[Closed] We've done planes on elevators...

No

Is it going down a hill?

Is it on a conveyor by any chance? If so, then probably yes

Are really strong magnets involved?

all things are possible if you believe

If the wind is gusting then technically yes but if it is steady wind on flat ground then no.

Yes, simply attach a large funnel to the vehicle to collect the wind and direct it at the sail

Yes. The wind can slow down, but wind powered vehicle will continue at a faster speed. Unless it's not on wheels.

It's possible for a vehicle to travel faster than the wind. Not directly in the direction of the wind, but the filled sail acts like a wing and generates 'lift' the magnitude of this force is what governs the speed of the vehicle. (of course other bits as well)

Some clarification:
the vehicle is moving in exactly the same direction as the wind.
the wind is blowing at a constant speed, and doesn't ever slow down or stop.
it's on flat ground with no conveyor belts, magnets or any other external forces.

Then no. Sails work most effectively in lift. is this a theoretical question or a problem that needs solving?

Has it got any booster rockets ?

No booster rockets, the only force involved is due to the wind (and a bit of friction).

Yes. 2.8 times faster than the prevailing wind.
http://boingboing.net/2010/11/05/downwind-faster-than-2.html

Yes. 2.8 times faster than the prevailing wind.
http://boingboing.net/2010/11/05/downwind-faster-than-2.html

My guess is that going 2.8 times faster than the wind has everything to do with the fishing rod strapped to the front of the car..

[img] [/img]

yes - its all about apparent wind

seriously!!

Wind car sponsored by Joby

Trick question? The "wind" is from cows farts and is caputured in a big tank and used to power two jet engines?

Instinctively I would have said no, but after reading [url] http://dwfttw.blogspot.com/ [/url], I'd say yes.

Actually reading the OP's question again, I would say no, as he uses the term 'sail', which a wheel driven prop isn't, by any conventional definition of 'sail'
So the OP needs to define more clearly what 'sail' means.

I think you need to read the question again again. Not only did I use quotes round the word "sail", it's also being used as a verb. I don't suppose the propulsion device used on the latest US Americas Cup yacht is what most people would call a sail - does that mean the yacht wasn't sailing? I'd argue that by any conventional definition of the word that vehicle is sailing, as it's using energy from the wind to move.

Surprised how long it took anybody to google!

Now having established what did happen, does anybody on here understand how it works?

Or explain this:

does anybody on here understand how it works?

It works on [i]exactly[/i] the same principle as a propeller on a plane or boat does ? The only difference being that it is powered by wind, as opposed to being powered by internal combustion/steam.

I guess you could have a huge "wind farm" type turbine on a ship which drives a ships propeller to the same effect. You could also have the turbine drive an electrical generator which powers the propeller (or wheels)

You're suggesting the prop is powered directly by the wind? How does that explain the treadmill video where there is no wind?

That Joby wind vehicle has so many jokes going for it, I don't know where to start

AFAIK the faster than wind vehicle uses the tailwind to turn the prop that is linked to the wheels by a very clever and very strong gearbox. As the vehicle accelerates the forwards rotation force of the wheels overtakes the roation force of the wind driven propeller and this starts to turn the propeller via the same gearbox, making the car go faster, I think there's a flywheel effect in there too...Its not a perpetual motion machine as it uses an external force. It cant sustain this acceleration and it will slow down after a short while.

How does that explain the treadmill video where there is no wind?

You mean the air wasn't stationary.......it was travelling at the same speed as the treadmill ?

Because that's the only way I can figure out how there would be "no wind".

In the same way, there wouldn't be any "wind" if I opened my car window at say 30mph, if the wind/air outside was travelling at the same speed and in the same direction.

I'm not ure what the point of the OP question was. Further i would question if wind-powered was sufficient for a definition of sailing. I've not had a close look, but it looks like the turbines are still acting in lift. Clearly the car is not being blown downstream. I can see how the car would accelerate with a tail wind, but once the car approaches the speed of the wind, the power generated would be so small that i imagine it would have trouble reaching wind speed and further, accelerating beyond it. of course once faster than the wind the turbine can be powered by the now headwind. It's the transition which I'm struggling with.

In the treadmill, it looks like the treadmill is driving the fan, which in this case is acting as a prop, rather than a turbine

It cant sustain this acceleration and it will slow down after a short while.

I can see how the car would accelerate with a tail wind, but once the car approaches the speed of the wind, the power generated would be so small that i imagine it would have trouble reaching wind speed and further, accelerating beyond it. of course once faster than the wind the turbine can be powered by the now headwind. It's the transition which I'm struggling with.

Check out this vid, no trouble at all with the transition which it accelerates smoothly through! See from about 1:20 if you can't be bothered watching the whole thing.

In the treadmill, it looks like the treadmill is driving the fan, which in this case is acting as a prop, rather than a turbine

Not entirely sure what you are trying to lead me to with those questions.

I'm familiar with the idea of reference frames, though I'm not sure how they explain my underlying concern about how the car can accelerate past the 'no wind' situation, without an external motive force, be in stored inertial, or other.

I can see how the treadmill replicates the 'no wind' with velocity situation. But there is external power too.

It would then seem that once the car is going faster than the wind, it will continue to accelerate unaided 'until the wheels fall off and burn'.

Not that the idea of external / stored power is problematic. Clearly on a still day all you would need to do is push the car and off you go.

I don't care what you all say, I want on of those cars!

On a day like today I could be at work in MINUTES!! (and with no emissions). Why can I not buy one in the shops?

Yes it's possible, but you need a non-fixed sail to do it (kite). With a fixed sail I don't think the combination of apparent wind and real wind vectors would ever be possible to achieve >1.

And I'm slightly confused by that second vid, unless my eyes are deceiving me the prop is turning the wrong way to start with - with the blades in the orientation they are they should be turning the opposite way, at least according to just about every experiment I've tried with props in the wind!

The wee carts in the treadmill video above (and the full-size one, which is exactly the same thing) are very cool and they can run "faster than the wind", but it's not a perpetual motion machine, they can only go a wee bit faster than the wind is blowing.

The propeller on them is just that - a propeller. It's NOT a turbine - the fan is being driven by the wheels, not the other way around.

I have my doubts about whether they could theoretically accelerate from a standing start, but in reality I think they probably could - just the wind acting on the (stationary) fan and the vehicle body would be enough to get them started.

That video has to be a fake.

Anything travelling downwind without gusts, cannot go faster then the wind is blowing.

Crosswind is an entirely different matter, and the speed depends on the overall lift/drag ratio of the machine.

That video must be a fake. The prop is acting as a propellor (look at the blade angles), so could only be driven round if geared to the wheels of the machine which itself is getting blown slowly downwind because of it's parasitic drag.

Sounds like a hopeless case to me, and then to keep on accelerating even further after the apparent wind reverses is impossible.

Actually, thinking further. By utilizing the wind gradient, something like this might just be possible. The wind at the top of the blade arc is faster than the wind at the bottom because of it's friction with the ground. I haven't got my head fully round the implications in this example, but I am sure there are some useful ones!

If there are other model flyers out there who have done any "dynamic soaring" in a wind gradient you will know what I mean.

I was kinda sceptical of the claims initially, but am coming round to thinking that it probably does work, although I am still waiting to hear a satisfactory explanation of why it works...

What I have gleaned so far is this:-

The wheels drive the prop, there is a ratchet mechanism that stops the prop driving the wheels.

This is not some kind of free energy machine, and it won't move when there is no wind.

Apparently this effect has been known about since the '60s.

At standstill, the wind pushes against the whole car, prop included, and this is enough to get the car moving slowly, at this point the prop is turning very slowly, and not really acting as a prop at all. The wind still keeps the same pressue on the car though, so it continues to accelerate slowly.

As the speed of the car increases, the prop will turn faster and faster, until at some point the prop will start to provide forward 'lift'... at this point the car should start accelerating faster. I'd guess the limiting factor in how fast it can accelerate here would be the losses due to machanical friction in the drive train and aero drag, both by the buggy and the prop.

Now, the problem I have is working out how it still takes energy from the wind while travelling faster than the wind... I can't quite see the mechanism for how this happens... and it's bugging me!

What's the tide/current doing?

Nothing.. it's a land vehicle.

@funkynick.

My thought is this: Consider the craft equalling the windspeed measured (for example) at the altitude of the prop hub. The top of the blade arc will still be in a small tailwind, whilst the bottom will be experiencing a small headwind. (There is no wind at zero altitude because of friction, and there will be something in between at the bottom of the arc).
So with the vehicle at "windspeed" (at the arc centre), the bottom of the arc is in a headwind and the blade down there will be driven by the forward movement of the vehicle, the top is still experiencing enough tailwind for the blade there to be able to drive the car a little faster than the wind speed.

So energy is gained from the wind gradient and the rotating "sail" effectively mixes the slow moving air near the surface with the faster moving air up high. This mixing increases entropy (ie mixing of the wind gradient) and thus extracts the energy from the wind gradient itself.

Similar principles allow an non-powered aircaft (ie a glider) to remain airborne indefinitely by flying repeatedly across a wind gradient boundary. This is easy to prove and fun to do if you are a model aircraft flyer.

I think this might allow it to propel itself very slightly faster then the wind.

It's the only way I can think it could ever work.

Why are the top and bottom of the arcs in headwind and tailwind?

Above all else, I cant' see how the pressure balance equations work out if the car is moving forward in headwind

@charlieMungus. Air is viscous, and friction will always mean the bits of air at ground level won't be moving at all, whilst high up you have the greatest windspeed. Naturally in between the wind varies with height. It does this most over the first few metres.
Sea birds such as the Albatross and to a lesser extent even seagulls use this wind gradient all the time to soar the turbulence.

here is a video (happens to be me) flying a model aircraft on the lee-side) (ie the back) of a hill. All the air is going downwards and the model has no engine. - yet it's quite possible to fly the model indefinitely in circles. This model happens to be quite inefficient, it's not very good and is loosing energy all the time. So where has this come from? it's come from causing mixing of the two horizontally moving airmasses.
[url]

If you imagine the model is in fact the rotating blades of the air-car thing, then those blades will be driven round and can extract energy from the CHANGE in the windspeed - not the actual windspeed per se.

Air is viscous, and friction will always mean the bits of air at ground level won't be moving at all, whilst high up you have the greatest windspeed

Ok, so you mean boundary layer effects? Forgot about them.

Do you still call this "boundary layer" on the big scale? I guess so.

I mean that the wind at 1m above the ground is less than the wind at 5m above the ground. - There is energy to be gained by mixing these two airmasses. (In this case with the propeller).

Now, the problem I have is working out how it still takes energy from the wind while travelling faster than the wind...

It's clearly possible, since you can tack downwind and arrive at your destination faster than the hypothetical balloon drifting in the wind - this is done every day by sailors.

Also there does not need to be a correlation between ambient windspeed and the velocity of your vehicle. Imagine a hypothetical device of non specific design for catching energy from wind. It could be very large and catch a lot of energy. However the vehicle itself could be very aerodynamic and not require much energy to slice through air.

So you see how energy harvested from wind is not necessarily directly linked to energy required to overcome wind resistance - unless you are a square rigged sailing ship.

@molgrips "You can tack downwind and arrive at your destination faster than the hypothetical balloon drifting in the wind"
.. Can you really? - even when the destination is directly downwind of the starting point?

I was not sure on that one, so I didn't suggest it!

I can see how the treadmill replicates the 'no wind' with velocity situation. But there is external power too

The treadmill video is the key. There's no wind to speak of but it simulates the point at which the real wind-driven vehicle reaches the same speed as the wind. So no relative wind. And you see the model accelerate.

Can you really? - even when the destination is directly downwind of the starting point?

Pretty sure you can. Did you read the comments after the original article/vid?

http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html

No I didn't read the comments. Thing is this actual vehicle is having it's blades driven by the wheels. There MUST be another source of energy input. Otherwise, when it reaches windspeed, this becomes a perpetual motion machine.

The corollary would be an aircraft which flies forever with one propeller (windmill) driving another (tractor), but in this case the "mill" is the wheels on the ground. Both clearly impossible, so it must be getting it's extra energy from the wind gradient.

Otherwise, when it reaches windspeed, this becomes a perpetual motion machine.

No it doesn't. It's getting energy from the wind. It just so happens that the energy harvested from the wind is greater than the energy required to push it forward. The two aren't linked necessarily.

The corollary would be an aircraft which flies forever with one propeller (windmill) driving another (tractor)

Not really the same. A plane travelling the same speed as the wind is effectively dead still. This car travelling at the same speed as the wind has ground travelling under it, which adds a different energy system.

Focus on the model on the treadmill...

I'm going for lunch now, and then I have to do some work. I'll have a think and get back to you with the definitive physical analysis later on 🙂

My thought is this: Consider the craft equalling the windspeed measured (for example) at the altitude of the prop hub. The top of the blade arc will still be in a small tailwind, whilst the bottom will be experiencing a small headwind

BTW where do you fly - I'm just getting into model aircraft and curious to see others on here.

I'm familiar with the idea of reference frames, though I'm not sure how they explain my underlying concern about how the car can accelerate past the 'no wind' situation, without an external motive force, be in stored inertial, or other.

I can see how the treadmill replicates the 'no wind' with velocity situation. But there is external power too.

You could if you wanted to put the treadmill in an enclosing bubble on the back of a vehicle. Accelerate the vehicle up to the same speed as the wind, and also make the treadmill go (backwards) at the same speed as the external wind. Nothing has changed for the treadmill experiment from it's inertial reference frame so it will still work just the same.

Clearly at this point you could remove the bubble and it would make no difference to the experiment as the apparent wind is zero. You could then take the vehicle off the treadmill and put it on the ground and that would make no difference either, as the top surface of the treadmill is stationary relative to the ground. Now you have the cart in zero apparent wind moving forwards at the same speed as the wind and wanting to accelerate...

Agreed aracer, hence you need another energy source!

With linear wind, once the craft reaches windspeed, it's effectively dead calm. There is simply no where for any energy to come from to accelerate it further - unless you accept that parts of the craft are experiencing differing windspeeds, ie wind gradient. But do look up "dynamic soaring". The propeller on this craft must effectively use "Captive dynamic soaring" (I just inventend that term). -Watch my vid and explain how that model flies in circles!

Once the craft is going faster than the wind, then I believe it [I]CAN[/I] be driven, and losses made up by using the wind gradient.

On the other point, I read the sailing comments link, and I'll certainly believe that a boat sailing broad reaches could get downwind faster than the same boat sailing directly downwind. But but I'm still not yet convinced it could do it faster than the windspeed.

I've thought about this long and hard for the last day or so. I can conclude that the actual answer is 'its magic'.

HTH

Wee bit of maths to put on it:

The power used by the propeller is the propeller thrust x the airspeed

Pp = T x Va

The power "generated" by the wheels is the resistance to forward motion x the ground-speed.

Pg = R x Vg

Now, the resistance to forward motion includes the torque required from the wheels to power the propeller and also the aerodynamic drag, rolling resistance of the tyres, etc. But hopefully we can agree, for the sake of argument, that these drag forces are relatively small. Let's lump them in with "transmission losses" and call it efficiency (m), which will be a number less than 1.

So, the power available at the propeller is the power "generated" at the wheels x the efficiency.

Pp = Pg x m

So T.Va = R.Vg.m

Now, if you're travelling just a little bit faster than the wind, what does that do for the equations above? Let's say there is a wind-speed of 19m/s and you're travelling at 20m/s. You therefore have a ground-speed of 20m/s and an airspeed of just 1m/s.

So T.Va = R.Vg.m gives you T = 20 x R.m

So, assuming m > 0.05 , then Thrust is bigger than resistance, so the vehicle accelerates.

We'd expect m to be pretty high, certainly much higher than 0.05, 'cos transmission losses in a mechanically-geared system will be pretty low, aerodynamic drag will be pretty low (you only have an airspeed of 1m/s remember) and rolling-resistance is never terribly big.

If there was no wind, then there would be no difference between airspeed and ground-speed, so there would be no unbalanced force available to drive the vehicle.

Clear as mud?

So hang one... This works with a propeller type arrangement, but would the same sort of motion/energy capture be possible with a cylinder-type turbine? (i.e. one that rotates around a vertical shaft, rather than blades around a horizontal one)

I only ask because I got blown to shit on the way home and back this lunchtime on my bike, and the idea of using some sort of wind-powered vehicle on this sceptered isle as a means of transport appeals to me.

Just put a sail on the bike! (BTW I did once and it worked OK. Quite exciting till I ended up in a ditch).

And a vertical axis mill would never allow you to go faster than the wind whilst downwind according to my concept of mixing the vertical wind gradient - Which is the main point being made here.

Stevo, power is a good way of looking at it, that's where I was struggling over lunch trying to use forces 🙂

There's a slight difference between the real world car and the model on the treadmill. The model is held still until it gets up to speed, which means it's forced into the situation where apparent wind = 0. The real car would never reach that condition because there'd need to be enough relative wind to match rolling resistance.

So I think that the tower the prop is mounted on is an aerofoil to provide a thrust vector with just enough forward component to overcome this. Wouldn't have to be much.

Mountaincarrot - it's got nothing to do with vertical wind gradient. Otherwise, how would the model accelerate?

The model initially accelerates because of prasictc drag from the superstructure.
The main point is that in a linear wind there is simply no energy input to make it move faster than the wind. It's a perpetual motion machine UNLESS you have another energy source. So what is that if it's not the wind gradient?

If this thing can work by any other means, then if you put it on a moving converyor belt, it would drive off the front on it's own. THAT won't happen.

There. I had to get a conveyor belt in somehow.

in a linear wind there is simply no energy input

Energy is extracted from the difference between the windspeed and groundspeed. If it worked with no wind (relative to the ground) that *would* be perpetual motion.

Something that might help get your head around it: as was pointed out above an ordinary sailing boat can outrun a tailwind by tacking across it. In this case the propellor blades are, in effect, also tacking across the wind as they rotate.

If this thing can work by any other means, then if you put it on a moving converyor belt, it would drive off the front on it's own. THAT won't happen.

Is that not exactly what the model video shows above?

The treadmill video is the key. There's no wind to speak of but it simulates the point at which the real wind-driven vehicle reaches the same speed as the wind. So no relative wind. And you see the model accelerate.

The problem with this is that, in this scenario, the treadmill is turning the wheels at a rate fast enough to generate lift on the propeller. I think at zero, wind or to be more accurate, when the balance of forces is such that there is no overall force (some balance between the motive force on the wind and system friction) I cant see where the acceleration comes from to 'tip' it over. The treadmill works, because it is already moving fast enough to provide the energy to propel the car forward. In effect the car is being dropped into headwind.

I'm not sure I see Mountai carrots reasoning, but it is consistent with the idea that the car would not work if there wasn't wind.

For comparison-

Speed record

The world land speed record for a wind powered vehicle was broken on 26 March 2009 by Richard Jenkins in his yacht Greenbird with a speed of 126.1 mph (202.9 km/h).[b]Wind speeds were fluctuating between 30–50 mph (48–80 km/h) at that time[/b].

The previous record of 116 mph (187 km/h) was set by American Bob Schumacher on March 20, 1999 driving his Iron Duck vehicle. Both records were set on Ivanpah Dry Lake near Primm, Nevada, USA.

But this would not have been directly downwind but on a very broad reach. A kitesurfer has also just popped the magic 100kmh mark to set a new water based record. When I was windsurfing a lot 40mph had just been beaten - never thought I would see the day that record had been beaten by over 50% by a bloody tea-bagger!

Yet to get my head around this bladed nonsense.

I cant see where the acceleration comes from to 'tip' it over.

If you look at my worked-example above, you can see that you actually get the biggest thrust to drag ratio when the speed of the car is a very small amount greater than the wind speed (therefore biggest ratio of ground speed to airspeed).

In a very theoretical steady wind, then this might be an issue. In the real world it's not as the slightest drop in wind speed will give you a decent amount of thrust to accelerate away. Equally, if you're already accelerating as you approach windspeed, you may have enough grunt to carry you over.

I cant see where the acceleration comes from to 'tip' it over

That thing the prop is on is an aerofoil I reckon. I bet it's moveable to gain just a smidge of forward thrust.

That thing the prop is on is an aerofoil I reckon. I bet it's moveable to gain just a smidge of forward thrust.

Not in the direction of (or opposite) the wind, i don't think.

Yet to get my head around this bladed nonsense.

Do you mean aerofoils etc?

The best way is to think of an aeroplane wing. The flow over it generates enough lift to pick a jumbo jet up. now imagine that instead of a jumbo, you hand a windsurfer attached to it. The force would be pretty much the same but with much less mass, the acceleration is governed by F=ma. So there would be huge acceleration, till the drag (and weight) force, was equal (and opposite to the lift force). As such the speed the wing travels at (in general perpendicular to the flow direction) is sort of not generally dictated by the speed of the wind.

Ex pilot so happy with the concept of aerofoils - it's the possibility of a vehicle acting in the manner shown on the vid that I am yet to comprehend.

Ex pilot so happy with the concept of aerofoils

Phew!

Design team's own explanation here:

[url= https://docs.google.com/View?docID=0AdRsKX7aaZTPZGRnbjhkajdfMTY0aGRzNWtnaGM&revision=_latest&hgd=1 ]Thin Air Designs FAQ[/url]

That's almost identical to mine! 😀

5 years of Mechanical Engineering weren't wasted after all!

They keep saying 'tailwind' but they must mean 'headwind' surely? At least in our cyclist terminology.

They keep saying 'tailwind' but they must mean 'headwind' surely? At least in our cyclist terminology.

It's a tailwind when you are stationary, but when you are travelling faster than [s]light[/s] the wind speed you have a headwind, no?

It's a tailwind when you are stationary, but when you are travelling faster than light the wind speed you have a headwind, no?

In cycling terms, No. On a bike no matter which direction you go or how fast, there will always be a headwind.