Author Topic: Mess with your minds: A wind powered craft going faster than a tail wind speed.  (Read 147260 times)

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Online fourfathom

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* There is no wind
* The propeller is powered by a battery

If the speed of the propeller wash is 1m/s, do you agree that the vehicle will move forward at around 1m/s?
Replacing the moving sail with a propeller will not be the same thing if the propeller is not powered by something.

This is a thought-experiment, OK?  The propeller is powered by a battery in this experiment.  The question is what will happen in this experiment.
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Offline Labrat101

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So propeller will still rotate but slower and slower same thing that actually happens after the vehicle used all the pressure differential and vehicle starts to slow down.
You keep forgetting 1 thing . This type of propeller has an integral clutch . Which means it can spin Free
with just wind force . only . so it can be used as a sail .(kind of ) When the clutch engages it can ether give
power to the wheels or from . Depending on the setting of the pitch angle of the Blades .
 
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Online fourfathom

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You keep forgetting 1 thing . This type of propeller has an integral clutch . Which means it can spin Free
with just wind force . only . so it can be used as a sail .(kind of ) When the clutch engages it can ether give
power to the wheels or from . Depending on the setting of the pitch angle of the Blades .

I believe we can analyze, or at least demonstrate this with a fixed-pitch, fixed-gearing propeller.  That's how the treadmill model is built.  I don't know if this model can start from a full-stop, but it does accelerate once past windspeed.
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Offline electrodacus

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I believe we can analyze, or at least demonstrate this with a fixed-pitch, fixed-gearing propeller.  That's how the treadmill model is built.  I don't know if this model can start from a full-stop, but it does accelerate once past windspeed.

That model will be able to start from a full stop assuming high enough wind speed so that power based on vehicle equivalent back area can overcome vehicle friction.
The thing you (all) seems to be ignoring is that air speed relative to vehicle is what can power the vehicle and so when air speed / wind is equal with vehicle speed in exact same direction then there is no way for the air to push the vehicle.
And somehow you imagine that propeller can push air backwards but that will require an energy source and wind is no longer there to help when vehicle is at or above wind speed.
You just think that you can take energy from the wheel but the problem with that is that when above wind speed that wheel no longer takes energy from the wind but will need to take energy from the vehicle kinetic energy and so vehicle will just slow down if you ignore a energy storage device like the pressure differential witch continues to push the vehicle basically taking the role of the wind for a few seconds or minutes depending on the design.
 

Offline Labrat101

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You keep forgetting 1 thing . This type of propeller has an integral clutch . Which means it can spin Free
with just wind force . only . so it can be used as a sail .(kind of ) When the clutch engages it can ether give
power to the wheels or from . Depending on the setting of the pitch angle of the Blades .

I believe we can analyze, or at least demonstrate this with a fixed-pitch, fixed-gearing propeller.  That's how the treadmill model is built.  I don't know if this model can start from a full-stop, but it does accelerate once past windspeed.
I am not sure how this can be done as both these propellers have totally different properties .Your know about boats . It would be like saying a life raft is the same as a boat or worse still a Ship . They both travel or go in water . (spot the difference)
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Offline Kleinstein

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I believe we can analyze, or at least demonstrate this with a fixed-pitch, fixed-gearing propeller.  That's how the treadmill model is built.  I don't know if this model can start from a full-stop, but it does accelerate once past windspeed.

That model will be able to start from a full stop assuming high enough wind speed so that power based on vehicle equivalent back area can overcome vehicle friction.
The thing you (all) seems to be ignoring is that air speed relative to vehicle is what can power the vehicle and so when air speed / wind is equal with vehicle speed in exact same direction then there is no way for the air to push the vehicle.
And somehow you imagine that propeller can push air backwards but that will require an energy source and wind is no longer there to help when vehicle is at or above wind speed.
You just think that you can take energy from the wheel but the problem with that is that when above wind speed that wheel no longer takes energy from the wind but will need to take energy from the vehicle kinetic energy and so vehicle will just slow down if you ignore a energy storage device like the pressure differential witch continues to push the vehicle basically taking the role of the wind for a few seconds or minutes depending on the design.

Yes the propeller to push back against the air and thus the wind does require some power and an energy source.  So this sounds like you agree that with a given power (from a battery or whatever source) the prop can drive the vehicle a little faster than the wind speed or at exactly windspeed (this is the case that is a bit easier to calculate).

Now comes the slightly tricky question: how much power is needed for the prop to create a given thrust (force) to overcome friction ?
 

Offline electrodacus

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Yes the propeller to push back against the air and thus the wind does require some power and an energy source.  So this sounds like you agree that with a given power (from a battery or whatever source) the prop can drive the vehicle a little faster than the wind speed or at exactly windspeed (this is the case that is a bit easier to calculate).

Now comes the slightly tricky question: how much power is needed for the prop to create a given thrust (force) to overcome friction ?

This sounds like a familiar question.  Why is important how much power is needed to overcome friction (not much for such a low friction vehicle like blackbird or even the treadmill model).  The important fact is that some power is needed and since wind can not do that when vehicle is above wind speed the only remaining option is the pressure differential energy storage.
And because friction is so low the vehicle will not start to slow down sooner and is able to be powered with that relatively small amount of energy for 2 or 3 minutes.

Offline Kleinstein

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The amount of power is important, as this decides if a movement at that speed is possible.

A suitable approximation for the power needed, when the vehicle goes at the speed of the wind would be a prop power proportional to the trust (may need a larger prop for more thust). This would be independent of the wind speed, as the prop does not see the wind speed.
 

Offline Alex Eisenhut

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The thing you (all) seems to be ignoring is that air speed relative to vehicle is what can power the vehicle and so when air speed / wind is equal with vehicle speed in exact same direction then there is no way for the air to push the vehicle.


The thing you seem to be ignoring is that the propeller is part of the vehicle and is blowing its own wind backwards. The effective wind speed is both.

Do you understand 2+2? Then why don't you understand 2 - (-2)?
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Online fourfathom

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[... fixed-pitch propellers ...]
I am not sure how this can be done as both these propellers have totally different properties .Your know about boats . It would be like saying a life raft is the same as a boat or worse still a Ship . They both travel or go in water . (spot the difference)

I'm not sure what you're saying here.  The fixed-pitch propeller drove the simple vehicle against the treadmill belt, faster than the (still air) wind.  Why can't this be used to analyze the principles?
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Offline IanB

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Do you understand 2+2? Then why don't you understand 2 - (-2)?

This is the problem. I made a post earlier, where I asserted something to the effect that two plus two equals four. electrodacus immediately responded by quoting that post and asking if I therefore agreed that two plus two does not equal four?

The guy is being vexatious, contrary, and nonsensical. He is playing games with everyone. There is no point trying to educate the guy with logic or rational arguments, because he is not trying to learn. He is just winding everyone up to see how long he can keep the thread going.

If you even get close to showing how a vehicle can sail downwind faster than the wind, as was happening a few posts back, electrodacus immediately changes the subject, or brings in something irrelevant to divert the thread.
 

Offline PlainName

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No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.

OK, so now we go back to scenario 2 and replace the moving sail with a static propeller. Recall that:

* There is no wind
* The propeller is powered by a battery

If the speed of the propeller wash is 1m/s, do you agree that the vehicle will move forward at around 1m/s?

Replacing the moving sail with a propeller will not be the same thing if the propeller is not powered by something.
And since you do not have a battery and you do not accept there is an energy storage device (pressure differential) then there is nothing that can power the propeller other than the kinetic energy of the vehicle but taking energy from that will result in reduced vehicle speed. So propeller will still rotate but slower and slower same thing that actually happens after the vehicle used all the pressure differential and vehicle starts to slow down.

Mate, why did you create a strawman? Well, obviously, it was to knock him down but, really, it's so blatant. You even quoted me where I said:

Quote
OK, so now we go back to scenario 2 and replace the moving sail with a static propeller. Recall that:

* There is no wind
* The propeller is powered by a battery

Can you grasp that? The propeller is using B.A.T.T.E.R.Y. power to turn, so the rest of your post knocking down your strawman is completely irrelevant.

So, once again and PLEASE don't jump ahead, make things up or change the subject:

With a propeller fixed in position, powered solely by battery and with no wind, if the prop wash is thrust back at 1m/s, do you NOW agree that the vehicle will move forward at around 1m/s?

Also, please bear in mind that this actual experiment is carried out across the world at all times of day an night, so if you still disagree I will be expecting an extraordinary explanation as to why.
 

Offline cdev

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Its the total power that matters I wouldthink. A strong wind can drive a turbine very fast, and then something like an automatic transmission might be used to propel a small payload very fast. Suppose it was running a generator and you were using that to power an eectric motor and wheels, its quite possible. There is even a kind of boat that drives a cylindrical vertical rotary shaft made of two offset cylinders acting as a wind turbine. These boats are able to propel themselves in any direction, just like a motor boat could, as long as the wind is blowing in any direction. (Does anybody know the name for this invention, which is at least 100 yrs old.

Against the wind already would be possible with a craft like this, as long as the drag force of the blades is higher than the craft, it would be working like a windmill,
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Offline electrodacus

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The thing you seem to be ignoring is that the propeller is part of the vehicle and is blowing its own wind backwards. The effective wind speed is both.

Do you understand 2+2? Then why don't you understand 2 - (-2)?

What powers the propeller when vehicle is above wind speed ? It is a simple question for with you have no answer.

Offline electrodacus

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Mate, why did you create a strawman? Well, obviously, it was to knock him down but, really, it's so blatant. You even quoted me where I said:

Quote
OK, so now we go back to scenario 2 and replace the moving sail with a static propeller. Recall that:

* There is no wind
* The propeller is powered by a battery

Can you grasp that? The propeller is using B.A.T.T.E.R.Y. power to turn, so the rest of your post knocking down your strawman is completely irrelevant.

So, once again and PLEASE don't jump ahead, make things up or change the subject:

With a propeller fixed in position, powered solely by battery and with no wind, if the prop wash is thrust back at 1m/s, do you NOW agree that the vehicle will move forward at around 1m/s?

Also, please bear in mind that this actual experiment is carried out across the world at all times of day an night, so if you still disagree I will be expecting an extraordinary explanation as to why.

What are you even talking about ?
Why include a battery in the equation when you claim no energy storage is available to vehicle.
Maybe you need to add the battery as else you have no explanation?

For the vehicle to maintain 3m/s with a 2m/s tail wind the battery will need to provide enough power to cover all frictional losses.
Now please explain what will replace your fictional battery in the real blackbird?

Offline PlainName

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Quote
Why include a battery in the equation

Because we are taking this little step by little step. No massive jump from A to 99, so when we disagree it will be a small specific thing that is easily resolved one way of the other. But we can't do that if you keep taking a running jump to the other playing field, see.

Now, the purpose of having this thing run off batteries is so we are not up against the problem of what powers the thing. We will resolve that later, step by step, but the purpose of these steps right now is to cut away all the distractions.

Quote
For the vehicle to maintain 3m/s with a 2m/s tail wind the battery will need to provide enough power to cover all frictional losses.

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?
 

Offline electrodacus

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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?

It is obvious I will agree. You can drive at almost any speed you want if you have a battery.
So what is next ?

Online fourfathom

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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?

It is obvious I will agree. You can drive at almost any speed you want if you have a battery.

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.
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Offline electrodacus

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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.

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.

Offline Domagoj T

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What powers the propeller when vehicle is above wind speed ? It is a simple question for with you have no answer.

The energy which is extracted from slowing down the wind.

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.
 

Offline Kleinstein

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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.

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.

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.
 

Offline IanB

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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.

« Last Edit: December 26, 2021, 05:30:11 pm by IanB »
 

Offline PlainName

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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?

It is obvious I will agree. You can drive at almost any speed you want if you have a battery.
So what is next ?

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.
 
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Offline electrodacus

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The energy which is extracted from slowing down the wind.

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?

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.

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.

Offline electrodacus

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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.

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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