Re: OT Here is an example of pseudo science.

[Ronald Raygun]

You strung a lot of words together, but in a way which didn't make clear what you were trying to say. I'm unable to follow your argument because your words don't convey a clear statement of what facts you're establishing and what conclusions you're drawing from them.

The main thing is that you need to get the energy budget to balance. It doesn't matter which frame you do it in, but for each balancing exercise you have to stay in the same frame.

It would help if you used terminology which is obvious to understand and appropriate to the frame you're using. For example "real wind" or "true wind" is something which really only makes sense in the ground frame. The corresponding term for the cart frame would be "apparent wind" or "relative wind".

If you have a cart going at 30mph in a 20mph wind, i.e. the cart is

10mph faster than the wind, then the real wind is 20mph and the relative wind is -10mph. OK?

When you were saying the wind was not being slowed, did you mean the prop was just freewheeling? So that it was not changing the speed of the air at all? Then it would be consuming no power. That's not interesting.

What we want the prop to do is to provide thrust. To do this it has to slow down the wind, for example from its incoming -10mph to an outgoing -15 or even -20mph. You see that -20 is less than -10, which is why I call it "slowing down". You may prefer to think of it as speeding up from 10 to 20 (both backwards). That's fine, because after all for kinetic energy purposes we square the velocity so any minus sign will disappear.

It is clear that in this situation, and in the cart frame of reference, you are not reducing the relative wind's kinetic energy, but increasing it. So you can't harvest any energy from it. To make it work, the balance of kinetic energy has to come from somewhere else, and as I explained elsewhere, it comes from slowing down the ground.

That's why it's easier to work in the ground frame, where the ground does not move, but the real wind is slowed down (for example from 20mph to 15mph, or even to 10mph, or even to zero).

[Ronald Raygun]

I think he's pulling the legs up to make them defy gravity.

[Ronald Raygun]

But now let's go back a step.

I've shown that our hero can keep his 120W lamps lit by putting in only 75% of the power himself, and harvesting the other 25% by stealing

30J of kinetic energy from one pleb each second. But these ratios are specific to the assumption that he's reducing each pleb's speed to zero.

I've been a victim of my intuition by thinking that if I take as much kinetic energy as I can from the pleb (i.e. all of it), then that's the best I can do in terms of getting the pleb to contribute to the energy budget.

It's easy to see that if the hero were to push each pleb for 2s instead of

1s, that this would reduce the pleb's speed from 1 m/s to -1 m/s, and so the pleb ends up with the same kinetic energy he started with. That means there's no contribution available from the pleb, and the hero would need to do 100% of the work himself. He might as well leave the plebs alone and crank his generator by hand.

It gets worse. If he pushes for 3s, the pleb's KE ends up *more* than it was, and the hero has to do 125% of the work needed to keep his bulbs lit.

So let's go the other way. Push for only half a second. The pleb's speed is reduced by half, but his kinetic energy therefore by 3/4 and the hero needs to do only 62.5% of the work. And if we push for only 1/4 s, the hero needs to do 56.25% of the work.

So, if instead of stealing 100% of a pleb's speed each second, we instead stole 50% of the speed from two plebs each second, we'd get more energy out. If we take 1/N of the speed away from N plebs each second, we get the same thrust, but get more power from them, and in the limit, when we let N go to infinity, the hero ends up needing to do only 50% of the work.

But of course this would mean letting delta V go towards zero and there may not be enough plebs in the hero's immediate vicinity, and he'd need to spread his arms ever wider. This corresponds to making the propeller bigger and bigger.

With the benefit of hindthought (is there such a word?) this is obvious, since given that KE varies with the square of speed, the optimum *rate* of energy extraction is if the speed difference spans the steepest slope of the energy curve. That's near your starting speed.

In conclusion, gentlemen, I believe I've now shown that you were right and I was wrong, and so I apologise again for the tone I took.

It's true what they say. The best (well perhaps not the best, but a good) way to learn about something is to write about it.

[dennis]

That's because i am trying to convey simple physics to people that obviously have no understanding of physics. If they did understand physics I wouldn't need to do it.

Rubbish. Stick to one frame, you obviously have trouble when you change frames just like rick does.

Just like I said in mine except I chose 20 and 10. That is -10 and -10 referenced to the cart and hence to the prop. See the problem now? they are both the same, you are travelling at twice the wind speed and are not slowing the wind. You are not extracting any energy. This is possible with a perfect prop, there are no losses.

Now if you speed up the prop and the cart (I don't care how it doesn't matter) and you add 1 to the speed what has happened?

Of course its interesting, you are travelling at twice the wind speed and not consuming energy to do it, this is what a perfect prop does no losses. Its the equivalent of a machine that just accelerates air through a tube creating no turbulence, etc. (They don't exist but lets not stop reality getting in the way of physics).

No we want the prop to extract energy from the wind, we don't care how it does this.

So you feel its OK to just switch frames of reference to get the answer you want then? Do you go by the name rick or thin air when things are going bad then?

Well if it doesn't work in the frame relative to the cart we can always create a new one where it does. Shame that that is breaking the laws of physics.

Anyway now we have established that to do it you have to change frames of reference I think we can just forget it and find some amusement elsewhere.

[dennis]

That's because i am trying to convey simple physics to people that obviously have no understanding of physics. If they did understand physics I wouldn't need to do it.

Rubbish. Stick to one frame, you obviously have trouble when you change frames just like rick does.

Just like I said in mine except I chose 20 and 10. That is -10 and -10 referenced to the cart and hence to the prop. See the problem now? they are both the same, you are travelling at twice the wind speed and are not slowing the wind. You are not extracting any energy. This is possible with a perfect prop, there are no losses.

Now if you speed up the prop and the cart (I don't care how it doesn't matter) and you add 1 to the speed what has happened?

Of course its interesting, you are travelling at twice the wind speed and not consuming energy to do it, this is what a perfect prop does no losses. Its the equivalent of a machine that just accelerates air through a tube creating no turbulence, etc. (They don't exist but lets not stop reality getting in the way of physics).

No we want the prop to extract energy from the wind, we don't care how it does this.

So you feel its OK to just switch frames of reference to get the answer you want then? Do you go by the name rick or thin air when things are going bad then?

Well if it doesn't work in the frame relative to the cart we can always create a new one where it does. Shame that that is breaking the laws of physics.

Anyway now we have established that to do it you have to change frames of reference I think we can just forget it and find some amusement elsewhere.

[Ronald Raygun]

Why are saying stuff which makes no sense? To my objection that you are not being clear, you respond that this is because you want to make it simple. But the best way to make it simple is to be clear!

Again you are saying stuff that makes no sense. I say stay in one frame, and you say "Rubbish, stick to one frame". If you're agreeing with me, why do you say "rubbish"?

What do you mean -10 and -10? Whose speed is the first -10 and whose is the second?

If the cart is going at 30 and the wind at 20, then in the cart frame the wind is indeed going at -10. That's the wind coming from the front and going into the propeller from the front. What else is going at -10? Do you mean the wind coming out of the back of the prop?

If I may assume that you are indeed saying that air is coming out of the prop at the same speed as it's going in, then the prop is freewheeling (spinning at just the right speed to let the air through without either speeding it up or slowing it down), it is neither extracting energy (which would be acting as a turbine) nor expending energy (which would be acting as a propeller). This consumes zero power from the electric motor, but gives no thrust, assuming it's not infinite in size.

No, no, no. We don't want to speed up the cart. We want to analyse the situation of the cart going *and staying* at twice the air speed to see if it can self-sustain this speed.

The whole idea, which I don't think you've grasped yet, is that the prop is really acting as a propeller and not as a turbine. It is putting energy *in* to the relative wind, not taking it *out*, it is

*speeding up* the relative wind, *not* slowing it down. The prop on the cart is operating in exactly the same way as that on an airplane in flight, it is pushing backwards on the air in order to get forward thrust on the plane.

Our cart wants the propeller to provide the thrust to keep going at its existing speed, without getting faster. The need for the thrust comes from the need to balance the drag caused by the generator coupled to its wheels, and also the air drag caused by the headwind acting on the chassis.

So if you analyse the energy budget in the cart frame, you lose energy by speeding up the wind, but gain energy by slowing down the ground.

If instead you analyse the energy budget in the ground frame, you gain energy by slowing down the wind.

[Roger Chapman]

When did Denis ever do anything else? ;-)

snip

[Roger Chapman]

Unfortunately his behaviour over the 10 years or so he has been infesting this ng indicates that if anything he is even more stupid than his recent activities would indicate.

[dennis]

See what I mean about you not understanding relative frames. this is really simple and I have no idea how to describe it in a less simple way. You leave me no choice but to give up.

[Rick Cavallaro]

I've been trying to post for 2 days. I think it's still broke for me.

[Ronald Raygun]

Well this post of yours has managed to get through, at least. I've been looking forward to your assessment of my analyses.

Since my analyses confirmed that I was wrong before, and, I hope, showed where I went wrong, I presume your assessment will be approving. I'd welcome any additional comments you'd care to make.

I think one of the things which confused me initially was the notion of propeller efficiency. I thought what was meant was something similar to the efficiency of any other device or machine intended to convert one form of energy into another, like the generator, the motor, and any associated bits and pieces of couplings.

Any inefficiency would mean a "loss" of energy, a conversion into undesirable form, usually heat. I assumed that in the same sense a less than 100% efficient propeller would also be losing energy somehow.

That's why working in terms of the skater analogy helped. By leaving behind any mystery associated with propellers, I could just pretend that our hero's arms were 100% efficient machines, and yet he suffers a kind of inefficiency, but it's not really an "engineering" kind, it's more "strategic".

Our hero can *choose* to take 100% of the kinetic energy of each pleb he pushes, or he can choose to take a smaller fraction of each pleb's KE, but from more plebs each second, while getting the same thrust, and in so doing he can convert (harvest) almost twice as much energy.

The inefficiency is not due to a conversion loss into the wrong form of energy, it's due to failing to convert as much KE as in available to be converted.

[Rick Cavallaro]

Well, let's see if this gets through.

[Rick Cavallaro]

Alrighty then... it looks like I'm back. Yes, I agreed with each part of your analysis.

Unfortunately, I don't recall what comments I did attempt to make over the past few days. Both JB and I were shut out from posting for some reason.

But that is exactly what is meant by propeller efficiency.

That's right. Consider a Cessna sitting still in the run-up area with brakes locked and prop spinning at full revs. It's blowing a lot of air, but the plane is not moving forward. All of that air motion ultimately becomes waste heat.

[Ronald Raygun]

But not in the sense of what I said below, that energy is lost to heat *during* the actual conversion.

Yes, ultimately, but we're not really interested in what happens ultimately, we're interested in what happens initially. Perhaps I should say "I" rather than "we", because I think I was interpreting efficiency differently from the way you were, and are. Remember, what I'm trying to do now is explain how I think my understanding of prop efficiency differed from yours, so if I say "I thought it was X" it confuses me more if you then say "but that's what it does mean".

Let's change the skater analogy to match your Cessna. Suppose our hero has a rope tied around his waist which is tied to a stake in the ground behind him. This time the plebs are coming towards him at 1 m/s from the front, and he pushes back on one of them for a second at the same 60 N as before, accelerating him to 2 m/s. During the second of contact the pleb travels 1.5 m, so our hero does 90 J of work. The 60 kg pleb's kinetic energy increases from 30 J to 120 J.

The hero's arm may conceivably have been 100% efficient at converting 90 J of chemical energy into a 90 J kinetic energy increase for the pleb. In the same way, the Cessna's prop could conceivably be 100% efficient at converting engine power into kinetic air power. We don't care what happens to the accelerated pleb after he's left the grasp of the hero's arm, in terms of how that pleb might vaporize when it collides with another pleb. Likewise we shouldn't need to care whether the accelerated air gets hot as it slows down a few tens of yards downstream.

I was thinking about the prop's efficiency as how its kinetic output power relates to the mechanical input power. I think you're considering prop efficiency in terms of what the plane gets out of the deal. In your Cessna with the brakes on, I presume you are considering the prop as running at 0% efficiency because its thrust is doing no work *on the plane*. I guess you don't care whether the loss ends up as heat or not, you simply consider any work done on the air as wasted. But that won't help you calculate how much fuel the engine's going to need.

If I cut the skater's rope so that he can accelerate too (corresponding to your Cessna releasing the brakes), he will travel forward 0.5 m during the second, and so he will have done 120 J of work. 90 J of that is still going into the pleb, and the remaining 30 J will have accelerated himself. In my sense his arm is still 100% efficient, in another sense he could be thought of as 25% efficient because, out of 120 J expended, 30 J have gone into useful KE, and 90 J into useless KE. But in your sense I think he's being

50% efficient because 30 J/s (30 W) is 50% of the best he could get by exerting 60 N at 1 m/s (60 W). Am I right?

[Doctor Drivel]

The balloon defies gravity. It goes up against the pull of gravity.

[Doctor Drivel]

A balloon goes up against the pull of gravity.

[Doctor Drivel]

Roger is an idiot. Sad but true.

[Roger Chapman]

ng indicates that if anything he is even more stupid than his recent activities would indicate.

Censored information reinstated.

Dribble isn't clever enough to come to that conclusion about anyone but himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another ng for an explanation of latent heat and at much the same time on this ng asked if metres cubed was a measure of speed. Such a display of ignorance made his parallel claim to be a heating engineer absolutely preposterous while his inability to understand that constantly posting a mixture of garbage and insults reinforces the impression that he is terminally stupid.

[Ronald Raygun]

Take a playground see-saw. Small child on one end. Along comes big child, sits on other end. Big child's end goes down, small child's end goes up.

Would you say the small child is defying gravity? No, of course you wouldn't. The fact is that gravity is *making* the small child go up as a side effect of pulling the big child down.

It's the same with the balloon. Gravity pulls down on the heavier air around it, into the space underneath, and this is what forces the balloon up.

Without gravity the balloon would not go up.

[The Natural Philosopher]

I never like getting involved in these 'does the mortar hold the bricks together, or keep them apart' arguments..

;-)