Downwind Faster than the Wind - Successful Run by manned cart

[RAI]

I still cant see how the machine can be taking energy out of what is relative to itself, stationary air.

Is that your question?

With the cart at wind speed, the energy is being taken from the relative movement of the propeller blades in the air. Air is being accelerated rearwards. Its kinetic energy is reduced or reversed. Conversely, the propeller generates thrust accelerating the cart forwards.

 

[Bosun Higgs]

So how does the cart start moving Mikemonty? The angle of the blades would encourage the cart to go backwards from a standstill unless the torque at the wheels is less than the force generated by the air resistance of the cart. Which I suppose has to be the case. Then once the cart is moving, the gain in thrust from the prop is greater than the drop in thrust from air resistance until we finally get to wind speed. At this point the thrust from the prop is sufficient to continue to accelerate the cart and does so until prop thrust equals wind resistance which is now from ahead. Presumably the system reaches steady state when wind resistance to going forward matches prop thrust

In which case all the energy required to do the work of pushing the cart forwards comes from deflection of the air flow at the blades.

None of which proves that its possible because the question depends on the balance of so many different factors from bearing and tyre rolling resistance to wind resistance in both directions and gearing losses and prop characteristics . In the absence of all this data I suppose we will either have to trust the video as proof positive.

 

[snowleopard]

Cant take heat energy from the road. Cant take chemical energy from it either. Tarmac isnt magnetis. The road doesnt go any lower so no potential energy loss. So your postulation must be that the vehicle takes energy from the road which will show itself ( so small as to be unmeasureable in practise) as a slowing of the rotation of the earth. In which case of course, the gadget would only work in one direction

The wheels exert a drag on the road (and vice versa). That means there is a force on the road in the direction of movement of the cart. That force produces an acceleration in the road and indeed the whole Earth. Because the mass of the earth is so great relative to the vehicle and observer the change in velocity is infinitesimal.

The force on the wheel is used, through the gears, to rotate the prop. The rotation of the prop imparts a backwards force on a the cylinder of air that passes through it. After the air has passed through the prop it is travelling in the original direction (from behind) but more slowly. The difference in velocity (delta V) of that cylinder of air relative to the ground is what powers the cart.

The energy extracted from the wind is:

0.5 x (Mass of air passing through the prop per second) x (delta V)²

 

[Pierrome]

MM.
Firstly I'm not sure of your position. I think you are saying it's possible, but your explanation is not the same as the more usual one? Since your post is so long I will have to answer it in parts. (I don't have all day!)

Pierrome,

The wheel is no more being driven by the ground than the boat is by the water. No energy is being transferred from water to boat or ground to wheel, other than by frictional losses.

Your wrong here the boat IS being driven by water, have you forgotten Newton's third law? "Every action has a reaction equal in magnitude and opposite in direction". The boat is being driven by an interaction between two mediums - it depends on your point of view which one is doing the driving.

 

[Pierrome]

Pierrome,

The same principle applies to the cart with the prop blade(s) as sails. The sail/blade is still going diagonally down the wind, except this time the force is being tranmitted to the wheel, not the keel

!

MM.
Wrong here. The prop blades are not analagous to the sails of the boat.
On the boat, the sails are the collectors of energy.
On the cart, the wheels are the collectors of energy.
You have fallen into the obvious trap that because prop blades look like sails, they must be doing the same job. They're not - its like the difference between a wind-turbine and a propeller - they look the same but they are doing opposite jobs.

 

[saltwater_gypsy]

Breaking news levitation achieved!!!

The same talented team who achieved Sailing Faster than the wind Downwind have gone on to achieve another amazing breakthrough...LEVITATION!
They have taken a standard sailing dinghy and by attaching a large HORIZONTAL propeller they managed to lift the boat clear of of the water by WIND POWER ALONE and were last seen disappearing over the horizon ,happily singing songs from "Mary Poppins".
What will these crazy College kids do next?

 

[RAI]

MM.
Wrong here. The prop blades are not analagous to the sails of the boat.
On the boat, the sails are the collectors of energy.
On the cart, the wheels are the collectors of energy.
You have fallen into the obvious trap that because prop blades look like sails, they must be doing the same job. They're not - its like the difference between a wind-turbine and a propeller - they look the same but they are doing opposite jobs.

There I entirely disagree with you.

 

[snowleopard]

They have taken a standard sailing dinghy and by attaching a large HORIZONTAL propeller they managed to lift the boat clear of of the water by WIND POWER ALONE

Next you're going to tell me a heavier than air machine can fly.

Ironically, the joke is on you. The device is called an Autogyro and it will 'levitate' using the power of the wind alone as long as you use something to stop it blowing away downwind as soon as it leaves the ground. An engine is usually used but a cable will do. It was first flown in 1923.

 

[Pierrome]

Pierrome,

As it passes windspeed the apparent wind starts to take over and starts to DRIVE the wheel(s).

!

No, you seem to be saying that it is working as a wind-turbine in the apparent wind. It's not, it's working as a propeller and accelerating the apparent wind.
What you're saying is that the propeller of a plane must be driven by the apparent wind. It's not - its the other way round. The prop is driving the wind resulting in an apparent wind to the whole plane.

 

[Pierrome]

Cant take heat energy from the road. Cant take chemical energy from it either. Tarmac isnt magnetis. The road doesnt go any lower so no potential energy loss. So your postulation must be that the vehicle takes energy from the road which will show itself ( so small as to be unmeasureable in practise) as a slowing of the rotation of the earth. In which case of course, the gadget would only work in one direction

You still haven't explained to a "simpleton" like me how a bike can take energy from the movement of the road using a dynamo to power the lights, but the cart can't take energy from the movement of the road to power it's prop. Please explain the difference.

 

[mikemonty]

So how does the cart start moving Mikemonty? The angle of the blades would encourage the cart to go backwards from a standstill unless the torque at the wheels is less than the force generated by the air resistance of the cart. Which I suppose has to be the case. Then once the cart is moving, the gain in thrust from the prop is greater than the drop in thrust from air resistance until we finally get to wind speed. At this point the thrust from the prop is sufficient to continue to accelerate the cart and does so until prop thrust equals wind resistance which is now from ahead. Presumably the system reaches steady state when wind resistance to going forward matches prop thrust

In which case all the energy required to do the work of pushing the cart forwards comes from deflection of the air flow at the blades.

None of which proves that its possible because the question depends on the balance of so many different factors from bearing and tyre rolling resistance to wind resistance in both directions and gearing losses and prop characteristics . In the absence of all this data I suppose we will either have to trust the video as proof positive.

As you say...

It is Aero drag that gets the cart started. (wheel driving prop).
It is possibly a combination of aero drag and prop thrust that gets it close to windspeed.
The difficult phase is at transition between true-wind driven and apparent wind driven.
At some point around windspeed there is no further aero drag available to push the cart and I can only assume that is possibly prop thrust, or lift from aerofoiling blades that takes the cart through the critical "same speed as the wind" phase.
Quite how this bootstrapping process is achieved is beyond me.

In this state the momentum of the cart SYSTEM (wheel neither driving nor driven) surely takes it through to FTTW - if you dont have enough angular and linear momentum you can't keep thrust going long enough to get to the third "prop driving wheel" phase.

AT SOME POINT the prop will be neutral pitched, neither thrusting nor providing force to the wheels - I assume that if this coincides with windspeed you are buggered.

Forget thrust as a concept once you are over windspeed - there is no appreciable thrust on your boats sails, it is lift that pulls you along - I'd assume that at faster than windpeed the prop on the cart is "sailing". (prop is driving the wheel)

Forget energy from deflection of flow - forget energy - that's not important, deal with the forces. On your sails, the force is produced by an aerofoil, not a turbine impeller.

Interestingly, as I think about it, as long as the prop and the ground are geared together by the wheel, the prop pitch will stay constant to the apparent wind for a varying cart speed - but would have to be adjusted for varying wind speed.

If I'm beginning to sound like a convert I apologise - I remain a sceptic - though not a la la fingers in ears one.

 

[Pierrome]

As you say...


AT SOME POINT the prop will be neutral pitched, neither thrusting nor providing force to the wheels - I assume that if this coincides with windspeed you are buggered.
.

MM
Why you should think this is a mystery. The prop is obviously pitched (that means the angle of the blades relative to the airflow) so that this is not the case. Also by the time it reaches windspeed it is rotating fast, which also increases the apparent angle of attack of the blades relative to the air.

How does a plane start its takeoff run then? - please answer that.

 

[Pierrome]

An alternative explanation to help the sceptics.

They have been talking about this on the AYRS (Amateur Yacht Research Society) mailing list.
One prominent member put forward the following way of thinking of it:

Not many would have difficulty in agreeing with the following.

1.
That you could charge up a battery by attaching a dynamo to the wheels of a cart being blown downwind.

2.
That you could use an electric motor powered by a battery to drive a fan to push a cart downwind faster than the wind.

3.
You could try to do both at the same time - charge your battery, drive your fan. (but it probably wouldn't work very well because of the considerable losses of such a setup)

4.
So why not cut out the dynamo - battery - motor and connect the wheels directly to the fan?
There you have it.
It works and has been demonstrated many times.

 

[snowleopard]

As you say...

It is Aero drag that gets the cart started. (wheel driving prop).
It is possibly a combination of aero drag and prop thrust that gets it close to windspeed.
The difficult phase is at transition between true-wind driven and apparent wind driven.
At some point around windspeed there is no further aero drag available to push the cart and I can only assume that is possibly prop thrust, or lift from aerofoiling blades that takes the cart through the critical "same speed as the wind" phase.
Quite how this bootstrapping process is achieved is beyond me.

I think you're over-complicating it. All you need to do is look at the apparent wind experienced by the prop blades and the thrust that produces.

Here's the same old diagram....

When the vehicle and prop are stationary the true and apparent winds are the same, the blade is stalled and the thrust is directly forward.

As the cart starts to move, the blade starts to move along the red track (combined forward and rotational movement) and the apparent wind starts to move in an anticlockwise direction and increase. As the apparent wind draws forward the efficiency of the prop increases and it goes from stalled to laminar flow.

As the speed increases relative to the true wind the apparent wind draws further forward and the angle of attack of the blades gets smaller and the lift drops off. That sets a limit on the speed that can be achieved.

Here is a plot of lift against angle of attack for a simple aerofoil.

The angle of attack starts way off to the right of the graph at ca. 110° with the foil stalled. Laminar flow kicks in at around 16° with lift at a maximum and dropping from then on.

I'll leave out the changes in Lift/drag ratio for now.

 

[shaunksb]

So why not cut out the dynamo - battery - motor and connect the wheels directly to the fan?
There you have it.

No you don't because that doesn't get rid of all the losses - dohhh

 

[[32511]]

Not a proper post, as no theorising, but all the headbanging going on is great- keep it up, chaps!
By the way, the little test model did some convincing faster than the wind runs this morning; apart from me, the only witness a bemused French tourist.

 

[RAI]

Not a proper post, as no theorising, but all the headbanging going on is great- keep it up, chaps!
By the way, the little test model did some convincing faster than the wind runs this morning; apart from me, the only witness a bemused French tourist.

Thank goodness some of us get out.

 

[saltwater_gypsy]

RE "Snowleopard"......That " Same old vector diagram" may be incorrect.
I havn't the graphics to draw the correct version at the moment but if you draw a proper parallelogram of vectors then the apparent wind is still behind the prop, pointing FORWARD. The component of the apparent wind in the direction of the cart motion is then of course smaller than the true wind
Come on ...back to school Physics!!

 

[snowleopard]

RE "Snowleopard"......That " Same old vector diagram" may be incorrect.
I havn't the graphics to draw the correct version at the moment but if you draw a proper parallelogram of vectors then the apparent wind is still behind the prop, pointing FORWARD.
Come on ...back to school Physics!!

Well you'd better get out your protractor and demonstrate because I am quite happy that my diagram is correct.

 

[RAI]

RE "Snowleopard"......That " Same old vector diagram" may be incorrect.
I havn't the graphics to draw the correct version at the moment but if you draw a proper parallelogram of vectors then the apparent wind is still behind the prop, pointing FORWARD.
Come on ...back to school Physics!!

SL's diagram shows the wind over ground going straight up the page. That's the true wind.

It shows a section of on blade of a propeller being carried forward over the ground by the cart and moving sideways by rotation of the propeller. (There is another blade going to the opposite side). So its a vector of the blades speed over the ground.

Wind over ground vector and blade over ground vector create an apparent wind vector.