I thought I posted a link to ice boats - they can go multiples of wind speed when tacking...and water yachts can also go faster than the wind - when they are tacking...
Solar sails are a different issue. Most of the momentum comes from solar wind particles, which AIUI mostly stick to the sail, so you only get 'downwind' force regardless of sail orientation.
Radiation bounces off a silvered sail, so you potentially get twice the momentum transfer, and you can tack.
Cheers
Phil Hobbs
What does tacking have to do with it??? You would tack if you were using a sail. I've tried to explain to you the propeller is turning the OTHER way, pushing back against the wind, not being turned by the wind, turned by the gearing from the wheels. It is only at the very beginning that the prop acts as a sail, catching enough wind to set the car in motion. After that the rotation is like on an airplane, pushing air backwards as the car moves forward.
One last time, tacking has nothing to do with it unless you wish to consider the blades of the propeller being angled against the wind to be tacking and dragging the car along. The video shows this with two boats tacking on a cylindrical earth essentially forming an earth sized propeller.
I'm not convinced any of the math is right. They have a formula for the effectiveness of the prop (using a word for this I'd never heard of before) that has a term (w-v) in the denominator which makes the equation blow up as the car reaches the wind speed. I'm not sure why w-v is in there to begin with. When w-v is zero, it's just the force generated by a stationary propeller. Yeah, they generate plenty of force. In the second video the author shows an alternate equation which does not put a naked (w-v) in the denominator. The point is the math is all off the cuff. But you can't argue with the video that shows it working unless you want to say they are cheating.
A physics professor lost $10,000 on the bet. Do you really think he would let cheating slide?
Then there are the small models that work on a treadmill. They pretty clearly have no batteries or motor.
These things should be reversible. If you turn it around, the wheels will turn the other way and the prop turns the other way making it work down wind still. Seems the models would be more stable with the prop in front.
When do you think they will market these for use on the highways? Kinda like balloon or glider flights. One way with a carriage back.
Has anyone tested an upwind version? If the wind is pushing it downwind the wheels will turn the prop in the opposite direction to go downwind. If the car is pushed upwind everything turns the right way, but how does it get started? Even then, I'm not convinced the details work. The only way I could make the idea work in my head for the downwind car is to consider the gearing to turn the prop slower producing more force than the force on the wheels to start the whole thing. If you reverse that gear ratio I don't see how the thing can work. The prop would exert less force because of the constant power through the gear train.
I'm faily sure it's been done.
I searched "propeller car upwind"; this was on the first page:
Yeah but the gearing between the "fan" and the wheels is such that the fan has machnical advantage over the wheels (for downwind the wheels have the mechanical advantage). thus the "propeller" (actually impeller or turbine in this case) turns the wheels driving the vehicle into the wind, and at the same time increasing the amount of wind encountered.
The wind turns the propeller
Consider the case where 1000 turs of the propeller cause 1 rotation of the axle, clearly it's at-least possible to travel upwind at some (low) speed
Now think aboput it this way, there is some gear ratio where the converted propeller torque exactly matches the axle torque, with this gear ratio the vehicle is immune to the wind and doesn't need a parking brake. you can push or pull on the front of the car and only feel the friction (and innertia), not the wind.
If the ratio is increased the axle torque produced by the propeller is insuficient to hold the car stationary and it travels down wind with a mechanical assist from the wheels driving the propeller
If the ratio is reduced instead, the propeller now drives the wheels and the car travels upwind, with a wind enhancement due to its direction of travel.
Ideal shapes for impellers and propellers are similar but probably not the same, but there is presumably some compromise shape that can travel in both directions given suitable gear ratios (or perhaps adjusatable blade pitch).
I'm faily sure it's been done.
I searched "propeller car upwind"; this was on the first page:
Yeah but the gearing between the "fan" and the wheels is such that the fan has machnical advantage over the wheels (for downwind the wheels have the mechanical advantage). thus the "propeller" (actually impeller or turbine in this case) turns the wheels driving the vehicle into the wind, and at the same time increasing the amount of wind encountered.
The wind turns the propeller
Consider the case where 1000 turs of the propeller cause 1 rotation of the axle, clearly it's at-least possible to travel upwind at some (low) speed
Now think about it this way, the wind blows on the propeller and creates a torque in the propeller shaft and also a force on the vehicle, the force on the vehicle creates a torque in the wheel axles. There is some gear ratio where the converted propeller torque exactly matches the axle torque, with this gear ratio the vehicle is immune to the wind and doesn't need a parking brake. you can push or pull on the front of the car and only feel the friction (and innertia), not the wind.
If the ratio is increased the axle torque produced by the propeller is insuficient to hold the car stationary and it travels down wind with a mechanical assist from the wheels driving the propeller
If the ratio is reduced instead, the propeller now drives the wheels and the car travels upwind, with a wind enhancement due to its direction of travel.
Ideal shapes for impellers and propellers are similar but probably not the same, but there is presumably some compromise shape that can travel in both directions given suitable gear ratios (or perhaps adjusatable blade pitch).
It's not so much the gearing specifically, it's the prop pitch being almost feathered. This creates very little drag and the force downwind is very slight. So the force rotating the blades can move the car forward.
Only if the wind force on the prop (drag) is less than the force from the wheels. I see where this is possible. I suppose that once again, the speed is only limited by the losses in the system. This also shows the vehicle can be moving in any direction relative to the wind. Interesting. It would require a lot of phone and electric lines to be buried or raised, also streets widened. Tough to pass. lol
I wonder if that guy on eevblog has figured out any of this. He sure seems dead certain none of this works. He is invoking pressure storage of energy and claiming you can only go a short distance. At one point I was trying to get him to not say people were incorrect in talking about "rate" being a time factor in regards to power vs. energy and he completely refuses to understand. I got into the conversation and compared power:energy to speed:distance at which point he started talking about relativity... Really? Relativity???
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