DDWFTTW - Directly Downwind Faster Than The Wind

I see how it doesn't work. You can't have the turbine inputting energy into the system and at the same time being a propeller and outputting energy. The equation is wrong; one of the energies needs to have a negative sign. If the wind power is acting on the system inputting energy it has a positive sign. The energy applied to the propeller to make the cart move is negative. They cancel out.

 

It isn't, that is the problem with your design.

 

So, you're familiar with the design spiral.
This is the reason/react spiral.

 

Gonzo, this is not my design.
Sadly, you are mistaken.

 

I don't think so, it is basic dynamics. Is the kind of problem for students on their first semester.

 

No, this is not at all what I explained. This is what I called an illusion. DCockey explains why. Your response to that is again based on the misleading intuition I tried to debunk. It doesn't matter that the wind velocity relative to the vehicle is zero. It only matters whether the media move relative to each other.

You are again thinking of one medium as providing the energy. If you must think of one medium as providing the energy which is transferred to the other medium, then for the DDFTTW cart, the medium from which energy is extracted is the ground, and the medium to which it is transferred is the air. I expect your next question will be why you can't extract energy from just standing on the ground, without wind, and that is where this way of thinking then becomes a little more complicated, and likely has been answered earlier in the thread. It certainly has been answered, with the relevant mathematics, in articles in Catalyst. The point of my post was to offer a way of thinking about this problem that sidesteps that misleading but seductive intuition without any mathematics at all. But if you need the mathematics, follow the references others have provided.

Do you accept that the windmill boat in the video can go upwind, that there is no hidden motor, or strings, or other trickery? If yes, consider that it starts at exactly the point that gives you problems, namely zero relative velocity between water and propeller. The DDFTTW cart's propeller does the same job as the windmill boat's propeller, and the cart's wheels to the same job as the boat's wind turbine. Zero velocity relative to the medium that provides your thrust is not an issue.

 

Learn to read. Almost every contributor has literally spelled out that there is no turbine. So your comments about turbines are irrelevant wastes of everyone's time.

 

I read that you are incorrectly calling the turbine a fan. If it is absorbing energy from the wind, it is a turbine.
How Do Wind Turbines Work? https://www.energy.gov/eere/wind/how-do-wind-turbines-work

 

According to you, energy is transferred from the air to the ground. In that case, that energy is somewhat stored in the ground, which there is no evidence of. It is not complicated or counterintuitive, simply wrong. The math is wrong too. There is a positive sign where a negative should be. My question is not why you can't extract energy from just standing on the ground. It is the same setup as using a generator to power an electric motor to turn the generator. There is a net loss.

 

The wind pushes the cart from 0 velocity to a point where the propeller takes over. The initial forward motion isn't from the turning of the propeller, but from the wind pushing on the cart. In the video, they mentioned the pilot's control of the propeller blade's pitch. That would be to maximize cart's resistance to the wind, not to turn the blades by wind power. The wheels drive the propeller, not the other way around.

 

There is a fundamental misunderstanding in your statement. A propeller, like in an airplane, transfers energy from the system to the environment. The cart is an open system where energy is transferred both ways: in and out. Unless the propeller is locked, the wind won't be pushing on it, but it will turn acting as a revolving foil and will be a turbine since it is transferring energy from the environment into the system. Then the energy is transferred from the system into the ground with all the losses that entails. There will be a percentage of the energy loss to friction, noise, vibration, etc. Since there is always a net loss, and there is the Betz limit to how much energy a turbine can extract from wind, the cart will never reach the speed over ground to equal the speed of the wind. You state the propeller takes over, however the only energy available to transfer energy back to the (now outputting energy) propeller is the kinetic energy of the cart which through friction makes the wheels turn and then through gearing turn the propeller. Each step has a net loss of energy, which would only work if the system could create energy. This violates the Laws of Thermodynamics. Nowhere in the description of the cart is any explanation on how this energy is created. The math does not account for it. The math does not indicate any of the mechanical, etc, losses either. In short, it does not work as described.

 

It is never a turbine. Did you watch the videos?

lets take a snapshot. Wind is 5m/s to the right carts is already past the windspeed going 6m/s to the right.
Apparent wind is 1m/s to the left.
From cart’s point of reference the ground s passing by at 6m/s and the experienced wind is only 1m/s.

The wheels take power from the ground (to spin the prop but we can imagine a generator too for illustrative purposes). Lets say the power is 600W so with P=FV we get that the braking force is
F=P/V
600W / 6m/s = 100N

Ok so the wheels try to slow down the cart with 100N, to the Left btw. All should be pretty clear up to this point.

now if we see what kind of thrust can we get from 600W with an air propeller operating at 1m/s airflow (our apparent wind).

using same P=FV
F=P/V
F = 600W/1m/s = 600N.

One interface (wheels to ground) has much larger speed difference than the other (prop to air) so braking force at high speed can be leveraged to higher force at lower speed. Boom: Newton is happy, hippies didn’t get free energy device and universe is saved. But the cart goes downwind faster than the wind.

at slightly over wind speed the leverage is significant so there is plenty of power to be wasted in: inefficiency of the prop, resistance of the mechanism, rolling resistance air drag of the cart etc.

As we pick up the speed the ratio of (speed of ground to cart)/(air to prop) starts to get smaller and smaller. In the example above it was 6/1

if we accelerate to 7m/s it becomes 7/2, further at 10m/s cart speed it is 10/5 (= 2/1) and as we accelerate it gets closer and closer to 1. Because of inefficiencies you don’t get in practice very close to one. But almost 3x windspeed by blackbird shows that their cart was quite well optimized. An I recall that they actually hit the brakes at that point.

 

And to address your point if prop in the wind at the start:

yes the wind, when the cart is static for example, does put a torque to the prop and tries to use it as a turbine. But the gearing is such that it will not. If the gearing or the pitch (different sides of same equation in a way) was different the prop would start spinning the other way ground and the cart would start moving into the wind powered by the wheels.

as is the torque from the wheels beats the prop torque. At the start the whole system acts more or less like a bluff body. But quickly the prop gets enough speed to produce thrust.

 

Ah, I see the confusion. There are actually three transfers of energy taking place, and you are correct that the two which we have been explicitly mentioning are negative.

There is a certain amount of energy required to spin the fan. Clearly there are some losses in this and those are a negative source of energy (sorry my language is clunky there).
Then there is the energy taken from the wheels (so ultimately from the forward motion) which is used to spin the fan.
But there is also energy transferred from the wind to the cart because the tailwind blows on the airstream coming from the fan. This is positive and it can be larger than the other two.

To see how the wind can transfer energy to the cart in this manner, lets consider a slightly different system. Imagine a cart with freewheeling wheels and an electrically powered fan. In still air it would move at some speed x. Now imagine it is in a tailwind of x/2. It would move faster than before, despite moving faster than the wind. This is because the wind can push on the airstream coming off the fan, which in turn pushes on the disc of the fan, and so via the bearings to the body of the cart. Given the cart is moving faster than in the still air case it has more kinetic energy, and where has that energy come from? From the wind. So we have established that a tailwind can transfer energy to a cart with a fan, even if that cart is moving faster than the wind.

Of course we need some of that energy which is transferred from the wind to the cart to drive the fan, via the wheels, but I have shown previously why we don't need all of it.

 

There is a component of energy from the wind on the cart. However, that goes to zero when the cart reaches wind speed
There is no "disc of the fan". If it is a propeller, there is slip on the foil, which generates thrust. Again, if the cart moves at the speed of the wind, there is no relative velocity. The wind does not push on the airstream of the "fan". There are plenty of sources online that show the fundamentals of fluid dynamics; check them out.

You have not done that. If there is no relative velocity, the turbine will not turn. Further, if you have an electric motor, it is a completely different system.
There is no energy transferred to the cart from the wheels. The kinetic energy from the cart gets transferred through gearing to a propeller, which can not be the same mechanism as the turbine; or at least not operate simultaneously if the same. That is a net loss of energy from the system to the environment. Please go online and read some basic thermodynamics which explain energy transfer. It is too long to explain in a post.
Another confusion is using the term "fan". It is either a turbine or a propeller. Each has a particular mode of operation. Proper terminology is crucial to have a clear discussion.