DDWFTTW: Looking for the least confusing explanation

[A.T.]

In a downwind configuration, the relative headwind is passing slowly and the ground is passing rapidly. You win by using a propeller to gain high thrust from the wind with a low power requirement while the wheels provide high power from the ground at an expense of low drag.

Well, the physics professors think it cannot possibly be that simple:

 

[jbriggs444]

I just watched a new Veritasium video where he announces the winning of the bet and does a very good job of describing the situation.

 

[anorlunda]

I think it is beneficial to everyone to debate things like that in public. I'll disagree with Veritasium on the last thing he said in the video. "Let's hope that this video puts the issue to rest, once and for all." In fact, I'll wager that more words, both for an against, will continue in this thread.

 

[jbriggs444]

I think it is beneficial to everyone to debate things like that in public. I'll disagree with Veritasium on the last thing he said in the video. "Let's hope that this video puts the issue to rest, once and for all." In fact, I'll wager that more words, both for an against, will continue in this thread.

Hope springs eternal, but that proposed wager looks pretty darned safe for you.

 

[cjl]

Well, the physics professors think it cannot possibly be that simple:

He may think that, but he's wrong. From the frame of the cart, the prop is acting exclusively as a prop. I suppose you could make an argument that it's acting as a "turbine" in the frame of the ground because the wind in its wake is traveling slower than the surrounding air, but that argument would also make the propeller of every powerboat traveling downriver also a "turbine" by the same logic (and every airplane with a tailwind).

 

[jbriggs444]

He may think that, but he's wrong. From the frame of the cart, the prop is acting exclusively as a prop. I suppose you could make an argument that it's acting as a "turbine" in the frame of the ground because the wind in its wake is traveling slower than the surrounding air, but that argument would also make the propeller of every powerboat traveling downriver also a "turbine" by the same logic (and every airplane with a tailwind).

The argument was more nuanced than that. The [mistaken] claim was that due to wind gusts, the propeller was intermittently acting as a turbine, harvesting energy and then as a prop, generating thrust. Together with an argument about measurement error (wind gradient with height and selective reporting), the claim was that it was all effectively smoke and mirrors.

 

[cjl]

Ah. Well, that's also incorrect (as has been explained here many times by many others).

 

[A.T.]

He may think that, but he's wrong. From the frame of the cart, the prop is acting exclusively as a prop. I suppose you could make an argument that it's acting as a "turbine" in the frame of the ground because the wind in its wake is traveling slower than the surrounding air, but that argument would also make the propeller of every powerboat traveling downriver also a "turbine" by the same logic (and every airplane with a tailwind).

Kusenko is not using an energy based definition of "turbine" (which would be frame dependent as you note). He is explicitly saying that the aerodynamic torque on the rotor has the same direction as the torque on the rotor transferred from the wheels and the rotation direction of the rotor:

https://docs.google.com/presentatio...KDkhEfN898K4/edit#slide=id.gb6e540a45a_618_51

When the car is moving faster than the wind, the passing air pushes the propeller in the same direction as the wheels push it. This has been a subject of discussion, and Blackbird has a ratchet to prevent the propeller from actively spinning the wheels, but the ratchet does not keep the propeller’s wind power from spinning the propeller itself, adding the torque in the same direction as the wheels.

I show why this is impossible here:
https://www.physicsforums.com/threa...ast-confusing-explanation.896869/post-6507170

 

[A.T.]

The argument was more nuanced than that. The [mistaken] claim was that due to wind gusts, the propeller was intermittently acting as a turbine, harvesting energy and then as a prop, generating thrust. Together with an argument about measurement error (wind gradient with height and selective reporting), the claim was that it was all effectively smoke and mirrors.

That's his initial arguments before the bet. But after conceding the wager he developed this prop & turbine at the same time (not intermittently) theory. See slide 6 here:
https://docs.google.com/presentatio...KDkhEfN898K4/edit#slide=id.gb6e540a45a_618_51

Kusenko still claims that the pure propeller explanation is wrong. and that he merely lost the bet on a technicality:

Technically, my wager with Derek Muller stated as part of the claim that “the propeller works like a fan rather than a wind turbine”, which is incorrect. The propeller acts as both a fan and a turbine, and the power is contributed by both moving media in a somewhat symmetrical manner.

 

[hmmm27]

Trying to wrap my head around it with the propeller proved confusing, so I thought up an alternative...

and somebody beat me to it.

 

[A.T.]

Derek has posted the entire discussion with Kusenko that preceded the wager. I'm linking to the timepoint, where Derek asks him about connecting two boats with downwind velocity component > windspeed (which Kusenko accepts as achievable by a boat), and thus creating a craft that goes DDWFTTW. Kusenko just waves his hands, and claims this has no relation to the propeller vehicle, because there is no wheel in the sailboat analogy.



Here is an animation that explains how the coupling to the wheels forces each section of the propeller blade to move across the wind:



Just like the keel forces the sail to move across the wind when downwind velocity component > windspeed:



I posted the vectors for both situations here:
https://www.physicsforums.com/threa...ast-confusing-explanation.896869/post-6499006

 

[rcgldr]

I've been searching in vane for the parts list people were using to build the treadmill models. Was it included in one of the treadmill test videos, or was it included here?

 

[A.T.]

I've been searching in vane for the parts list people were using to build the treadmill models. Was it included in one of the treadmill test videos, or was it included here?

There is a part list in the video description here:

 

[pbuk]

I've been searching in vane

Freudian slip?

 

[rcgldr]

There is a part list in the video description here.

I meant the old ones from 10 years ago. No 3d printing required, and they were lighter. Several of these were made by different people here at Physics Forums, and a parts list posted somewhere.



"searching in vane" - this pun was caught a bit sooner that I thought it would be.

 

[A.T.]

I meant the old ones from 10 years ago. No 3d printing required, and they were lighter.

Here

 

[anorlunda]

I just read those threads from 2008. Wow! There were a lot of posts back then. Most of the posts in this thread repeat points made 2 or 3 times before.

 

[rcgldr]

Here

Thanks.

 

[OmCheeto]

No, I think we're all good and in agreement. Device works, Propeller explanation is good.

Well, I'm not in agreement.
The device does not work.
Even Kusenko said so.

 

[hutchphd]

All we have is definitive theoretical and experimental evidence. Seems pretty weak to me.

 

[Merlin3189]

[Moderator Note -- sub-thread merged into this main thread]

Having seen some discussion of FTTWDDW in the past on PF, I thought people might be interested in Veritasium's latest videos:
Original (land car) experiment
Enhanced "proof" to settle a $10k bet with physics prof
No doubt it's a bit light for PF, but the second one certainly convinced me he could be right.

Edit: Apologies for this post. I did search for this thread, but did not find it, perhaps because I used the wrong acronym. But now that I've looked at recent posts, I can see you've already discussed it and some of my searches should have found those posts?

Thanks to Mods for moving it, but they are welcome to delete it since it adds nothing to what's gone before.

 

[.Scott]

You could use this to power an airplane - until it left the ground.

 

[DaveE]

Plus this one: How to build your own.

 

[Ivan Seeking]

It seems to me that the reason the math blows up in the solutions discussed in the Veritasium videos as well as the physics papers referenced is that everyone ignores the increased velocity of the air due to the fan. They use the speed of the air through the fan as V-W and therefore the fan power as F(V-W), where V is the speed of the car relative to the ground and W is the speed of the wind relative to the ground. But they ignore the delta V induced by the fan motion. It should be V + delta V - W. Add this and you no longer have the divide by zero problem without any need for complex fan efficiency equations. And you have the correct power when the wind speed matches the vehicle speed.

The increased velocity of the air molecules then manifests as increased pressure behind the fan.

Right?

 

[anorlunda]

It seems to me that the reason the math blows up in the solutions discussed in the Veritasium videos

Have you read this entire thread? Have you watched this video?

 

[Ivan Seeking]

Have you read this entire thread? Have you watched this video?

Yes I was quoting their equation that fails.

Derek justifies it by referencing a more complex equation needed for the prop efficiency but that doesn't seem to be necessary to make sense of things. It seems to me that they have missed a key term - the energy added to the air by the fan.

 

[jbriggs444]

Yes I was quoting their equation that fails.

Derek justifies it by referencing a more complex equation needed for the prop efficiency but that doesn't seem to be necessary to make sense of things. It seems to me that they have missed a key term - the energy added to the air by the fan.

If one hypothesizes (as the simplistic equation requires) an arbitrarily large propeller rotating at an arbitrarily small rotation rate with an arbitrarily large torque from a transmission with an arbitrarily high gear ratio then an arbitrarily large thrust can be obtained, even without 100% efficiency.

The larger you make the prop, the larger the deflected air mass becomes, the lower the imparted delta V needs to be and the problem with energy loss due to induced drag can be made as small as one pleases.

[Obviously, there are practical limitations with material rigidity and the finite depth of the atmosphere]

For any fixed finite propeller size, infinite torque means zero rotation rate and you get zero propulsion from an infinite gear ratio.

 

[rcgldr]

It should be V + delta V - W. Add this and you no longer have the divide by zero problem without any need for complex fan efficiency equations.

That just shifts the divide by zero problem to some speed slightly less than wind speed, where V + delta V - W = 0.

 

[A.T.]

It seems to me that the reason the math blows up in the solutions discussed in the Veritasium videos as well as the physics papers referenced is that everyone ignores the increased velocity of the air due to the fan. They use the speed of the air through the fan as V-W and therefore the fan power as F(V-W), where V is the speed of the car relative to the ground and W is the speed of the wind relative to the ground. But they ignore the delta V induced by the fan motion. It should be V + delta V - W. Add this and you no longer have the divide by zero problem without any need for complex fan efficiency equations.

As @rcgldr notes, it doesn't help at all, because you just have another point where the airspeed you use is zero. And additionally you now need to know delta V. The math doesn't blow up at this point, when you include the propeller efficiency, which also goes to zero, when the airspeed does.

But all this is not needed to show that steady state above wind-speed is possible, regardless how it got there. For this the simple idealized formula that the video shows is enough, since it shows excess thrust, which can account for the loses.

The static thrust estimation is only needed to predict the acceleration phase. But if you want to do that, then you also have to consider the propeller working in reversed flow below wind speed, basically with negative efficiency.

 

[Ivan Seeking]

But all this is not needed to show that steady state above wind-speed is possible, regardless how it got there. For this the simple idealized formula that the video shows is enough, since it shows excess thrust, which can account for the loses.

The static thrust estimation is only needed to predict the acceleration phase. But if you want to do that, then you also have to consider the propeller working in reversed flow below wind speed, basically with negative efficiency.

But the equation is wrong. Where does it show the energy added to the air moving through the prop area?