| General > General Technical Chat |
| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| fourfathom:
--- Quote from: IanB on December 23, 2021, 01:54:14 am ---Well, obviously you can compress a spring by pressing on only one end of it, while leaving the other end free... :-// --- End quote --- The other end of the spring is being pushed by the wind itself. Obviously. Or something. |
| gnuarm:
--- Quote from: IanB on December 21, 2021, 05:03:27 pm --- Actually, if you look at the car/wheel/belt system carefully, and make the following assumptions: 1. The system is in steady state (no acceleration) 2. The system is ideal (no friction losses from bearings or air resistance) Then from these assumptions the power everywhere is zero, as power is only required to overcome losses or produce acceleration. Since the power is everywhere zero, it is not possible to analyze this system in terms of power flows, and no conclusions can be drawn using that approach. To analyze the system correctly, a different approach is required. --- End quote --- Yes, don't assume zero losses. Assume some losses from the air resistance and rolling resistance. It doesn't need to be complex. Just consider that you can't move through the air with zero loss. |
| gnuarm:
--- Quote from: electrodacus on December 21, 2021, 05:59:43 pm ---I have never claimed blackbird is not going faster than the wind. What I was saying is that it can only do so using energy storage and in case of blackbird that energy storage is the pressure differential created by the propeller. A gear can not amplify power thus output power will always be lower than the input. --- End quote --- What does energy storage accomplish? The vehicle moves faster than the wind continuously. In the examples where you talk about microslipping or some such nonsense, the vehicle is clearly moving faster that the relative wind continuously because your microslipping is such a short time duration. |
| Brumby:
--- Quote from: gnuarm on December 23, 2021, 04:01:22 am ---Yes, don't assume zero losses. Assume some losses from the air resistance and rolling resistance. It doesn't need to be complex. Just consider that you can't move through the air with zero loss. --- End quote --- I suggest it is better to not complicate the key discussion with real world minutiae. It is far more practical to work out the basic principles using ideal elements. That is, to work out if the desired result is possible. Once that is established, then add in the real world losses to see what sort of experimental result you can expect. If it can't be found in the ideal situation, then there's no point in even considering losses. |
| IanB:
Actually, the analysis does not change if you introduce friction losses. All that happens is that any losses due to friction are overcome by the power of the motors driving the belts. You just have to assume that the motors are powerful enough to achieve the stated belt speeds, and that the wheels of the cart do not slip on the belts. |
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