Wind driven cart goes downwind faster than the wind

[ceptimus]

Most people are familiar with the fact that ice yachts, sand yachts and even some water-born craft like windsurfers sometimes go faster than the wind that drives them - but this is when the vehicles are reaching (sailing crosswind).

What is less well known is that ice yachts are able to sail downwind faster than the wind - for example in a 15 mph northerly wind, an ice yacht can 'tack' downwind (south) - its speed over the ice might be 30 mph and the southern component of that speed might be 20 mph.

So a small balloon can be released from a stationary ice yacht and allowed to blow away at the speed of the wind - the ice yacht can then set off after the balloon and overtake it!

Some guys set out to build a cart that would drive DIRECTLY downwind, faster than the wind, and they've now succeeded! It all seems a bit counter-intuitive at first but the physics, and the actual results are sound. So far they've managed over twice the speed of the wind, and they're now aiming to go three times as fast.



You can read their blog and view the photos here.

[Sock Puppet]

Oh crap, now that interminable thread from TR will bleed over here.

[Deadlokd]

I'll keep this short and sweet then. So the wind turns the propeller which turns a turbine which provides electricity to turn the wheels?

A simple yes or no will suffice.

[Sock Puppet]

Oh, I'm not complaining about the discussion of the cart itself. There are just some ... odd people and some amazingly tenacious flamewars going on over there.

[Deadlokd]

I haven't read the thread. Megathousand posts is too many to catch up on. I'm only up to page 1747 in the Countdown thread here for example.

[ceptimus]

Originally Posted by Deadlokd

I'll keep this short and sweet then. So the wind turns the propeller which turns a turbine which provides electricity to turn the wheels?

A simple yes or no will suffice.

No.

[Deadlokd]

So how does it work?

[ceptimus]

What happens is that power is extracted from the wheels (so the wheels tend to slow the cart down) and fed to the propeller which pushes the cart forward.

With no wind and a 100% efficient propeller and transmission system, the drag of the wheels would equal the thrust from the propeller and the cart would neither accelerate nor slow down.

Of course the propeller and transmission aren't 100% efficient, so the cart would quickly come to a standstill in no-wind conditions. But when a wind is blowing, it adds to the thrust generated by the propeller and the cart accelerates.

[Ensign Steve]

Why don't you crawl in a hole and die, Sockpuppet?!

[mickthinks]

Originally Posted by ceptimus

It all seems a bit counter-intuitive at first but the physics, and the actual results are sound.

It is certainly counter-intuitive and frankly, I don't believe it. Do you have a link to the physics? I'm wondering which direction the propeller turns as the cart starts overtaking the wind? Does it change direction, and if not, why not?

[ceptimus]

The propeller always turns the same way - in the direction that pushes the cart forward through the packet of air in which the cart sits. There is a simple chain drive linking the rear axle of the cart to the propeller shaft.

From a standstill, the cart is just blown forward like any ordinary cart would be when standing in a wind - the gearing between the prop and the wheels is not sufficient to drive the cart INTO the wind. The wheels turn and this makes the propeller turn pushing the cart forward faster.

If the propeller were replaced by a sail, the cart could only ever reach the speed of the wind, and with friction losses would always go slightly slower than that.

One way to think of the propeller is that it is sweeping out a virtual surface (a helical plane) that is always moving backwards relative to the cart. You can think of the wind 'pushing' against this virtual surface - and when the wind is able to push this virtual surface at or near wind speed then the cart will be going faster than that due to the surface always moving backwards relative to the cart.

[F-X]

Originally Posted by Sock Puppet

Oh crap, now that interminable thread from TR will bleed over here.

Probably not. There are about 20 threads about this all over the place, so it isn't that big a danger.

[Doctor X]

--J.D.

[F-X]

Well the official trials are over. They got at least 3 times windspeed. Richard Jenkins - World Land Speed Record Holder, the dude that ran the Greenbird to fame, he heard about this and showed up. Video he shot, and a guest blog entry here

Ride Like the Wind (only faster)

I love how he calls them idiots and shit like that.

A few months later I actually met the idiots in question and, to my surprise and concern we not only have a few mutual friends, but they seemed to be rather technically credible. But, everyone makes mistakes, and I let them off as decent people with a blinkered view of fundamentally flawed engineering....

Looks like spork and JB will be heading to Letterman and beyond. They certainly deserve it.

[naturalist.atheist]

Originally Posted by ceptimus

Most people are familiar with the fact that ice yachts, sand yachts and even some water-born craft like windsurfers sometimes go faster than the wind that drives them - but this is when the vehicles are reaching (sailing crosswind).

What is less well known is that ice yachts are able to sail downwind faster than the wind - for example in a 15 mph northerly wind, an ice yacht can 'tack' downwind (south) - its speed over the ice might be 30 mph and the southern component of that speed might be 20 mph.

So a small balloon can be released from a stationary ice yacht and allowed to blow away at the speed of the wind - the ice yacht can then set off after the balloon and overtake it!

Some guys set out to build a cart that would drive DIRECTLY downwind, faster than the wind, and they've now succeeded! It all seems a bit counter-intuitive at first but the physics, and the actual results are sound. So far they've managed over twice the speed of the wind, and they're now aiming to go three times as fast.



You can read their blog and view the photos here.

Of Course! They are tacking sideways!

[F-X]

You have GOT to go over to TR and post that in the thread.

[Dingfod]

This wind-driven wessel did not go downwind faster than the wind, but it might have eventually.

[Ensign Steve]

"wessel"

[F-X]

It's official.

[Ensign Steve]

Hmm, that doesn't sound right. Are you sure? I watched one or two episodes of that show late night on Sci-Fi Channel this one time when I was really drunk, and I'm pretty sure I would remember something like that.

lol, nice edit.

[ChuckF]

Originally Posted by Ensign Steve

"wessel"

Ensign Steve I thought you might like to know that this is actually a reference to a television program called The Original Star Tracks where there was a character who said "wessel."

[Ensign Steve]

Okay, now you guys are just messing with me.

[F-X]

YouTube- The Nuclear Wessels?

[ChuckF]

Robin Williams was good in that.

[Ensign Steve]

Originally Posted by F-X

YouTube- The Nuclear Wessels?

WTF is this shit? According to wikipedia, the Star Tracks is about outerspace, not San Francisco in the 80's.

Nice try, guys, but you have to get up pretty early in the morning to put one over on me!