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Mess with your minds: A wind powered craft going faster than a tail wind speed.

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bdunham7:

--- Quote from: electrodacus on December 21, 2021, 06:07:52 pm ---I have never claimed blackbird is not going faster than the wind.

--- End quote ---

But you are claiming that it cannot sustain the faster-than-wind travel.  Or are  you beginning to realize that you are wrong?


--- Quote ---I say that available wind power is defined by this equation  0.5 * air density * area * (wind speed - vehicle speed)^3

--- End quote ---

As I said earlier, the 'available wind power' does not depend on the vehicle speed where the vehicles mechanisms can use the ground as a fixed reference, only the windspeed relative to the ground matters.  So the prediction is that a properly configured vehicle can travel both upwind (at some rate) and downwind (faster than the wind) indefinitely.  And, it does exactly that experimentally.

electrodacus:

--- Quote from: Kleinstein on December 21, 2021, 06:23:27 pm ---One may call the pressure that drives the prop forward stored energy, if one really wants too. However this a energy in a dynamic balance: loosing energy to drive the vehicle and to the less than perfect aerodynamics and getting new energy from the rear wheels.

--- End quote ---

I do not see how else you can call the pressure differential created by the propeller other than a form of energy storage.
There is loop but it is as follow (proportions are just an example).
Wind power available to vehicle will be split say in two equal parts.  One half accelerates the vehicle directly thus ends up as kinetic energy the other half will be taken from wheels and sent to propeller witch then increases the pressure differential (you can look at this as increasing the apparent wind speed relative to vehicle).
So for a sail only vehicle all available wind power is converted to kinetic energy thus it will accelerate much faster than blackbird.
Since there is no other energy storage device other than kinetic energy all potential wind energy is converted to kinetic energy in case of sail cart and so that can never exceed wind speed unless it is not driving directly down wind.
For blackbird since it takes in this example half of the available power and stores that as pressure differential basically increasing the available potential energy it allows the blackbird to exceed wind speed for some limited amount of time as when above wind speed direct downwind there is no longer any wind power available to vehicle and it is starting to use the energy stored as pressure differential but it will continue to use just half of the power provided by the pressure differential to accelerate (increase kinetic energy) and then the other half it will put back in to increasing the pressure differential.
Obviously since only half is put back the overall pressure differential will drop and vehicle acceleration rate will continue to drop until there is not enough and it will need to start to slow down.



--- Quote from: Kleinstein on December 21, 2021, 06:23:27 pm ---The point is that the driven prop can get you more drag than a sail. It can do it a zero speed (e.g. no wind and not moving and thus zero drag for the sail) - this case is the abvious one I think.  Similar it can do it when the vehicle is driving at the speed of the wind and thus zero relativ speed between the vehicle and the wind.

When starting at low speed the slow moving prop would still have some drag, not much, but it should be enough to get it starting. The prop would need not extra power and at low speed the prop would actually generate extra forward power and act as a windmill. It would need some minimum wind speed to get it going.

I don't think they needed the adjustable pitch during driven. This is likely just a thing of finding the best setting to get the highest speed. Once set right it should be OK to keep it there. The prop is not very heavy and not going very fast, as it is optimized to work at a relatively low relativ air speed. Remember it only sees the difference, so normally not the full wind speed. So the kinetic energy in the prop is low compared to the kinetic energy in the main vehicle. The model on the treadmil works with a fixed prop.

--- End quote ---

Yes as shown by the treadmill model the variable pitch is not needed for this to work.

electrodacus:

--- Quote from: bdunham7 on December 21, 2021, 06:30:46 pm ---But you are claiming that it cannot sustain the faster-than-wind travel.  Or are  you beginning to realize that you are wrong?

--- End quote ---

Yes that is correct the acceleration rate as seen in all tests drops and at some point acceleration rate will be zero and the deceleration phase will start.


--- Quote from: bdunham7 on December 21, 2021, 06:30:46 pm ---
--- Quote ---I say that available wind power is defined by this equation  0.5 * air density * area * (wind speed - vehicle speed)^3

--- End quote ---

As I said earlier, the 'available wind power' does not depend on the vehicle speed where the vehicles mechanisms can use the ground as a fixed reference, only the windspeed relative to the ground matters.  So the prediction is that a properly configured vehicle can travel both upwind (at some rate) and downwind (faster than the wind) indefinitely.  And, it does exactly that experimentally.

--- End quote ---

Please provide the equation describing what you are saying. If you can not do that you can not claim you understand how this vehicle works.
It is super obvious that vehicle speed while driving directly down wind affects the amount of available wind power since the wind speed relative to vehicle drops (wind speed - vehicle speed).

fourfathom:

--- Quote from: electrodacus on December 21, 2021, 06:42:32 pm ---
--- Quote from: bdunham7 on December 21, 2021, 06:30:46 pm ---But you are claiming that it cannot sustain the faster-than-wind travel.  Or are  you beginning to realize that you are wrong?

--- End quote ---

Yes that is correct the acceleration rate as seen in all tests drops and at some point acceleration rate will be zero and the deceleration phase will start.


--- Quote from: bdunham7 on December 21, 2021, 06:30:46 pm ---
--- Quote ---I say that available wind power is defined by this equation  0.5 * air density * area * (wind speed - vehicle speed)^3

--- End quote ---

As I said earlier, the 'available wind power' does not depend on the vehicle speed where the vehicles mechanisms can use the ground as a fixed reference, only the windspeed relative to the ground matters.  So the prediction is that a properly configured vehicle can travel both upwind (at some rate) and downwind (faster than the wind) indefinitely.  And, it does exactly that experimentally.

--- End quote ---

Please provide the equation describing what you are saying. If you can not do that you can not claim you understand how this vehicle works.
It is super obvious that vehicle speed while driving directly down wind affects the amount of available wind power since the wind speed relative to vehicle drops (wind speed - vehicle speed).

--- End quote ---

The applicable equation is some_factor * air density * area * (wind speed - ground speed)^3 

Since ground speed = 0, the equation becomes:
some_factor * air density * area * (wind speed)^3

The difference is that your equation assumes that the vehicle has no contact with the ground, and behaves something like a balloon.  This is certainly not the case with the DDWFTTW vehicles we ae discussing.

bdunham7:

--- Quote from: electrodacus on December 21, 2021, 06:42:32 pm ---Please provide the equation describing what you are saying. If you can not do that you can not claim you understand how this vehicle works.

--- End quote ---

I have and I just did.  Simply remove '-vehicle speed' from your equation.  And I can claim anything I like, so I'm going to claim that power has very little to do with how it works.


--- Quote ---It is super obvious that vehicle speed while driving directly down wind affects the amount of available wind power since the wind speed relative to vehicle drops (wind speed - vehicle speed).

--- End quote ---

Yes, at first glance it seems 'super obvious', but that's the whole point that makes it interesting--the 'super obvious' answer is wrong.  And that has been demonstrated and explained in detail--and understood clearly by everyone but you, as far as I can tell.  Do  you think that YouTube and EEVBlog, or the entire world for that matter, is gaslighting you? 

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