88 replies to this topic

[Wuzak]

More explanations

 

 

Even includes a demonstration of gruntguru's ceiling analogy.

Edited by Wuzak, 01 July 2021 - 13:37.

[gruntguru]

After watching the moving ceiling part of the video (at 13:48) I thought it would be fun to check the chart I posted on page 1. Actually I checked the formula used to create the chart:

 

Vcar/Vwind = 1/(1-Ratio) where "Ratio" = prop displacement/wheel displacement

 

Estimating gearing from the screen (diameter of white cylinder (axle))/(diameter of road wheel) = 0.36 gives Vcar/Vwind = 1.57

 

The cart in the video travels 6 units along the floor, while the board (ceiling) travels 4 units ie Vcar/Vwind = 6/4 = 1.5 . . . . close enough.

[Kelpiecross]

In that case, your statement 

is incorrect.

in DWFTTW mode, the prop acts as a sail.

In UWFTTW mode, the prop acts as a turbine.

The chart shows what gear ratios are required to achieve a desired vehicle speed relative to wind speed (zero-slip prop). Gear ratio is (prop displacement) / (wheel displacement)  For example:

 - to travel downwind at 10 times the speed of the wind requires a gear ratio of 0.9

 - A gear ratio of zero (locked prop, free wheels) produces vehicle speed = wind speed.

 - Counterintuitively, negative ratios still produce downwind motion. vehicle speed < wind speed. But with inreased thrust - the prop is acting as a sail AND a turbine. The wheels are helping to propel the car.

 - Gear ratios > 1 drive the vehicle upwind

 - to travel upwind at 10 times the speed of the wind requires a gear ratio of 1.1

 - A ratio of 2.0 produces vehicle speed = wind speed (upwind)

 

 In what sense can the prop act as a sail?  Why would a sail  allow downwind travel faster than the wind?

 

 Surely logic would say the same mechanism operates both upwind and downwind   - that is - as a turbine.   

[Kelpiecross]

More explanations

 

 

Even includes a demonstration of gruntguru's ceiling analogy.

 

 His explanation of how it works - is (I think) a bit juvenile.  The wheels drive the prop and it acts as a fan driving the vehicle along?   No it doesn't - the pitch is in the wrong sense to do this. 

 

 You can make up a slightly complex (but possible)  thought experiment arrangement that can  demonstrate how a turbine on the vehicle  can always see the wind speed as constant no matter what the vehicle's ground speed.

 

 Imagine the vehicle has caterpillar tracks and not wheels (possible but silly)  - where the tracks contact the ground they are stationary relative to the ground - imagine that along the tracks there are mounted wind turbines  - these  turbines , free to turn only when touching the ground - they  would always see the wind speed as constant  no matter the vehicle's speed.   

 On the real vehicle a similar (but slightly harder to see)  effect happens -the prop is screwing its way back the way it came -  effectively  (like the imaginary tracks) remaining motionless relative to the ground  - thus the prop always sees the wind speed as constant no matter the vehicle speed.   

Edited by Kelpiecross, 02 July 2021 - 07:03.

[gruntguru]

  . . .  The wheels drive the prop and it acts as a fan driving the vehicle along?   No it doesn't - the pitch is in the wrong sense to do this. 

Really??? Look again.

[Kelpiecross]

Really??? Look again.

 

 I looked again - still the wrong pitch.  

[MatsNorway]

More explanations

 

 

Even includes a demonstration of gruntguru's ceiling analogy.

That example at 14:00 is just beautiful.  A touch of genius, and every course on the topic should start with that as it removes any doubt about it being possible in a zero drag/loss environment.

 

 

You can even add drag as a friction angle i believe(for the system?), by angling the plank as you push it.

Edited by MatsNorway, 03 July 2021 - 07:03.

[Wuzak]

 His explanation of how it works - is (I think) a bit juvenile.  The wheels drive the prop and it acts as a fan driving the vehicle along?   No it doesn't - the pitch is in the wrong sense to do this. 

 

You could always build your own and test it with the prop on both directions.

 

[BRG]

What it really needs to nail this one is some scientific institution to undertake some proper research under properly controlled conditions.

 

Not some random YouTubers in their own backyards.

[Greg Locock]

I'd argue all it needs is a whiteboard and a jug of coffee, since it has been demonstrated to work.

[gruntguru]

Yep. . . . and don't forget the physics Prof who bet $10,000 that it doesn't work has now conceded the bet.

[gruntguru]

 I looked again - still the wrong pitch.  

Then you are tone deaf.

[Kelpiecross]

You could always build your own and test it with the prop on both directions.

 

 

 This may come as a great surprise to you (or it may not) -  but turning a prop over doesn't reverse the airflow it produces - the pitch is still in the correct sense.  The airflow is reduced because the camber etc. on the prop is the wrong way around - like a wing being put on a 'plane back to front - it will still work but not very well.  To reverse the airflow  you need a prop manufactured with the correct reverse pitch. 

 

 With free-flight models (powered by a small engine but no radio control etc.) -  beginners with first flights of their models  are often advised to fit the prop backwards to reduce the thrust so that the model doesn't fly off into the distance.    But the backwards prop still blows the same way. 

 

  Even though it was interesting I don't know what the girl was trying to demonstrate.   This debate is about  energy from the wind not from a treadmill.   

Edited by Kelpiecross, 04 July 2021 - 05:13.

[Kelpiecross]

Then you are tone deaf.

 

 I will try and explain this in terms even my dog can understand - if I can get him to listen and stop reading the paper for a minute.  

 

 Imagine you have the vehicle with wind coming coming from behind (ie. - the downwind case)  The driven wheels are jacked up off the ground  - the prop starts to turn, the wheels start to turn.  The wind direction is from the rear towards the front.

  Now the wind speed drops to zero.  The jacked-up up wheels are turned by a motor in the same direction as previously - this starts to move the air - which way does  the air move?  The same way - from the rear towards the front.  Which direction does this produce thrust?  Towards the rear - like a normal "tractor" prop on a 'plane.   

 

  Ergo - the prop can never turn into a fan propelling the vehicle.  

Edited by Kelpiecross, 04 July 2021 - 05:09.

[Kelpiecross]

I'd argue all it needs is a whiteboard and a jug of coffee, since it has been demonstrated to work.

Exactly.

  

 If you do accept that it works -  that it does run downwind  about three times faster than the wind - then logic says that clearly in some odd way the the prop on the moving vehicle must be "seeing" the wind as if the vehicle was stationary.  The problem then is to work out why this happens. 

 

 This is a similar argument  to the Michelson - Morley  experiment - if you accept  that their results were accurate - that the speed of light was the same no matter what the movement of the observer  -  then the basic conclusions of the Theory of Relativity must follow fairly simply and must also be true.

 

 I mean - this endlessly repeated crap about, at high speed, the wheels driving the prop and turning it into fan - have I entered some kind of Bizarro  World?  How can anybody seriously suggest this?            

Edited by Kelpiecross, 04 July 2021 - 04:52.

[gruntguru]

If you watch the video, the prop turns clockwise viewed from the rear and the blades are angled such that clockwise rotation will move air from front to back of the vehicle. ie it is pitched to act as a fan.

 

Here's another thought experiment.

Disconnect the drive so the wheels and prop can spin freely. Tow the vehicle downwind at 3x wind speed. The wheels are now turning at 3x and the prop is turning at 2x (the apparent wind is from the front at 3-1=2x). Now connect a 2:3 transmission (0.666 ratio in the chart below). Nothing happens - the prop and wheels continue at the same speed and there is no torque in the transmission. Now increase the ratio slightly to say 2.1:3 (0.7 ratio in the chart). This ratio turns the prop faster so additional thrust is generated by the prop at the cost of some drag on the wheels. Fortunately the thrust is greater than the drag (about 50% greater due to the gearing) so the vehicle will accelerate to the new equilibrium speed on the chart (3.333 x wind speed)

 

[Wuzak]

 This may come as a great surprise to you (or it may not) -  but turning a prop over doesn't reverse the airflow it produces - the pitch is still in the correct sense.  

 

Yes, I didn't think that through.

[Wuzak]

 I will try and explain this in terms even my dog can understand - if I can get him to listen and stop reading the paper for a minute.  

 

 Imagine you have the vehicle with wind coming coming from behind (ie. - the downwind case)  The driven wheels are jacked up off the ground  - the prop starts to turn, the wheels start to turn.  The wind direction is from the rear towards the front.

  Now the wind speed drops to zero.  The jacked-up up wheels are turned by a motor in the same direction as previously - this starts to move the air - which way does  the air move?  The same way - from the rear towards the front.  Which direction does this produce thrust?  Towards the rear - like a normal "tractor" prop on a 'plane.   

 

  Ergo - the prop can never turn into a fan propelling the vehicle.  

 

My understanding is that the prop never operates as a turbine.

 

The prop is always driven by the wheels.

[Wuzak]

Even though it was interesting I don't know what the girl was trying to demonstrate.   This debate is about  energy from the wind not from a treadmill.   

 

That the thrust from the propeller can be greater than the rolling resistance of the wheels. But only if the gearing is correct.

[gruntguru]

Even though it was interesting I don't know what the girl was trying to demonstrate.   This debate is about  energy from the wind not from a treadmill.   

Frames of reference. To an observer standing on the treadmill belt, the model is travelling faster than the wind.

Same as an observer standing beside the track watching a DWFTTW vehicle. That observer and the track itself are not stationary when viewed from space.

[Greg Locock]

Last time around i built a generator into the propeller, and put a motor on a platform at the base to drive the wheels. Then I could think about things without being distracted by gearing.

[gruntguru]

Yes the electric-transmission analysis is very elegant and well explained here.  https://www.youtube....pnSJyV8&t=4198s

[Kelpiecross]

My understanding is that the prop never operates as a turbine.

 

The prop is always driven by the wheels.

 

 What video are you referring to?   The "treadmill" videos are only a confusing distraction  - I am only referring to the  real-life  salt flat videos where the only energy source is the wind via the prop.  It seems to me that a lot of people (I nearly wrote "idiots")  are claiming that  in the "salt flats" videos the energy comes from the wheels - which sounds a little like a scheme that Feliks would come up with.    

[Kelpiecross]

Yes the electric-transmission analysis is very elegant and well explained here.  https://www.youtube....pnSJyV8&t=4198s

 

 I think the only thing revealed in the video is that he has no idea how it works.   A better electric analogy would be to have a stationary turbine turning a generator and the power transmitted to the car by a live rail or overhead wire or etc.   And this is how it all works -  it is as if  the turbine on the car remains stationary.   

[Kelpiecross]

Then you are tone deaf.

 

 Get two ordinary household fans - point one at the back of the other - turn the front fan on and off.   I think you will find the "dog" explanation is correct (I tried it).   The pitch is wrong - it can never act as a fan to push the vehicle along - and it is ridiculous to suggest that it could.  .    

[Wuzak]

[Kelpiecross]

 

 Interesting - but I think the prop/wheels gearing is reversed.  The fan is blowing the contraption along and overpowering  the torque from the prop.  

[Kelpiecross]

Frames of reference. To an observer standing on the treadmill belt, the model is travelling faster than the wind.

Same as an observer standing beside the track watching a DWFTTW vehicle. That observer and the track itself are not stationary when viewed from space.

 

 The prop is always operating in still air  -  the arrangement doesn't test  the basic puzzle of how the  vehicle can move faster than the wind.   

[gruntguru]

No. An observer standing on the treadmill belt will assert that the prop is operating in a wind.

 

Your emphatic assertion that the prop never operates as a fan has been disproven many times here. Try watching the video I linked earlier from start to finish. It is a presentation by the builder of the FTTW machine in the OP - I think he might know which way his prop turns. At one point he states - the prop operates as a fan in DWFTTW mode and as a turbine in UWFTTW mode. He presents 11 different explanations of the concept and says that each explanation is the "light-bulb" moment for different people - one of them might work for you.

 

https://www.youtube....pnSJyV8&t=4198s

[gruntguru]

Have you thought about how a "wind turbine" powered vehicle can operate downwind at wind speed? The "turbine" is seeing zero wind - the vehicle must slow down.

[Kelpiecross]

Have you thought about how a "wind turbine" powered vehicle can operate downwind at wind speed? The "turbine" is seeing zero wind - the vehicle must slow down.

 

 Isn't this what we have been debating at great length?   In some way the prop does see  the wind as if the vehicle was stationary.   In post 54 above I  showed that with a slightly ridiculous imaginary example that a turbine on a moving vehicle would see the wind speed as constant.  This effect occurs because the  bottom of the wheel (or track)  is moving backwards in the opposite sense to the vehicle's travel and at the same speed  - so it appears stationary.   Because of the prop's pitch and its connection to the wheels it is also going backwards at the same speed as the vehicle is going forwards - so the prop, like the bottom of the wheels appears to be stationary- and sees the wind as unchanging.

 

  Do you agree that the pitch is the wrong way around  for it to become a fan?   The only source of energy is the wind -  how can the wheels possibly  power the prop as a fan - where is the energy  coming from? 

[Kelpiecross]

No. An observer standing on the treadmill belt will assert that the prop is operating in a wind.

 

Your emphatic assertion that the prop never operates as a fan has been disproven many times here. Try watching the video I linked earlier from start to finish. It is a presentation by the builder of the FTTW machine in the OP - I think he might know which way his prop turns. At one point he states - the prop operates as a fan in DWFTTW mode and as a turbine in UWFTTW mode. He presents 11 different explanations of the concept and says that each explanation is the "light-bulb" moment for different people - one of them might work for you.

 

https://www.youtube....pnSJyV8&t=4198s

 

  I have watched the video a couple of times - he has no idea how it all works. 

[Kelpiecross]

 The prop should have been mounted on the other side of the pylon to get it away from the driver's head.   

[gruntguru]

 In some way the prop does see  the wind as if the vehicle was stationary. 

Correct. So if the vehicle is travelling downwind faster than the wind and the prop is not rotating, the prop will see air coming from the front of the vehicle. To see that air as stationary, the prop will need to screw its way forward at wind speed.

[gruntguru]

Do you agree that the pitch is the wrong way around  for it to become a fan?   The only source of energy is the wind -  how can the wheels possibly  power the prop as a fan - where is the energy  coming from? 

No. By simply looking at the video you can see the prop is screwing forward in the same direction the vehicle is travelling.

 

The wheels get the energy to power the fan from the "push" of the wind acting on the prop - which appears and acts like a sail.

[Kelpiecross]

 I have to admit that I may got things slightly arse-up.  This brings me right back to the start of this puzzle - how does it go faster than the wind?      

[gruntguru]

The apparent wind at the prop is always slower than the speed of the wheels across the ground. (The difference between the two apparent speeds is the wind speed). This speed difference means any power harvested by the wheels will requires less force (drag) than the thrust produced motoring the prop with that same power.

 

Upwind is the opposite. The apparent wind is always higher than the roadspeed, so you harvest from the wind and motor the wheels.

[Kelpiecross]

The apparent wind at the prop is always slower than the speed of the wheels across the ground. (The difference between the two apparent speeds is the wind speed). This speed difference means any power harvested by the wheels will requires less force (drag) than the thrust produced motoring the prop with that same power.

 

Upwind is the opposite. The apparent wind is always higher than the roadspeed, so you harvest from the wind and motor the wheels.

 

 That explanation may make sense to you - but it certainly doesn't make sense to me.  This going to require some serious thought. 

[gruntguru]

"Apparent wind" - how the wind appears to someone standing on the vhicle beside the prop.

 

If the car is travelling 30 m/s with a tailwind of 10 m/s, the apparent wind at the prop is a 20 m/s headwind. If you can spin the prop at 21 m/s it will be starting to push the vehicle. P = V x F so if the prop has a thrust of 5N, the power required to do this is 20 x 5 =  100 W. To harvest 100 W from the wheels F = P/V = 100/30 = 3.33 N so the vehicle can accelerate.

Edited by gruntguru, 09 July 2021 - 22:51.