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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| fourfathom:
--- Quote from: Brumby on December 22, 2021, 08:10:01 am ---Again, you are aware of the phenomenon, but you continually misrepresent it! The "pressure differential" is thrust. This thrust is not stored energy - it is continuously created moment by moment. Disconnect the drive shaft and it will cease as soon as the rotational momentum of the propeller is spent. --- End quote --- In the electrodacus theory, this pressure differential is stored and accumulated while the vehicle is below wind speed (sort of like inflating a balloon behind the vehicle). As the vehicle reaches wind speed, the balloon begins to deflate, blowing the vehicle above windspeed for a while. Once the balloon is deflated the vehicle slows down until it is again under windspeed. I don't buy it. There is no stored pressure differential. |
| IanB:
--- Quote from: Brumby on December 22, 2021, 02:28:56 pm ---Then you are saying there is a fundamental problem with our friend's original formula? --- End quote --- You can see in post #1012 where I showed the more correct formula for wind power on a simple sail going downwind. Surprisingly, it turns out the maximum efficiency is only about 15%. With a wind turbine, the common calculation is to consider the total rate of wind kinetic energy flowing through the swept area of the turbine blades (the energy flux), and then consider how much of that energy can be captured by the blades, which leads to the Betz limit. With a sail, no kinetic energy can flow "through" the sail, so the calculation instead needs to consider the wind force on the sail multiplied by the speed that the sail is moving (power = force x velocity). With a sail, the transferred power is zero when the sail is stationary (no velocity), and it is zero when the sail is moving at the same speed as the wind (no force). Maximum power transfer appears to occur somewhere around one third of the wind velocity. |
| fourfathom:
An irrelevant question that has no bearing on the problem: Is the "sail" in that equation just a planar surface? Actual sailboat sails, even those used for DDW sailing, are actually cut to create a three-dimensional airfoil, and are trimmed (tensioned with ropes) to optimize airflow and create lift. So the efficiency numbers may be different than the Bernoulli numbers. |
| Kleinstein:
--- Quote from: Brumby on December 22, 2021, 02:28:56 pm ---Then you are saying there is a fundamental problem with our friend's original formula? --- End quote --- The problem is in what the formula stands for: The (w-v)³ type formular is for the power available to a moving wind turbine (ignoring the power needed of vailable form the movement of the turbine). It is the the same as the normal ergy availabel to a wind turbine, just with a different wind speed. This is different from it is totally different from the power from a sail on the vehicle: that power is force times vehichle velocity as also shown in the calculation of IanB. So the 2 parts together may make more sense. Anyway the equation for the power a sail vehicle could use is not even relevant for the calculation that shows that the Backbird vehicle can work. With only using a formula for a sail based setup on can not calculate the prop driven vehincle, as this is something different. When you do the calculation for the simple (e.g. 1 D world directly downwind) sail driven vehicle the result is that it can not go faster than the wind. This still does not say anything about a sail driven vehicle going zig-zag. This is known to be able to go faster. I only took a more detailed look with the case of zero and low speed, to show that the 0.5 * air density * area * (wind speed - vehicle speed)^3 form is obviously wrong for a sail based vehicle. For some reason electrodacus has difficulties in accepting that he can be wrong with his equation. It is not the only point he is wrong, but I had hope he would recognize the error in a equation that is not really related (at least I don't see a good way to use it one way or the other) to the main question. With a failure to recognize even such a simple point, I think I will give up on convincing electrodacus. Not sure if he does not know better or is acting as a troll just to keep the discussion running in circles, repeating his wrong claims over and over again. The sail in the calculation is just the planar surface with wind perpendicular (down wind). So not sophisticated areodynamics included. |
| IanB:
--- Quote from: fourfathom on December 22, 2021, 05:01:29 pm ---An irrelevant question that has no bearing on the problem: Is the "sail" in that equation just a planar surface? Actual sailboat sails, even those used for DDW sailing, are actually cut to create a three-dimensional airfoil, and are trimmed (tensioned with ropes) to optimize airflow and create lift. So the efficiency numbers may be different than the Bernoulli numbers. --- End quote --- Yes, that derivation is for a "square" sail, perpendicular to the wind direction, sailing directly downwind. That is not, of course, representative of real sails on real boats with wind angles and keels and all sorts of other cleverness. It is just a theoretical exercise. Having said that, all analysis of real sails will involve force triangles and resultant vectors, and real and apparent wind velocities. In fact, not much different from the engineering analysis of the blades on a wind turbine. |
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