Downwind faster than the wind. Poll

[mono]

I know you're new here, and probably haven't had time to read all the posts but ... so what? A propeller works perfectly well in still air, and the important thing to realise is that the propeller is blowing back against the wind.

Let me give an example of this mechanism. Imagine a helicopter climbing at 1kt vertically above a field on a hot summer's day. A thermal kicks off in the field, giving a vertical air velocity of 10kt. The helicopter is now climbing at 11kt ... faster than the wind.

That is the crucial mechanism here: a propeller working against a bulk air movement produces more thrust than one operating in still air.

Not particularly new here - I've been on and off these forums since they started more than ten years ago. I've also read all the posts before wading in.

I have a problem with your helicopter analogy. The blades are driven by an engine - in the same way that the prop on the cart was driven by an engine i.e.the wheels being held down onto the treadmill. However, remove that engine and the helicopter will fall to earth and the cart will slow down.

The thermals bit is just another load of hot air. It's a movement of air, otherwise known as wind. A helicopter rising at 11k in an updraft of 10k is moving 1k faster than the wind, not 11k.

Now, back to essentials: the original post asked whether an object can run downwind, under the power of the wind (not under the power of a rolling road or other engine) at a greater speed than the wind that is driving it.

 

[Ubergeekian]

Not particularly new here - I've been on and off these forums since they started more than ten years ago. I've also read all the posts before wading in.

Sorry, I just assumed you were since you were raising points dealt with a hundred posts that (^) a way.

I have a problem with your helicopter analogy. The blades are driven by an engine - in the same way that the prop on the cart was driven by an engine i.e.the wheels being held down onto the treadmill. However, remove that engine and the helicopter will fall to earth and the cart will slow down.

Sure. But that wasn't the point of the analogy. Several people have got hung up on conservation of energy. The helicopter analogy shows that, for the boat, the work done by the wind on the boat can be greater than the shaft work turning the propeller. Of course the reaction work done at the water turbine is also greater than the shaft work, thanks to propeller inefficiencies. The Big Question for any given implementation is whether the benefit from the moving wind is enough to overcome the losses of propeller inefficiencies. That's a design issue, though, and not a physical fundamental.

The thermals bit is just another load of hot air. It's a movement of air, otherwise known as wind.

Yes, that was exactly my point.

A helicopter rising at 11k in an updraft of 10k is moving 1k faster than the wind, not 11k.

That's why I wrote, "11kt ... faster than the wind" and not "11kt faster than the wind".

Now, back to essentials: the original post asked whether an object can run downwind, under the power of the wind (not under the power of a rolling road or other engine) at a greater speed than the wind that is driving it.

Yes, of course it can. We've all seen the treadmill video. If you have read the thread you'll have seen, by the way, that the power source is the difference in speeds between wind and ground/sea. A treadmill running in still air is precisely the sames as a still piece of track and a headwind.

 

[mono]

In reply to my post saying "Now, back to essentials: the original post asked whether an object can run downwind, under the power of the wind (not under the power of a rolling road or other engine) at a greater speed than the wind that is driving it." you say:

Yes, of course it can. We've all seen the treadmill video. If you have read the thread you'll have seen, by the way, that the power source is the difference in speeds between wind and ground/sea. A treadmill running in still air is precisely the sames as a still piece of track and a headwind.

Yes - we've all seen the treadmill video but no, we don't all agree that "of course it can". In fact, last time I looked at the poll, only 25 out of 88 people thought that the video proved the proposition.

You say that my points have been dealt with 100 posts earlier - no doubt to the entire satisfaction of the person(s) refuting the points raised - but this doesn't mean that the points are wrong, merely that someone disagrees with them.

 

[Ubergeekian]

Yes - we've all seen the treadmill video but no, we don't all agree that "of course it can". In fact, last time I looked at the poll, only 25 out of 88 people thought that the video proved the proposition.

That says more about the poor state of science education in Britain - and poor thinking skills - than the video. Against stupidity, as they say, teh gods themselves contend in vain.

You say that my points have been dealt with 100 posts earlier - no doubt to the entire satisfaction of the person(s) refuting the points raised - but this doesn't mean that the points are wrong, merely that someone disagrees with them.

It's not that they are wrong - just that you raised perfectly good questions which have been answered before.

 

[mono]

Thanks for the argument - I enjoyed it. Now I must do something about the house or I'm in deep trouble.

 

[PaulJS]

Sure. But it illustrates the two important principles here:

1. The wind can do work on a propeller with a relative velocity downwind and
2. The amount of work the wind can do is completely independent of the shaft work needed to turn the propeller

Once the helicopter is climbing (or hovering) relative to the air, vertical air motion can do work on the helicopter without any additional fuel being used. And that's the key with the cart or boat: getting reaction work out of the prop for a lot less shaft work.

In still air, you see, it's easy. If the propeller exerts a thrust T and is moving at v through the air, the reaction work T x v can't be more than the shaft work needed to turn the propeller - and in the case of the hovering helicopter, the reaction work is zero.

However, when the propeller is working against wind, the game changes, and it's possible to get more work out than you put in. Or, before the conservation of energy zealots wet their knickers, it's possible to have the wind do some work on the propeller too.

I'll leave it as an exercise for the reader to calculate the 'efficiency' (power out over power in) of a gyroglider climbing in a thermal ...

As several of us have posted, there is absolutely no problem with fundamental laws regarding travel downwind faster than the wind. Those who mutter about conservation of energy or the second law of thermodynamics just haven't thought about it clearly.

Practical implementation is a different matter, of course, and is clearly not going to be easy.

Sorry Ian,
I'm definitely a luddite (No reference to Mr. Ludd of this parish!), so I guess that I'm sitting in soggy knickers. I disagree that you can get more energy from a system than is put in.
Your point about the helicopter being held up by vertical air motion is only valid if the air is moving vertically upward at the same speed relative to the helicopter as the helicopter would descend to earth in non rising air when autorotating, I'm afraid friction is, and will always be the killer - even if we assume a frictionless drive train friction must be generated at the propellor blades by the air molecules impinging on them to create lift.
By the way I completely agree that a gyro glider can climb in a thermal, as can a glider, but not faster than the air in the thermal is rising except of course after diving to build up forward momentum and energy... and then dissipating that energy in converting forward motion into vertical.

I think that we will have to agree to disagree here.

 

[Pierrome]

Sorry Ian,
I'm definitely a luddite (No reference to Mr. Ludd of this parish!), so I guess that I'm sitting in soggy knickers. I disagree that you can get more energy from a system than is put in.

Nobody, but nobody has claimed in this discussion that you can get more energy out than you put in.

The helicopter illustration although pertinent, has further confused the issue.

Here is a new explanation:

First you have to agree that it is possible to obtain energy from the difference in velocities between two mediums, such as air/water or air/land.
(If you say that is not possible then you are saying that sailing is impossible so nobody should deny that!)

Consider the man carrying downwind cart. You are sitting aboard. Before you start you notice that the wind is blowing on your back at 10mph. The brakes are released. The cart gathers speed. The roadspeed increases but the windspeed decreases, at all times during this acceleration the speed of the wind + the speed of the road adds up to 10mph.
For example, when the cart reaches 5mph on the road, speed of the wind on your back would be 5mph = total 10mph.
Similarly, when the speed of the cart reaches 8mph then the speed of the wind behind you is 2mph - total 10mph.
Anyone should agree this, it makes perfect sense.
Finally, the cart reaches the speed of the wind. The speed of the road is 10mph and the speed of the wind on your back where you are sitting on the cart is zero.

So, the point is that the difference in velocities between the two mediums STILL EXISTS whether the cart is standing still or travelling at windspeed.

If you agreed at the beginning that it is possible to obtain energy from the difference in velocities between two mediums, then you must agree that the cart can still use this energy to obtain progress forwards through the air (which is static at this stage from the point of view of anyone riding on it).

If it can make progress through the air (using its propeller) from the point of view of anyone riding on it,
then from the point of view of an outside observer it is travelling faster than the wind.

Please refer to post 229 about different points of view, because this is the secret of understanding how it is possible.

Note: If you want to ask "how does the cart reach exactly windspeed" the answer is that the process of transferring energy from the road to the propeller starts to take place before the cart reaches windspeed.

 

[Bosun Higgs]

DOWNWIND ... write out 100 times ...
The device is travelling DOWNWIND DUFUS!

Wrong Fireball. Once you are going downwind faster than the wind you are by definition going upwind.

 

[Bosun Higgs]

If you agreed at the beginning that it is possible to obtain energy from the difference in velocities between two mediums, then you must agree that the cart can still use this energy to obtain progress forwards through the air (which is static at this stage from the point of view of anyone riding on it).

No. You can possibly transfer energy from one medium to another when velocities differ, but in this case there is no velocity difference once the vehicle reaches wind speed except the difference between vehicle and road. And there is no mechanism for a transfer between these two.

 

[RAI]

No. You can possibly transfer energy from one medium to another when velocities differ, but in this case there is no velocity difference once the vehicle reaches wind speed except the difference between vehicle and road. And there is no mechanism for a transfer between these two.

But that is exactly what the gearbox on the cart is doing, transferring torque from the wheels to torque on the propeller. When the cart is travelling at wind speed, the propeller is still driving air backwards. It doesn't stop doing that until the cart reaches a speed where the incidence angle on the blades is too low to produce enough thrust to further increase the cart's speed.

 

[Pierrome]

No. You can possibly transfer energy from one medium to another when velocities differ, but in this case there is no velocity difference once the vehicle reaches wind speed except the difference between vehicle and road. And there is no mechanism for a transfer between these two.

First you say there is no velocity difference. Then you say there is a velocity difference between the vehicle and the road. What do you mean?

Then you say there is no mechanism for transfer. But there is a mechanism for transfer, which is the drive between the prop and the wheels.

It's not very easy to see what you mean.

 

[Bosun Higgs]

First you say there is no velocity difference. Then you say there is a velocity difference between the vehicle and the road. What do you mean?

Then you say there is no mechanism for transfer. But there is a mechanism for transfer, which is the drive between the prop and the wheels.

It's not very easy to see what you mean.

There are three "mediums" if you want to call them that. The road, the vehicle and the air. You have no mechanism for transferring energy from the road to the vehicle not least because the road doesnt have energy in a transferable form. Potential energy except it cant fall. Chemical energy maybe if you managed to burn the tarmac. Some heat energy but nothing to receive it on the vehicle.

The moving air does have energy to transfer and to the extent that the air loses velocity as it goes through the prop, this is exactly what it does. But once the air is moving at the same speed and in the same direction as the vehicle, it has no means of transfering energy. So ignoring minor losses like friction in the bearings we achieve a steady state when vehicle velocity equals that of the air.

 

[Bosun Higgs]

But that is exactly what the gearbox on the cart is doing, transferring torque from the wheels to torque on the propeller. When the cart is travelling at wind speed, the propeller is still driving air backwards. It doesn't stop doing that until the cart reaches a speed where the incidence angle on the blades is too low to produce enough thrust to further increase the cart's speed.

You're mixing things up here. The governing principle is that of conservation of energy - the torque or forces involved will change at various poins up and down the drive train. They arent constants.

To the extent that the vehicle is moving and overcoming friction ( in bearings etc) it is using up energy and it has to get that from somewhere. That is does by modifying the flow of air through its prop - slowing it down and changing its direction. The sum total of the energy involved remains constant.

If the vehicle is moving at the velocity of the air, it cannot extract energy from the air which is still relative to it. Can you sail in a flat calm? No you cant.

Think of it this way. On a flat calm day at the equator, the air on the surface of the sea is moving through space at roughly 900 mph. You dont see that because you are moving at the same speed and so is the surface of the sea. Neither does your boat which just sits there and wallows, and your boat doesnt have the ability to extract any movement out of that air because relative to the boat the air isnt moving.

 

[bbg]

I think the problem is that the believers don't realise that, in order to use the wheels to drive the prop, the wheels will be loaded and therby slowing down the cart.
What happens when you are in your car, coasting down a very slight hill with your foot on the clutch. Let's say the hill is just steep enough to maintain your speed - no accelerating, no slowing down. Now the engine stalls.

Instead of turning the ignition key, you decide to pop the clutch. After all, your car is moving relative to the road, so you can extract the energy!

What happens? Car slows down. Now, instead of turning the motor you could use that energy to drive a stonking big propeller. I suppose the believers think that the thrust from the prop will more than offset the drag from the loaded wheels. I don't. I'll believe it when I see it.

The point is, the prop doesn't rotate for free. If it is providing thrust, it has to take it from somewhere, and if it is driven from the wheels that WILL slow the cart down. The believers just seem to ignore this.

 

[Pierrome]

There are three "mediums" if you want to call them that. The road, the vehicle and the air. You have no mechanism for transferring energy from the road to the vehicle not least because the road doesnt have energy in a transferable form.

So you're saying that if you are on the cart and the road is zooming underneath you at 10mph you can't get any energy from it? So if you put a wheel down on a moving treadmill you couldn't get any energy from that wheel? When a cog in a gearbox engages with another cog it cannot transfer energy?.........!

 

[Pierrome]

The point is, the prop doesn't rotate for free. If it is providing thrust, it has to take it from somewhere, and if it is driven from the wheels that WILL slow the cart down. The believers just seem to ignore this.

The thrust from the driven wheels would slow it down if it was only a brake, but the "slowing down" energy is being fed into the prop and we know from the example of the upwind windmill boat that a prop can give more thrust than it's drag (because of its l/d ratio).
If you say it's not possible, then you also say the upwind windmill boat is not possible.

 

[RAI]

You're mixing things up here. The governing principle is that of conservation of energy - the torque or forces involved will change at various poins up and down the drive train. They arent constants.

I think you are missing the important point that the balance of forces on this machine does not occur at cart-speed-minus-wind-speed but at a possibly higher speed.

To the extent that the vehicle is moving and overcoming friction ( in bearings etc) it is using up energy and it has to get that from somewhere. That is does by modifying the flow of air through its prop - slowing it down and changing its direction. The sum total of the energy involved remains constant.

The energy in this system is not constant and energy is not being conserved. The energy comes from the wind over ground speed and the mass of the air passing the cart. There is an excess of it and most of it is wasted. Mark Drela's analysis provides an illustration of the relationships involved.

 

[[32511]]

Gulls already exploit wind shear; it's called dynamic soaring, I seem to remember. Into wind, you can climb, because the momentum keeps you going, and you can trade the increasing wind speed for height, at the loss of some airspeed. Turn downwind, and as you descend, your momentum and glide angle will see a relative increase in airspeed as you descend through the slower moving layers. You can now turn into wind, and trade this increased speed and momentum for height again.

 

[Ubergeekian]

I'm definitely a luddite (No reference to Mr. Ludd of this parish!), so I guess that I'm sitting in soggy knickers. I disagree that you can get more energy from a system than is put in.

Have I stopped beating my wife. I have never said that you can get more energy out than you put in. Here's what I wrote: However, when the propeller is working against wind, the game changes, and it's possible to get more work out than you put in. Or, before the conservation of energy zealots wet their knickers, it's possible to have the wind do some work on the propeller too.

My point is that you can get more energy out in reaction work than you put in in shaft work. In other words, the wind does more work in pushing the propeller along than it takes to turn the propeller.

If that worries you, think about a spinnaker. Work done by wind: lots. Work done to keep spinnaker there: nil. Think of the propeller as a big thing which needs to be turned in order to act like a spinnaker. Or even better than a spinnaker, because a spinnaker needs a tail wind to work and a propeller - if it's going round fast enough - works fine in a headwind.

You might like to consider the parallels with a Flettner rotor. That's a big vertical axis cylinder which is spun round by a relatively small motor - Flettner's prototype on the Buckau only took about 25hp to spin them. The spinning cylinder caused circulation as it dragged air round and circulation + moving air -> lift, which drove the ship. With much more than 25hp worth of power.

The point, you see, is not that you are getting energy from nowhere, but that by putting in one sort of energy (shaft work) you can harvest another (reaction work).

Simple but true: all other things (pitch, rpm) being equal, a propeller in a tailwind will produce more thrust than a propeller in a head wind.

 

[Ubergeekian]

Wrong Fireball. Once you are going downwind faster than the wind you are by definition going upwind.

It depends on your frame of reference.