Haven't seen anything of denverdon lately - unless he disguised himself as shroder.

What I find disappointing is that the thread was locked because of a quarrel between the moderator and shroder. We'll see if it gets unlocked, I was having fun exploring different ideas.

It appears to be the same problem with people deciding it is a hoax and going to great lengths and very poor logic to "prove" their point. Never fails to generate heated discussions!

I had asked:

quote:

If you mounted a battery operated fan on the cart to provide the downwind component the cart should move downwind faster than the wind?

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Originally posted by Nanomyth:
I had asked:

quote:

If you mounted a battery operated fan on the cart to provide the downwind component the cart should move downwind faster than the wind?

That was not the point of the exercise, and would be considered cheating.

My point was that if the fan were blowing into a sail the cart would not go anywhere. It would be no different than putting the cart in a wind tunnel.

No, the point of putting the cart into a wind tunnel is to show that it works. A wind tunnel gives a measurable and repeatable wind, with no variance (cross winds, gusts, etc.). The treadmill should have been a good enough test but there are still some silly skeptics out there who won't believe their eyes and would rather try to twist the idea of frame of reference to defend their mistaken beliefs.

quote:

Originally posted by Nanomyth:
My point was that if the fan were blowing into a sail the cart would not go anywhere.

Surprisingly enough it actually would if the sail were of the proper shape.

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It would be no different than putting the cart in a wind tunnel.

Actually it would be different. A wind tunnel would provide a steady wind relative to the ground. Your fan would provide a steady wind relative to the cart - and at the "expense" of the cart.

I have been doing some thinking on the full scale version. One question does this vehicle run afoul of the square/cube law? The smaller version of the two seemed very spry.

So far I've seen the results of two medium sized carts and two small ones.

Break even speeds for the big prop carts: 4 mph (Jack) and 10 mph (spork and JB)
Break even speeds for the small prop carts: 2.7 mph (JB and spork) and 8.5 mph (Mark)

Considering that full sized iceboats seem to be capable of the required efficiency, a man-sized prop cart should work just fine.

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Surprisingly enough it actually would if the sail were of the proper shape.

I thought I had read in other threads and elsewhere that it would not work.

I had expected there to be some loss in thrust if it did work. Thank you for your response.

The prop on the cart only has to blades. Would the addition of a third or fourth blade help any?
That would be easier than adding a contra-rotating blade.

Hi, I'm new here and don't want to get up anyone's nose but after reading this thread I just can't help but to steer you guys straight.

The propeller design seems plausible at first glance, however testing on a treadmill is flawed. The vessel is treadmill powered and not wind powered.

The air forced through the propeller is moving in relation to the ground. The air is the driving force of the vessel therefore velocity also needs to be measured in relation to the ground. It is pointless to have the treadmill as reference for the vessel, and the ground as reference for the air.
All that you have designed is a way to conserve the momentum which is transferred between the propeller and the wheels. It is not possible for any wind powered vessel to travel faster than the wind consistently when heading directly downwind. You can prove this to yourself by using a windmeter and a stop watch and experiment outside.

I am an avid sailboarder and know it is impossible to go faster than the true wind when heading directly downwind at least on a sailboard (or iceboat by association). There is no aerodynamic lift in the sail when heading directly downwind therefore the apparent wind is zero, power is solely from the true wind. The iceboat is able to travel faster than the wind when reaching but the data linked to from this thread doesn't show the iceboat ever heading directly downwind. Iceboats are significantly faster than a sailboards due to the almost frictionless surface but the principle is the same.

If this ever makes it to mythbuster status then the title of the thread is confusing.
"sailing downwind faster than the wind" the answer is CONFIRMED. If it were "sailing *directly* downwind and *maintain* a consistent speed more than the wind" then the answer is BUSTED.

The only way I would deem this worthy of mythbuster is if they incorporate it into another myth. Strap a stick of dynamite to it and blow it up. This is not meant to be offensive, I really like the design of the propeller craft, but if it is on mythbusters it *must* get blown up and show on high-speed camera or else the show would be boring. Maybe Adam and Jamie would blow up an iceboat would be pretty cool.

Thanks for your input. Here's another thread to read:

http://community.discovery.com/eve/forums/a/tpc/f/9701967776/m/4561931559/p/1

Let us know what you think about that. By the way, the treadmill test has been confirmed as a legitimate test by several physicists and some guy with an MS of aerodynamics - but thanks anyway.

And it has been tested outdoors and manages to go directly downwind faster than the wind.

Maybe you could volunteer to blow up your sailboard instead!

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The propeller design seems plausible at first glance, however testing on a treadmill is flawed. The vessel is treadmill powered and not wind powered.

You might want to look up inertial frames of reference.

The cart can't tell what's moving and what isn't. All it can "see" is the difference between the wheels and the air. Mathematically, the ground moving and the air still is *equal* to the ground still and the air moving. Yes, semantically the cart is now "treadmill powered" and not "wind powered", but it is still effefcting propulsion from the *difference* between the air and the wheels.

The only reason the treadmill was chosen was that it is easier to control the slope and constant strength and direction, instead of finding a perfectly flat spot with winds that are constant and in one direction. Just because it's an "easier" demonstration to effect, doesn't mean its mathematical equivalent is invalid.

This the the exact same as a boat in still air in a current. Once it starts to drift with the current, the sail "sees" a wind from the front, and the boat can tack into this relative wind for a total component of velocity that exceeds the velocity of its power source, the current.

The treadmill just seems to be confusing everybody, no-one is considering the fundamental laws of physics.

There is a law of CONSERVATION OF ENERGY.

ENERGY IN = ENERGY OUT

ENERGY IN = Kinetic energy from the treadmill
ENERGY OUT = Change in momentum + Heat loss from friction

The treadmill only needs to provide enough energy to overcome the energy lost from friction (very little) and the change in momentum of the vessel.

People seem to focused on the speed of the wheels and not in the change in momentum of the vessel.

quote:

Originally posted by Stickywulf:
It is not possible for any wind powered vessel to travel faster than the wind consistently when heading directly downwind.

In light of the outdoor test that proved that it is possible, and the treadmill tests that use an equivalent from of reference to also prove this, I would suggest a little more research before making blanket statements. This is explained quite carefully in the thread that I linked. You obviously haven't read it.

Everyone who understands this knows that it doesn't go against the fundamental laws of physics. Do yourself a favour and catch up on your reading.

quote:

Originally posted by Stickywulf:
It is impossible to go faster than the true wind when heading directly downwind at least on a sailboard (or iceboat by association).

This is true for a sailcraft. A sailcraft needs a forwards and/or crosswind component to move forwards. The apparent crosswind is equal to the speed of the wind times sin(angle between wind direction and sailcraft direction), and since it's perpendicular to the sailcraft direction, it's independent of the sailcraft's forward speed, so the sailcrafts maximum speed is limited by it's effective lift to drag ratio for a given apparent crosswind.

Ignoring the analogy of the prop blades moving in a spiral path, the DDWFTTW carts work because the power input comes from the wheels which interact with faster moving ground, while the power output comes from the prop which interacts with slower moving air. This allows the force from the wheels to effectively be geared down to multiply the force and divide the speed, and even though there are losses, the resultant force from the prop is greater than the driving force from the wheels, it just occurs at a slower speed.

Power equals force times speed, so the wheel force times wheel speed can be "geared" so that prop force is greater while prop speed is less.

In a no wind situation, this doesn't help. But if there is a tailwind, then the reduction of speed at the prop is compensated for by the tailwind.

From my post at physics forums:

Note the speed factors here are relative to the medium the wheel and prop interact with. The wheels interact with the faster moving ground and the prop interacts with the slower (relative to cart) moving air.

If the cart is moving at the same speed of the air, then the only speed at the prop is due to it's own induced wash, which is 1/2 the sum of the entry and exit (when air pressure returns back to ambient downwash of the prop) speeds, while at the wheels, the relative gound speed is equal and opposite to the wind speed (if the cart is going the same speed as the wind).

The power input is the force the ground applies to the wheels time the forwards speed of the cart. The power output is the force the prop applies to the air times the backwards speed of the air flow through the prop.

Vw = speed of wind
Vc = speed of cart
Vp = speed of induced wash from prop
Fp = force from prop
Fc = force from cart wheels
Fd = overall losses (drag) related to forward speed of cart

The net force on the cart = Fp - (Fc + Fd)
The power input = Fc x Vc
The power output = Fp x (Vp + Vc - Vw)
The power loss = Fd x Vc

The point here is that Vc can be > Vw, Fp can be > Fc, with power output still well below power input as long as Vw is greater than zero (a tailwind), because the cart uses effective gearing to multiply the force and divide the speed from the ground to the prop, which works because the prop interacts with the air (wind), as long as the reduction of speed by the gearing is less than the difference than the speed between the air (wind) and the ground, and the force is mutliplied enough to overcome any loss factors.

quote:

The air forced through the propeller is moving in relation to the ground. The air is the driving force of the vessel therefore velocity also needs to be measured in relation to the ground.

Yes, and with a tailwind, the air is moving at 10mph forwards without any prop interaction. If the prop generates 5mph of thrust, then that 5mph of thrust is in addition to the 10mph of tailwind, for an effective thrust rate of 15mph. The induced wash required to generate the thrust is subtracted from this 15mph. If the required induced wash is 3mph, then the cart goes DDWFTTW at 12mph, 2mph faster than the tailwind.

I was just about to post when I saw what you wrote jeffareid. Give me some time to think through what you have written. For now, this is what I prepared - mostly about debunking the treadmill experiment.

FACT is, treadmills remove the concept of momentum and therefore this experiment is useless. I have no desire to watch youtube videos of treadmills knowing that the experiment is flawed. A stationary object on a treadmill exhibits virtually no change in momentum. I read through some of your previous posts oldguy1 and you seem to have a good grasp of physics and it's concepts, why is this so difficult to understand? The vessel changes the direction of motion, but the magnitude of velocity is small with the vessel creeping along the treadmill slowly. The small velocity does not correspond proportionally to the amount of energy applied through the treadmill belt.

The treadmill experiment is debunked. There is nothing more to say about this. Ask the physicists and aeronautical engineer if they understand the most basic of all fundamental physics - conservation of energy. I am worried about my children's future if this is the level of education today.

The outdoor tests? Hmmm, where are they. All I see is a guy chasing a propeller across a carpark - which doesn't seem to be moving any faster than a beachball. Where is the stopwatch? What is the windspeed? This doesn't prove anything! Then there is the original video, again with no measure of windspeed or the vessel's velocity, the only indication of speed is the ribbon attached to the top which shows the motion is at right angles to the wind - it would be faster with a sail!.
You need to help me find these outdoor videos which prove something because I scanned that thread twice for links and couldn't find anything significant. All I see is speculation based on treadmill tests (debunked), iceboats and unmeasured outdoor tests.

My intent is not to disrup, only to clarify the discussion rationally. I am making an attempt to try and see your point of view, if you could offer the same courtesy and at least attempt to consider the change in momentum relevant, would be appreciated thanks. I really don't want to see this myth go to air, since it would be as boring as POAT, I would rather see something get blown up. If we can rationalise this here then more explosions on tv.

If you know the guy who built the vessel then a simple experiment will clarify this. Measure the force of the vessel when it is on the flat treadmill. Then disconnect the propeller from the wheels and measure the force again. The two forces should be almost equal and opposite - almost because some energy is lost through the gears spinning the fan.

quote:

I would suggest a little more research before making blanket statements

Hundreds of years of technology and there is no documented evidence to show anything contrary. Since it is almost impossible to disprove something, then I can safely say it is not possible until proven otherwise. Or do I have to also prove that there isn't a teapot orbitting the sun?

The reason the treadmill is used is that, in the real world the wind strength and direction are constantly changing, as I am sure you have experienced yourself. This means that a video done in real world conditions inevitably gets responses that it wasn't directly downwind, or in a gust or downhill etc. The treadmill test is equivalent to driving in a car downwind at the same speed as the wind, holding the cart on the road then letting go. If going directly downwind faster than the wind is impossible then then every time you let go of the cart then it will go backwards relative to the car or relative to the treadmill. This of course ignores how the cart would get to the same speed as the wind. There is little controversy that a wind powered vehicle can travel downwind slower than the wind so the wind speed is assumed to be the limiting case, if the cart accelerates downwind when it is travelling at the same speed as the wind then it is assumed to be able to accelerate at speeds less than that of the wind. I'm not sure I understand what momentum has to do with this. Surely the change in momentum from 10mph to 11mph is the same as the change in momentum from 0mph to 1mph.

How is this possible? I am sure you are comfortable with the idea that you can sail across the wind at a speed much greater than wind, this is due to an increase in the apparent wind due to the motion of the sail. I'm not sure whether this applies to sailboards due to their relatively poor downwind performance, but other high performance sailing boats sailing at an angle to the wind can achieve downwind VMGs much greater than they would sailing directly downwind due to the same apparent wind effect. Iceyachts can exceed the velocity of the wind when sailing at an angle to the wind, hence the use of iceyachts in the discussion.

What if you could continue to have your sail go across the wind while your board goes directly downwind? The energy gained from the sail would surely exceed that required to make the board go downwind faster than the wind. It is suggested that the prop cart is just that, sails going across the wind while the rest of the cart goes directly downwind. Others choose to model the cart as a propeller gaining extra thrust due to the tailwind which I am sure is equivalent.

Out of interest, if you believe that conservation of energy makes this impossible what do you propose is the maximum energy input that can be gained from the wind?

"Treadmills remove the concept of momentum"???

You do get that the cart runs "steady state" in a fixed wind??

I.e. there is no change in momentum of the cart, whether on the treadmill or running with the wind?


What is the change in momentum of anything moving steady-state?

Stickywolf, here's a link to an article by the maker of the big prop cart that you and many others claim is running at a significant angle to the wind.

http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

He lists the specs for his cart, documents the treadmill test and presents his method of testing.

quote:

The treadmill experiment is debunked. There is nothing more to say about this. Ask the physicists and aeronautical engineer if they understand the most basic of all fundamental physics - conservation of energy. I am worried about my children's future if this is the level of education today. Eek

I'm assuming that your next statement indicates a change of direction in your investigation of this.

quote:

I am making an attempt to try and see your point of view, if you could offer the same courtesy and at least attempt to consider the change in momentum relevant, would be appreciated thanks.

The point is that there is a lot more energy available to be harnessed than is used by a cart with a sail mounted on it. You can see how much more by the way the cart accelerates when a cart is placed in a tailwind and released. It moves out smartly but it can't use all that energy to go faster than the wind with just a sail. There is still a lot of energy in the wind that is there for the taking if a method can be thought up and implemented. That's exactly what the prop carts do, they sidestep the issue by using a prop to move the cart faster than the ground speed of the wind while still remaining in "contact" with the wind. Most people, like yourself, think that the cart is either outrunning its power source or somehow getting more energy than is available. A cart with a sail would be. A cart with a prop doesn't. That's the whole reason for the prop.

Read some more about why a treadmill test duplicates that air to ground speed difference in this thread:

http://www.physicsforums.com/showthread.php?t=272437

Warning: in this thread, one person stubbornly refuses to think about what anyone else is posting and manages to get the thread shut down before much progress is made. Frames of reference were discussed ad nauseum in an attempt to correct this person's thinking.