Remember the debate about downwind vehicles

[PaulS1950]

The cart shown in the picture - is it a wind sail cart?
I have seen boats that use airfoils instead of sails - this looks like it could be the same thing.

 

[sophiecentaur]

None. We've only operated this as a downwind vehicle to date. Thus we've only ever used the one propeller. We're considering building a turbine to demonstrate that it can go UP wind faster than the wind. The prop we have has been operated fixed pitch and was later converted to variable pitch.

I have a feeling that the drive to an upwind vehicle would have to be reversed but, judging from my previous intuitions (!) I could be wrong. A 'force times speed' argument is more difficult for me to apply here.

But we still get "slippage" with our prop - right?

I can see that but i suppose that would just limit the power available.

Not necessarily true. We can set the prop to a pitch such that the cart won't self-start, but it works great once underway. Keep in mind that the entire prop is stalled while at rest (or worse yet, producing significant lift in the wrong direction at the roots). As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.

So a variable pitch prop would / could be a good thing. If it were feathered at rest then the wind would always push it forwards (?).

 

[A.T.]

But a DWFTTW device finds a way to operate with the wind being the stationary frame of reference and the ground being the moving frame. This is made clear by the fact that most, iirc, have to be pushed to get them started. They start from "stationary" in the frame of reference of the wind.

As spork said: They can start from rest relative to the ground:



The key of the physics is: Wind power always comes from reducing the true wind (velocity difference between air & ground). If there is true wind, then no matter how you move relative to airmass & ground, you always see the velocity difference and can potentially reduce it (harvest wind energy).

Here an animation showing the different reference frames and force vectors:

 

[kjl]

Nicely done video. I got a little kick out of the notion that you think it's unfakable. You try and make something foolproof, and I can bring you a better fool *every* time.

That's not my video - I do not think it is "unfakable" in that there is always somebody willing to believe a bigger trick or conspiracy. ( ) Where I think that video could help people to understand is that the wind->propeller->gear->wheels system is non-intuitive, especially as used in your cart. The little lego car vehicle, on the other hand, is quite easy to look at and visualize, and it has the same "wtf" brain bend. If you can understand why the lego car works, it is then easy to see why your prop cart works. (Replace the top gear with a worm gear with exactly the same gearing and now it is rotating the same direction as a propeller, and now it's an easy visualization jump from a worm gear to a prop).

As an added bonus, anybody with a basic technic set of Legos in their closet (which, frankly, any physics nerd should have :) ) can go make it, while not everybody can put together a version of even the little cart you used in the treadmill video.

Physics, and why even some very highly educated people substitute their own faulty intuition and call it physics.

To be fair, I think you'd have to admit that 1) intuition plays a very important part, even in formal science, to skip past all the stuff you've already proved to yourself to a reasonable degree (otherwise, you'd be stuck debunking perpetual motion machines all day long) and 2) the gearing in your cart is not very intuitive, and also triggers all the same red flags that perpetual motion machines do, though in this case they are false positives :) . What was most surprising to me, I think, was the vitriol and ridicule you were subjected to...


I am curious, though; when you first thought of the problem, did you initially intuitively believe downwind faster than the wind to be impossible or not?

As an aside, I hear you met my friend Garrett Lisi the other day - he coincidentally brought up your cart in conversation just this morning!

 

[rcgldr]

To go against the wind efficiently we have to swap our prop out for a turbine. I expect we might build one over the winter and try it next season. From 20' away you won't notice any difference at all, except I expect it will go directly upwind faster than the wind.

Depending on the range of pitch you can adjust on the prop, you may only need to increase the pitch to convert the prop into a turbine.

For an upwind vehicle, the pitch of the prop just needs to be increased so that the effective gear ratio is > 1, so that prop "wash" speed is greater than wheel speed. This will change the situation so the prop acts as an upwind turbine, driving the wheels, although I'm not sure how efficient the current prop will be if used in turbine mode. My guess is that a prop "wash" to ground speed ratio between 1.5x to 3.0x would be a good starting point. (The closer to 1.0, the faster the cart goes upwind, limited by the efficiency of the cart). Using the 3.0x as an example, say wind speed is 10mph, at a cart speed of 5 mph, the prop speed would be 15 mph, which would be the upper limit on speed with the 3.0x ratio with a 10 mph wind, for 0.5x upwind speed. With a 2.0x ratio and 10mph wind, upper limit would be cart speed of 10 mph, prop speed of 20 mph for a 1.0x upwind speed. If 1.5x worked, then upper limit would be cart speed 20 mph, prop speed 30 mph, a 2x upwind case. You need a 1.0 < ratio < 2.0 to achieve greater than wind speed upwind.

 

[robinson]

As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.

Have you thought about writing a book? Or at least a documentary? I would contribute to the cause. This is nerd heaven to me.

 

[Mercstein]

For an upwind vehicle, the pitch of the prop just needs to be increased so that the effective gear ratio is > 1, so that prop "wash" speed is greater than wheel speed. This will change the situation so the prop acts as an upwind turbine, driving the wheels,

Just to be clear, this cart, in its record breaking configuration (effective ratio < 1) will not travel forward when presented only with a direct headwind from standstill.

 

[mender]

That's because as configured it is a downwind vehicle. If placed in an "upwind" (in other words pointing the wrong direction), it will do what it was designed to do and move in the same direction as the wind, though not as effectively as when facing the proper way.

Just to be clear, this cart, in its record breaking configuration (effective ratio < 1) will not travel forward when presented only with a direct headwind from standstill.

And to be perfectly clear, the cart as configured will not travel forward into a headwind whether from a standstill or otherwise. It will always try to move with the wind; if towed into a headwind, it will promptly slow to a standstill and then go with the wind.