I googled DDWFTTW and still don't get it.

Discussion in 'Boat Design' started by Squidly-Diddly, Nov 20, 2011.

1. Joined: Sep 2007
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Squidly-DiddlySenior Member

Any help?

I assume we are talking about straight down wind, sustained steady state.

Once it goes as fast as the wind, what makes the prop turn?

I'd say the prop loses energy as the craft approaches wind speed and would become zero if craft matches wind-speed, and would require energy to spin the prop once past wind speed.

What don't I understand?

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NoEyeDeerSenior Member

IIRC the prop is driven by the wheels at high speeds. I'll find the explanation.

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Jim_HbarJunior Member

Directly
Down
Wind
Faster
Than
The
Wind

You left yourself wide open on that one!

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NoEyeDeerSenior Member

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Squidly-DiddlySenior Member

Wheels turn prop? thats diff. Still a brain teaser.

Any equations on all this? With LOTS of pictures and children's book style examples???

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daiquiriEngineering and Design

These are the facts you have to consider, in order to understand how it works:
1. When starting from the zero ground speed, the driving force is the aerodynamic drag of the vehicle (prop + mast + all the rest of it). So it is not difficult to comprehend how it can start moving downwind.
2. When moving at any speed different from zero, the wheels-to-prop gear ratio is such that for every unit of distance covered by wheels, the through-air distance of the propeller is much higher than a unit. This is what allows it to get through the "windspeed barrier". When the vehicle is moving at the speed of the wind, the propeller is rotating at such speed that it still produces thrust. In this particular moment, the propeller gives a bollard pull, to use the nautical terminology.
It is convenient to remember a simple fact that, at any speed, the rotation of the propeller is driven by wheels through the higher-than-unit gear ratio.
In other words, the wheels provide the torque for the prop and the prop provides the thrust for the vehicle.

It is somewhat (just to make you comprehend it through a nautical example)) similar to a situation when forward throttle is applied to a boat moving sternwise (negative speed). The propeller will give positive (forward) thrust in every moment (even at negative speeds), while the speed will gradually change from negative to positive. In the moment when the boat moves through the zero speed, the prop will give the thrust equivalent to the bollard pull, and will keep accelerating to a positive-speed regime.

Hope it makes things more clear for you.

Cheers

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CloxxkiJunior Member

It's like this.
Imagine a 15m² sail, or turbine, or even rear end of a big truck. It can extract a serious amount of energy from the wind.
Say, E amount of energy as wind speed V.

This amount of energy would propel a smaller vehicle, say a 1m² go-kart, to 3*V. If only you could get that big sail to transfer power to the quicker go-kart. ..

That's where the prop comes in.
The kart starts rolling in a tailwind, just like a landyacht would. The wheels are turning.
The wheels' axle drives the propellor, it turns much as on an airplane, hovercraft, swampboat, etc. It's geared such that it's thrusting air, throwing it back into the tailwind.
Due to the propellor "crawling" through the wind at an angle, it's going quicker than the wind. It doesn't have the speed limit V for a full open sail going directly downwind. It's like a gearbox for the sail. Each blade being a sail.

[The gearing between the wheel axle and the prop, pitch, etc, greatly affects the acceleration and top speed. In fact, if you change the gearing (and ideally mount a turbine-specific designed spinner, the cart becomes able to drive straight into a headwind. This is well-established, international races ar held for this.]

You'll see that the DDWFTTW cart itself is kept aerodynamic. It will be facing fierce apparent headwinds as it reached 3*V. It only has the power from the whole vehicle worth of tailwind. And there's some rolling resistance to deal with. The wheels may even skid a bit, due to the drag the prop tries to put on the wheels. On a slippery ice track, this won't work without spike tires, it needs the "braking" traction!

Let me know if I can help further. I *think* I understand it, and really, REALLY want to apply this stuff for the betterment of mankind, or general scientific fun projects.

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NoEyeDeerSenior Member

Nope.

The cart relies on the prop moving less than one unit through the air. From the link I posted:

Squidly: try Spork's jello block explanation in the next post after that one.

(Incidentally, Spork is one of the team that built the Blackbird. I assure you he knows how it works)

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daiquiriEngineering and Design

Oh, it's so nice to discover that I didn't get a heck about it till now. Not that I've ever really applied actually, but still...
Thanks for the correction. Apart the wheel-to-prop gear ratio, is the rest of my post correct?

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yipsterdesigner

think above under the search button of this good old forum you'll find the most info on "ddwfttw"
as well as the much easy'r to comprehend sailing straight upwind mills on the net

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CloxxkiJunior Member

Harder to explain, but the Blackbird team is now aiming to reach >1* windspeed, directly upwind. www.fasterthanthewind.org
It will experience 2* wind speed worth in apparent wind when matching windspeed. Ride your bicycle upwind and'll hear and experience the wind blow harder than when stationary, right?
In their DDWFTTW attempts, at times they reached the same parameters. When going 3* wind speed DDW, the apparent wind is also 2*.
So, with the turbine specific blades and lowered gearing, it looks like they'll make it. Advantage will be that the rolling resistance will be only 1* wind speed, where they had nearly 3* in the DDW record.

Last edited: Nov 22, 2011
12. Joined: Oct 2002
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yipsterdesigner

thx for the pointer Cloxxki
i did not check that blackbird before

13. Joined: Sep 2007
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Squidly-DiddlySenior Member

OK, now I'm getting it after remembering my idea I had

as a kid to sail 'straight up wind' by connecting at least two sail boats with some sort of scissors so they would tack upwind towards and away from each other, but the frame would be going straight upwind.

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Tiny TurnipSenior Member

I think this is the video which most clearly shows the transition between DDWSlowerTTW to DDWFTTW. Watch the streamers. If you haven't got the patience for the whole thing, the meat of it happens between about 2.50 and 3.30

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Squidly-DiddlySenior Member

I thinking of it as backblast from prop is getting pushed

by the wind as it is still slower(groundspeed) than the wind.

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