Challenge: MythBusters and Cruisers Forum

[Little Otter]

Look Scotty This is actually very possible and I can tell you how it is possible,
First the propeller is not driving the car it is turning the weels and that is what is driving the car

second if you are going forward and air is being pushed through the blades then it will keep doing so untilit hits something or the prop breaks off

thrid if you look at racing sailboats do they not go faster than the wind speed?

also this does not infer perpetual motion but rather an unlimited powersource that can be and is feasible and being used now,

If you don't beleive me then go back to highschool and pay more attention in physics class =)

[dacust]

Originally Posted by Little Otter

Look Scotty This is actually very possible and I can tell you how it is possible,
First the propeller is not driving the car it is turning the weels and that is what is driving the car OK, but something has to be spinning the propeller, right? The wind, right? However, in the video it's the other way around, the treadmill is powering the wheels which then is then powering the propeller.

second if you are going forward and air is being pushed through the blades then it will keep doing so untilit hits something or the prop breaks off But what pushes air through the blades when the car reaches the speed of the wind it is going downwind with?

thrid if you look at racing sailboats do they not go faster than the wind speed? Not directly downwind, they don't. And that's what is specifically stated as the issue. Going directly downwind faster than the air.

also this does not infer perpetual motion but rather an unlimited powersource that can be and is feasible and being used now, unlimited?

If you don't beleive me then go back to highschool and pay more attention in physics class =) Maybe that'd be a good idea for you.

I added some comments in blue.

The video with the treadmill doesn't convince me. It's a flawed experiment.

They don't show a fan. The power looks to me to be coming from the treadmill. It is using the friction of the wheels, coupled with the added friction of the wheels having to spin the propeller. It's just enough friction so it doesn't coast back down the ramp and fall off. So it slowly tries to climb. It's friction. The power was supplied by that big electric motor in the treadmill.

So, I think what they did was flawed. Not saying I definitely know it can't work, but that video just hasn't proved it to me.

The outside segment looks better (both designs), but neither one shows a measured windspeed or vehicle speed. That one doesn't prove it to me either.

-dan

Let's do a thought experiment....

I have a propeller mounted on my little cart. It is a left handed propeller, geared so that when the propeller turns counter clockwise the wheels drive the cart forward.

My cart is at rest. There is a 10 knot wind blowing from behind the cart. The propeller starts to turn, counterclockwise, and the cart moves forward, slowly at first, then faster and faster. the apparent wind gradually slows down. When the cart is moving at 10 knots the apparent wind switches from behind the cart, to forward of the cart...

NOW.... with the apparent wind from IN FRONT of the cart my propeller now starts to turn CLOCKWISE and the wheels turn backwards...

Right?

[Ex-Calif]

I am at a disadvantage because I got no you tube at work - darn IT guys - and haven't seen the video.

It sounds like you are trying to compare some sort of land based mechanical wheeled dievice with a propellor mechanically linked to wheels to a sailboat propelled by rotating blades (propellor).

If so there is no comparison.

[anotherT34C]

I don't think I understand the problem. I saw (in the video) the little cart held down on the treadmill until the prop reached speed. Depending on the gear ratio coupling the wheels to the prop, the prop can achieve very high rpm's. As for the cart on the road, why is the prop blade not simply like a sail on a tack (like the iceboat)? I didn't think the prop was coupled to the wheels on the outdoor cart.

[Hampus]

Also, there is no wind on the treadmill, not apparent nor any other sort.

[Robertcateran]

I have been thinking a bit about the situation. It won't work. The energy has to come from slowing down air molecules relative to the water. The air ends up traveling even faster if the boat is travelling faster than the wind dead downwind. This is very different from ice boats cutting through a very large area of moving air and gaining energy from it in the form of kinetic energy, and then using that energy to go directly into the wind with minimal interference, and then bearing away again to cut across the wind again to get some more energy.
(And please don't ask me to go back to school and pay attention in physics-I have retired from teaching it to trainee Physics teachers)

[Hud3]

I used to regularly sail faster than the true windspeed on my Hobie 16. But it was on a reach, not dead downwind. HOWEVER, when starting off, the sails were sheeted out. As my boatspeed picked up, the apparent wind shifted forward, and I had to sheet the sails in tighter and tighter as boatspeed picked up, even though my heading didn't change. The only thing changing was boatspeed and apparent windspeed and direction. At max boatspeed, the sails were set as if I were sailing close-hauled, hard on the wind, but I was actually on a broad reach, sailing across the wind.

Now, the cart has a prop, not sails. But a sail is a prop in a fixed position, right? And a prop is a sail that can move. They're both foils. When the cart starts off, headed DDW, the wind is pushing it. But then the prop gets spinning, and the apparent wind across the individual prop blades changes. The individual prop blades will see wind from a different direction than dead behind the cart, and accelerate until they are at the point of "pinching", then slow a bit, then speed up a bit. Just like you sail your boat close-hauled--pinch up, fall off, in small increments.

Forget about the wind over the cart. It's the apparent wind over the individual prop blades as they spin that causes the cart to move faster than the true wind.

p.s. I'm not sure what the treadmill experiment proves...

[anotherT34C]

I think the makers of the video were confused. They probably didn't realize they were testing two different physical principles with the outdoor cart and the indoor cart on the treadmill. I think they meant the indoor treadmill test to be a frame of reference control. Of course, if you think about it for a few seconds, and imagine making the prop arbitrarily large, you'll see why it fails as a test. A large prop, even at low rpm (or small prop with appropriate gear ratio to provide high rpm) can easily provide enough force to push the cart forward against the treadmill.

I also noted they repeat (in the video) the urban legend that bumblebee's can't fly according to the laws of aerodynamics. Sorry guys, no new physics here either, but it's a nice lab for a freshman mechanics class.

[Little Otter]

The tread mill was turning the opposite way the wheels were turning and therfore can't be powering the car.

[anotherT34C]

The wheels were coupled to the prop. The treadmill was powering the prop. Now imagine a much larger prop switched in (same rpm, defined by the treadmill velocity) and you'll understand exactly what's happening.

[Robertcateran]

Originally Posted by Hud3

I used to regularly sail faster than the true windspeed on my Hobie 16. But it was on a reach, not dead downwind. HOWEVER, when starting off, the sails were sheeted out. As my boatspeed picked up, the apparent wind shifted forward, and I had to sheet the sails in tighter and tighter as boatspeed picked up, even though my heading didn't change. The only thing changing was boatspeed and apparent windspeed and direction. At max boatspeed, the sails were set as if I were sailing close-hauled, hard on the wind, but I was actually on a broad reach, sailing across the wind.

Now, the cart has a prop, not sails. But a sail is a prop in a fixed position, right? And a prop is a sail that can move. They're both foils. When the cart starts off, headed DDW, the wind is pushing it. But then the prop gets spinning, and the apparent wind across the individual prop blades changes. The individual prop blades will see wind from a different direction than dead behind the cart, and accelerate until they are at the point of "pinching", then slow a bit, then speed up a bit. Just like you sail your boat close-hauled--pinch up, fall off, in small increments.

Forget about the wind over the cart. It's the apparent wind over the individual prop blades as they spin that causes the cart to move faster than the true wind.

p.s. I'm not sure what the treadmill experiment proves...

It is the prop that is moving faster than the wind. Once the boat reaches windspeed dead down wind the only forces on the prop are resistance forces. If you go any faster the forces on the prop can only slow the boat down

[Ex-Calif]

Originally Posted by anotherT34C

The wheels were coupled to the prop. The treadmill was powering the prop. Now imagine a much larger prop switched in (same rpm, defined by the treadmill velocity) and you'll understand exactly what's happening.

Ok I saw the video. This is an energy question clearly but the treadmill test is completely different than the outdoor test.

Treadmill Test - Energy is being introduced mechanically via the gearing between the treadmill, which is powered, through the wheels, gearbox and into the propeller.

Conclusion - Of course this can outrun the treadmill. You have unlimited (basically) horsepower to turn the prop. You put a higher pitch prop, a bigger prop or even smaller wheels on the treadmill and the device will take off. Imagine the treadmill is an airplane engine. If the device could not "fly" downwind faster than the wind speed, airplanes couldn't fly. Note that he has to impart resistance to the device as he puts in on the treadmill. This build up the kinetic energy in the device. If he started with the device stationary the device would fall off the back of the treadmill. Once the prop has developed thrust to overcome the rolling resistance of the wheels the device can stay put - with optimized gearing or treadmill speed - or climb forward with higher treadmill speed or fall back with lower treadmill speed.

This test is completely different than the outdoor wind test.

Hud, thank you for so eloquently stating my point. In fact, if a variable pitch prop was used, it could easily be set to act as a more efficient foil, and allow the prop to travel forward faster than wind speed. Calculating the zero point where the prop would cease rotation, and gearing the wheels to drive the vehicle just below that speed would produce a land vehicle that would exceed wind speed.
I do not pretend to know the math on this, but using random numbers to illustrate the point, if the prop was capable of maintaining a forward rotation at 2% over the wind speed due to the low pressure created on the front of the blades, and the wheels were geared to drive the vehicle at 1% over wind speed, the prop would never reach the zero point. In theory, the vehicle should be able to exceed wind speed by 1%. It would not likely remain constant, as the prop would try to reach the zero point, but when it reaches that point, it would decelerate, and begin to accelerate again.
To go a step further, if the cart was fitted with a smaller prop, pitched to disengage the wheels just before the drive prop reached the zero point, the speed would likely remain constant.
I know we have at least a couple of pilots on here. Would be good to have someone jump in here who has a better understanding of aerodynamics.

[GordMay]

Originally Posted by Kai Nui

... I know we have at least a couple of pilots on here. Would be good to have someone jump in here who has a better understanding of aerodynamics.

Although Air and Water are both Fluids, and thus might share many common properties of Fluid Dynamics; Water is practically incompressible (air is compressible) and nearly 1,000 times denser than Air.

Might not these significant differences give rise to differences between Hydrodynamic & Aerodynamic calculations?