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spork, I always agreed that the cart can sail directly downwind faster than the wind, but my reason why was different from yours.

O.K. ...

You'll have to remind me what your reason was.

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Reluctantly, I finally agreed with you. But now, after thinking this through, I'm going to challenge your idea of why it works. I'll post my reasoning later tonight.

I think my vector analysis lays it out pretty simply. I look forward to your post.

If you guys are talking about sailboats, there’s more pushing the boat than just the wind. As you’ve mentioned before there are currents. But, there is also the design of the hull. Most are configured to be 1 and 1 half times the length of the wake they make (or some multiplication of). That way the wake of the boat itself will help propel the boat through the water. Also, there are the waves. Boats tend to have flat sterns that get pushed by the waves and as the wave passes by the boat the boat tends to surf a bit.

As a kid, sailing around Prince William Sound, I would often watch the spinnaker go slack while sailing ‘down wind’ and I was of the opinion it was because of what was happening in the water and not that our boat was ‘magical’.

Perhaps a better question would be, ‘After factoring out friction, and under the same ‘down wind’ circumstances, would a land based sail craft perform better than a one that is sea based that also enjoys the benefits of current, design and waves’?

Or you could just continue doing what you do so well and forget everything I’ve written. I don’t care.

Take care,
Jeff

Jeffery A Rogers:

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Perhaps a better question would be, ‘After factoring out friction, and under the same ‘down wind’ circumstances, would a land based sail craft perform better than a one that is sea based that also enjoys the benefits of current, design and waves’?

Hey, that's a great question! You should start a thread titled: ‘After factoring out friction, and under the same ‘down wind’ circumstances, would a land based sail craft perform better than a one that is sea based that also enjoys the benefits of current, design and waves’?

JB

spork, my original idea was that the cart is pushed forward, the tires, geared to the prop, start to roll, thereby making the prop turn. Once the prop turns, a rear pushing wind speed from the prop is created. The downwind speed of the wind hits this rear pushing wind speed from the prop, driving the cart faster, turning the wheels faster, turning the prop faster, creating more rear pushing wind speed from the prop, creating more cart speed.

If I remember correctly, the rear pushing wind speed from the prop is equal to the forward speed of the wind. So, if the wind is 10 MPH, the rear pushing wind speed from the prop is 10 MPH. This creates a 20 MPH collision of air. About 5 MPH is lost to friction, giving the cart an overall top speed of 1.5 X that of the wind.

And yes, in that scenario, if the wind is not exactly, directly, downwind, the cart loses efficiency.

spork, going back to your idea, you said to imagine an ice boat, sailing around a cylindrical Earth at a constant 45º angle, with the wind speed blowing directly down the cylinder. Will the ice boat outrun the wind and get to the other end of the cylindrical Earth before a baloon, released at the start would? Here, we both agreed the ice boat would win.

Then, you said to imagine 2 of these ice boats, directly opposite of each other, but attached to each other, and both tacking at the same 45º angle down the cylindrical Earth. Would outrun the wind and beat the balloon? Again, we both agreed they would.

Then, you said these 2 ice boats represent the prop on the cart, and that they are on a constant 45º tack, Kinematically constrained to the wheels of the cart.

I agreed, but now that I think about it, in order for that to work, the tailwind would have to blow against the prop tips, that are on that 45º tack, spinning the prop, which is geared to the wheels, and the wheels would then pull the cart forward.

Now, going back to my original idea. Let's take a cart, with an electric powered prop only, and let's say the weight & drag ratios were equal to the prop cart in question. If the prop were blowing rearward at 10 MPH, and we had a 10 MPH tail wind, how fast would the cart go? Due to friction loss, I'm going to guess 15 MPH, or 1.5 X the wind speed, as about 5 MPH is lost to friction.

I'm thinking this is what makes the cart outrun the wind. The tailwind colliding with the rear pushing wind from the prop.

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I'm thinking this is what makes the cart outrun the wind. The tailwind colliding with the rear pushing wind from the prop.

Well, one thing is for sure... if that's the case then the same is true for an ice-boat on a frozen lake on a 45 degree downwind tack at twice the wind speed. The two situations are essentially identical.

My vector analysis describes exactly what happens. Perhaps there's a way to look at it as wind pushing against a tailwind, but I don't think so.

I understand what your saying, but with the ice boats on the cylindrical Earth, they are constantly on a 45º tack, spinning around the cylinder. They are moving around & down the cylinder & outrunning the wind in the process. If that is applied to the prop on the cart, then the spinning of the prop, just like the spinning around the cylinder of the ice boats, will mean that the prop is powering the cart's wheels and the wheels are then powering the cart.

As I understand it, the wind is pushing the cart, not spinning the prop. When the cart is pushed, the wheels power the prop. If the wind were to power the prop, like in the ice boat example, then the cart would go against the wind, not with it. You could reverse the gearing, but in the video, you can tell by the angle of the prop tips that the wind is not powering & spinning the prop.

And... if the wind were spinning the prop, then once the cart matches or outruns the wind speed, the prop tips will no longer "feel" the tail wind. Like a sailboat, it will run out of wind, while sailing directly downwind. In that case, a cross-wind would help the cart, instead of hurt it, like in my example.

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...just like the spinning around the cylinder of the ice boats, will mean that the prop is powering the cart's wheels and the wheels are then powering the cart.

Nope. The force diagram is the same for both. In both cases the wind acts directly on the wing (sail or prop), and the skates, or the wheels & transmission, provide the kinematic constraint. That constraint simply says "to move forward x feet you must move sidewards y feet".

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As I understand it, the wind is pushing the cart, not spinning the prop.

Well, that's true. The wind is "pushing" the prop, and the wheels are spinning the prop. No other part of the cart has to have any sort of aerodynamic property (drag etc.) at all.

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When the cart is pushed, the wheels power the prop.

Well, that's close to true. When the *prop* is pushed by the wind, the prop pushes the cart, which causes the wheels to spin, which in turn causes the prop to spin. But to say the wheels "power" the prop is not accurate. No more than to say the keel of a sailboat powers the boat. Yes the keel causes the boat to move forward when the wind pushes the sail to the side, but we wouldn't say the keel powers the sailboat.

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If the wind were to power the prop, like in the ice boat example, then the cart would go against the wind, not with it.

While it's possible to set the cart up in such a configuration to do exactly that, that's not what happens in this configuration. Yes, the prop would turn the opposite way if it were mounted to a fixed tower, but it's not. It's mounted to a moveable cart. As such it's easier for the entire cart to move downwind (initially much slower than the wind), even though the prop has to move opposite the direction you'd expect. As the speed picks up, the prop starts to see relative wind from a different direction - just as the sail of an ice-boat on a downwind tack does.

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You could reverse the gearing, but in the video, you can tell by the angle of the prop tips that the wind is not powering & spinning the prop.

"Powering" yes - "spinning" no - at least not directly. In the end, the wind provides every bit of the power, at least from the typical reference frame of the fixed ground.

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And... if the wind were spinning the prop, then once the cart matches or outruns the wind speed, the prop tips will no longer "feel" the tail wind.

Just as the ice-boat on a 45 degree downwind tack feels no tailwind component when it's downwind velocity component equals the wind-speed, neither does the prop feel the downwind velocity component when the cart reaches the wind-speed. But in the case of the boat, it has a significant cross-wind component. This still provides ample relative wind on the sail. Similarly the prop is spinning and therefore still feels plenty of relative wind to power it. Just as the wind doesn't "spin" the prop, the wind doesn't push the ice-boat forward. The wind pushes it sideward and the skates translate that into forward motion. Same for both the cart and ice-boat.

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Like a sailboat, it will run out of wind, while sailing directly downwind.

No - it won't. Because the prop blades are never going directly downwind - just like the sail on the ice-boat is never going directly downwind.

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In that case, a cross-wind would help the cart, instead of hurt it, like in my example.

The only important aerodynamic part of the cart (the prop) ALWAYS has a crosswind at all times.

Not to put too fine a point on it - but I designed the darn thing. I know how it works. Yes, it's true that I later learned that others designed and built them before me. To my knowledge the only others to do so were Bauer and Jack Goodman. But for all I know, there may have been 100's of people to independently arrive at this design.

I know for a fact that Fosset had considered using this design to set a land sailing speed record.

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Originally posted by spork: The only important aerodynamic part of the cart (the prop) always has a crosswind at all times.

Again this ignores the induced wash. The crosswind component on the propeller is much less than that of sails on a landsail or icesail, because the air is being rotated and acclerated before reaching the blades of propeller. By the time the air reaches the blades, it's already rotating in the same direction as the blades and it's already moving linearly towards the blades.

I'm not claiming it makes the device impossible, just that it's not going to have anywhere near the downwind peformance that a land sail, or the example of a pair of land sails tacking back and forth while attached to a common cart.

A link explaining why it takes more power to hover than than to move forwards in a helicopter, which is similar to the situation here, although the angular apects of induced wash is ignored in this artcle, except for tip vortices:

heli hovering

I'm still waiting to see a treadmill video of one of these devices actually working.

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Originally posted by jeffareid:
A link explaining why it takes more power to hover than than to move forwards in a helicopter, which is similar to the situation here:

heli hovering

although the angular apects of induced wash is ignored in this artcle, except for tip vortices

Looking at the diagram more carefully, I see that the the induced wind vector is angled to the side as well as downwards, so the angular induced wash is being shown, but it's small as shown in the diagram.

>>
A link explaining why it takes more power to hover than than to move forwards in a helicopter, which is similar to the situation here...
<<

I've been flying R/C helis for many years, and have a little time in full scale helis. I also have a B.S. and M.S. in aero, and am very familiar with both the dynamics and aerodynamics of helicopter flight.

While there are some effects from induced wash, I think you're significantly overestimating them. Keep in mind that a helicopter hovers in still air while a normal propped vehicle pulls itself continuously into clean air. Helis experience translation lift not only in forward flight, but also when ascending. In both these cases they are biting clean air. The latter case is more like this cart in all but the case of going downwind at wind speed or less.

In any event, the entire topic goes to the efficiency of the vehicle, not whether it can do as it claims - and certainly not whether any vehicle can satisfly the claim of DWFTTW (which of course is the only aspect that particularly interests me).

jeffaried:

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I'm not claiming it makes the device impossible, just that it's not going to have anywhere near the downwind peformance that a land sail,

I agree with you that the L/D of the cart will never equal that of an iceboat, but disagree as to the reasons.

The drag of a sharp skate as it glide across the ice (or rolling tire in dry lake bed) will always be less than the pullies, belts and bearings required by the cart. Both mechanisms provide the exact same service, but one has far more losses.

Like spork, I believe your overestimating the wash issue. As it moves forward relative to the wind, obviously this device is not hovering -- and it is in that hover mode that a heli experiences their most pronounced related effect.

In the end,we at least do agree on the various effects, if not the degree.

JB

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Originally posted by spork: I've been flying R/C helis for many years

RC helis have huge power to weight ratios, hovering or descending into induced wash is not an issue for them.

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Keep in mind that a helicopter hovers in still air while a normal propped vehicle pulls itself continuously into clean air.

Propellers also operate in their own induced wash: "The airspeed through the propeller disk is simply the average of the free stream and exit velocities": propeller analysis. The air ahead of the prop accelerates from V0 (free stream speed) to Vp by the time it reaches the blades of a propeller. Once the air reaches the blades, there is little change in speed, and mostly a change in pressure.

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Originally posted by HUDMAN:
spork, my original idea was that the cart is pushed forward, the tires, geared to the prop, start to roll, thereby making the prop turn. Once the prop turns, a rear pushing wind speed from the prop is created. The downwind speed of the wind hits this rear pushing wind speed from the prop, driving the cart faster, turning the wheels faster, turning the prop faster, creating more rear pushing wind speed from the prop, creating more cart speed.

Hudman, you have to watch out for those flashbacks! I mentioned that I had them occasionally after I finally understood what spork was describing. You managed to make the crossover once you realized the significance of the sideways component of the relative wind that the prop rotation provides.

Just for a quick reality check, compare what is happening in a rocket and a jet engine. Do either of these "push" against the air behind it to any significant degree? Or do they depend on F=MA - developing a force by accelerating a mass?

Go back and reread the last few pages of that previous thread. I'm sure you'll "get it" again!

I did reread it, right before I posted my thinking on this thread.

To me, and I may be wrong, thinking that the prop tips are tacking sails would be the same as saying an airplane flying in a 10 MPH tailwind now has a ground speed of 10 MPH faster because the prop tips are tacking to that 10 MPH tailwind. The reality is that it's flying in a 10 MPH medium, so it's ground speed is 10 MPH faster than its air speed. There is no friction loss because the plane is not touching the ground.

The cart is also trying to go 10 MPH faster than the tailwind, if that tailwind was 10 MPH. But, 5 MPH is lost to friction.

I agree, an ice boat tacking at 45º, is pushed sideways by the wind and the skates translate that into forward motion. But, at 45º to the tailwind, it will still "feel" the tailwind. Once the cart outruns the wind, there is only a relative headwind, and no tailwind can be felt, because the cart is going directly downwind.

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RC helis have huge power to weight ratios

Generally true for most R/C helis. Not for all.

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hovering or descending into induced wash is not an issue for them.

Absolutely not true.

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Propellers also operate in their own induced wash...

The extent to which this is true depends entirely on things like the RPM of the prop and the rate at which it's moving through the air. I don't understand where you're going with this. Are you suggesting the cart cannot go DWFTTW?

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...thinking that the prop tips are tacking sails would be the same as saying an airplane flying in a 10 MPH tailwind now has a ground speed of 10 MPH faster because the prop tips are tacking to that 10 MPH tailwind.

Nope. Not at all the same thing.

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The cart is also trying to go 10 MPH faster than the tailwind, if that tailwind was 10 MPH. But, 5 MPH is lost to friction.

How do you arrive at this?

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I agree, an ice boat tacking at 45º, is pushed sideways by the wind and the skates translate that into forward motion. But, at 45º to the tailwind, it will still "feel" the tailwind.

No more or less than the blades of the prop will. Remember the ice-boat has a downwind velocity component faster than the wind. So how can it "feel" the tailwind?

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Once the cart outruns the wind, there is only a relative headwind, and no tailwind can be felt, because the cart is going directly downwind.

The cart is going directly downwind. The prop tips are not. They are doing exactly the same thing as the ice-boat sail. For every foot they go downwind, they go 1 foot cross-wind. Same as the ice-boat.

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How do you arrive at this?

Just a guess, based on the cart I built.

Given the prop pitch and gear ratios, you can easily compute how much faster than the wind the cart is "trying" to go. The frictions and inneficiencies account for the difference between this and the real results.

Hudman, how do you explain an iceboat achieving a speed up to 6 times higher than the wind speed, and five times faster in terms of VMG? The iceboat definitely doesn't "feel" any tailwind and also isn't providing an "air cushion" effect behind it. It is outrunning the wind by a very large margin - and yet it moves.