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

--- Quote from: bdunham7 on December 27, 2021, 04:51:55 am ---
--- Quote from: electrodacus on December 27, 2021, 04:36:41 am ---This is not a question of general knowledge is a question of logic alone. So you did not need to learn this in school to understand the two cases are the same as far as drag is concerned.

--- End quote ---

I don't know what you should and shouldn't learn in school, but I do agree that the question is extremely simple.  And you have simply gotten it wrong.  The drag is the same, the power is not.  Drag is a force and is the same in each case.  Thus when I pedal the bicycle, I will have to apply a certain amount of force to the pedals to counter that drag.  If I'm going 20km/h in still air however, I have to pedal twice as fast with the same force as I do pedaling an identical bicycle at 10km/h into a 10km/h wind.  Same force, twice as fast.  Twice the power.  Extremely simple.  No calculus.  Everybody understands it except you.

--- End quote ---

Ah, just saw this part that emphasized "power" vs. force.  Got it.
IanB:

--- Quote from: gnuarm on December 27, 2021, 03:58:13 pm ---I don't want to go through the entire history of this.  Why would that be true?  Are they taking into account the other losses like tire drag?  The air drag would only be a matter of bike vs. air, no?

--- End quote ---

This is not about rolling resistance or any other extraneous factors.

There is a simple formula for the mechanical power of a moving vehicle:

  power = force ("volts") x speed ("amps")

In SI units you get force [N] x speed [m/s] = power [Nm/s = J/s = W]

The force in this case comes from resistance to movement of the vehicle, such as aerodynamic drag. The speed is how fast the vehicle is moving (no movement, no power).

The aerodynamic drag is proportional to the square of the effective wind velocity as seen by the vehicle. The effective wind velocity is the sum of the vehicle speed and the wind speed.
gnuarm:

--- Quote from: IanB on December 27, 2021, 06:20:19 am ---
--- Quote from: electrodacus on December 27, 2021, 01:10:09 am ---Thus this sort of wrong understanding is fairly common as it ended up in Wikipedia.

--- End quote ---

But every physicist in the world, every engineer in the world, every textbook in the world, agrees with that formula in Wikipedia. So are you going to tell the whole world they have been getting it wrong for the past 200 years and re-write all the physics textbooks?


--- Quote from: electrodacus on December 27, 2021, 04:36:41 am ---This is not a question of general knowledge is a question of logic alone. So you did not need to learn this in school to understand the two cases are the same as far as drag is concerned.

--- End quote ---

This is interesting. That formula that appears in Wikipedia is not derived from logic. It is derived from experiment. Many experiments. Over the years, scientists and experimenters measured the amount of power required in different situations, and they found that all of their experiments match the formula given by Wikipedia. What is more, you can repeat those experiments yourself, and if you do so, you will also obtain results matching that formula.

--- End quote ---

Yeah, but it doesn't take into account the stored energy...  ;-)
gnuarm:

--- Quote from: Kleinstein on December 27, 2021, 09:19:51 am ---
--- Quote from: electrodacus on December 27, 2021, 04:55:42 am ---Also if you double the speed drag power increases 8x not 2x or even 4x

--- End quote ---

It is darg force, not power.


--- Quote from: electrodacus on December 27, 2021, 04:23:04 am ---
--- Quote from: bdunham7 on December 27, 2021, 04:20:18 am ---
No, they got it exactly right here and we've all been telling you the exact same thing....

--- End quote ---
OK then why that second formula that should represent the same thing provide a different value ?

--- End quote ---
The problem is not with the 2nd formular, but with the 1st.
If to claculations don't agree one (at least one) has to be wrong (or using different approximations). If you get one of the formulas from text books and many sources chances are that that's the good one and the other is the bad one.
For the second formular there is a simle derivation:  The drag force is proportional to relative wind speed squared and the mechanical power is force times relative speed (in this case bicycle relative to ground).

--- End quote ---

Why is anyone concerned about drag?  I'm being serious.  On the blackbird the speeds are low enough that the drag has to be pretty minimal.  The issue is more one of showing the forces to create acceleration once you reach wind speed.  No?  The forces from the propeller are going to be much greater than wind drag at 15 mph. 
IanB:

--- Quote from: gnuarm on December 27, 2021, 04:15:43 pm ---Why is anyone concerned about drag?  I'm being serious.  On the blackbird the speeds are low enough that the drag has to be pretty minimal.  The issue is more one of showing the forces to create acceleration once you reach wind speed.  No?  The forces from the propeller are going to be much greater than wind drag at 15 mph.

--- End quote ---

You are right to ask this. When examining a wind powered vehicle, it is necessary to consider the balance of forces on the vehicle. The resulting net force will show what direction it moves and how fast it will eventually go. The limiting speed is where drag comes in, later on.
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