Is a passive vehicle exceeding wind speed from rest possible with momentum conservation?

Question

People have heard of the Blackbird vehicle that can exceed wind speed. But active control is required to control the pitch of the blades. It's an either/or process, either the wind is pushing the vehicle, or the vehicle is pushing the wind back. From an article on it,

"getting going in the first place might seem like a chicken and egg problem. Which comes first, the wheels turning or the prop turning? Cavallaro says that is not an issue. He has some control over the pitch of the blades, and by flattening them at the start, the car gets the push that it needs."

https://www.bbc.com/future/article/20120727-the-wind-beneath-my-wheels

Are they wrong? Is controlling the pitch of the blade not needed for acceleration from rest? How would momentum add up to exceed wind speed in a passive system?

If there is evidence, please link to it.

Whether one would call this fully wind powered is another question (like if a gliding bird is wind powered, why doesn't a dead bird fly), and whether leaving this info out is a fair bet for the prof who bet against veritasium on it. The model made that does something similar needed to be accelerated by hand.

Edit: if you say this is an engineering question, then you would be saying the inventors themselves are wrong, because they say it's not possible.

Edit: this is about a constant wind speed. A higher initial wind speed of course could make an object move faster than the slower wind speed after.

Edit: if it works sometimes without a variable pitch, the question is why not always? What are the varying conditions?

Answer 1

The BBC article is wrong. There is endless evidence for these DDWFTTW devices operating without variable pitch.

// "The hidden dirty little secret is that the vehicle cannot go from traveling a bit below wind speed to a bit above without changing pitch or pointing the prop a different direction, or getting a gust around the transition spot. I’m not claiming that makes it unimpressive nor anything else. Just that it is left out."//

The reason it's left out is you are incorrect. There is no special transition between below windspeed and above windspeed. The Blackbird (and similar devices) can sit at exactly windspeed indefinitely as demonstrated here:

https://youtu.be/5YNjXKpalc0?t=357

Answer 2

The variable pitch is not required. The prototype of the Blackbird didn't have a variable pitch control for the pilot, and could still self-start and accelerate beyond windspeed, if the fixed pitch was chosen appropriately before the run. This is demonstrated in the following videos:

• BUFC very first run

• BUFC second run with full audio

Note that they did increase the fixed pitch between these two runs, which results in a much slower initial acceleration, but higher final speed in the second run.

On the third run they increased the pitch even further, so the vehicle could not self-start (in the given wind condition) and had to be initially pushed. Then it reached even higher speeds, but since the power transmitted through the chain grows non-linearly with speed, the upper sprocket eventually failed:

• BUFC blowup run with full audio

For a demonstration of the whole acceleration from zero to above windspeed, with fixed pitch, under controlled wind conditions, see the various circular treadmill tests:

• DDWFTTW Demo at the physics department of the Freie Universtät Berlin
• DDWFTTW - More efficient
• Direct down wind faster than the wind my first experiment

The linear treadmill models are also able to self start and produce sustained net thrust at windspeed with the same fixed pitch:

• Example 1: Self-start, Net thurst at windspeed

• Example 2: Self-start, Net thurst at windspeed

Answer 3

The secret they never let out is that the vehicle cannot go from traveling a bit below wind speed to a bit above without changing pitch or pointing the prop a different direction, or getting a gust around the transition spot. I’m not claiming that makes it unimpressive nor anything else. Just that it is left out. No: a passive system does not do it. Many disagree but nowhere will provide any info about the workings of that.

In all the videos, they show it in steady state pointing the prop in the direction of the wind travel, which is moving backwards relative to the vehicle, and occasionally briefly show the start-up with relative wind going the opposite way.

I do not think it is coincidence that no one we are chatting with on here has any idea how these transitions are made. There is never any information provided about any of this. Not in write-ups or vehicle videos or online build-your-own instructions or model videos on treadmills, etc.

Here you see the gust between 0:30 and 0:50 https://youtu.be/7SYvg40NHtc which changes prop direction. Initial positioning is important based on wind patterns.

Clearly if gearing doesn’t change, prop direction doesn’t change, pitch doesn’t change, and wind doesn’t change.. then it won’t work for both directions of relative wind.

None of this would be all that big of a deal; we aren’t talking about a ton of energy to get rolling. Except the deception makes it bigger.

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Edit: Some other thoughts/questions for OP:

1. Maybe the question is, “Does it really qualify as ‘fully wind powered’?”

2. The answer to that is partially semantics. Is steering, or throwing a switch, not wind powered? Would reliance on even tiny gusts, which are inevitable at some point, not be wind powered? (I’m not sure whether that’s the case, needing only tiny gusts; just asking. It’s frustrating because they don’t ever tell us. And I again claim it’s intentional.)

3. Finally, are you sure the bet said that? I thought they said basically “maintaining vehicle speed above wind-speed for a long period using only wind power”.