I am. Jon's remark amounts to, "I don't understand it, so you should pretend you don't understand it, too."
But I do understand it. The theory makes perfect sense and is--as is so often the case--obvious and intuitive once you do understand it.
Some will hate this!!!
- Thread starter rgranger
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But I do understand it. The theory makes perfect sense and is--as is so often the case--obvious and intuitive once you do understand it.
I agree with Jon that the demonstration could’ve been better. It was a little more about drama than physics. The small difference in speed could have been from gusts or boundary effects like Jon mentioned. But he did say afterward they’ve gotten to 2.8 times wind speed, which would exceed those kinds of measurement issues, and I think the physics supports it.
well. once again i am wrong. they made it work. they claim. i am sure they are honest. his explanation of they wind was simplistic at best. at what height they measured the wind speed, unknown.
so be it.
their thrust blasting into the wind seems to be how it works
so be it.
their thrust blasting into the wind seems to be how it works
Actually I think the prop is a fan at all speeds. When the prop is turning, it is increasing the pressure rearward. Even though air might be faster than the forward motion, if the prop is turning, it is pushing against the forward moving air. For the purposes of understanding, think of the cart as stationary. The air blows against the fan, but the fan is always blowing against the wind. While the fan may not spin fast enough to push the air backwards, it is still pushing against the wind - more than it would if it was not turning. And that is the beauty of the idea. Even if the cart does not exceed the downwind speed, for the given surface area, it increases the force against the cart.
Now if I put a prop in the water and use that to propel a fan on my mast, can I sail downwind faster than the wind?? Fun thought, but a boat moving through the water is going to be way too inefficient to convert forward motion into reverse flowing air.
Lastly, did anyone have Bob Seagar's Against the Wind going through their head while watching the video?
It answers the age-old question with both a "yes" and a "no". The car is going downwind faster than the wind so "Yes" The prop blades are turning such that they are like a reaching sail creating apparent wind. This is what makes it possible so "no". I need a drink.
I found it a cool demonstration "the aerodynamics of an airfoil". There was so much not shown or reviewed.
I was just hoping that they were not using one or more of those tiny electric motors on the bicycle wheels that supported the body, like they did on the bikes in the Tour d'France a couple of years ago.
I was just hoping that they were not using one or more of those tiny electric motors on the bicycle wheels that supported the body, like they did on the bikes in the Tour d'France a couple of years ago.
It would be interesting to see a car that had a gear box to change from pushing to pulling so it could do down wind faster than the wind, as these gents were doing, then switch to powering itself back upwind. Maybe it could be done with only a prop that could switch the pitch to the opposite direction to go from driving to driven, although I think the efficiency may not be great in the "opposite" mode.
Maybe the propeller winds a spring and the spring drives the vehicle. If the prop is as efficient at winding the spring as pushing against the wind, similar performance parameters should be achievable in any direction. Like a self winding watch. When the wind blows, the spring gets tighter, when the car's going, the spring drives the car. Both could even happen at the same time.
-Will
-Will
The statement is that you cannot sail "dead down wind faster than the wind speed" which is true in all cases. This vehicel is "traveling" dead down wind and if it did not have anything else could not exceed wind speed. The trick here is that the two blades are NOT trevling dead down wind. They are on a continuous jibing angle that wraps into a helix.
The wheels turn the prop, not the other way around. The prop is a fan, blowing the car forward. The wind blows the car, which turns the wheels, which drives the chain, which turns the prop, which blows air, which moves the car faster.
You can think of it is pressure differential. Initially, the wind pushes the car forward because there is a higher pressure behind than in front, just like standing on a skateboard with your jacket open. Then the car gets moving which turns the fan blades. The fan creates a low pressure in front and a high pressure behind which pushes the car forward. This pressure differential doesn't really care if you are going faster than the wind or not, it just wants to balance the forces. Eventually the rolling resistance and aerodynamic drag of the car catches up with the pressure differential and the car cannot go any faster.
It would take some experimenting to get the gear ratio right, but they seem to have got that sorted out.
Here's a follow-on video:
Spoiler: Veritasium wins the bet.
I think the analogy near the end with the cart and the board is a nice way to give an intuitive understanding of what's going on--the best I've seen, so far.
This should put an end to the debate. Note that I'm saying this should put an end to the debate, not that it will!
I think the analogy near the end with the cart and the board is a nice way to give an intuitive understanding of what's going on--the best I've seen, so far.
This should put an end to the debate. Note that I'm saying this should put an end to the debate, not that it will!
I like how, at about 4:30 in the video, Alex is trying to explain how the experimenter is inducing a subconscious off-the-wind bias with his spork. Notice how Alex smiles as he's talking. It looks like he's realizing he's sounding a little far reaching, even as he's explaining it.
-Will
-Will
That's exactly what I was thinking. Unless the you're discussing psychology, someone turning to motive or unconscious bias as an explanation is a pretty reliable indication that they're floundering--whether they know it or not.
Something to think about. The wind is carrying lots of power, but not much speed. The question becomes how to you convert that power into speed. In your car, you shift gears. As you up shift, you loose torque, but you gain speed. Knowing this is what proves it is possible. All that was needed was a device to make the conversion. To work, the force provided by the prop needs to be greater than the friction losses in the drive mechanism.
It is a difficult concept to get your head around even if you have a background in science and engineering.
Fluid Dynamics Lesson (Or Long Winded Rant): The main ingredient here is conservation of energy or, more specifically, the Bernoulli Effect. The blades act like a reaching sail in that they are not going directly downwind. They are off the wind and, thus, the apparent wind relative to the moving blade is much higher than the wind relative to the car. But why does that matter? It is the same reason we can sail upwind. When sailing upwind we are going against the wind and yet we move forward. How? Conservation of energy. The sail shape is such that the velocity of the air moving over the leeward side of the sail is higher than that on the windward side. The faster the air moves the lower the air pressure against the sail (this is the conservation of energy tradeoff) and, thus, the pressure on the windward side, where the airspeed is slower, is higher than that on the leeward side. This net difference is what drives the boat (or blade) forward.
The propeller is driven by the wheels so the anology is not accurate. It is a fan blade pushing the car forward, just like if the propeller had a motor, only the motive energy comes from the wheels, not a motor.
Think about the car not moving, with a tail wind. The propeller doesn't move because there is a direct drive between the wheels and the propeller. The propeller presents a bluff body to the wind which creates a force pushing the car forward. As the car begins to move, the propeller starts to turn. The tricky part is what happens when the car approaches wind speed. Apparent wind drops as you approach wind speed going downwind so a bluff body would have less force from pressure differential and rolling resistance provides losses which would prevent you from ever reaching wind speed. BUT you have a propeller being turned by the wheels which increases the pressure differential which pushes the car forward (thrust or force pointing forward). With no losses this would create a limitless speed increase, however in the real world there are inefficiencies which limit the speed.
That’s what I thought was happening at first too, but watching the video more closely I saw that it’s not. The blades, driven by the wheels, are spinning to push air backwards, not pinwheeling allowing air to pass forward like your model would suggest. AC boats that sail downwind faster than the wind use the principles you mentioned, but that’s different than this geared arrangement.