Directly UPWIND faster than the wind

[spork]

There is an easy way to understand the feasibility of the upwind cart.. That's to understand that individual blades of the prop do not move directly upwind. They are "tacking" in 3D.

This is true as well - and is true of both the upwind and downwind cart. In fact this is how I originally conceived of the downwind cart (though it turns out I was not the first to do so).

To understand that the blades are on a continuous helical tack, it's of course important to understand that their gearing to the wheels replaces the constraint that would otherwise be provided by the keel of a sailboat.

 

[r-j]

You are correct.

There's something I rarely see. ;)

What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?

 

[Llyricist]

There's something I rarely see. ;)

What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?

The advance ratio between the rotor and the wheels sets the hard theoretical limit.

For instance, in the downwind case, a propeller pitched and geared to advance half as far through the air as the wheels go over the ground, will be limited by that to twice wind speed. Of course, since a propeller can never achieve "hard gearing" with the air to do that (there will always be "slip"), the limit will be less than that. An example of a very similar situation is:

But in real terms, it is mostly aerodynamic drag force, as well as the other efficiencies, that catches up to the the thrust force and prevents further acceleration.

 

[spork]

There's something I rarely see. ;)

What causes the cart to reach a maximum speed? If the faster it goes the more wind it 'feels', is there a limit to how fast a craft could eventually reach?

The vehicle has a "design speed" given by the turbine pitch and gearing. This is what we call the "vehicle speed ratio" (VSR). It's the rate at which the turbine would advance through the air divided by the rate at which the wheels advance over the ground - keeping in mind the two are connected by a transmission.

If that number is greater than 1.0, you have a vehicle that can theoretically go directly into the wind faster than the wind. If it's less than 1.0, you have a vehicle that can go directly downwind faster than the wind. The closer you get to 1.0 from either side, the greater a multiple of wind speed you can theoretically achieve (this can be worked out through simple kinematics).

But... the closer you get to 1.0, the greater the required efficiency needed to make the vehicle work at all at that VSR. You can change the VSR by changing the gearing or the prop/turbine pitch. With the Blackbird, we do have the ability to change the prop/turbine pitch on the fly. We can change the gearing, but only while stopped.

That being said, there is no theoretical limitation on maximum multiple of wind speed. It's just a matter of building a vehicle with high enough efficiency - and operating in a low enough wind.

The reason you need to operate in a low wind if you want to achieve very high multiple of wind speed is that you start fighting compressibility effects of air at higher speeds. This will inherently reduce your efficiency, and therefore limit your multiple of wind speed.

 

[Llyricist]

I see that the term has changed from "Advance Ratio", which also refers to something else related to props, to vehicle speed ratio. Fair enough :)

 

[spork]

I see that the term has changed from "Advance Ratio", which also refers to something else related to props, to vehicle speed ratio. Fair enough :)

Yes, JB and I initially did refer to it as "advance ratio". I think this would be the ideal term if not for the fact that it's too easily confused with the advance ratio of a propeller.

 

[rcgldr]

There is an easy way to understand the feasibility of the upwind cart.. That's to understand that individual blades of the prop do not move directly upwind. They are "tacking" in 3D.

Although propellers and turbines are more efficient than other aerodynamic devices, tacking isn't required for directly downwind or upwind vehicles. It could be possible to make a vehicle using a paddle wheel like setup or a treadmill with collapsable sails (no taching involved),, and although it wouldn't be as efficient, in principle it could work.

 

[spork]

 

[A.T.]

NALSA confirmed 2.1 windspeed, directly upwind:
http://www.nalsa.org/

They are at it again! A new record has been set, this time sailing dead upwind...that's right, directly into the wind (not tacking). The NALSA Board of Directors has ratified the following two records achieved by Rick Cavallaro on New Jerusalem Airport near Tracy California on June 16, 2012, with the wind turbine driven sailing craft, Blackbird. Mr. Cavallaro achieved a maximum boat speed to wind speed ratio of 2.1:1 while sailing directly into the wind and a maximum speed in a wind turbine driven sailing craft of 22.9 mph on a different run.

https://www.youtube.com/watch?v=F7PNSyAfCjk

 

[r-j]

This should be on television!

 

[A.T.]

This should be on television!

The downwind part was on Discovery:

https://www.youtube.com/watch?v=DPvGTjmn9y0

The upwind record was mentionend briefly on CCN:

https://www.youtube.com/watch?v=b8MWCvKIi7E

They tried to get on Mythbusters, but there is not enough fire and explosions in it.

 

[r-j]

I know somebody in SF that would so fund a documentary on this, do they have a media contract yet?

 

[spork]

I know somebody in SF that would so fund a documentary on this, do they have a media contract yet?

We'd be happy to have someone do a documentary on the project.

They tried to get on Mythbusters, but there is not enough fire and explosions in it.

Just a slight correction... We tried like crazy to get Mythbusters to take on the myth that you can't make a wind powered vehicle go downwind faster than the wind. We never entertained ideas of getting our ugly mugs on Mythbusters. We finally gave up on the idea that they'd do it, so we built one ourselves.

 

[rcgldr]

I recall that CNN clip. They mentioned 2x faster than wind, but failed to mention it was upwind.

 

[spork]

I recall that CNN clip. They mentioned 2x faster than wind, but failed to mention it was upwind.

Yup. The whole idea that we built a wind powered cart that could simply go faster than the wind makes it sound even more pointless than it was.