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Science Experiments / Re: Can we go downwind faster than the wind?

« on: 26/06/2021 04:01:42 »

What does it take to make it move slower than the wind?

For this the advance ratio A must be negative, so you have to get rid of the blue wheel or apply the ruler at the bottom of the blue wheel.

My analysis shows that to go faster than the wind,...

Just to clarify: Above I was answering the question about making it move slower than the wind.

... the bottom wheels must spin in reverse direction than the top.

Well, it depends on which side of the wheel the ruler/belt is interacting with it (see the single spool). But if you stick to:
- top-side for the top wheel
- bottom-side for the bottom wheel
then yes.

The advance ratio A is the ratio of relative velocities, that your gearbox is trying to achieve:

A = air_relative_velocity / ground_relative_velocity

When moving with the air but faster, or against the air, the air_relative_velocity and ground_relative_velocity have the same sign so A > 0. When moving with the air but slower, they have opposite signs so A < 0.

I'm attaching a plot of advance ratio vs. maximal speed. For a gear box with no slippage you can assume the ideal velocity (blue line). With slippage and drag (like the propeller cart) you get something like the red velocity line.To make sense of the diagrams in the table below, you should watch the video on the bottom.


rotor_carts_01.png (209.85 kB . 812x1000 - viewed 2736 times)

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Science Experiments / Re: Can we go downwind faster than the wind?

« on: 25/06/2021 11:23:30 »

Then Coco from the video would be right: It can't work at all. It would slide or lock or break.

In my diagram, one of the conveyors is slippery. Nothing has to break.

Then it will slide.

What does it take to move in reverse direction?

Just turn it upside down.

But if that's not allowed then look at my diagram. The formula for this case is:

V/W = 1 / (1 - r/R)

To make V/W negative, you must have:

r > R

But note that r still touches the upper wheel, while R still touches the ground/lower belt. To make this work requires a double rail / split belt on which the now smaller R parts roll. So turning it upside down is really easier.