How can an vehicle move faster than the wind that is powering it?

[DaveC426913]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

 

[ThinAirDesign]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

At this point I couldn't care less about any analogies --- I just simply want to know the level of physics understanding of the folks in the exchange. If they don't understand simple inertial frames, I have to approach any explanation much differently.

JB

 

[schroder]

schroder, before you walk away from the conversation, humor me and answer the below question. I'd really appreciate it.


You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

Thanks

JB

I don’t know why I bother, but I will answer your question, and then ask one of my own. If the headwind is a true wind relative to still water, then the boat will get to port before the buoy. In fact, the buoy will never get to port! But, if the wind is a relative velocity caused by a ten knot current flowing towards the port, then the buoy will absolutely reach port first, regardless of how high performance the yacht is or how clever the skipper. Remember, there is no wind, only the relative wind to the current! And the discussion I had with spork was in reference to the latter scenario.

Now, let me ask you this: Suppose you have a very well designed auto rotation glide copter, the very best there is. You drop it in still air under the force of gravity. As it descends, the blades start to turn due to the relative velocity of the air as gravity pulls it towards the earth. Is there ever a point where the lift from the blades can stop the downward fall, or even reverse it so the copter goes up?

 

[ThinAirDesign]

I don’t know why I bother, but I will answer your question, and then ask one of my own. If the headwind is a true wind relative to still water, then the boat will get to port before the buoy. In fact, the buoy will never get to port! But, if the wind is a relative velocity caused by a ten knot current flowing towards the port, then the buoy will absolutely reach port first, regardless of how high performance the yacht is or how clever the skipper. Remember, there is no wind, only the relative wind to the current! And the discussion I had with spork was in reference to the latter scenario.

Thanks schroder. I will answer your below question and then I'll get back to the above.

Now, let me ask you this: Suppose you have a very well designed auto rotation glide copter, the very best there is. You drop it in still air under the force of gravity. As it descends, the blades start to turn due to the relative velocity of the air as gravity pulls it towards the earth. Is there ever a point where the lift from the blades can stop the downward fall, or even reverse it so the copter goes up?

I believe I understand the question, but let me be sure -- I assume by "glide copter", you are referring to say the equivalent of a gyrocopter without a motor. Am I right?

Assuming "Yes" to the above:

In a steady state situation, there is absolutely no version of this craft which could halt its descent or reverse it. The better designed the machine ... the slower the descent could be, but it will never get to zero or be able to reverse.

Hope I got that right.

Thanks for the quid pro quo. I hope it can continue with my next post.

JB

 

[schroder]

Thanks schroder. I will answer your below question and then I'll get back to the above.




I believe I understand the question, but let me be sure -- I assume by "glide copter", you are referring to say the equivalent of a gyrocopter without a motor. Am I right?

Assuming "Yes" to the above:

In a steady state situation, there is absolutely no version of this craft which could halt its descent or reverse it. The better designed the machine ... the slower the descent could be, but it will never get to zero or be able to reverse.

Hope I got that right.

Thanks for the quid pro quo. I hope it can continue with my next post.

JB

Thank you for your response, which is correct. Now can you please explain how a cart which is powered (through wheels and drive belt and propeller) running on a treadmill, can possibly reverse direction and move against the movement of the tread?

 

[ThinAirDesign]

Ok schroder, thanks again for the exchange (feel free to ask your own questions as before and I will do my best to answer them). I have posted my original question at the bottom of this post, just for reference.

Another question related to the original:

After you and I decide that we will tack upwind toward shore (South) and we drop our marker bouy, we set sail as planned. We start with a ~45 degree tack (or reach depending on whose terms we're using) to the SW and after a few hundred yards we tack back to the SE and return to a point directly South of our bouy. Just for conversation, let's say that the progress me made directly south was 500ft relative to the bouy.

Do you agree that the above progress relative to the bouy is possible?

Thanks

JB

You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

 

[ThinAirDesign]

I have to begin my commute schroder. I will continue a bit later (and will do my best with your question)

Thanks again.

JB

 

[uart]

Here's one train of thought that might help convince that the "treadmill in still air" situation is a least feasible.

Imagine for a moment that the rotating a propeller was replaced by a large “cork screw” (or similar). That is, the wheels were coupled through a suitable drive train to turn this large “cork screw”.

Now imagine that the corkscrew is started into a large block of soft foam attached to the front of the treadmill. If the treadmill is run then the wheels turn and the corkscrew turns and the vehicle will move forward as the corkscrew screws into the foam.

Perhaps we can think of the propeller in a similar was as "screwing" itself forward into the stationary air. I’ve got to admit that when I first saw the video I thought it was a hoax/impossible but right now I certainly think it’s at least feasible.

BTW. I also sail (just windsurfer ) and I’ve found this a very interesting discussion.

One last thing. Way back in this thread I posted a mathematical derivation of the maximum downwind speed as a function of the acute angle “phi” that a (sail or ice) boat could head into the apparent wind. The post was effectively lost to the forums current Latex problem so I’ll just briefly repost the main results here.

From the vector diagram attached,
W = wind velocity.
V = craft velocity.
A = relative (or apparent) wind velocity

Applying the sine rule (and simplifying) you get.

V / W = cos(theta) + cot(phi) sin(theta)

(note that “theta” is your boat heading off the wind and "phi" is your boats highest heading into the apparent wind)

From the above the VMG is

V cos(theta) / W = cos^2(theta) + 1/2 cot(phi) sin(2 theta)

Differentiating wrt theta the optimum heading theta was found to be,

theta_best = 45 – phi/2 degrees (assuming phi is acute)

And the resulting maximum VMG was found to be,

VMG_max = (1 + cosec(phi)) / 2

Clearly this is greater than W and I believe this proves that as long a phi is acute (which is the same as saying that the craft is capable of tacking up wind) then the maximum downwind VMG is greater than W.If anyone wants to see more details of the derivation take a look at post #62 in this thread when the latex bug is eventually fixed.

 

[spork]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

The sailboat tacking into the wind is not an analogy at all. It IS the device we seek to build. If you don't understand intertial reference frames, you won't see that.

However, you claim the sailboat can sail circles around the buoy. And I agree with that of course. In that case we know that the moving current can move the boat in the current's direction - faster than the current.

If the headwind is a true wind relative to still water...

We're in the middle of the ocean - 100's of miles from land, thousands of feet from the ocean floor. What IS "true wind". Do you honestly believe that what the air is doing relative to something so far away has any bearing on anything? Why not measure the wind speed relative to the moon in that case? Einstein tells us quite clearly that if we put a giant box around our boat (the fog for example) we have no way at all to distinguish between moving wind and still water vs. moving water and still wind. In fact Einstein would point out that even referring to either one as "moving" ONLY even makes sense relative to the other. After all, how fast is the "true" wind moving in an absolute sense? How fast is the Earth moving in an absolute sense? Ans: there is no answer - velocity is not absolute - it's relative.






I'll ask again. Are most of us on-board that the ice-boat can do as claimed by the ice-boating commnity (and backed up by GPS data and analysis)? At the start of this thread, most if not all said the ice-boat could never achieve a downwind VMG faster than the wind. I'd like to believe the evidence

 

[uart]

Hey I've always believed they could a achieve a VMG of W( 1 + cosec(phi) )/2

 

[LURCH]

I can't egt the video to play anymore. Can anyone else still see it?

 

[ThinAirDesign]

I can't egt the video to play anymore. Can anyone else still see it?

No problems for me.

JB

 

[spork]

New offer: the bet is now 10:1. I'll put up $100K against anyone's $10K, and I will prove this is possible.

It seems there are easily 10 people on here that don't buy this. That's only $1K each. Wouldn't that be a much better way to end this thread?

 

[schroder]

We're in the middle of the ocean - 100's of miles from land, thousands of feet from the ocean floor. What IS "true wind". Do you honestly believe that what the air is doing relative to something so far away has any bearing on anything? Why not measure the wind speed relative to the moon in that case? Einstein tells us quite clearly that if we put a giant box around our boat (the fog for example) we have no way at all to distinguish between moving wind and still water vs. moving water and still wind. In fact Einstein would point out that even referring to either one as "moving" ONLY even makes sense relative to the other. After all, how fast is the "true" wind moving in an absolute sense? How fast is the Earth moving in an absolute sense? Ans: there is no answer - velocity is not absolute - it's relative.

I’m not Einstein, but I find it very easy to tell the difference: a boat that is drifting with the current, in still air, will have no water breaking against the bow and it will leave no wake behind it. A boat that is drifting with the wind, in still water, will have water breaking against the bow and it will leave a wake. Very simple.

 

[ThinAirDesign]

Thank you for your response, which is correct. Now can you please explain how a cart which is powered (through wheels and drive belt and propeller) running on a treadmill, can possibly reverse direction and move against the movement of the tread?

schroder, I appreciate the frank exchange and I'll do my honest best but I fear that this explanation won't be any more productive than the others. Forgive my lack of ability to explain it.

In as simple as terms as I know: When it comes to the treadmill as compared the aircraft, you're forgetting that in the former there are two operating mediums moving relative to each other (air and a solid surface). In your 'glide copter' example, there is only one medium and that is still air.

Where two objects are moving relative to each other, there is energy to be extracted and utilized. This principle is well established -- rivers move and we extract energy, wind blows and we extract energy, even something as simple as a bicycle tire moving past the fork provided the opportunity to mount a headlight generator .

The copter only has it's own kinetic/potential energy upon which to capitalize. The cart on the treadmill (and sailboat and ice-boat and land yacht and wind turbine, and, and, ...) has the opportunity to extract energy from the motion of the two relative surfaces and use it however it sees fit.

In the case of the wind turbine, the energy extracted from the wind/tower interface is used to power the grid. In the case of the cart, it uses the energy extracted from the treadmill/air interface to move itself forward on the belt -- useless I know, but still entertaining since folks say it can't be done.

That's my best. I'll answer any question you wish regarding the above as best I can.

Thanks for listening schroder.

JB

 

[spork]

I’m not Einstein, but I find it very easy to tell the difference: a boat that is drifting with the current, in still air, will have no water breaking against the bow and it will leave no wake behind it. A boat that is drifting with the wind, in still water, will have water breaking against the bow and it will leave a wake. Very simple.

Then you're smarter than Einstein. Because he makes it very clear that he can't tell the difference.

 

[uart]

I’m not Einstein, but I find it very easy to tell the difference: a boat that is drifting with the current, in still air, will have no water breaking against the bow and it will leave no wake behind it. A boat that is drifting with the wind, in still water, will have water breaking against the bow and it will leave a wake. Very simple.

You should re-think that, it's definitely not true.

The boat drifting in the wind in still water will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move correct. I only say this becasue for some inexplicable reason you chose to imagine that in the still water plus wind case that the boat is "feeiling the wind" wheareas in the current plus no wind case you chose to assume that the boat does not feel the apparent wind. Tell me, do you understand the concept of apparent wind?

 

[ThinAirDesign]

I’m not Einstein, but I find it very easy to tell the difference: a boat that is drifting with the current, in still air, will have no water breaking against the bow and it will leave no wake behind it. A boat that is drifting with the wind, in still water, will have water breaking against the bow and it will leave a wake. Very simple.

(schroder, I've again included my orginal question at the bottem of this post for reference)



Considering that you have the ability that you describe above, I'm going to ask you a simple question:

In the below scenario, before we raise our sails I ask you to look over the side and tell me what you see -- water breaking against the bow, or no water against the bow.

Which do you see?

Thanks.

JB

You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

 

[ThinAirDesign]

You should re-think that, it's definitely not true.

Yes he should, and it's most definitely not true ... However, I have a bit of an issue with your explanation.

The boat drifting in the wind in still water will only have bow wave and wake if it is under sail.

That depends on the definition of "bow wave and wake". With no sails up, the drag of the hull, mast and other will definitely move the boat downwind through the water. This movement will cause some "wave and wake", just not as dramatic as when the sails are up.

I'm sure we're on the same page, but just wanted to clarify that small point.

JB

 

[Trond]

I’m not Einstein, but I find it very easy to tell the difference: a boat that is drifting with the current, in still air, will have no water breaking against the bow and it will leave no wake behind it. A boat that is drifting with the wind, in still water, will have water breaking against the bow and it will leave a wake. Very simple.

It's probably just semantics again, but if the boat is drifting with the current in air which is still (lets say relative the moon...) there will be a wind relative the sails. Exactly like a wind relative the still water. So in both cases you would actually sail if you have your sails up hence there will a wake..

 

[spork]

With no sails up, the drag of the hull, mast and other will definitely move the boat downwind through the water. This movement will cause some "wave and wake", just not as dramatic as when the sails are up.

Yes, and as I'm sure both of you understand, that will be identical whether in a 5 knot current with no wind or vice-versa.

 

[Topher925]

We're in the middle of the ocean - 100's of miles from land, thousands of feet from the ocean floor. What IS "true wind". Do you honestly believe that what the air is doing relative to something so far away has any bearing on anything? Why not measure the wind speed relative to the moon in that case? Einstein tells us quite clearly that if we put a giant box around our boat (the fog for example) we have no way at all to distinguish between moving wind and still water vs. moving water and still wind. In fact Einstein would point out that even referring to either one as "moving" ONLY even makes sense relative to the other. After all, how fast is the "true" wind moving in an absolute sense? How fast is the Earth moving in an absolute sense? Ans: there is no answer - velocity is not absolute - it's relative.

Just curious, why is it Einstein would know so much about this and other physicists wouldn't? For example why not Planck, Tesla, or Hawking, etc. Or did you just use him because it was the first name that popped into your head?

So according to your "relativity" analogy, you can define your frame of reference from the sail of your ice boat. Assuming the iceboat has the ability to accelerate to a downwind component equal to the wind speed, its apparent wind velocity will be directly orthogonal to its downwind component. In this case you then have an apparent wind equal to the perpendicular downwind velocity, correct? This apparent velocity flowing over your "airfoil" like sail is what produces lift and allows you to actually go faster than the wind. Is this correct?

 

[spork]

if the boat is drifting with the current in air which is still (lets say relative the moon...) there will be a wind relative the sails...

Yes, the wind relative to the sails will be at least hundreds of miles/hr given that the surface of the Earth moves at about 1000 mph and the moon is moving in orbit a bit above 330 mph. I'd think about battening down the hatches.

 

[spork]

..... Is this correct?

No. But thanks for asking.

 

[uart]

Yes he should, and it's most definitely not true ... However, I have a bit of an issue with your explanation.
That depends on the definition of "bow wave and wake". With no sails up, the drag of the hull, mast and other will definitely move the boat downwind through the water. This movement will cause some "wave and wake", just not as dramatic as when the sails are up.

I'm sure we're on the same page, but just wanted to clarify that small point.

JB

You should re-think that, it's definitely not true.

The boat drifting in the wind in still water will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move correct. I only say this becasue for some inexplicable reason you chose to imagine that in the still water plus wind case that the boat is "feeiling the wind" wheareas in the current plus no wind case you chose to assume that the boat does not feel the apparent wind. Tell me, do you understand the concept of apparent wind?

I made it perfectly clear that I was taking the ideal case of zero wind resistance on the entire craft! I can't see how you missed that.

My point was that schoder seemed to be under the impression that in the other case (no wind and boat in current) that the apparent wind had zero influence on the boat. I was saying that if you take this erroneous assumption and apply it to the no current + wind situation then you'll also get no bow wave or wake. I hope that makes it clear :)

 

[ThinAirDesign]

It's probably just semantics again, but if the boat is drifting with the current in air which is still (lets say relative the moon...) there will be a wind relative the sails. Exactly like a wind relative the still water. So in both cases you would actually sail if you have your sails up hence there will a wake..

Trond has hit the problem schroder appears to be having right on the head.

In BOTH cases there is the same wind over the boat. If the air is moving 10mph and the water still, the boat *sees* an ~10mph wind. If the air is still and the water is moving 10mph, the boat *sees* a ~10mph wind.

With the wind the same in both scenarios, any 'wake and wave' will be the same. Any windsock will show the same. Any physics experiment will be the same. The occupants of the boat will be powerless to determine which is moving and which not -- they only know they have wind.

JB

 

[Topher925]

ThinAir,

In BOTH cases there is the same wind over the boat. If the air is moving 10mph and the water still, the boat *sees* an ~10mph wind. If the air is still and the water is moving 10mph, the boat *sees* a ~10mph wind.

Can you explain how this is different then what I stated in my last post.

Spork, do you care to explain WHY it is incorrect.

 

[ThinAirDesign]

I made it perfectly clear that I was taking the ideal case of zero wind resistance on the entire craft! I can't see how you missed that.

I didn't miss it uart, but you still didn't make it perfectly clear by any means ...

Here's the quote:

The boat drifting in the wind in still water will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move correct.

If it has zero wind resistance and the water is still, it won't be "drifting" now will it? If it's "drifting", sails up or down, it will have wakes and waves to varying extents.

You clearly state above that the boat is drifting and you clearly state that it won't have wake with the sails down. Those points are mutually exclusive. Any drift or relative motion creates wake.


uart, I know *you* understood your point (as did I), I was only trying to make sure that it was technically correct for those who are having difficulty with the inertial frame references.

Best wishes.

JB

 

[uart]

"Spork, do you care to explain WHY it is incorrect."

No I think it was basically correct. When the downwind component of velocity is equal to the wind velocity then yes the apparent wind will be in a direction perpendicular to the actual wind direction. And yes in this direction it may be possible to still produce lift.

 

[uart]

If it has zero wind resistance and the water is still, it won't be "drifting" now will it?
JB

Congratulations you finally got it. That was actually the point I was trying to make, that it wouldn't be drifting! That's what the words "then it would not move" mean.

I was trying to show the falsity of schoders argument so I used his words (drifting) and then went on to explain why it wouldn't drift unless you assumed some interaction with the wind.

I'm presuming that if I had of punctuated it better as in,

The boat "drifting in the wind in still water" will only have bow wave and wake if it is under sail. If you could make the hull etc to have zero wind resistance then it would not move, correct.

then you wouldn't have a problem.

Sorry but I find that level of nit picking very annnoying when you're trying to make a quick post

 

[ThinAirDesign]

ThinAir,

Can you explain how this is different then what I stated in my last post.

I'm not sure what you need me to explain. My post was not in response to yours -- I hadn't even seen yours when writing mine. I was writing my explanation for schroder.

Same or different -- can't say as I wasn't referencing your comments.

JB

 

[ThinAirDesign]

I absolutely love the "still air relative to the moon" bit. If I'm picking a frame of reference for sailing -- that's the one I'm using every single time. And hey, if the wind is still, relative to the moon, we'll only need a one foot mast and a hankerchief -- saves a lot of room down below.

This "physics" forum is a real kick.

JB

 

[spork]

Someone made a comment about how diagrams are so "second degree university" - but there hardly seems to be 3 people here that understand the most basic high school physics concept of inertial reference frames. Discussing the prop-cart going downwind faster than the wind is beyond hopeless.

Heck, we've presented every possible type of evidence of what ice-boats are capable of - and all that has resulted in is silence.

 

[Trond]

I wonder why...let me know when you are ready to discuss the cart

 

[rcgldr]

Assuming the formula stated for a device with a Beta of 14 degrees, I made a graph of Vmg/Vt versus heading offset from true down wind. It peaks at about 2.56 at 38 degrees. http://jeffareid.net/misc/dwvhdg.gif

I missed the obvious flaw that when the heading is 0 degrees offset from the wind direction, Vmg/Vt is 1.0, where it would have to be less than 1.0 because of drag. The Beta factor needs to vary based on heading.

Does anyone have an actual table of heading versus Vmg? Another issue is lift versus drag as it applies to sails. When heading downwind, aerodyamicd and ground lift are zero while aerodyanmic drag = ground drag. Also unlike wings, sails aren't designed to divert the air flow by 90 degrees. I need to do a rethink on this.