DDWFTTW Turntable Test: 5 Min Video - Is It Conclusive?

[spork]

You need to avoid using sources with "longer path" or "equal transit" theory, if you want to talk about how lift is generated and be taken seriously.

Aside from the fact that the "equal transit" theory is complete nonsense, the image shown has the stagnation point entirely in the wrong location.

 

[ThinAirDesign]

Jeff, I'm pretty sure that we both have the same understanding regarding how a sail works and are just discussing semantics, but again I just have to repeat ... your statement:

No sailcraft extracts energy from an apparent headwind

... is just plain wrong.

The "sailcraft" is the entire system, not just the sail -- and inarguably (rationally at least) sailcraft CAN and DO extract energy from headwinds, apparent and true.

Perhaps you wish to argue that when analyizing a specific component of the sailcraft (like perhaps the sail) there is a different conclusion, and I'll have to think about that before responding -- but the "sailcraft" wording just doesn't fly.

JB

 

[vanesch]

Yes!

1002 reached ! Brilliant. You guys have been great. With a special thanks to Schroder

 

[rcgldr]

Perhaps a clarification of terms?

I did that in post #999 (congrats on having the 1000th post). I separate the apparent wind into components in the direction of and perpendicular to the direction of the sailcraft (wrt to the ground). The component in the direction of sailcraft is an apparent headwind or apparent tailwind (or zero). The component perpendicular to the sailcraft is an apparent crosswind (or zero).

work done by an airfoil, in this case a sail based airfoil. The sail will generate lift.

I prefer to separate the components with respect to the direction of the sailcraft. In this case, the purpose of a sail is to generate thrust (wrt to sailcraft). The "lift" component (wrt to sailcraft) only results in an equal and opposing force from the ground. It doesn't directly contribute to the forward speed of the sailcraft. If a sailcraft experiences an apparent headwind, then the thrust from the sail is related to the apparent crosswind, not the apparent headwind. The sail also needs to divert (accelerate) the apparent wind (both headwind and crosswind components) to true upwind sufficiently so that the true wind (wrt to ground) is decelerated.

Maybe a wiki airfoil picture is helpful:

Equal_transit-time_NASA_wrong1.gif

There's a reason that diagram is named "wrong". It's sufficient to note that the pressure "above" an airfoil is less than the pressure "below" in order to generate lift (how it does this can be a discussion in another thread). In the case of a sailcraft, "lift" from an airfoil, wrt to the apparent wind, isn't interesting, only the component of "lift" that translates into thrust wrt to the sailcraft.

Perhaps you meant that the craft would fail to do work ... lift vector is perpendicular to the vessel.

This is a case of my terminology, not the physics of the airfoil. I should have made myself more clear. I tried to cover this in post #999.

 

[spork]

I prefer to separate the components with respect to the direction of the sailcraft...

Let's consider a direct-upwind prop-cart. This would be basically the same as our DDWFTTW cart only we change the gearing so it will advance directly into the wind. This "sailcraft" will in fact use an apparent (and true) headwind to propel itself directly upwind. Of course the blades of the prop are not going directly upwind (nor directly into an apparent headwind), but as JB says we're talking about the sailcraft - not the sail.

No sailcraft extracts energy from an apparent headwind

I tend to agree with JB that this probably has more to do with semantics than physics, but I still struggle with this statement because we definitely feel a very significant effect of apparent wind while kiteboarding. When going 90 degrees to the true wind the pull of the kite and performance overall definitely improves as our speed increases. Clearly the only thing changing in such a situation is that the apparent headwind is increasing (working from your definition), yet we definitely seem to extract energy from it.

 

[schroder]

Good to see you’re still “with us”. Don’t see how two videos where everything is the same except the efficiency of the cart can give the same mathematical results when the observed results are so different.

Wheel running Radius of TT = 60cm. Wheel diameter = 8cm.

Thanks for that. I will do a few calculations and let you know what I find.

 

[schroder]

A smaller cart wheel (5.5cm dia). Sorry about the background glare and the now unbalanced TT (stupid tiered demo).

The bouncing around is sure to have an effect on the drop out and it certainly does. However, the longer it runs, the faster the cart is going. Right at the end I had the cart going from 8 o’clock to 3 o’clock CCW or a displacement of 5 hours for one CW revolution of the TT. That is 2.4 : 1

 

[rcgldr]

No sailcraft extracts energy from an apparent headwind.

Let's consider a direct-upwind prop-cart.

I already did.

Upwind carts extract energy from an apparent headwind, but not downwind carts.

I wasn't considering an upwind cart as a sailcraft. If an upwind cart is to be considered as sailcraft, then upwind sailcraft are the exception to the rule.

The main point I was getting at is that the true wind has to be decelerated (wrt ground) in order to extract power from the wind. For a downwind cart, an apparent headwind has to be accelerated upwind, reducing thrust from the propeller, and increasing drag; the apparent headwind is an overhead for a downwind cart, not a source of energy.

Of course the blades of the prop are not going directly upwind

Sails on a front to back track on the cart could be used instead, with no crosswind component required, just movement of the sails relative to the cart. Slower backwards relative to cart speed for a downwind cart, faster backwards for an upwind cart. (The sails would retract and redeploy at the ends of the track, sort of a linear like paddle wheel).

I still struggle with this statement (sailcraft don't extract energy from a headwind) because we definitely feel a very significant effect of apparent wind while kiteboarding. When going 90 degrees to the true wind the pull of the kite and performance overall definitely improves as our speed increases. Clearly the only thing changing in such a situation is that the apparent headwind is increasing (working from your definition), yet we definitely seem to extract energy from it.

The pull will get stronger, but only in the direction perpendicular to travel. The thrust will diminish as speed increases.

Perhaps I should restate what I'm getting at: The apparent headwind component of the apparent wind on an airfoil results in lift and drag. Only the apparent perpendicular component of the apparent wind on an airfoil can result in thrust.

Where the apparent headwind comes into play is that the sail diverts the apparent wind, both headwind and crosswind, with a signifcant true upwind component, decelerating the true wind, even though the sailcraft's downwind component of speed is faster than the wind.

 

[schroder]

Ok this really takes the cake. Are you just straight out lying or do you really expect us to believe that you don't understand the "strobe" effect of low frame rate movies (as per the old wagon wheels appear to turn backwards on movie films etc).

Here's the original outdooor test that Schroder is referring to.


Note that the device will be acting as a propeller if it turns counter-clockwise looking from behind and that it will be acting as a wind turbine if it turns clockwise when looking from behind. You can very clearly see that it turns counter-clockwise (from behind) when it is running very slowly, both at the start and at the end of the video. Now Schroder is telling us that this prop is actually rotating clockwise (from behind) because it appears to turn this way due to strobe effect at about 2.55 in the video. If you watch the whole video you can clearly see that it appears to change rotation direction many times throughout the clip, obviously due to strobe effect at different speeds.

Is there anyone else here that's seriously in doubt about the direction of rotation here (counter-CW from behind), or is this just another instance of Schroder being unbelievably silly?

By all means please carefully watch that video. Wait with great patience towards the end to see which way the propeller is turning. Don’t get caught reaching for the popcorn at the moment of truth when the prop is slow enough to just see…..OH crap! What happened there? The cart goes off camera! How strange is that anyway? Right at the very point where we can see the direction of rotation! Accident or hoax? You be the judge. And if you believe this video, I have a bridge for sale in Brooklyn, very good price special for you!

 

[schroder]

The fastest cart so far is only going 1.6 times wind speed, say 16 mph in a 10 mph wind. From the cart's frame of reference, apparent headwind = 6 mph, apparent ground speed = 16 mph, an overall efficiency factor of a paltry 37.5%.

Do you have any evidence for this? The best I have seen is Swerdna’s turntable during the heterodyne run with a ratio of 2.4 : 1 which equates to 1.41 x turntable velocity. And I stress 1.41 x Turntable velocity, NOT wind velocity! In the wind, I have seen exactly Zero evidence of anything greater than wind velocity.

 

[schroder]

Correct. These carts extract energy from the tailwind by slowing it down. The headwind is an overhead, as explained in my previous post, but doesn't prevent DDWFTTW.



No sailcraft extracts energy from an apparent headwind (except when it's using the wind to decelerate the sailcraft). A sailcraft extracts energy from an apparent tailwind or apparent crosswind. The highest speeds are achieved when a sailcraft extracts energy from an apparent crosswind. Note the apparent crosswind is independent of the sailcrafts speed (by definition, it's perpendicular to the path of the sailcraft), and is equal to the true wind (wind_speed - ground_speed) times sin(angle between wind and direction of sailcraft). The maximum speed of the sailcraft depends on how much apparent headwind and ground related drag the sailcraft can achieve for a given apparent crosswind.

Agreed. After going DIRECTLY downwind, if ever the craft exceeds wind velocity, it will be going Directly upwind! Going Directly upwind there is no crosswind, it is the same as going downwind again, in REVERSE! Going directly upwind means the craft is at the same angle to the wind as the angle of the wind to the water. The sine of zero is zero, meaning you have no crosswind component. The craft would be pushed backwards. All the talk of tacking at an angle is not relevant to this discussion. Let us please leave sailboats and iceboats that are tacking at an angle, out of this discussion.

 

[schroder]

Yes!

1002 reached ! Brilliant. You guys have been great. With a special thanks to Schroder

It's always nice to be appreciated! But this thread can stop as soon as you admit to the following:

1) There is NO evidence at all that any cart has ever or will ever go directly down wind faster than the wind.

2) That what is happeneing on the turntable, while interesting, has NO relationship to what happens in the wind.

 

[rcgldr]

no crosswind component.

The DDWFTTW carts don't need a crosswind component.

If the craft exceeds wind velocity, it will be going directly upwind!

For DDWFTTW carts, the only thing that has to move slower than wind speed is the air from the prop wash. The craft will experience an apparent headwind, but the prop will be accelering the apparent headwind faster still, so that the prop wash still opposes the true wind, slowing the true wind down as required for any wind powered device.

As mentioned before once at terminal speed, power output related to thrust = thrust times apparent headwind (wrt cart), and is less than power input = thrust time apparent ground speed (wrt cart). The rest of the power ends up as heat.

The thrust is related to the change in speed of the true wind, with the prop acting as any prop, where thrust = (mass flow / unit time) x (Vout - Vin). As long as Vin, the apparent headwind, doesn't exceed the capabilities and efficiency of the cart and it's prop, the cart can go DDWFTTW.

 

[spork]

OH crap! What happened there? The cart goes off camera! How strange is that anyway? Right at the very point where we can see the direction of rotation! Accident or hoax? You be the judge.

O.K. I will be the judge. The prop is geared direclty to the drive shaft via a timing belt with a 90 degree twist. There is NO WAY for it to change directions. Why not watch the beginning of the video where this is abundantly clear!?

And if you believe this video, I have a bridge for sale in Brooklyn, very good price special for you!

I've built my own carts. I've spoken to Jack Goodman many times. I've done my own analysis. Yes I believe the video is real. In fact I have NO doubt about it. In fact I'm willing to bet you it's real. You name the amount. Let's both put our money where our mouths are. Whadayasay?

Do you have any evidence for this? The best I have seen is Swerdna’s turntable during the heterodyne run with a ratio of 2.4 : 1

You should really keep in mind that Swerdna has never done a "heterodyne" run. You know why that is? Because that's simply a bunch of nonsense that you made up.

I have seen exactly Zero evidence of anything greater than wind velocity.

Of course you haven't. How could you? Any evidence of faster than wind velocity is discounted by you by definition. It must either be a hoax, or doesn't fit within your extraordinarily limited understanding of physics.

Let us please leave sailboats and iceboats that are tacking at an angle, out of this discussion.

We're going to be talking about a lot of things we don't expect you to understand. Just pretend you're waiting for a friend at a rocket science convention. Don't let the big words bother you 'mmKay?

this thread can stop as soon as you admit to the following:

1) There is NO evidence at all that any cart has ever or will ever go directly down wind faster than the wind.

2) That what is happeneing on the turntable, while interesting, has NO relationship to what happens in the wind.

Let's add:

3) There is NO evidence that man has ever stepped foot on the moon

4) Global warming is a hoax being perpetrated by the government to control us.

5) Dr.'s don't want us to be healthy - just alive, so they can continue to bill us.

6) Of course they have cures for cancer and AIDS. But they'd never release them because treatment is such big business.

7) If "they" wanted to they could give us 200 mpg carbeurators, but big oil has paid off anyone that uncovers this truth - and then kills them.

8) The "chem trails" we see behind commercial jets are used by the government for mind control - and it's working.

9) The 9/11 attacks never really happened. And if they did, the U.S. government was behind the whole thing.

10) What holocaust?

 

[spork]

1) There is NO evidence at all that any cart has ever or will ever go directly down wind faster than the wind.

2) That what is happeneing on the turntable, while interesting, has NO relationship to what happens in the wind.

You know schroder - I refuse to believe you're as confused as you make yourself out to be. I think you're intentionally messing with people on this thread just to perpetuate this silliness. If I'm wrong - prove it to me. Prove you really believe what you say by taking my bet. Otherwise I'll have to conclude you're simply trolling.

 

[ZapperZ]

I believe that we all have had our fun, and this thread has been allowed to run for quite a while with this many posts. It is time to give it a rest already.

Zz.