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 Notices Sadly I have to announce that Locknar is leaving the moderating team. He's contributed massively to keeping this place going over the years. Thanks for all your hard work especially dealing with the new registrations (yeah really thanks for leaving me with that!)

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 14th October 2010, 10:57 AM #3241 quarky Banned   Join Date: Oct 2007 Posts: 20,121 Or, is my math wrong? Any reply appreciated. I hate to walk around with a delusion in my head.
 14th October 2010, 02:03 PM #3242 Ririon Cool cat     Join Date: Jan 2005 Posts: 2,059 Originally Posted by quarky Or, is my math wrong? Any reply appreciated. I hate to walk around with a delusion in my head. There was no math in that post. It was in your head, so it is somewhat difficult to decide how correct it was. As for very high speeds.. You get more air resistance (proportional to the square of the speed of the cart through the air.), so my WAG would be that the process would be less efficient, and you would end up with a lower multiple of the wind speed. That had a little math in it. __________________ Engineer by day, scientist by night.
 14th October 2010, 05:51 PM #3243 quarky Banned   Join Date: Oct 2007 Posts: 20,121 Originally Posted by Ririon There was no math in that post. It was in your head, so it is somewhat difficult to decide how correct it was. As for very high speeds.. You get more air resistance (proportional to the square of the speed of the cart through the air.), so my WAG would be that the process would be less efficient, and you would end up with a lower multiple of the wind speed. That had a little math in it. Thanks. My unspoken math was the power/speed ratio (cubic), against the squared losses.
 19th October 2010, 06:24 AM #3244 quarky Banned   Join Date: Oct 2007 Posts: 20,121 Still not sure if its wrong-think. I'm missing something.
 19th October 2010, 11:15 AM #3245 Ririon Cool cat     Join Date: Jan 2005 Posts: 2,059 Originally Posted by quarky Still not sure if its wrong-think. I'm missing something. If I read you correctly, you are saying that the efficiency of the propeller is (roughly) proportional to the cube of the speed. We seem to agree that the losses to air resistance is (roughly) proportional to the square of the speed. (Not getting into the finer points of which speeds we are talking about right now.) I'd like to see an explanation for the cube factor. I think that is where the dog i buried. (Is that an expression in English?) __________________ Engineer by day, scientist by night.
 19th October 2010, 01:21 PM #3246 my_wan Graduate Poster     Join Date: Feb 2007 Posts: 1,074 The question asked is rather an interesting one, and I don't have the answer. What can be said is that a craft speed between 0 and wind ground speed you actually gain energy, by the wind/ground speed difference. This is because the craft has a tail wind up to that speed, with a wheel coupled to the prop so the effective power doesn't fall off as the craft approaches wind ground speed. The effective tail wind even increases with speed up to ground wind speed, because the prop air speed is counter to the wind. Once the craft exceeds wind ground speed, it begins expending that energy differential against a head wind. It doesn't expend it linearly because the increased speed also increases the prop exhaust speed. Presuming a simplistic conservation effect I presume this is why it exceeds twice wind speed. So I don't know what a maximum craft speed would look graphed against variations in ground wind speed, at various cross sectional wind resistance efficiencies. Even the efficiency of the prop varies with speed. It's far from as simple as it looks from squaring wind velocities. To many feedback and variable efficiency variances. My best guess is that, if you changed gearing ratios to maximize for a given wind ground speed, the higher the wind speed the greater multiple of that speed you could exceed. Just a guess based on the prop continuing to increase wind output speeds even during the power use phase, i.e., at or above ground wind speed. Also note that, without drag, any small force over enough time can accelerate any large object to any speed respecting Special Relativity. __________________ Peace to all people of the world. The evidence indicates that this is best accomplished through a skeptical approach.
 19th October 2010, 08:33 PM #3248 RossFW Muse     Join Date: Dec 2008 Posts: 896 Originally Posted by my_wan The question asked is rather an interesting one, and I don't have the answer. What can be said is that a craft speed between 0 and wind ground speed you actually gain energy, by the wind/ground speed difference. This is because the craft has a tail wind up to that speed, with a wheel coupled to the prop so the effective power doesn't fall off as the craft approaches wind ground speed. The effective tail wind even increases with speed up to ground wind speed, because the prop air speed is counter to the wind. Once the craft exceeds wind ground speed, it begins expending that energy differential against a head wind. It doesn't expend it linearly because the increased speed also increases the prop exhaust speed. Presuming a simplistic conservation effect I presume this is why it exceeds twice wind speed. So I don't know what a maximum craft speed would look graphed against variations in ground wind speed, at various cross sectional wind resistance efficiencies. Even the efficiency of the prop varies with speed. It's far from as simple as it looks from squaring wind velocities. To many feedback and variable efficiency variances. My best guess is that, if you changed gearing ratios to maximize for a given wind ground speed, the higher the wind speed the greater multiple of that speed you could exceed. Just a guess based on the prop continuing to increase wind output speeds even during the power use phase, i.e., at or above ground wind speed. Also note that, without drag, any small force over enough time can accelerate any large object to any speed respecting Special Relativity. My Wan, There is no change of "Mode" between less than wind speed and greater than windspeed. You correctly state that below windspeed, the cart takes energy from the differential between the speed the wheels go over the ground and the speed the propellor goes through the air. That differential still exisit above wind-speed. At all times, the cart's groundspeed is greater than it's airspeed, and that is where, by the gearing between the wheels and the advance ratio of the prop, it gets the energy to provide thrust. Above windspeed the only thing that changes is drag, both from the increased friction of the mechanism, and the aerodynamic drag of the now present headwind.
 20th October 2010, 11:36 AM #3249 my_wan Graduate Poster     Join Date: Feb 2007 Posts: 1,074 Originally Posted by RossFW My Wan, There is no change of "Mode" between less than wind speed and greater than windspeed. Yes, you are correct. The "mode" change was merely a useful definition, not any sort of change in the mechanics of the system. As a power graph to speed ratio, it merely marked the point were the difference in power used to power available was at a maximum. Which corresponds to a speed at which wind ground speed is (closely) matched. Merely a useful fiction to call it a change of mode, since it's in some ways easier to model if the math model starts at that point. __________________ Peace to all people of the world. The evidence indicates that this is best accomplished through a skeptical approach.
 21st October 2010, 10:25 AM #3250 quarky Banned   Join Date: Oct 2007 Posts: 20,121 I remain uncertain, but I feel a little better about it.
 21st October 2010, 10:54 AM #3251 Skwinty Philosopher     Join Date: Jun 2008 Posts: 5,593 Originally Posted by quarky I remain uncertain, but I feel a little better about it. Listen to some Depeche Mode, it may help. __________________ What is reality? Nothing but a collective hunch. --Lily Tomlin

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