General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Kleinstein:
--- Quote from: electrodacus on December 26, 2021, 03:13:20 am ---
--- Quote from: fourfathom on December 26, 2021, 03:02:34 am ---
That's not the point. The question is *how* *fast* will the vehicle move under the conditions above. 3 m/s, right? We are trying to find agreement on forces and speeds.
--- End quote ---
We just agreed on an arbitrary number of 3m/s and I said yes it will be possible. What else will you want me to answer ?
Battery is an energy source and since there is no electrochemical battery on Blackbird I look forward to see what that will be replaced with.
--- End quote ---
The next step is slightly more tricky: how much power is needed to drive the prop to create the 1 m/s air speed ?
The power would obvious depend on the size of the prop. In ideal world this would be comparable (maybe smaller by some factor somewhat smaller than 1, as neither the wind turbine nor prop are 100% efficient) to the power a wind turbine could produce.
IanB:
For my own interest, I made a geometrical model to show how a cart with appropriate gearing can travel in the opposite direction to the moving belts it is sitting on.
When the belts are moving to the right the cart moves to the left, and vice versa.
The works because one belt is moving twice as fast as the other, and the two wheels are the cart are connected by a geared pulley, so that both wheels turn together. The difference in belt speeds is used to make the wheels of the cart turn in the opposite direction of the belts.
PlainName:
--- Quote from: electrodacus on December 26, 2021, 01:22:17 am ---
--- Quote from: dunkemhigh on December 26, 2021, 12:51:51 am ---
Trust me, it is a fucking HUGE battery. But if it pleases you we can make it nuclear-powered instead, just in case you're worried about the thing running out of charge.
So, are we at this point agreed that, ignoring how the propeller is powered, the vehicle with a 1m/s prop and a 2m/s tail wind will move forward at close to 3m/s?
--- End quote ---
It is obvious I will agree. You can drive at almost any speed you want if you have a battery.
So what is next ?
--- End quote ---
OK. Well sorry we have to take this a small step at a time but with all the distractions and other comments possibly confusing and obscuring things we do need to be able to reflect on where we were in order to make the next move.
Which, right now, is realising that the propeller is essentially equivalent to the moving sail. That is, just as the wind pushed against the sail moving backwards, the wind is pushing against the prop wash similarly moving backwards. Still with me?
The prop wash moving at 1m/s from a prop of radius r is pretty much the same as the sail moving backwards at 1m/s of area πr2. You OK with that? Speak up if not.
The other thing to ponder at this point is that the vehicle in our scenario is moving at 3m/s ground speed with a 2m/s wind. It follows, then, that if the vehicle was moving at only 2.5m/s there would be 0.5m/s of wind available to power things. Note that we are still using a battery as a power source so this is 'spare' wind power. Nevertheless, it shows that the vehicle as a whole can be moving faster than the wind yet still make use of wind power.
We can change this: travel at 2.1m/s and we have 0.9m/s available wind despite still going faster than the wind. But, again, we are still using battery power to turn the prop so this IS NOT powering the prop from the available wind. You will be dying to point out that the 0.9m/s we have available isn't quite the 1m/s the prop is putting out to make that available. In other words, this is not the explanation for the Blackbird.
OK so far? If you're still happy at this point we will make a sideways move and press on, but if you're not then it's important that we sort these two issues out here.
electrodacus:
--- Quote from: Domagoj T on December 26, 2021, 08:29:28 am ---
The energy which is extracted from slowing down the wind.
--- End quote ---
Yes that is a perfectly fine explanation for when vehicle is below wind speed. But how you do that when you are above wind speed and apparent wind speed changes direction?
--- Quote from: Domagoj T on December 26, 2021, 08:29:28 am ---On blackbird, the propeller is not a windmill, it's a literal propeller exactly like on an airplane that pushes the air backwards. Effective airspeed speed at the propeller is higher than windspeed, causing the airmass to slow down, which results in excess energy which is used to push the vehicle forwards.
You agreed that the yoyo toy can move faster than the string. Earlier I replaced a hand pulling the string with a sail system that is basically infinite. Sails always move slower than the wind, but the vehicle itself is faster. That model should be perfectly clear.
--- End quote ---
An airplane that has a propeller has also a internal engine and fuel to provide the energy needed.
Yoyo has always access to energy trough the string as long as the string is under tension and it can only do so as long as there is enough string so it has a limited range of motion.
It will be very similar to a Blackbird powered by a stationary generator (can even be a wind turbine) and then have a very long extension cord so it can only move as much as the extension cord allows.
Air is a fluid trough with your vehicle moves and while driving directly down wind with vehicle speed lower than fluid speed the fluid can push your vehicle but as soon as fluid speed and vehicle speed are the same the fluid can no longer provide any power to the vehicle and if by some chance your vehicle is above fluid speed the fluid will oppose your vehicle motion thus not only there is no way for your vehicle to accelerate but your vehicle will be slowed down.
And mathematically tat will be seen as the direction of the fluid motion will change relative to vehicle.
All that moves is the fluid relative to ground and vehicle also relative to ground and as soon as fluid and vehicle have the same speed there is no way for the vehicle to be powered by the fluid. What happens in case of blackbird is that instead of using all the available wind power to accelerate it splits that in two parts one for acceleration (so slower acceleration) and one to power the propeller witch will then store energy by creating a pressure differential (possible since air is a compressible fluid). This pressure differential is like a bubble that moves with the vehicle since it is maintained by the spinning propeller and this is what powers the vehicle even before wind speed is exceeded and will continue to do so above wind speed for a limited amount of time of course since the pressure differential will drop as there is no more wind power to continue to power that.
If you where to understand what air is exactly and how it is different from water (non compressible fluid) then you will understand why energy can be stored in air but not in water.
electrodacus:
--- Quote from: Kleinstein on December 26, 2021, 09:06:18 am ---
The next step is slightly more tricky: how much power is needed to drive the prop to create the 1 m/s air speed ?
The power would obvious depend on the size of the prop. In ideal world this would be comparable (maybe smaller by some factor somewhat smaller than 1, as neither the wind turbine nor prop are 100% efficient) to the power a wind turbine could produce.
--- End quote ---
Your question is not super clear. There is some energy needed to accelerate the vehicle to 1m/s based on vehicle weight and then after vehicle is at constant 1m/s speed for a real vehicle there will still be power needed to cover the friction losses. So the power for a real vehicle will never be zero.
That is even before considering the efficiency of the propeller.
So my answer is even at steady state you still need power to maintain (not continue to accelerate) 1m/s.
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