DDWFTTW - Directly Downwind Faster Than The Wind

For the same effort, you could have learned to tango.

 

From the video "Faster than the wind directly downwind - its about leverage" it is implied that the way the faster than wind cart works is that the connection between the wheels and the propeller has a gear ratio which drives the propeller faster than the wheels are driven. So the propeller pulls the cart along faster which turns the wheels faster, and so on in a feedback circuit.

Is that how it works?

 

not quite really. Your description sound like a perpetual motion which this is not.

What does it mean for the propeller to be faster is bit relative anyway but it actually is rather gearing down than up.
It is about the speed that the wheels see A (in terms of the ground) and the speed the propeller sees B (in terms of the air). Pretty close to the wind speed (we can be going faster than the wind) the difference between A and B is very large - allowing for using "gearing down". Ie. Same power results in higher force. As the speed goes up both A and B grow linearly yet their ratio gets smaller and smaller thus ability to "gear down" diminishes and eventually the excess matches the losses and air resistance etc. and an equilibrium is achieved. It is powered by the wind despite the unintuitive nature of it all.

(not sure if you followed the thread but I made the video)

 

copying from old comment elsewhere:

Let's ignore rolling resistance as it is minimal. For the sake of argument lets assume that we don't use mechanical means of transmission of the force from wheels to the propeller but electric generators and motors. (in reality they would be about 90% efficient best case each but we can pretend they are 100%).

Before we even get to the wind cart: think of a cart with with its wheels hooked to a generator pulling 600watts at 6m/s.
You would start pushing this thing, at slow speed the resistance would be moderate but to push it generating 600W would take a really fit person to do it even for a short to moderate distance. I think there is a force there and I think anyone reasonable would agree with that. A force resisting forward movement, trying to slow down the cart.

From P=FV we get that force is 100N by the way. Producing 100N at slow speed is easy but at higher speeds it becomes increasingly hard. P=FV already tells us that.

As does familiarity with levers, creating big force but doing little work is easy. Big force and a lot of work is hard - or takes a lot of time. Power is work done in a unit of time.

Now lets put that same generator on a wind cart. And further assume that there is 5m/s back wind and let's get back to a 6m/s second situation. Cart is whizzing on the ground at 6m/s. The Generator - via the ground contact puts a 100N force trying to slow the cart down. For the cart to keep moving we need more than 100N force pushing it forward or it will slow down.

P=FV gave us the power budget of 600W. Now in this situation the cart has passed the "true wind" (ie. measured in relation to the ground) and is already operating in 1m/s headwind. As we have 600W to use, the question is: how much thrust can we get from the propeller with this power (run by an electric motor in this case)?

P=FV -> F=P/V -> Force = 600w/(1m/s) = 600N. This assuming no losses, in reality is naturally less. But we have a ratio of 6:1 which gives us ample excess for losses in propeller efficiency and transmission. This is how the cart's principle works.

For sake of fun let's check the earlier cart at 10m/s speed (same 5m/s wind). Now the ground speed is 10m/s and wind speed is 5m/s. The ratio isn't 6:1 (6) anymore but only 2:1 (2) Further at 15m/s it would be 3:2 (1.5), at 25 m/s it would be 25:20 (1.25).
ie. The faster we go the less speed difference there is in those two medium interfaces [cart to ground] vs [cart to air]
(or [wheel to ground] vs. [prop to air])

 

After reading
Downwind Faster Than the Wind by Veritasium: How Does it Work? https://rosemary-barnes.medium.com/downwind-faster-than-the-wind-by-veritasium-how-does-it-work-536a3e43939e

The analysis is:
After reaching higher than wind speed the sequence is now:

1. The wheels turn the prop at half the revolutions of the wheels.
2. The prop always produces a thrust no matter what the apparent air speed is.
3. That thrust adds to the speed of the cart and the wheels.
4. The wheels turn the prop faster than before.
5. And so on…

You can make it more efficient by changing the pitch angle in real time, but that’s not necessary to make it work,

Now, does it take energy to produce that thrust? That energy has to come from the chain drive from the wheels doesn’t it?

I haven’t heard of any evidence that it’s extracting more energy from the wind than what is normally in the wind itself.

 

I am not sure I follow what you mean by the "more energy" than is normally in the wind. It doesn't extract more energy than what the wind has. And really actually it extracts it from the ground - it is matter of point of reference really.

Let's swap the thinking for a sec: If we set the point of reference to the air our vehicle is traveling "up ground", and ground is passing by quickly. Certainly we can pick some energy from it - just like a sailboat can go upwind (not directly) we can go up-ground. Our mechanical arrangement is different than the sailboat so we don't have to tack.

Same exact cart can be changed from downwind to up wind by simply changing gearing or propeller. Now the propeller would act as a turbine and not a thrust generating device and the wheels would be the propulsion. This is widely accepted as possible - but the downwind cart isn't really any different from the "macro-physics" point of view - just that the point of reference needs to the be the air and not the ground to understand the underlying similarities.

btw. half the rotations on the propeller mean little without knowing the propeller pitch. Change of propeller equals changing gearing. And yes - it would benefit from variable pitch / variable gearing as one setup is really only optimized for one speed (but works less optimally over broader range of conditions).

 

It can not extract energy from the ground. The only energy input is from the wind. You are confusing yourself by using two moving frames of reference.

 

... from the speed difference.

the rest is matter of point of reference.

 

Exactly; you are using two different points of reference that are moving with respect to each other.

 

To extract energy you've got to have two mediums moving with respect to one another. A balloon can't extract energy from the air it is travelling in because there is no relative movement.

 

Does it extract ALL the energy from the ground?
Does it extract SOME energy from the ground and SOME energy from the wind?
Does it extract the energy from the difference between the wind speed and the ground speed?.

What is this "difference" between the ground movement and the wind movement that gets extracted?
Are you saying you can extract energy when there is two different reference frames?

 

Since the ground is stationary, there is no energy input from it into the system.

 

In this image a boat is sailing faster than the wind more-or-less downwind, not uncommon for fast skiffs and catamarans. Imagine that it tacks (not gybes because the apparent wind is ahead of the mast) several times so that its' average course is directly downwind. It is then sailing DDFTTW. Seems plausible, and easy to understand. I haven't bothered to calculate, but if the vector diagram is to scale then the boat would be doing about 15kt DD. If it slows to 14kt in the middle of the tack then the apparent wind is 4kt going from bow to stern. Image from University of NSW.AU found online.

 

Alan, you are right of course. but unfortunately except this is, argueb my many, to be impossible. it is very common to see claims that the max speed of a sailboat downwind is limited by downwind component maxing out at wind speed.

Even Wikipedia article about sailing faster than wind has parts removed because of "controversies".
Land yachts with their smaller resistance frequently reach distance made good downwind significantly faster than the wind. But people still don't accept this reality.

Would be better to try to understand why instead of arguing it isn't possible.

 

Downwind means moving in the same direction as the wind. In that case, the wind speed is the maximum theoretical speed. However, because of friction and the force approaching zero as the vessel approaches wind speed, it is not attainable. Alan's diagram shows a typical technique to increase VMG. That is not the same as going downwind; it is broad reaching.