Directly UPWIND faster than the wind

A.T.

The Blackbird Landyacht which went 2.8x faster than the wind directly downwind, has been outfitted with a turbine and is now going directly upwind, faster than the wind. The official numbers are not in yet, but estimates are 1.5 - 2x windspeed.

The DUWFTTW blog:
https://fasterthanthewind.wordpress.com



For comparison: At the Aeouls races the upwind vechicles achieve max upwind speeds of 0.75 windspeed. But their rotor size is limited by the rules:
http://www.youtube.com/watch?v=gf2A3-TRf1A

The old threads about directly downwind:
http://www.physicsforums.com/showthread.php?t=421733
http://www.physicsforums.com/showthread.php?t=562993

sophiecentaur

Very humbling when I am struggling to make any distance at all when 45 degrees off the wind in my dear ol' Westerly Centaur sailing cruiser, 'Sophie'. But you can't go below in that machine above and make a cup of tea!

A.T.

Some made boats like this. They also go directly upwind, but not as fast:

sophiecentaur

Not available on eBay yet?

spork

Some raw video from the runs. It looks like we'll be submitting data for a run at 2.1X wind speed directly upwind.

http://www.metacafe.com/watch/867923...than_the_wind/

CWatters

Nice work.

It's amazing how many people still don't think it's possible to sail directly up wind. Even when videos are available.

cosmik debris

Yes, no one can fake a video, right? BTW I'm not saying that these are fakes only that a video is not the best of proofs.

spork

That's why we documented everything about the project to the best of our ability and posted it in our build blog. We also set a record - not because we wanted a record, but because it would necessarily involve a trusted and disinterested 3rd party scrutinizing our efforts.

Finally, I posted a set of build videos so anyone could make their own working model for just a few bucks and demonstrate it for themselves on a treadmill in their own living room.

I tried to do everything but go door-to-door. Some people find that creepy.

A.T.

In Australia ME students have to build DUW carts in introduction courses:



One can also buy build sets:



However, going 2.1x windspeed directly upwind would be significant. A paper by Blackford from 1978 claimed that 2x directly upwind would be the absolute theoretical limit. The response by Ruina was never published:
http://ruina.tam.cornell.edu/researc...e_pull-you.pdf

spork

Andy Ruina and I have become friendly acquaintances over the last couple of years. Interestingly, he's done some really good work on another topic that I've debated hotly on internet forums - namely bicycle dynamics (and particularly the need to counter-steer at all speeds). He's a very very sharp, friendly, and interesting guy.

Don't be too surprised if you see a paper on wind powered vehicles from him published after all.

rcgldr

I seem to recall something in one of the previous threads that with similar efficiency, upwind would be about 1x slower than downwind, so if 3x downwind, then about 2x upwind.

A.T.

That is true for low total efficiency. The difference gets smaller for higher total efficiencies.

Check out equation 9 (upwind) and 20 (downwind)

http://orbit.dtu.dk/fedora/objects/o...748519/content

Total efficiency -> wind-speed multiple:



I assume the Blackbird reaches around 0.7 total efficiency at max. speed. Note that his total efficiency already includes aero-drag and rolling resistance.

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harrylin

When I heard about these things I first though" no way", based on wrong (simplistic) intuition - and it took me some time to figure out how/why it can work.
Link please?

rcgldr

Doing the math for (downwind - upwind):

1 / (1-x) - 1 / (1/x - 1) =
1 / (1-x) - 1 / (1/x - x/x) =
1 / (1-x) - 1 / ((1-x) / x) =
1 / (1-x) - x / (1-x) =
(1-x) / (1-x) =
1

A.T.

Indeed.

On the other hand: for 2x upwind you have 1.5 times the relative headwind, of 3x downwind, so 2.25 times the drag. This is why that total efficiency is tricky, because it not only affects max. speed but also depends on the speed.

It is also not clear if the 2.8x (3.4x unofficial) was the Blackbird's limit for downwind. In stronger winds it might have gone even faster efficiency-wise, but was prevented by transmission power limits (chain broke).

A.T.

http://www.instructables.com/id/Down...The-Wind-Cart/

rcgldr

I thought I already removed that part from my previous post. You have less ground speed but more wind speed for the upwind case, but I assume the formulas in that pdf article already took that into account, so I removed the comment about ground speed in the upwind case.
I'm thinking that 3.4x was closer to it's limits. I'm not sure if a stronger wind helps. There's probably an ideal wind speed, above which reduces efficiency, perhaps due to drag being related to speed². The ice boats seem to get their best ratio of boat speed VMG (the directly downwind component) versus wind speed with a wind speed around 10 to 16 mph (16 to 26 kph), or at least the ratio at 10 to 16 mph is better than the ratio above 30 mph. Maybe the issue is the wind tends to be more gusty if the average wind speed is over 30 mph.