Here is an article on the Blackbird wind powered cart. Apparently it able to go 2.86X faster than wind blowing downwind. This seems a bit baffling since, at some point in its acceleration, it will match the speed of the wind. How then at that point would it be able to extract the needed additional wind energy to continue its acceleratation? This is the puzzle you are tasked with solving. This is a key issue that the article seems oblivious to.

Here is a video of the Blackbird in action. At 1st the video is fairly underwhelming, so you need to be patient, but eventually it picks up significant speed.

Maybe with a side wind?


PairTheBoard

In the video, he appears to be moving forward the whole time. This means you would never have that problem.

Sailers and windsurfers can go faster than the wind, and even sail against the wind. If you've done any waterskiing you know that it is possible to pass the boat. In both cases a component of the force is developed in a direction other than the direction of the wind or boat by turning the sail (tacking) or the skis. Here's a thorough analysis of sailing faster than the wind by Terence Tao:

http://terrytao.wordpress.com/2009/0...than-the-wind/

A wild guess: The props are attached to the wheel-motion. The wind blows this light car forward and at a decent speed it generates some extra forward lift. Something like this?

From another related article:

I didn't google this ... I thought of generating lift and saw the props turning very slowly at the start.

I've done a lot of sailing myself and know about sailing faster than the wind. The issue is sailing faster than the wind when it comes from directly behind you. At some point, you might match speed with the wind, but then you are effectively becalmed. How then do you go even faster? I can think of ways where the cart follows a curved path or makes use of a fly wheel, etc., but that is not what appears to be happening in the video.

One issue you might want to consider is that in transitioning from lower than wind speed to higher than wind speed, the propeller will want to change its direction of rotation. The only ways I can think of avoiding that are:

1. Rotate the propeller assembly along a vertical axis.
2. Invert the camber of the propeller.

#1 does not occur in the video. #2 would make the propeller shape inefficient in at least one of the 2 directions of rotation. In either of #1 and #2, you still have the problem of accelerating when your velocity relative to the wind is zero.

I think I see one possible source of confusion.

The headline of the article is not the thing you're talking about.

2.86 times would be too fast by that mechanism alone anyway. Tao shows how it can only be double the wind speed if you are not using what he calls "water sails". But with those he shows that in a simplified theory you can go any number of times wind speed.

This seems to explain it well enough.

Even though the car is relatively windless, the propeller is turning based on the motion of the wheels. All you need is to be accelerating quickly enough to "push through" the stationary wind situation and get into the "upwind" driving situation.

Maybe think "escape velocity"? [No -- that analogy doesn't really work.]

This sounds perpetual motionish to me. It sounds like on a windless day you could just give the cart a push to get it started and it would take off accelerating on its own from there.


PairTheBoard

The wheels will not be spinning fast enough to cause the propeller to generate enough thrust to do anything interesting.

http://en.wikipedia.org/wiki/Momentum_theory
http://www.public.iastate.edu/~aero442/unit2.pdf

Think how a helicopter functions creating thrust due to power applied on the rotating fan. Now the power can be coming from the wheels of the car that the wind pushes and accelerates (P=Fwind*u). Obviously its proportial with (some losses) to the supplied power by the wind on the vehicle as a total and air resistance (and friction on wheels) eventually kicks in too. Once the motion starts the fan operates with the above equation in the link that Thrust=force~P^(2/3). As a result the fan itself accelerates the system further (Bernoulli effect) which then gives the wheels more power that goes back to the fan to rotate it faster. Obviously there is a limit to all this as you also create air resistance. Maybe we can write down a differential equation of the system for a given wind speed and other parameters of the system. Its an interesting exercise.

Notice the video seems to agree with the idea of a differential equation feedback system that gradually accelerates the system but of course up to a point. I see as maximum speed a result that can be almost anything given proper fixing of all the parameters involved and strong enough starting wind and small friction.

Originally Posted by PairTheBoard
"This sounds perpetual motionish to me. It sounds like on a windless day you could just give the cart a push to get it started and it would take off accelerating on its own from there."


Push harder. Why should it work when the wind does the intitial pushing but not when you push it manually?


PairTheBoard