Saturday, December 4, 2010

Downwind faster than the wind

In July 2009 I wrote a post about wind-powered vehicles that travel directly downwind faster than the wind, giving links to videos explaining why this surprising idea is in fact possible.
I've now noticed that in July 2010 a fantastic team of enthusiasts indeed demonstrated a single-person wind-powered vehicle that goes more than twice as fast as the wind, directly downwind. Don't you just love engineers?!


Unknown said...

Dr. MacKay, I do love engineers, and I'm honored to be named among the "fantastic team of enthusiasts".

Also, I downloaded your book last night. I'm extremely glad there are a few rational people worrying about the things that are going to be extremely important to the future of intelligent life. I sure hope it catches on with the folks it will surely affect.

Your dedication says it all:
"to those who will not have the beneļ¬t of two billion years’ accumulated energy reserves"

I wish people understood just how scary that sentiment is.

Best regards,

Rick Cavallaro

Nick Palmer said...

Has to be a scam. They start out from a standstill, wind coming from behind. They accelerate up to "twice the wind speed".

Surely someone of your ability should realise that there comes a point after they start at which the relative wind experienced at the prop is zero? - at which point no further acceleration can happen let alone acceleration into what would thereafter become a head wind!

If this device works, then they could run it on a circular track in zero wind by giving it a push to start it off and it would then be a perpetual motion machine. Come on!

Unknown said...

This is great, and feels like light relief when the discussion about efficient and sustainable energy use can be heavy going!

We often ask questions like "is it greener to boil water using an electric kettle or gas stove?" - read for the answer!...

So the idea you can use the wind's energy to travel faster than the wind, is terrific. (Would be great for my sailing!)

Thank goodness for the engineers. We apply our own to push the uptake of renewable energy via solar panels, and we sincerely hope can make a practical difference in making energy use greener. Imagine if we could harness more energy from the sun, we'll keep trying.

Richard, Homesun

David MacKay FRS said...

Dear Nick Palmer, "has to be a scam"? - no. Yes, it sounds impossible. Obviously I have had all the thoughts you mentioned about why it sounds impossible. But then I thought carefully, listened to the explanations, and: I think it is possible, and not a scam. Imagine that you didn't know that "sailing upwind" is possible! (eg by tacking). I bet you would have exactly the same reaction - "how can you possibly accelerate upwind into a headwind?!" But you would be wrong. Tacking is possible. Some ships (with turbines) can sail directly upwind. And sailing directly downwind faster than the wind is possible too. To understand, You have to have a change of view of what the turbine is doing. Once the speed is greater than the wind, it's not acting like a standard wind turbine, getting work from the wind - it is acting as a thruster. There are lots of good explanations out there on the net, so try reading them before you shoot from the hip.

Nick Palmer said...

I wasn't shooting from the hip. I have seen the videos and they don't quite get reality.

The deceit is obvious if you consider the point where the cart is travelling at the same speed as the wind

I repeat:

If this device works, then they could run it on a circular track in zero wind by giving it a push to start it off and it would then be a perpetual motion machine

Your idea about getting energy from the "roaring road" to get the device over the hump (where the relative wind at the prop/turbine is zero) sounds rather like the magic magnet motor guys who give their devices a "kick" to reset the magnetic flux so it can attract the rotor again - they don't work because it always takes more energy to overcome the hump than one gets back from doing it. Basic conservation of energy!

In your original post you linked to a youtube video "Under the ruler faster than the ruler" which is more deceit. The presenter is not doing what he claims he is doing - he is actually "squeezing" the cart by pressing down with a tilted ruler to achieve his misdirection.

Perhaps I have a advantage. You will see from my avatar that I am a hang glider. I live or die from my knowledge of relative wind, stalling and air flow.

This device cannot be blown downwind from a standstill with the wind drag and accelerate through the point at which the cart travels at the same speed as the wind (measured on the runway) then continue to accelerate to some speed higher than that

David MacKay FRS said...

OK, Nick Palmer, you claim you are not shooting from the hip. Well, fine - then you are simply wrong. Please either learn some physics and think very hard, or go and be wrong on someone else's website. :-)

Ed said...


Nick Palmer said...

David, Ed. I am not disputing that a propeller driven cart can accelerate directly upwind. Neither am I disputing that a prop driven cart can travel directly downwind faster than the wind. What I do dispute is the claim that a propeller cart can, from a standing start, be blown downwind by the drag on the prop and cart, accelerating all the time to the point where the prop magically flips into "turbine mode" and starts driving the cart wheels through the point where the relative wind at the prop is zero - which is when the cart is travelling at the same speed as the wind.

Perhaps you need to "think very hard" about the direction the air alters around a propeller blade during the transition from "being driven" to "driving".

I can go from flying my glider upwind, to flying down wind "faster" than the wind but if claims about this DWFTTW machine were real, instead of two different effects grafted together to deceive, my glider would be able to gain energy and climb instead of losing it and sinking.

Anonymous said...

In the case of downwind faster than the wind, it is the wheels which are driving the propeller,.. given the equation Power = Force x Speed,.. the higher speed of the wheels relative to the ground experiencing a lower drag, can translate to a greater thrust force at the propeller at a lower relative airspeed, giving a net forward thrust without violating the conservation of energy.
The confusion arises from thinking the wind alone is the usable power source when in fact it is the relative motion of the two media(air and ground) in which the vehicle is traveling.