Downwind Faster than the Wind - Successful Run by manned cart

I am probably one of the many who have limited knowledge/ability to get their head round this. I (think) I understand enough to see how it just may be possible - I just find it hard to imagine how there is enough windage to drive the prop in an oposite/thrust direction what with all the drag/losses, but , if it can be, I see that prop thrust against the headwind could cumulatively get you faster than the wind (I think). I would like to see all wheels lifted off the ground prior to the run (to exclude the possibility of stored energy) I would also like a good look over and in the vehicle instead of grainy distant video. I would also like to see a balloon released at the same time that motion started in the sled, as (and please don't shoot me down here) untill the released balloon has been caught up and overtaken.....has it really sailed DDWFTTW.....I want to believe this is true ...I'm just having a bit of difficulty accepting it!
 
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jokerboat said:
Yes if there is a wind blowing. I was responding to Woodlouse's comment about it working in zero wind. I think in that situation it should work with a shove.
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Won't work in zero wind. If you give it a shove, from the moment you stop pushing it'll start slowing down from drag and friction.
 
Woodlouse said:
Snowleopards drawings are wrong, as the whole point of this discussion is to show motion in the same direction as the true wind, where as the diagram shows a broad reach in relation to true wind and motion.
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Can you not understand the 3rd diagram?

The wind acts on the blades of the prop, not the chassis of the cart. The blade is moving along a spiral path created from the rotation of the prop across the true wind and the forward motion of the cart in the same direction as the true wind.
 
Keen_Ed said:
Won't work in zero wind. If you give it a shove, from the moment you stop pushing it'll start slowing down from drag and friction.
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Except for that whopping great propellor turning around, which will keep driving it forward. And since UG tells us that the shaft work required to turn the prop is less than the thrust you get out if it, it should keep going.
 
Woodlouse said:
What I'm saying is that once you exceed the true wind speed, whilst running dead down wind, the force that has initially pushed the cart has stopped acting upon it, which means that the energy is coming from somewhere else.
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No, the energy - relative to the ground - always comes from the same place.

So I'll try again, the wheels are turning the propeller, which then pushes the cart that turns the wheels
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The wheels turn the propeller, allowing the wind to push the propeller, and thereby push the cart and turn the wheels.
 
jokerboat said:
I'll confess I was very much in the No camp, but then started to waver.

Ignoring the mathe, which is beyond me, those videos clearly show it working. The "how it works" bit is still a bit confusing but then so is nuclear physics.

I don't need to know how something works to accept that it does.
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Just been reading through a bit of this thread ... and I'm probably wrong ... but ....

The truck initially just blows along the ground - ala Avon dinghy you forgot to tie off ... (or with a nudge for the big models)

The movement forward rotates the wheels which turns the prop - which pushes air backwards - but you've still got air coming forwards - so you're effectively creating a mini high pressure behind the cart and it's this cushion of air/high pressure that pushes the cart onwards ....

Take away the true wind and the truck will stop ...

??
 
fireball said:
The movement forward rotates the wheels which turns the prop - which pushes air backwards - but you've still got air coming forwards - so you're effectively creating a mini high pressure behind the cart and it's this cushion of air/high pressure that pushes the cart onwards ....
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Yup, for all practical purposes that's it. Just add that the cushion of air is blowing backwards, so can still do work against a headwind.

[quote
Take away the true wind and the truck will stop ...
[/QUOTE]

Yup.
 
fireball said:
The movement forward rotates the wheels which turns the prop - which pushes air backwards - but you've still got air coming forwards - so you're effectively creating a mini high pressure behind the cart and it's this cushion of air/high pressure that pushes the cart onwards ....

Take away the true wind and the truck will stop ...

??
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Strictly, like a sail, it's the higher pressure behind the blade and lower pressure in front but - yes, correct. All this talk about moving in still air is just as silly as the proponents intend it to be.
 
I still believe that, at low speeds, the wheels drive the propellor. At some point, whilst there is still an apparent tailwind for the cart, there is an apparent headwind for the prop, and the prop then starts driving the wheels. Once past the point of zero apparent wind for the cart, you are into a fully apparent headwind. I think it is agreed that a boat can progress directly into wind when a large airscrew drives a water propeller via gearing; this is the same, except that the wheels will have little slip, unlike the prop in the water.
Incidentally, as the wheels, via the aerodynamic drag's push, are driving the prop at low speeds, this will give some forward thrust up to the change-over point.
 
Another view from Spork. (one of the team, AIUI)

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The prop produces the thrust that pushes the cart, which turns the wheels, which provide the torque to turn the prop, which produces the thrust... Of course this sounds like perpetual motion, but it's really a simple feedback loop being powered by the wind (motion of the air relative to the ground).



This is priceless!
 
Keen_Ed said:
Won't work in zero wind. If you give it a shove, from the moment you stop pushing it'll start slowing down from drag and friction.
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Which is exactly what I think would happen as soon as the cart exceeds the true wind speed.

If what you are all saying is true, then what is stopping the contraption at only 2-3 times the wind speed? Surely it should just accelerate indefinitely as the faster it goes, the faster it can power its self along?
 
shaunksb said:
What limits it to "about 3 times wind speed"?
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The efficiency - lift/drag ratio of the prop. The overall drag - from friction in the bearings etc and aerodynamic drag.

In the same way when you're sailing a racing catamaran on a broad reach. You accelerate, so you pull the sail in to take account of the apparent moving foward and increasing. So you accelerate some more. So you pull in the sail some more. Eventually, you hit the lift/drag limit and you've reached top speed.
 
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