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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: BrianHG on December 16, 2021, 11:33:54 am ---How many here would like to see that happen?
--- End quote ---

I'd like a ticket for this, please. 

Are you building something?  Just don't use a rubber-band instead of gears or chain-drive.  We wouldn't want any of that "stick-slip hysteresis energy storage" to cloud the discussion.

electrodacus:

--- Quote from: Psi on December 16, 2021, 08:12:01 am ---
--- Quote from: electrodacus on December 16, 2021, 03:15:09 am ---No vehicle without an external energy source or energy storage device can exceed wind speed direct down wind.

--- End quote ---

Then your argument has nothing to do with blackbird, you're trying to argue against the accepted scientific facts about sailing boats.

The fact that the boat achieves sailing directly downwind faster than the wind from point A to B through a series of indirect zig-zags is irrelevant, since the length of each zig/zag has no effect on mechanism causing faster than wind travel. 
If the sail boat could change direction in an instant with no time needed to stop, rotate and move the sails, then it could move downwind faster than the wind through a series of zig-zags that were so tiny that they were imperceptible and it would, in effect, be a straight line downwind faster than the wind.

--- End quote ---

Notice the very important direct downwind or direct upwind in my statement.
While you zig zag a sailboat you can charge the kinetic energy of the boat at higher levels and then you can turn straight and continue for some limited amount of time determined by friction losses and amount of kinetic energy you acquired during travel at an angle to the wind direction.

It is just incorrect to think propeller blades can use wind because they are at an angle as the analogy with a sail traveling at an angle does not exist.
It is a problem of geometry. If you are traveling at 45 degree to the wind direction then wind can still get to your sail as while your speed is above wind speed your speed relative to the wind is not as you take the long route while the wind keeps the direct route.
In case of propeller on a vehicle traveling directly downwind since propeller is fixed to the vehicle no wind can catch any of the blades when vehicle is at wind speed and above.

IanB:
We have to put this in terms with no room for misunderstanding.

If the wind is blowing from east to west at 20 mph, then a boat can sail 20 miles due west in less than one hour.

electrodacus:

--- Quote from: Kleinstein on December 16, 2021, 09:21:06 am ---That is the power theoretical available to a wind turbine on the vehicle, but not the power needed to more the vehicle. So a "correct" formular. but for a different problem.

--- End quote ---
We are discussing the best case here and ignoring frictional losses. And so even in this ideal case no more wind power is available to vehicle than that is calculated with that formula.



--- Quote from: electrodacus on December 16, 2021, 03:15:09 am ---0.5 * 1.2 * 0.5 * (64)^3 = 78.6kW of drag with just 0.3kW on input power.
How much more extreme you need the values to be in order to seems shocking.

--- End quote ---
I am quite shocked by this result - not by the number, but by the units:  the drag is a force and not a power ! :horse:
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Feel free to use force if that is your favorite but that is how mistakes are made. I personally prefer to use power as it already includes the speed also and there can be no confusion.
The results are correct and they should be shocking as you need 78.6kW to drive at 230km/h with no wind and a 0.5m^2 equivalent frontal area
Same will be needed if you only want to move with 1km/h upwind and with a wind speed of 229km/h. There is no difference in power needed if air moves or you move through the air.
Every time wind speed doubles the power needed to overcome drag increases 8x
Why do you think a bike rider capable of 300W can only drive with no wind at 36km/h or so ?  It is the air resistance and if you could move that you can get to almost any speed on a bicycle.
There is no difference between you moving through air or heaving a similar speed headwind. Bicycles are mechanically super efficient as much as 95% so the limitation is mostly the air.

electrodacus:

--- Quote from: BrianHG on December 16, 2021, 11:33:54 am ---Ok, give me a vote to go ahead and flip 'electrodacus' head this weekend making him convince himself that a wind powered craft going faster than a tail wind speed with his own logic...

How many here would like to see that happen?

--- End quote ---

I will be very grateful to you if you can manage that as I will learn something that I got wrong.  Unfortunately for you chances are very slim you will be able to do that.
Keep in mind what we are discussing here is blackbird that claims to travel directly downwind faster than wind and so there is no zigzaging to be able to use kinetic energy as energy storage device.
I never claimed Blackbird can not exceed wind speed directly downwind as it clearly can in real world tests including the treadmill model.  My claim is that it can only do that because it can store energy while below wind speed and then that stored energy is what is used to exceed wind speed for a limited amount of time. All my equations perfectly explain what is seen in real world tests.
The formula Derek used is not even close in explaining the real world results as it will predict ever increase acceleration rate rather than the observed slowing down of acceleration rate as you will expect from a energy storage device that it is being discharged. 

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