Downwind faster than the wind. Poll

[snowleopard]

Just keep going until just short of mach 1!

My car can do 150 mph on wheels of 600mm diameter. If I increase the diameter to 2400mm it will do 600 mph.

So all I have to do is get a longer back axle and put tractor wheels on then I too can break the sound barrier.

 

[nmeyrick]

Well it's all down to the change in velocity of the air. Instead of stopping the wind, the spinning prop sends it back the way it came. If the prop pushes the wind back from its blades imparting a backward thrust of 6 m/sec, let's see what has happened to the kinetic energy in the wind:

The mass of air that has gone through the prop is the 6 cu m that have gone through the fan, i.e. 7.2 kg .

To start with it was doing 10 m/sec, now having been given a 6 m/sec push it is only doing 4 m/sec so its change in velocity is 6 m/sec The amount of energy extracted from that air is therefore

1/2 x 7.2 x 6^2 = 129.6 Joules.

And that is the energy that goes to overcoming frictional losses in the machine.

Sorry Snowleopard you're changing frames of reference. As you pointed out you can only have energy, speed etc in relation to something else.

From the point of view of the cart the wind is static, since both are travelling at windspeed. Your propeller accelerates the wind backwards by 6m/sec _expending_ 1/2 x 7.2 x 6^2 = 129.6 Joules. The clue is that you say you give the air a push - and call the blade a prop, not a turbine.

Of course I agree that accelerating the air behind you will cause an equal and opposite force on the cart, which will accelerate it downwind. But you still need to get the energy to turn the prop from somewhere, in this case the relative motion between the cart and the ground (since you told me the wheels are turning the prop). This results in a deceleration of the cart which, at best and assuming no losses, would be equal to the acceleration coming from the prop.

Your energy source, ie the velocity differential between the cart and the ground, is the kinetic energy of the cart. To increase the cart's velocity you have to increase its kinetic energy. That is where the conservation of energy problem comes in.

 

[halfway]

snowleopard, xxyyzz, Keen_Ed: I have given a non-friction issue as to why the speed is not limitless. The apparent wind angle on the blade foil.

You counter the argument by saying the friction is the ONLY limit to any system. You are saying doubling the prop size will cause more friction and NO more speed. Even if doubling only gave 1% extra speed I could just keep doubling in your world and eventually hit mach 1. To not reach mach 1 you would have to say that the speed would be identical if you used twice the air column energy. I.e the other column of air would be totally wasted!

So why not half the size of the prop? If it makes no difference to double, halving should result also in the same speed. So half it again. and again. and again Whoops!

You have invented limitless energy.


So the energy is not relative to the ground and you can not just collect that energy from a moving car. The argument is incorrect.

But the issue of going faster than the wind is not

 

[snowleopard]

Even if doubling only gave 1% extra speed I could just keep doubling in your world and eventually hit mach 1. To not reach mach 1 you would have to say that the speed would be identical if you used twice the air column energy. I.e the other column of air would be totally wasted!

How fast do you think my car would go with tractor wheels on the back?

 

[Keen_Ed]

snowleopard, xxyyzz, Keen_Ed: I have given a non-friction issue as to why the speed is not limitless. The apparent wind angle on the blade foil.

You counter the argument by saying the friction is the ONLY limit to any system. You are saying doubling the prop size will cause more friction and NO more speed.

Not what I'm saying at all. There are all sorts of limits on speed, including drag losses from the prop, drag losses from the cart, friction - in the gearing system, betwen road wheels and the road etc etc.

Or are you saying that there's no speed limit to a propellor driven aircraft?

 

[xxyyzz]

The apparent wind angle on the blade foil.

If by this you mean there's a limit to how much air the prop can push, that's true of course. Props have maximum speeds, they can fall apart and as the blades approach the speed of sound odd things happen, which I don't understand, but I believe they essentially stop working.

Even if doubling only gave 1% extra speed I could just keep doubling in your world and eventually hit mach 1.

Assuming no friction!

You have invented limitless energy.

No, actually you have, you have assumed:-
1) No friction.
2) Limitless material strength
3) That propeller blades work just dandy when the blade is moving at the speed of sound.
4) That you have a limitless runway and a constant wind.

4 especially is the definition of limitless energy.

If you have a constant source of energy, no friction, no material failures and limitless runway, would you struggle to accelerate something to the speed of sound?

Imagine a wind turbine with no friction, no material failures, endless constant wind what stops the blades from accelerating?

 

[Ubergeekian]

Sorry Snowleopard you're changing frames of reference. As you pointed out you can only have energy, speed etc in relation to something else.

From the point of view of the cart the wind is static, since both are travelling at windspeed. Your propeller accelerates the wind backwards by 6m/sec _expending_ 1/2 x 7.2 x 6^2 = 129.6 Joules. The clue is that you say you give the air a push - and call the blade a prop, not a turbine.

If you want to use a reference frame moving with the cart then (a) the cart is stationary and needs no kinetic energy and (b) the road is moving past you and can have KE extracted from it.

If you want to use a reference frame which is fixed to the road then (a) both the cart and the air have KE and (b) it's possible for the cart to extract KE from the air.

You can't mis those reference frames, though. You can't say relative to the cart the wind has no initial KE but relative to the road the cart needs KE. Either reference frame works fine as long as you apply it consistently.

Your energy source, ie the velocity differential between the cart and the ground, is the kinetic energy of the cart. To increase the cart's velocity you have to increase its kinetic energy. That is where the conservation of energy problem comes in.

The energy source is the velocity difference between the air and the ground. The cart merely exploits this, and any suitable reference frame, used properly, will allow this use to be analysed.

 

[colingr]

How fast do you think my car would go with tractor wheels on the back?

Probably slower, like putting a bigger prop on a boat does not increase the speed. The power needed to turn the wheels would have to increase as well otherwise formula 1 cars wouls all have massive wheels and go at mach 6.

 

[lw395]

All the pseudo-proofs of impossibility invoking conservation of energy etc etc, would appear to prove that 'apparent wind' craft cannot have a downwind VMG of greater than the windspeed.
It would seem that many people on here would be hard pressed to explain how a boat can sail upwind.

You need to make sure that your assertions are compatible with these observed phenomena.

It is also true that apparent wind boats craft were conceived a long time before being made to go downwind faster than the wind.

 

[nmeyrick]

If you want to use a reference frame moving with the cart then (a) the cart is stationary and needs no kinetic energy and (b) the road is moving past you and can have KE extracted from it.

Yes, that is exactly what I am saying. If you prefer it the cart has no kinetic energy and the road does have kinetic energy which can be extracted. But what are you trying to achieve?

From this frame of reference you want the road to move faster than the windspeed, so you need to increase the kinetic energy of the road. So how can you do this using the kinetic energy of the road as the source of your energy? It really is a conservation of energy problem!

Please note that I have not said at any point that DDWFTTW is not possible, only that this mechanism cannot provide it.

 

[snowleopard]

Please note that I have not said at any point that DDWFTTW is not possible, only that this mechanism cannot provide it.

It's getting confusing switching between threads. I'm beginning to regret starting the second one.

I think I can answer that question. Look at this mechanism....

The cart moves around 3 times as fast as the ruler above it. Now replace the upper wheel rolling along the ruler with a prop moving through the air - just another linkage but this time with an inefficient coupling.

We won't get 3x speed of course because of prop slip but with a good prop we will get more than 1x.

 

[lw395]

Kinetic Energy

When our arbritrary dwftttw machine is moving at a steady speed, its kinetic energy is constant.
The wind is losing kinetic energy.
This lost energy goes as heat from friction and air drag.
The ground is probably best thought of as having no kinetic energy, i.e. being still, but if you choose a different frame of reference, the speed of the ground is constant, so there is no exchange of energy from it.

A water based machine is different in that the water can move at different velocities at different points, so even at a steady speed the water can take away energy.

I think a lot of people do not get the distinction between energy, power and force. My chair is exerting a force on my backside, but it is not doing any work, because it is not moving.

To return to the 'Oracle' which way is the lateral resistance from the water to the centreboard acting? Answer an UPWIND component on the boat....take away that upwind force and BMW-coracle will cease to go downwind so quickly!

 

[lw395]

It's getting confusing switching between threads. I'm beginning to regret starting the second one.

I think I can answer that question. Look at this mechanism....

The cart moves around 3 times as fast as the ruler above it. Now replace the upper wheel rolling along the ruler with a prop moving through the air - just another linkage but this time with an inefficient coupling.

We won't get 3x speed of course because of prop slip but with a good prop we will get more than 1x.

In your picture, the cart is moving to the right. Therefore the wheels are rotating clockwise. Therefore the big circle is rotating widdershins as they say in Somerset. Therefore it's bollox.

 

[xxyyzz]

In your picture, the cart is moving to the right. Therefore the wheels are rotating clockwise. Therefore the big circle is rotating widdershins as they say in Somerset. Therefore it's bollox.

No, because the cart is moving.

 

[lw395]

If the big roller is rolling rightwards along the ruler, how does the ruler exert a rightwards force on the roller?
I don't see the relevance of this 'model'.

 

[snowleopard]

In your picture, the cart is moving to the right. Therefore the wheels are rotating clockwise. Therefore the big circle is rotating widdershins as they say in Somerset. Therefore it's bollox.

I'm sorry. What is bollox?

 

[snowleopard]

how does the ruler exert a rightwards force on the ruler?

?

 

[lw395]

Moving the ruler to the right against the roller will not drive the cart to the right.

Working relative to the cart, the ruler is acting on the roller with a force F, leftwards.
The roller and wheels act as gears. The force at the rim of the wheel is reduced by the ratio of the 'gear' radius to the rim radius. Therefore the force exerted on the road is about F/2. Rightwards.
So the nett force on the cart is about F/2 to the left.
So it will accelerate to the left.

 

[lw395]

?

Sorry typo now krekted.

 

[Ubergeekian]

Yes, that is exactly what I am saying. If you prefer it the cart has no kinetic energy and the road does have kinetic energy which can be extracted. But what are you trying to achieve?

From this frame of reference you want the road to move faster than the windspeed, so you need to increase the kinetic energy of the road. So how can you do this using the kinetic energy of the road as the source of your energy? It really is a conservation of energy problem!

The road is the source of energy, and you use it to do work on the wind.