General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Brumby:
--- Quote from: electrodacus on December 21, 2021, 06:07:52 pm ---I need to bring back the discussion to what is relay important.
What is the available wind power to direct down wind vehicle.
I say that available wind power is defined by this equation 0.5 * air density * area * (wind speed - vehicle speed)^3
If anyone disagree with that please provide the correct equation so we can compare the predictions.
--- End quote ---
For a passive sail, then you may be right, but for the Blackbird, I believe you need something extra:
0.5 * air density * area * (wind speed - vehicle speed + speed of propeller thrust)^3
bdunham7:
--- Quote from: Brumby on December 22, 2021, 05:09:19 am --- + speed of propeller thrust)^3
--- End quote ---
Don't go there! Thrust is force and force doesn't have a speed, although I can understand what you are getting at. I think what you want is the effective pitch of the propeller blades multiplied by its angular velocity. So if the pitch, which can be stated as distance per revolution, is 0.1m/rev and the propeller is spinning at 100rpm, then it's pitch speed (probably the wrong term) is 10m/s.
Brumby:
--- Quote from: bdunham7 on December 22, 2021, 06:12:39 am ---
--- Quote from: Brumby on December 22, 2021, 05:09:19 am --- + speed of propeller thrust)^3
--- End quote ---
Don't go there! Thrust is force and force doesn't have a speed, although I can understand what you are getting at. I think what you want is the effective pitch of the propeller blades multiplied by its angular velocity. So if the pitch, which can be stated as distance per revolution, is 0.1m/rev and the propeller is spinning at 100rpm, then it's pitch speed (probably the wrong term) is 10m/s.
--- End quote ---
WHOA! I'm just looking at the OUTPUT of the propeller - which is air moving at a non zero speed.
I'm not talking about how that is calculated. That is not important for this equation, just that it is the missing part of the formula which was presented. Our friend asked where that formula was wrong and I have obliged.
Brumby:
Besides, I use the word "thrust" here simply as a descriptive term. If you want something less controversial, then I could have said:
0.5 * air density * area * (wind speed - vehicle speed + speed of the air being blown backwards by the propeller)^3
electrodacus:
--- Quote from: Brumby on December 22, 2021, 06:41:54 am ---Besides, I use the word "thrust" here simply as a descriptive term. If you want something less controversial, then I could have said:
0.5 * air density * area * (wind speed - vehicle speed + speed of the air being blown backwards by the propeller)^3
--- End quote ---
There is nothing you need to do to that equation. The correct one is the one I presented.
The extra bit that is missing the the stored energy. The propeller was powered by wind power when well below wind speed then as the wind power decreased the stored energy started to provide most of the power to vehicle.
The way that stored energy is calculated is a bit complicated but it can be done as I already did in the spreadsheet calculator.
Say 60% of wind power is sent to propeller (from the wheel) then the other 40% gets to accelerate the vehicle increasing the vehicle kinetic energy and obviously speed.
The 60% that gets to propeller will be contributing to increased pressure differential (of course propeller may be 50% efficient so half of that 60% ends up as stored energy) You can see this increase in pressure differential about the same way as if natural wind speed has increased.
In the calculator all that energy put in the propeller increases the potential wind energy and with such a mechanism 2x even 3x wind speed is not a problem but since this is stored energy as soon as it is all used up vehicle will slow down below wind speed.
All that it will need to be done is to run the experiment fully not stop the experiment just before vehicle will start to slow down.
So if anyone has a treadmill and one of those small blackbird models you can test by increasing the vehicle weight. If vehicle weight is increased enough so that vehicle starts to slow down before vehicle gets to the end of the treadmill then you can show the entire cycle of charge peak speed and then slow down.
Likely some people that did this test and saw the vehicle slow down very soon after release where thinking their vehicle is just not build well enough.
As for direct upwind that can be tested with wheels only no propeller or air is needed to demonstrate that but a relatively high speed camera at least 120fps will be good to see any direct upwind vehicle and the fact that there is a charge discharge cycle multiple times per second no matter if it uses a belt or gears.
I showed those in my video but for those that think my test setup was not done properly do your own setup and take a high speed video.
You may never understand the theory or equations but you can at least see the real test.
It is a shame that this problem is wrongly presented in some schools and I wish I could do something about it. I was much more optimistic about being able to explain this but it seems so far I'm failing to do so.
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