Viewing 40 posts - 1 through 40 (of 82 total)
  • We've done planes on elevators…
  • aracer
    Free Member

    How about: is it possible for a wind powered vehicle to “sail” directly downwind faster than the wind is blowing?

    TheFunkyMonkey
    Free Member

    No

    TheSouthernYeti
    Free Member

    Is it going down a hill?

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    TheFunkyMonkey
    Free Member

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

    TheSouthernYeti
    Free Member

    Are really strong magnets involved?

    iDave
    Free Member

    all things are possible if you believe

    falkirk-mark
    Full Member

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

    Oggles
    Free Member

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

    ernie_lynch
    Free Member

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

    CharlieMungus
    Free Member

    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)

    aracer
    Free Member

    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.

    CharlieMungus
    Free Member

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

    ernie_lynch
    Free Member

    Has it got any booster rockets ?

    aracer
    Free Member

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

    CountZero
    Full Member

    Yes. 2.8 times faster than the prevailing wind.

    Downwind faster than the wind: Blackbird sets a record

    can-uk
    Free Member

    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..

    iht4
    Free Member

    yes – its all about apparent wind

    seriously!!

    mcmoonter
    Free Member

    Wind car sponsored by Joby

    [video]http://www.youtube.com/watch?v=kmpRQLaxF60[/video]

    damo2576
    Free Member

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

    IanMunro
    Free Member

    Instinctively I would have said no, but after reading http://dwfttw.blogspot.com/, 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.

    aracer
    Free Member

    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:
    [video]http://www.youtube.com/watch?v=7xL8gRJ5F6k&feature=related[/video]

    ernie_lynch
    Free Member

    does anybody on here understand how it works?

    It works on exactly 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)

    aracer
    Free Member

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

    nickc
    Full Member

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

    muddy@rseguy
    Full Member

    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.

    ernie_lynch
    Free Member

    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.

    CharlieMungus
    Free Member

    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.

    CharlieMungus
    Free Member

    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

    aracer
    Free Member

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

    Nope – it can keep going as long as it likes.

    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.
    [video]http://www.youtube.com/watch?v=5CcgmpBGSCI[/video]

    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

    You’re almost there – what do you think the relevance of the treadmill video is, and do you understand the concept of inertial reference frames? http://en.wikipedia.org/wiki/Galilean_invariance

    CharlieMungus
    Free Member

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

    CharlieMungus
    Free Member

    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.

    willard
    Full Member

    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?

    coffeeking
    Free Member

    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.

    coffeeking
    Free Member

    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!

    stevomcd
    Free Member

    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.

    mountaincarrot
    Free Member

    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.

    mountaincarrot
    Free Member

    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.

    funkynick
    Full Member

    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!

    DaRC_L
    Full Member

    What’s the tide/current doing?

    funkynick
    Full Member

    Nothing.. it’s a land vehicle.

Viewing 40 posts - 1 through 40 (of 82 total)

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