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PostPosted: Sun Mar 27, 2011 11:29 am 
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It is impossible if the wind were travelling like a single wave going 10 mph through an otherwise still medium then objects moving ahead of that wave in the same direction and faster would not be affected since the wave would never reach them. But a constant 10 mph wind is in the entire environment all around the vehicle not just behind it but also ahead, so it has an effect on the thrust of the propeller just like a fan blowing 10 mph wind can impact the output of another fan acting as the propeller by directing the 10 mph wind either into or away from the intake of the propeller fan.

Bob


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PostPosted: Tue Mar 29, 2011 3:06 pm 
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bobco wrote:
But a constant 10 mph wind is in the entire environment all around the vehicle not just behind it but also ahead,

That is the key observation: You cannot escape the moving airmass, even if you move faster than it. So there is always wind energy available (velocity difference between air & ground), regardless of your speed. You just need a clever way to harvest it.


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PostPosted: Tue Mar 29, 2011 3:13 pm 
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Gringo wrote:
The wind doing the pushing would not even be touching the object it pushes.

As bobco said, wind is not like the end of a stick pushing you. You don't lose contact with it, just because you are faster. The air mass is like a long object that pushes you, while you ride along it. Like the stripe of paper shown here:




Wind is a velocity difference between the surface and the airmass. And that velocity difference is there, and can be exploited, regardless of your speed.


Last edited by reksio on Tue Mar 29, 2011 5:12 pm, edited 1 time in total.

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PostPosted: Tue Mar 29, 2011 3:23 pm 
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Gringo wrote:
Sails do not rotate.

Traditionally no. But I don't think the general definition of a sail-craft rules that out. It's an unconventional sail-craft, but still a sail-craft. But that's just semantics.


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PostPosted: Tue Mar 29, 2011 3:41 pm 
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bobco wrote:
And here is the counter intuitive part - that apparent head wind now provides a force that tries to slow the vehicle down yet that same head wind also assists the prop since now reinforcing its rotation

This part seems wrong. The apparent head wind is NOT reinforcing the rotation of the prop above windspeed.

The prop is always turning against the torque from the air, and accelerating air backwards to:
- produce forward thurst
- slow down the true wind (energy input)

If the rotor would become a turbine, which is turned by the apparent head wind, it would accelerate air forward, and thus:
- produce drag
- accelerate the true wind (energy loss)
This wouldn't work in steady state. It can happen if the true tailwind suddenly stops or slows significantly. But then the cart would also stop.

bobco wrote:
it follows that if that tail wind suddenly shut off, the apparent head wind should increase instantly providing a sudden jump in the drag force which had just before been exactly balanced by the prop's thrust but now exceeds it

The key thing, that would stop it (if the true tailwind stops) is not the increase in aerodynamic drag at the chassis, but the decrease in propeller thrust (due to more apparent headwind). The wheels produce drag, to turn the propeller. The propeller can create more thrust, than the wheel drag ONLY if the apparent headwind is reduced by the true tailwind (airspeed < groundspeed).

Without the velocity difference between ground & air the propeller thrust cannot be greater than the wheel drag needed to turn the propeller, and will in fact be smaller due to transmission loses. That stops the craft even without any aerodynamic drag at the chassis.


Last edited by reksio on Thu Mar 31, 2011 1:35 am, edited 1 time in total.

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PostPosted: Wed Mar 30, 2011 6:46 pm 
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Thank you reksio for a clear explanation of a complex issue. I'm still uncertain about some aspects of why head wind helps to accelerate when it passes through the zero apparent wind but that same apparent head wind increase slows down the vehicle when the tail wind cuts off though I do recognize that the energy source has been cut off so it has to slow down - just struggling with the "how" it does it.

I like the part about the various mechanisms to harness the energy between the wind and the ground regardless of speed.

But I think the spool analogy while provocative doesn't fully capture the complexity of the vehicle's mechanism since the spool's coupling between small radius and larger radius is rigid whereas the wheels/prop versus the energy harnessed is soft.

The result is that the spool always goes at exactly the same multiple of speed determined by the ratio of the two radii whereas the downwind vehicle has a continuously variable speed from start to max steady-state. However it does demonstrate your point of capturing energy of environment relative to the ground regardless of speed.

Bob


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PostPosted: Thu Mar 31, 2011 1:22 am 
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bobco wrote:
I'm still uncertain about some aspects of why head wind helps to accelerate when it passes through the zero apparent wind

The apparent head wind never helps to accelerate. The reduction of the apparent head wind (due to existence of a true tailwind) allows to accelerate. The thrust force of a propeller depends on input power via shaft and the relative headwind (airspeed):

thrust = prop_efficiency * (input_power / airspeed)

If you reduce the airspeed (like the true tailwind does here) you get more thrust for the same input_power. But the input_power coming from the wheels depends only on the ground speed, and is not affected by the reduction of airspeed. This tips the energy balance in favor of the vehicle.

bobco wrote:
But I think the spool analogy while provocative doesn't fully capture the complexity of the vehicle's mechanism

At least it shows that such a mechanism doesn't violate any conservation laws. These laws are general and do not differentiate between rigid and soft masses.

bobco wrote:
since the spool's coupling between small radius and larger radius is rigid

This represents the rigid coupling between prop (small reel radius) and wheels (large reel radius). What is "more rigid" on the spool is the coupling between the small reel radius (propeller) and the paper (airmass). But efficient propellers can achieve up to 90% efficiency, so it is not that far away.

bobco wrote:
The result is that the spool always goes at exactly the same multiple of speed determined by the ratio of the two radii

Not really. If you move the paper first while holding the reel still and then release the reel, you will have a acceleration phase too: from zero, trough 1 x paper_speed up to 2 x paper_speed. The acceleration phase is just much shorter than with the wind cart.


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PostPosted: Thu Mar 31, 2011 7:55 am 
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Reskio, that was a great explanation comparing the mechanism of energy capture using the spool demo with the sailing craft without violating any laws of physics.

However, I'm still having problems with your claim that the apparent head wind would never contribute to the prop thrust. It seems that more mass is expelled at equal or higher acceleration if an apparent wind is blowing in same direction as the prop's expelled air mass than when an apparent wind is blowing in the opposite direction. Therefore the prop thrust is helped by the"same direction" head wind given that the force or thrust is proportional to the mass and its acceleration (f=ma). Thus intuitively you would expect that the apparent head wind is reinforcing the prop thrust and the initial tail wind at the start of the downwind run is inhibiting the prop thrust though less and less as the apparent tail wind decreases.

Even your equation seems to support this. Holding all else constant, the equation you gave says that thrust is inversely proportional to apparent air speed meaning more wind speed leads to less prop thrust if the vectors of the wind and prop thrust are aligned. It follows that if the apparent wind vector is opposite the prop thrust then it subtracts from the prop thrust so less of the apparent tail wind is better for the prop thrust. This would explain why it has such a slow start and then accelerates to faster and faster speeds then gets diminishing returns as the apparent head wind lessens the thrust to the point where it is balanced by that same head wind's drag on the vehicle at it reaches steady state at 2.8 times the true wind speed. Similarly, if the tail wind is cut off, the jump in apparent head wind decreases the thrust due to inverse relationship when wind vectors are aligned and the drag of that same apparent head wind slows the vehicle down to a stop.

So I'm still uncomfortable hearing that "The apparent head wind never helps to accelerate. " Yet I'm equally uncomfortable (though it is consistent with equation to say) that more of the head wind provides less prop thrust because of the inverse relationship. But that seems inconsistent with F=ma if more head wind means more mass expelled so more thrust and not less.


Bob


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PostPosted: Thu Mar 31, 2011 10:25 am 
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bobco wrote:
It seems that more mass is expelled at equal or higher acceleration if an apparent wind is blowing in same direction as the prop's expelled air mass than when an apparent wind is blowing in the opposite direction. Therefore the prop thrust is helped by the"same direction" head wind given that the force or thrust is proportional to the mass and its acceleration (f=ma).

The problem here is that the acceleration of the air will be less. The faster the air already moves backwards, the more difficult(more energy required) it becomes to accelerate it even more.

The relevant formula here is: power = force * velocity

With more velocity, you need more power for the same force.

bobco wrote:
Even your equation seems to support this. Holding all else constant,

This is tricky. The prop_efficiency is not constant but highly dependent on the conditions. It can achieve 80-90% at design conditions. But for most of the run it is far worse.

bobco wrote:
the equation you gave says that thrust is inversely proportional to apparent air speed meaning more wind speed leads to less prop thrust if the vectors of the wind and prop thrust are aligned. It follows that if the apparent wind vector is opposite the prop thrust then it subtracts from the prop thrust so less of the apparent tail wind is better for the prop thrust.

This is even more tricky. The prop formula (coming from the realm of aircraft propellers) offers a sensible definition of propeller efficiency in apparent headwind (the normal case for planes). To use it for airspeeds <= 0 you would have to assume negative efficiency, because the propeller is now slowing down the air (in the propeller's frame). The formula doesn't really help us to calculate thrust in the below windspeed situation, because it assumes a certain direction of energy flow, which now reversed. And there is little empirical data on propellers in this condition.

bobco wrote:
This would explain why it has such a slow start and then accelerates to faster and faster speeds

This is the variable propeller efficiency. Initially the blades are stalled (separated flow). When it comes closer to the design conditions, they unstall and work more efficiently.

bobco wrote:
So I'm still uncomfortable hearing that "The apparent head wind never helps to accelerate. "

In general, it would make no sense, if the relative headwind, that you create with your own motion, would help you to accelerate.

In general, in terms of optimally achievable performance:
relative headwind : bad
less relative headwind : good

But for a specific vehicle, for example with fixed propeller pitch, there is one optimal combination of airspeed and ground speed, that offers the best attack angle at the blades. So sometimes an increase in airspeed can increase the acceleration. But not by increasing thurst, but rather by increasing the prop efficiency and thus decreasing the wheel drag:

better attack angle at the blades ->
less induced drag at the blades ->
less reaction torque that brakes the wheels

bobco wrote:
But that seems inconsistent with F=ma if more head wind means more mass expelled so more thrust and not less.

See first paragraph.


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PostPosted: Thu Mar 31, 2011 6:17 pm 
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Good point about the lack of acceleration when wind is in same direction as prop thrust.

Now back to your statements that the head wind you create cannot accelerate you. Were you referring to down wind travel only? We all use apparent wind to get our fastest speeds and that comes from the wind we create vectored with the true wind.

Or perhaps there is some intrinsic property of props that they cannot exploit the head winds they create combined of course with the true wind to result in acceleration?

Bob


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PostPosted: Fri Apr 01, 2011 4:45 am 
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bobco wrote:
We all use apparent wind to get our fastest speeds and that comes from the wind we create vectored with the true wind.


Of course you use the apparent wind to generate the force at the sail, but this is only possible because of the true wind component in the apparent wind. The induced head wind component (negative boat velocity) itself can increase the magnitude of the apparent wind, but then it also makes the direction of the apparent wind worse (more frontal).

So the apparent wind with the best acceleration in practice depends on the specific sailcraft: Big cloth sails need less magnitude, but a good direction. Efficient thin airfoils need more magnitude but, can deal with very low apparent wind angles

bobco wrote:
Or perhaps there is some intrinsic property of props that they cannot exploit the head winds they create combined of course with the true wind to result in acceleration?

For the analogy to the propeller you would have to consider the apparent wind at the blades, which has a tangential component that allows them produce thrust. The axial component (relative head wind) makes it in general more difficult to produce thrust. But a propeller with a specific fixed pitch requires a certain head wind to operate efficiently.

Compare that to a sail craft accelerating from zero on a fixed broad reach course: If you are not allowed to adjust you sail angle, and have to set it for high speed with downwind VMG > 2 x windspeed from the very start, then your initial acceleration will be bad, and will eventually reach a peak at some speed.

Yet that doesn't mean that the induced head wind is a good thing for acceleration in general, but rather that your specific craft is not making the best possible form every given condition.


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PostPosted: Fri Apr 01, 2011 6:28 am 
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reksio wrote:
The induced head wind component (negative boat velocity) itself can increase the magnitude of the apparent wind, but then it also makes the direction of the apparent wind worse (more frontal).


I like the analogy of the rotor blade as a sail with its own induced wind since it could be different in both magnitude and direction than the apparent wind of the boat. So what is direct downwind to the boat may not be to the rotor blades. If the air over the blade is fast enough then the induced component could be so great that the negative effects of being more frontal when going down wind may be eliminated or at least reduced enough such that it benefits.

Ice boats can create such extreme induced wind up to 5 or 6 times the true wind that they can sail 135 degrees off the wind so the frontal aspect may not always be a negative since the ice boat is closer to downwind than upwind.

Similarly, the high speed of air flow over a properly oriented blade may also benefit from the blade's induced wind when vectored properly with the true wind. If so then the induced head wind may help the prop's thrust and not necessarily be a negative when going directly down wind.

I'm probably wrong here again but I'm trying to understand the fundamental logic of why not. Before DDWFTTW type craft were proven, many if not most experts claimed it could not be done due to the laws of physics.

Amazingly they were wrong, and proven so not just by experiment but more intriguingly by logic such as that you already gave - namely capturing the energy of the difference between the true wind and the ground will work regardless of speed even if faster than the wind provided that the drag and inefficiencies (energy loss) of the mechanism is less than the thrust of the capture device that is driven by that energy difference.

Bob


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PostPosted: Fri Apr 01, 2011 3:14 pm 
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bobco wrote:
Ice boats can create such extreme induced wind up to 5 or 6 times the true wind that they can sail 135 degrees off the wind so the frontal aspect may not always be a negative since the ice boat is closer to downwind than upwind.

Apparent wind that is more frontal is bad because the huge sail force generated by those 5 or 6 times the true wind, gets mostly canceled by the keel or sleds. Efficient sail-craft are not profiting from a more frontal apparent wind, but are rather able to work despite of it.

bobco wrote:
Similarly, the high speed of air flow over a properly oriented blade may also benefit from the blade's induced wind when vectored properly with the true wind. If so then the induced head wind may help the prop's thrust and not necessarily be a negative when going directly down wind.

If you want a properly oriented blade then more headwind means more blade pitch. But that means that less of the blades lift points forward and contributes to thurst, while more of it points tangentially and creates drag at the wheels.


bobco wrote:
I'm probably wrong here again but I'm trying to understand the fundamental logic of why not.

It not really a question of "yes" or "no" logic, but rather a more complex issue, where the answer is "you have to look at the numbers and vectors of the specific case".

The only general thing you can say: With more headwind you need more power to generate the same thrust.

bobco wrote:
Amazingly they were wrong, and proven so not just by experiment but more intriguingly by logic such as that you already gave - namely capturing the energy of the difference between the true wind and the ground will work regardless of speed

In theory it works regardless of your speed, because the velocity difference between the media is the same. However, with more speed, you have to transfer more power from the fast medium to the slow medium, to generate the same thrust. And more power means more transmission losses. So even with constant rolling friction and no aero drag at the hull, you cannot go at any speed. Your transmission will generate increasing losses or even blow up.This happened here:
http://www.youtube.com/watch?v=PhD_3vdIvu4
Prop at high pitch, so it didn't self-start, but achived very high speed, far beyond design parameters.


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PostPosted: Sun Apr 03, 2011 9:57 am 
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reksio wrote:
Apparent wind that is more frontal is bad because the huge sail force generated by those 5 or 6 times the true wind, gets mostly canceled by the keel or sleds. Efficient sail-craft are not profiting from a more frontal apparent wind, but are rather able to work despite of it.


Propeller driven sailing watercraft using only the wind can sail directly into the wind. So there must be sufficient net forward force components when using propellers to use a 100% frontal wind to go directly into the wind - granted that they go slowly compared to a conventional tacking sailboat probably due to the inefficiency you have discussed. So the issue remains, is there some more efficient way to harness the frontal winds?

You mentioned that keels cancel the lateral forces. But what if keels were adjustable foils and used sail powered undulatory propulsion like a undulatory flag, fish or snake? Could they provide not only the lateral canceling force but also forward thrust much like the wheels that power the prop for forward thrust in the DDWFTW craft? Imagine a TI sail powering the motion of an undulatory flexible foil "keel" in one of the TI wells now using a mirage drive.

Evolution tends to converge on the most efficient designs and yet it did not develop the wheel or propellers in any animal locomotion - with a few exceptions such as insects that curl up in a wheel to roll away from danger or the maple leaf seed as a spinning prop to drift down slower away from the parent's shade for a better chance of success,.... Rather in extreme environments where energy must be conserved, nature perfected the efficient penguin pendulum walk (left-right swing of the body and the forward back-swing of the legs) for its land locomotion and most likely more efficient than a wheel for its irregular environment. And more relevant to sailing, nature perfected the moving flexible foil of its body for submerged propulsion.

Therefore, one topic of interest to Tiers regarding the DDWFTW innovation might be to explore how we could go faster than the wind or directly up wind by better exploiting the insights gained by DDWFTW. Those innovators have or will shorty demonstrate the ability to go faster than the wind directly down wind as well as directly upwind, feats that water sailing craft no matter how efficient so far have not been able to achieve. But is there some fundamental logic of the physics of how the DDWFTW craft function that could be used to either suggest it is or isn't possible?

One idea to explore possibly on a separate thread would be going directly upwind in a sailing water craft without using propellers but rather moveable foils like in nature. After all, two boats tacking in opposite directions linked by a light stretchable cord move as one system directly upwind so it seems it may be possible with the right capture/propulsion mechanism using a sail. The reasoning used by the DDWFTTW innovators not only was used to prove that it was possible but the analysis also predicted its limits and thus made an equally valuable contribution of why everything outside those limits was not possible.

Is there some such reasoning that can be used to argue that a watercraft will never be able to sail directly upwind without using a propeller?

Bob


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PostPosted: Mon Apr 04, 2011 1:28 am 
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bobco wrote:
Propeller driven sailing watercraft using only the wind can sail directly into the wind.

For going into the wind the air-rotor is a turbine, while the underwater-rotor is a propeller.

bobco wrote:
Evolution tends to converge on the most efficient designs and yet it did not develop the wheel or propellers in any animal locomotion

Evolution develops most efficient designs within given constraints. How would you transport nutrition into a freely spinning body part of a complex species like a bird or fish, that is based on blood vessels? For primitive organism evolution did develop rotary propellers:
http://www.microbiologybytes.com/blog/2 ... -a-rudder/

And wheels would generally suck in the wilderness. Evolution didn't optimize for roads (at least yet). But you would have the same problem as above.

bobco wrote:
Those innovators have or will shorty demonstrate the ability to go faster than the wind directly down wind as well as directly upwind, feats that water sailing craft no matter how efficient so far have not been able to achieve. But is there some fundamental logic of the physics of how the DDWFTW craft function that could be used to either suggest it is or isn't possible?

No, there is nothing fundamental that prevents it from working on water.

bobco wrote:
One idea to explore possibly on a separate thread would be going directly upwind in a sailing water craft without using propellers but rather moveable foils like in nature.

Have a look here:
http://www.boatdesign.net/forums/attach ... part-1.pdf
http://www.boatdesign.net/forums/attach ... part-2.pdf


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