart and pull carefully. I can sort of get that to work with the same cart just putting the paper under the other wheel but it's hard to do without slip.
--- Quote ---If you want to see things in modified reference frame
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I would caution you against trying to think in multiple reference frames at once,. It's true that you can solve the problem in any reference frame you want and get the right answer but in practice it is an easy way to get led astray. This is because it is easy to come up with "invariants" that are not actually true in other frames.
One example is that the change of frame itself does not conserve energy. In fact in any reference frame other than the lab frame the kinetic energy of the ground is "infinite" so you basically can't use global conservation of energy. Also if you solve part of the problem in one frame and switch you will get inconsistent results. Another problem people have is that they accidentally use an accelerating reference frame such as a vehicle that isn't moving at constant speed.
All this is why I stick to the lab frame in all of these discussions.
electrodacus:
--- Quote from: Kleinstein on September 18, 2021, 05:00:28 pm ---The flyheel effect of the propeller can not be used to accelerate the vehicle, unless they have a variable gear box that they would switch. The speed is linkt to the wheels and thus the speed to ground. So like heavy wheels the flywheel part acts similar to additional mass of the vehicle, making it harder to accelerate. From that argument there is also no real way to osciallate energy between the pressure field around the vehicle and the propeller RPM.
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Yes correct the flywheel can not accelerate the vehicle above wind speed can only slow down the rate of deceleration. And yes having a gear box will have allowed vehicle to exceed wind speed using that stored energy alone. They actually have a gear box on blackbird is just less obvious and that is the fact that they can control the propeller pitch. By increasing the pitch angle they can use the flywheel stored energy to accelerate.
Obviously flywheel is not what accelerates this type of vehicle above wind speed since it was shown with the small treadmill model that wind speed can be exceeded without propeller pitch control.
--- Quote from: Kleinstein on September 18, 2021, 05:00:28 pm ---Keep in mind that the time scale for the pressure field is the speed of sound divided by the dimensions - so this is less than 1/10 s. Without a container, compressed air is not really effective storrage. So if at all you may get a very short time effect, hard to see on a normal video. So the air pressure around the vehicle can not be the magical energy storrage to drive the vehicle.
It is the stationary pressure field that builds up that drives the vehicle. The wind and the power from the wheels transferred to the prop keep this pressure field around the vehicle alife. Usually one uses a simpler pricture of the working the prop and calls it the prop driving the vehilcle.
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No container is needed you just need to have a feedback loop in this case the wheel of the vehicle connected to propeller.
That pressure field you are referring to is increased as vehicle accelerates initially (either pushed by wind or by humans) and from the moment vehicle is no longer pushed by humans or wind that pressure differential starts to drop as it is being used up to power the vehicle that is linked to propeller so propeller is also powered from that.
This link between wheel and propeller is what makes utilizing the pressure differential stored energy much more effective and why it lasts minutes instead of just seconds without that connection.
Same energy is stored in pressure differential with or without the connection to wheel but without the connection to wheel much more of the energy will end up as heat plus you can not just start the vehicle directly with wind or even just pushed you will need something to spin the propeller to proper speed.
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