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Downwind Faster Than The Wind - update

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Bille
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Re: Downwind Faster Than The Wind - update

Postby Bille » Fri Jun 28, 2019 1:57 am

spork wrote:
Thu Jun 27, 2019 5:01 pm
Long lines won't solve the straight downwind problem. The problem is that, to go directly downwind faster than the wind, you have to make the wind work against the water in the downwind direction.
I "GET" it now !

Thanks : Bille
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Re: Downwind Faster Than The Wind - update

Postby spork » Fri Jun 28, 2019 4:39 am

Are the physics behind it explained somewhere in this thread?
No, but if you have a couple of hours to waste on a really boring video, you can watch me try to explain it at the St. Francis Yacht Club...


I understand that when you are going the same speed as the wind, the apparent wind is 0, but because of the kinetic energy of the cart driving the propeller, it's basically the same as driving the propeller in no wind...
Yes and no...
It doesn't rely on the kinetic energy of the cart to get past wind speed. It's true that there is no wind relative to the vehicle at wind speed, but there's still wind relative to the ground. And even at exactly wind speed, we're still slowing that wind down. So we can go exactly windspeed all day long - even up a slight incline.

But it's true the the propeller is behaving as it would if it were spinning in zero wind, much like a small plane starting to taxi. So producing thrust is no problem, but many people find it counter-intuitive that the energy balance can work out in your favor at exactly wind speed.
I can imagine this only working in very little wind to reach more than 2x windspeed, am I right?
It's true that it becomes harder to achieve multiples of wind speed in higher wind. This is because it requires a very efficient overall process to get high multiples, and you lose lots of efficiency when you begin to get the tips of the propeller in the realm where the flow is compressible (around Mach 0.3). I think we achieved about 3.1X wind speed in 10-15 mph wind unofficially, and 2.8X wind speed in 10-12 mph wind for the record runs.
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Re: Downwind Faster Than The Wind - update

Postby cglazier » Fri Jun 28, 2019 6:28 am

Spork I have been following your project from the beginning and I have always been enthusiastic. It took me a while to appreciate how it actually worked. It is most counter intuitive and very impressive.

Even now when I show some people the video of you going downwind faster than the wind their eyes just glaze over not understanding that it can be possible.

Thanks for all your work on this.

😉
CG

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Re: Downwind Faster Than The Wind - update

Postby spork » Fri Jun 28, 2019 6:52 am

Thanks!

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Re: Downwind Faster Than The Wind - update

Postby Regis-de-giens » Fri Jun 28, 2019 7:38 am

Love this faster than wind thing and thinking about the math for a long time. I have not followed these 30 pages so maybe it has already been discussed :
To me the rotating wing is neither a propulsor nor a generator. It is "in between" actually, working as a spinacker sail pushed by the wind . Its rotation (combined to wing angle of attack) is just here to "divide" the cart speed at wing level so that the wing surface always go lower than the cart, hence still pushed by the wind.
Sorry if already discussed earlier, but it's such a strange arrangement... i love it
Last edited by Regis-de-giens on Fri Jun 28, 2019 12:37 pm, edited 2 times in total.

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Re: Downwind Faster Than The Wind - update

Postby Herman » Fri Jun 28, 2019 9:36 am

Watched and thoroughly enjoyed the video. What a way to prove a point! Congratulations!

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Re: Downwind Faster Than The Wind - update

Postby Laughingman » Fri Jun 28, 2019 6:41 pm

ooooooooh! so funny thing, I always thought it was the propeller that turned the wheels, up to a certain point and then at that point the wheels would turn the propeller, figured you could change the angle of the propeller to pull instead of push... if that makes any sense.... I am way less confused now.

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Re: Downwind Faster Than The Wind - update

Postby spork » Fri Jun 28, 2019 7:53 pm

Laughingman wrote:
Fri Jun 28, 2019 6:41 pm
ooooooooh! so funny thing, I always thought it was the propeller that turned the wheels, up to a certain point and then at that point the wheels would turn the propeller
Interestingly we have run the vehicle in two modes - upwind and downwind. In the downwind mode the propeller is *always* a propeller (which is to say that the wheels turn it) below, at, and above wind speed.

In the upwind mode, the big spinny thing acts as a turbine (which is to say that it turns the wheels) at all speeds.
...figured you could change the angle of the propeller to pull instead of push...
Initially we did not implement variable pitch on the propeller. It still worked fine going directly downwind faster than the wind, but the startup is very slow (which was to be expected). We later added variable pitch, and that saved us a lot of runway in getting up to speed. But we never move the pitch from pusher to tractor or vice-versa on any run.
Last edited by spork on Fri Jun 28, 2019 8:57 pm, edited 1 time in total.

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Re: Downwind Faster Than The Wind - update

Postby knotwindy » Fri Jun 28, 2019 8:46 pm

Great vid, not a waste of time or boring at all. I assume in the post above you meant “ ‘upwind’ mode, the big spiny thing acts as a turbine”?

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Re: Downwind Faster Than The Wind - update

Postby spork » Fri Jun 28, 2019 8:58 pm

knotwindy wrote:
Fri Jun 28, 2019 8:46 pm
Great vid, not a waste of time or boring at all.
Thanks!
I assume in the post above you meant “ ‘upwind’ mode, the big spiny thing acts as a turbine”?
Yes - that's exactly what I said (now that you caught it and I changed it :) )


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