Merged DDWFTTW in a “Real” Wind

[Llyricist]

We were talking about the situation below wind speed, that calculation was for 3.11 times the wind speed, a situation for which I do have access to the math.

I meant to quote 69Dodge's post about that situation to show the numbers were very close, but forgot.

 

[windgrins]

Reasoning Backwards!

Would you like to see a video of a UFO? Bigfoot? Ghosts? An elephant flying? I can direct you to the links, but I'm sure you can find them on your own.

This is physics, or so it is claimed, and mathematics rules. Where's the math?

Prof,

I think you have it backwards. Reality exists and phenomena occur. Physicists build theoretical and mathematical models to explain and match the phenomena. They then use the models to predict similar and unexpected behavior of systems that fit the models parameters.

It doesn't mean that if you can't write a mathematical model, the event/phenomenon doesn't occur. This is called sticking one's head in the sand.

Let's put the horse back in front of the cart (no pun intended.)

Regards,

WindGrins

 

[mender]

I don't say this often but when it comes to trolls, humber is truly is a class by himself; the others don't even come close.

The reason I say this now is that I crossed paths with a troll on another forum that wouldn't last one page with humber before he'd be in tears and running away. Not that I condone trolling in the first place.

 

[3point14]

Would you like to see a video of a UFO? Bigfoot? Ghosts? An elephant flying? I can direct you to the links, but I'm sure you can find them on your own.

This is physics, or so it is claimed, and mathematics rules. Where's the math?

The maths describes reality.

Sometimes a given mathematical model can't cope with a given situation.

In situations where the mathematics disagrees with reality, the maths is flawed, not reality.

There are any number of videos available on the internet. Were it just one, from distance, all grainy and badly lit, you may have a point. As there are lots, and many more that prove the concept (the little device on the edge of the record table is my favourite - don't know who did it, but kudos for that)

The long and the short of it is that if you're not prepared to believe the huge amount of video evidence available online, from CGI proof of concept to practical demonstrations of the concept to watching the streamers on an actual cart change direction as it goes from below windspeed to above windspeed, then all the badly applied maths in the world isn't going to help you.

 

[Subduction Zone]

ppg it is not just one video. It is a complete blog on how it was built. It is details on how to build a small scale version of this. It is many videos of the small carts working on the treadmill, and not only by spork but by several others as well. It is papers by Drela, MacG, and others that go over the math of this, sometimes specifically mentioning how the treadmill tests are obvious proofs that the cart can go DDWFTTW. It is the sanctioning by a world wide governing organization that could only lose if they were taken in.

Against this we have a few cranks who cannot understand the concept of a frame of reference, much less a Galilean Transformation.

 

[Llyricist]

Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.

If the cart moves at 28 mph, the propeller spins at 133.7 RPM or 2.228 rev/s. The airspeed of the cart is 18 mph, or 26.4 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 26.4/[(2.228)(16)] = 0.7406.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0834 and cq = 0.0120.

Then, the forward thrust of the propeller works out to be 61.32 lb, the torque driving the propeller is 141.2 ft-lb, and the backward force of the ground on the wheels is 48.11 lb.

That leaves 13.21 lb to overcome friction and accelerate the cart.

Using the same .365 Cd and fresh BMX tires, 90% drive train efficiency, and 89% efficient prop but with max at 92%, I get:

145 ft lb of torque, 3.3 HP, and 61.5 lbs of thrust, and a net acceleration force of 9.18 newtons

 

[3point14]

With a 90% drive train efficiency, fresh BMX tires, Cd of .365, the prop operating at 89% efficiency with a theoretical max of 93%. I'm getting:

26 horsepower at the shaft, 569 ft lbs torque, and 228.8 lbs of thrust, and a 10 newton net acceleration force.

That seems, to my inexpert eye, to be an impressively large amount of HP.

 

[Llyricist]

That seems, to my inexpert eye, to be an impressively large amount of HP.

yeah, that's the situation at 56 MPH in an 18 MPH wind, when JB told spork to slow the heck down before anything broke.

In fact here's the video of when it happened:
http://www.youtube.com/watch?v=5CcgmpBGSCI&feature=player_embedded

 

[CORed]

I prefer to show the real life result, since I don't have access to the necessary formulas to show the math.

http://www.youtube.com/watch?v=7SYvg40NHtc

Oh come on! If you can't explain mathematically how it happened, you must have been hallucinating when you saw it happen.

 

[spork]

There are any number of videos available on the internet. Were it just one, from distance, all grainy and badly lit, you may have a point. As there are lots, and many more that prove the concept (the little device on the edge of the record table is my favourite...

When it comes to science, I'll tell you which is my favorite... the series of build videos that specify part numbers, sources, and detailed instructions that allow anyone to build their own working model to reproduce our tests.

They aren't sexy, and the production value is crap. But that is science.

 

[Llyricist]

Oh come on! If you can't explain mathematically how it happened, you must have been hallucinating when you saw it happen.

yeah I guess it's not enough to understand the concepts involved in how it happened, If I can't run the numbers, it didn't happen.

 

[Llyricist]

When it comes to science, I'll tell you which is my favorite... the series of build videos that specify part numbers, sources, and detailed instructions that allow anyone to build their own working model to reproduce our tests.

They aren't sexy, and the production value is crap. But that is science.

What you think science is about reproducible results and rigorous tests against hypotheses?

Pishaw!

It ain't science until it's published in a peer reviewed journal!

 

[Subduction Zone]

That seems, to my inexpert eye, to be an impressively large amount of HP.

Since the cart is moving faster than the wind, in fact about three times the speed of the wind, the amount of air that it interacts with is much higher than it does when it is stationary, or even when it is going at the speed of the wind. I can see how this concept drives the H's nuts. The faster the cart goes the more energy it can get from the wind. It almost seems it should be the other way around.

 

[Subduction Zone]

Pi, here is a link with an animation that uses a similar technology to harvest extra wind power for a series of turbines on a structure similar to a box kite:http://www.jobyenergy.com/tech

Conspiracy warning, this was made by Joby Energy, one of the sponsors of the Blackbird. Since I don't want my internet capability cut off I won't say anything about the other sponsor

 

[CORed]

Oh yeah! A brand new chain in a clean lab environment, perfectly aligned and tensioned and run by computer controlled electric motors. I would say if they didn't get efficiencies in the high nineties, they screwed up. But your chain, hanging out in the wind, very long and twisted isn't going to get anywhere near 90%. Lucky if you get 80%!

While I don't think anybody would dispute that the efficiency of the chain drive on the blackbird was less than the 98% achieved under ideal laboratory conditions, the fact that you happened to extract 80% from some orifice does not make it the correct number.

 

[profpangloss]

What you think science is about reproducible results and rigorous tests against hypotheses?

Pishaw!

It ain't science until it's published in a peer reviewed journal!

I don't see any rigorous tests, just some good old boys having fun out in the desert.

Yes indeed, a peer reviewed paper would make all my doubts go away. Got one?

I didn't think so! So all you have is Internet wooo wooo! Do you have any math at all?

 

[Llyricist]

I don't see any rigorous tests, just some good old boys having fun out in the desert.

Yes indeed, a peer reviewed paper would make all my doubts go away. Got one?

I didn't think so! So all you have is Internet wooo wooo! Do you have any math at all?

Yes, the math that shows the cart will continue to accelerate once beyond wind speed.

 

[profpangloss]

Yes, the math that shows the cart will continue to accelerate once beyond wind speed.

Is it posted here? I didn't see it. I would be most obliged if I can examine it, thank you!

 

[Subduction Zone]

profpangloss the evidence that spork et al. have that the cart will work is massive. What evidence do you have that it won't? And don't mention the laws of thermodynamics without some math that backs you up. Creationists do that all of the time. You have to be able to understand the physical laws or theories that you cite to be able to use them. Remember Dr. Camp? He was the physicist who originally said it was impossible. He never did understand the treadmill test, but he obviously believes that spork succeeded. He said that the concept of DDWFTTW breaks no physical laws.

 

[humber]

You are merely confusing yourself by calling it this "retreating prop" argument.

Consider a point on the propeller's arc which is, let's say, midway along the blade's length and vertical to the shaft's centre line. Turn the prop so the leading edge of a blade is on this vertical line. Project a perpendicular point from the leading edge to the propshaft's centre line. Now turn the prop a few degrees so the blade's trailing edge is now on the vertical. Project another perpendicular point onto the propshaft centre line. Is this point further rearward than the first? Yes, it is.

For every point on the surface of the blade as you turned it, the point projected perpendicularly onto the centre line moved consistently backwards.

That is all.

Nonsense. All of the prop moves forwards at once. Anything else as written above, it the result of an over-active and erroneous imagination, brought about by the need to support a failed idea.