Saturday, November 6, 2010

Directly Downwind Faster Than The Wind

A recent article in Make:Online and soon to appear in Make Magazine discusses the successful attempt by Rick Cavallaro to create a wind-powered vehicle that can go faster than the wind. This "land yacht", called the Greenbird, has been verified by the North American Land Sailing Association to have run at 2.8 times the speed of the wind. They have done this with various sensors, the data for which can be found on their website.

Rick was continuing the work of Jack Goodman who released a video of his toy DDFTTW cart in 2006 and created a huge internet buzz. Unfortunately for Jack, it also created huge controversy. People issued cries of "Perpetual Motion", "fraud", "its all done with string". Physicists claimed if this video were true it would break the basic laws of physics so it is clearly a hoax. One contributor to Make Magazine tried to reproduce the results and couldn't, and therefore concluded that Jack Goodman was a fraud.

You would think that Rick's full-scale reproduction of Jack's work verified by an independent body would be enough to silence the critics, but people continue to lose their minds over the impossibility of using the wind to travel faster than the wind. The cries of perpetual motion and breaking fundamental laws of physics continue. I find the hysteria just a little bit baffling.

Understanding how DDFTTW is possible is actually fairly straightforward. Once you understand what is going on, it is clear that what is causing all the naysaying is simply a failure of imagination.



Ordinarily, harnessing power from the wind is straightforward and it is something that has been done for millenia. You can use a sail or a turbine, but either way you have the wind push against it and get energy as a result. Connect the sail or turbine to a boat or sled or cart and you get the vehicle being pushed by the wind.

The trouble is that the energy harnessed comes from the relative speed of the air compared to the vehicle. As the vehicle speeds up the push from the air drops, until the vehicle reaches the speed of the wind (a little less due to friction, actually) and no longer gets energy from the air.

The first thing that is different in Rick's land yacht is that it does not get energy from the difference in speed between the air and the vehicle. Instead it gets energy from the difference in speed between the cart and the ground. As long as the wind continues to blow, the cart can continue to roll. As soon as the wind dies down it comes to a halt. So you can see there is no free energy of the perpetual motion kind here, just well understood harnessing of energy based on the difference in speed between two mediums, the cart and the ground. No matter what speed the car is going (other than when it is halted), there is still a difference in speed that yields energy.

But now the question comes as to how you harness that energy in order to provide thrust. The answer is that you connect the wheels of the cart to a propeller. The wheels of the cart are in contact with the ground, the propeller is in contact with the air. The wheels provide the energy, and the propeller provides the thrust by moving air.

The real brilliance of Jack Goodman and Rick Cavallaro is that they not only connected the air to the ground, they also inverted the source of the energy and the thrust. In a typical air-powered design, you get your maximum thrust when the cart is stopped. In this design, you get the least amount of thrust when the cart is stopped. But as the wheels turn faster they power the propeller to provide more and more thrust. As the propeller provides more thrust, the wheels turn faster and generate more energy. Rather than having less energy as the cart approaches the speed of the wind, it has more.

Does that mean that the cart will go faster and faster until it approaches the speed of light? Of course not, that is just silly. It is only harnessing the energy in the speed difference between the air and the ground, and that is limited.

You might be thinking that it isn't possible to design the cart to harness more of the energy from the air/ground speed differential than would be required to accelerate the cart to the speed of the wind, but that is easily disprovable. Imagine a more traditional design where you use a sail. Can you imagine the sail pushing a cart that is significantly heavier than the Greenbird? If you can imagine that then you can see that more energy is available with the right design.

So the problem is not having a vehicle that goes directly downwind faster than the wind. You only need a design to extract enough energy from the relative speeds of the air and the ground to accelerate your cart that fast. The one thing I can't figure out is that there are claims that the vehicle can start itself. Although the cart provides some surface area for the wind to push against, it would be surprising if it were sufficient to overcome the friction of the wheels, especially since the wheels are turning a big propeller. You might think (as I originally did) that the propeller initially acts as a sail just to get things going, but the comments on the article from Make:Online specifically state that there is a ratchet in place to prevent the propeller from ever driving the wheels. The video clearly shows that Jack Goodman's design needed a push to get going, so I have to admit I'm flummoxed on that score.

As far as a wind-powered cart running faster than the wind, it is a fine bit of out-of-the-box thinking combined with excellent engineering. Kudos to all concerned, but it is hardly going to revolutionize physics.

7 comments:

henkdeleeuw said...

The reason that it can work is even simpler than in your explanation, and also explains how the cart can start up without being pushed first.

The propellor is used as a sail.
As long as there is pressue on the back of the 'sail' it can accelerate the cart.
If the sail were fixed w.r.t. the cart, it would stop receiving back pressure as soon as you reach wind speed. When you pass wind speed, you would get wind pressure from the front, and you would slow down.
So, what do they do: they drive the propellor from the wheels, so the surface of the propellor gets a forward speed (w.r.t the groud) that is lower than the forward speed of the cart itself.
In that way, even when the cart itself has passed wind speed, the surface of the propellor hasn't, and the wind can keep pushing the propellor forward, and thus, the cart.
The cart itself will feel the wind coming from the front, but the surface of the propellor, because it is turning, will still feel the wind force from the back.

The propelling force from the wind is there from standstill, and keeps working until your speed is so high that even the surface of the propellor blades have reached wind speed (or actually until the drag will be greater than the propelling force)

Bruce Atherton said...

Thanks for that explanation, henkdeleeuw, but I have to admit I am still surprised this cart can start moving from a standing start.

The rotation of the propeller is always restricted from turning the wheels, so that means that the whole motivating force at standstill is provided by wind pressure on the cart and the propeller. That has to provide enough energy not only to overcome the friction of the wheels on the ground, but also give the wheels enough energy to spin the propeller. I find that result surprising, but as they say the proof is in the pudding. I can either accept that it happens or decide that there is a vast conspiracy trying to prove something that isn't true. I'll go with the former.

henkdeleeuw said...

That is just a little bit of extra friction you need to overcome.
As soon as the force on the propellor blades is large enough to overcome the friction of the wheels and the friction of the propellor, you will start moving forward.
I think the propellor will not have much more friction than the wheels.
Once you start moving, the acceleration will be slower with a driven propellor because the propellor will need to be accelerated(against the wind!) as well, but the force needed to start moving is just the friction force in the wheels and the propellor, which need not be very high.

Bruce Atherton said...

I guess we just see the problem a little differently. I would agree that if the propeller blade were flat that it would not be much friction, but as the pitch increases the way that a volume of air is affected by the propeller increases, and thus the energy needed to rotate the blade increases. As I said, I am still surprised that the surface area of the cart and propeller provides enough energy, but am willing to accept the results of reality.

BTW, I realized that your explanation and mine are quite complimentary. I explained where the energy came from and why it didn't drop to zero as the vehicle approached wind speed by using the speed relative to the ground to collect energy.

The way that the propeller provides thrust at all speeds I handwaved away as an engineering problem, but your answer explains how that engineering problem was solved, and also what the limiting factor is on the speed of the cart. So between us I think we have a complete answer that answers both the science and the engineering questions.

henkdeleeuw said...

Indeed, due to the pitch of the blades, they would like to rotate in the other direction than we want them to when hit by wind.
We will need some extra force to overcome this.
So, we need to choose the gearing between the wheels and the propellor so, that the force on the blades is large enough. We know the force on the wheels at standstill: that is equal to the wind force on the blades + cart minus the friction forces. We know the force that is needed to make the propellor turn against the wind for a given wind speed and blade pitch, so we can calculate the needed gearing.
By the way: if we calculate the garing "wrong", and let the blades turn with the wind initially from standstill instead of against it, the cart will also move in the other direction. We will have created a cart that will ride against the wind instead of with it!
And when we choose a gearing in the middle of these two, we'll have a cart that will not move whatsoever, and remain still.

It's easy to make the gearing so 'light' that the cart will come into motion easily, but if you make it too light, then the propellor will not turn fast enough to reach above-windspeeds.

Anonymous said...

A small correction to this excellent description... "Instead it gets energy from the difference in speed between the air and the ground" should be "... the cart and the ground".

Graham

Bruce Atherton said...

Thanks, Graham. I agree that this phrase is misleading, given my simplified model.

Once henkdeleeuw explained how the standing start worked, it is clear that the cart is taking energy predominantly from the ground to cart speed difference, but also (in a minor way) the cart to wind speed difference based on the surface area of the cart and propellor. Eventually that energy gain turns to energy loss as the cart passes wind speed and the push becomes drag, but in the meantime it turns out, in a funny way, that my mistake was actually more accurate than the rest of the explanation.

It is a complicating factor in the whole explanation, though, and is only needed to explain why the standing start works. So I'll make the correction you suggest and otherwise leave things the way they are.