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Mess with your minds: A wind powered craft going faster than a tail wind speed.
gnuarm:
--- Quote from: IanB on December 23, 2021, 07:38:58 am ---
--- Quote from: electrodacus on December 23, 2021, 07:06:37 am ---That is exactly what it seems meaning there is no wind at all but since vehicle travel trough air at 120km/h the power that vehicle will need to overcome drag is the one I calculated using the same formula.
If vehicle was to drive at just 20km/h but with a headwind of 100km/h then it will need the same amount of power to maintain that speed.
--- End quote ---
Why do you keep going back to old things that we have proved to you are not true?
A normal person cannot ride a bicycle at 60 km/h. However, a normal person can comfortably ride a bicycle at 10 km/h against a headwind of 50 km/h. This is because the correct formula for power is (drag force) x (vehicle speed). If the vehicle speed is lower, the power required is less. This is common sense. It takes more power to go faster.
--- End quote ---
If you are talking about having no other losses than air drag and ignore ground effects of the air stream, you are completely right except that you are wrong.
If you are including other losses such as tire losses, then yes, there is an element of lower resistance since the ground speed is lower. The calculator you show below includes tire losses and has an input to select the type of tire.
--- Quote ---You can see this for yourself at the bicycle calculator we previously showed you: https://www.omnicalculator.com/sports/cycling-wattage
--- End quote ---
I think he is talking about only the wind effects, so he is right.
IanB:
--- Quote from: gnuarm on December 23, 2021, 05:10:47 pm ---I believe you said you could not analyze the power flow because it was non-existent in the ideal case. So I'm saying don't assume a perfectly ideal case and you can analyze the power flow.
--- End quote ---
I think you misunderstand. I said you cannot perform an analysis of that system using power flows as the basis, because that is not the correct analysis. The correct analysis is to use belt speeds, wheel speeds, and gear ratios, as I did. That analysis is the same regardless of any friction losses in the system.
Power requirements are an outcome of the calculations, not an input to the calculations.
IanB:
--- Quote from: gnuarm on December 23, 2021, 05:22:15 pm ---I think he is talking about only the wind effects, so he is right.
--- End quote ---
No. He said the power required to overcome wind resistance is the same when cycling at 60 km/h as it is when cycling at 10 km/h against a 50 km/h headwind. That is wrong. It takes much less power to go at 10 km/h.
(This has nothing to do with rolling resistance. This is only about wind effects.)
Kleinstein:
One can do the analysis based on power flow also with losses. It is also relatively easy. However one should not look at the power taken from the wind and the power transferred to the vehicle mass. The easy way is looking at the power to drive the prop and the power you can get from the wheels. The movement is possible if the wheels can generate more power than needed for the prop + the power needed to overcome friction.
The calculation is relatively easy for the vehicle going at the wind speed and the question is than of the wheels can generate more power than actually needed. With excess power available the vehicle could go faster than the wind too.
There is no need to care about energy storage - all is steady state.
There is no need to care about wind resistance / aerodynamics as the relative wind speed is zero in the calculation. Just take a given working point of the prop (a given thrust and the power needed for that).
There is no need to calculate the maximum available power (it is enough to show a way to get enough).
Diretly trying to calculate the maximum available power from the wind is tricky, as there are different ways to used the wind and not all have a well defined area and the like. Chances are that for a vehicle going faster than the wind this may be actually near infinite (e.g. getting to the speed of sound were some of the approximations fail), as more and more air mass is available with an ever faster speed.
Prooving that something is impossible can be quite tricky and would usually be of the type showing a violation of the laws or equavalence to something known to be impossible (e.g. a perpetu-mobile). The other way around is often much easier, as it only has to show 1 possible implementation and this does not even have to be a good one.
electrodacus:
--- Quote from: IanB on December 23, 2021, 07:38:58 am ---
A normal person cannot ride a bicycle at 60 km/h. However, a normal person can comfortably ride a bicycle at 10 km/h against a headwind of 50 km/h. This is because the correct formula for power is (drag force) x (vehicle speed). If the vehicle speed is lower, the power required is less. This is common sense. It takes more power to go faster.
You can see this for yourself at the bicycle calculator we previously showed you: https://www.omnicalculator.com/sports/cycling-wattage
If you keep going back and re-stating wrong things that have been corrected earlier, then this thread is going round in circles and cannot make progress. That is why we think you are trolling.
Insisting on something doesn't make you right, no matter how often you repeat it. In order to demonstrate the correctness of what you are saying, you have to be able to prove it with appropriate equations and logic, which is something you consistently fail to do. Moreover, you keep ignoring inconvenient facts when they go against your preconceptions.
--- End quote ---
Yes that bicycle calculator is incorrect (you are not the only one to get this wrong so that includes those people that did the calculator)
That calculator will say you can drive at 1km/h in 230km/h headwind with just 300W (easy for a cyclist)
This is maybe 80km/h no where near 230km/h and here is what happens
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