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Race a balloon then


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#1 MatsNorway

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Posted 01 June 2021 - 12:03

This is so dumb... or?

 


Edited by MatsNorway, 01 June 2021 - 12:13.


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#2 Canuck

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Posted 01 June 2021 - 20:20

Jesus...was that guy (builder, not the host) here once upon a time? Didn't we razz the daylights out of a propellers and beam axles gent a few years back?

 

I think it's rather interesting and it shows how "trapped in the box" our thinking can become. I wouldn't have believed it without seeing it.



#3 GreenMachine

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Posted 01 June 2021 - 21:59

Yes, I have seen that concept some time ago, can't be sure it was here though.

 

Seconding 'have to see it to believe it', quite counter-intuitive.



#4 gruntguru

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Posted 01 June 2021 - 23:06

Certainly is hard to get your head around. Here's what works for me:

Think of the wind as a threaded rod moving along above the ground. Think of the propeller as a nut that can run on the threaded rod. Now gear the nut to the wheels of the vehicle so the nut winds forward on the threaded rod as the wheels roll forward. Viola!


Edited by gruntguru, 01 June 2021 - 23:17.


#5 Greg Locock

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Posted 01 June 2021 - 23:46

What's even more amazing is that they reconfigured the record setting DWFTTW car (Blackbird) and demonstrated being able to go directly upwind at twice the wind speed. Incidentally a successful DWFTTW car was built in the 70s.



#6 Canuck

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Posted 02 June 2021 - 21:03

Yes, I have seen that concept some time ago, can't be sure it was here though.

Found what I was looking for. I don't believe our erstwhile Franklin Rafcliffe was talking about the same thing at all.

https://forums.autos...er#entry2455983



#7 desmo

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Posted 03 June 2021 - 02:51

Summary:
 
A successful attempt for a dead-upwind-faster-than-the-wind (DUW) record was made by Rick
Cavallaro on New Jerusalem Airport near Tracy California on June 16, 2012, with the wind
turbine driven sailing craft, Blackbird. Mr Cavallaro achieved a maximum boat speed to wind
speed ratio of 2.1:1 and a maximum speed in a wind turbine driven sailing craft of 22.9 mph on a
different run. The observers have reviewed the data from the June 16 trial, and we are confident
the relevant data is valid. We are equally confident that Blackbird, the measurement process and
the pilot (Rick Cavallaro) complied with Revision 4 of the NALSA Regulations for Speed
Record Attempts that were approved by the Board in early June 2012. We recommend that these
records be ratified by the NALSA Board of Directors. 
 

The upwind data are even more confoundingly counter-intuitive than the impressive downwind numbers for me. The Wiki Blackbird page has some good footnoted links where I found the above.



#8 gruntguru

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Posted 04 June 2021 - 00:13

Interesting to note that energy flow is reversed for the downwind vs upwind cases.

Running downwind, the wheels harvest energy (retarding the vehicle) and use it to drive the fan (propelling the vehicle), making it a kind of motorised "super-spinnaker".

Running upwind the fan harvests energy (retarding the vehicle) and uses it to drive the wheels (propelling the vehicle).

 

This would suggest that the same fan cannot be optimised for both tasks. Even if was turned to face the opposite way its design would need to be compromised if it was to perform both tasks.



#9 Greg Locock

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Posted 04 June 2021 - 05:28

It's variable pitch, from memory. Oddly to me the upwind result is easier to understand intuitively.



#10 gruntguru

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Posted 04 June 2021 - 22:59

Even variable pitch doesn't solve the issues of transforming a motored fan into a generating turbine. Blades with zero camber would be one solution but of course that necessitates an efficiency compromise.

 

Same for me - upwind is easier to visualise.



#11 Greg Locock

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Posted 05 June 2021 - 00:18

They used a different transmission setup.



#12 Fat Boy

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Posted 05 June 2021 - 19:45

When I initially looked at the propeller, I thought it was an autogyro of sorts. Once I realized the prop is driving the wheels, it became much easier to visualize how and why it works.



#13 gruntguru

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Posted 06 June 2021 - 01:18

They used a different transmission setup.

Neither variable pitch nor transmission setup will solve the efficiency compromise resulting when you re-purpose a propellor as a wind turbine (or vice versa).

The need for a thicker leading edge, camber and blade twist mean that a design that does both jobs will be necessarily a compromised design.


Edited by gruntguru, 06 June 2021 - 01:19.


#14 Wuzak

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Posted 06 June 2021 - 04:01

When I initially looked at the propeller, I thought it was an autogyro of sorts. Once I realized the prop is driving the wheels, it became much easier to visualize how and why it works.

 

I thought they said that the wheels drive the propeller, which produces thrust.



#15 Fat Boy

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Posted 06 June 2021 - 20:17

I thought they said that the wheels drive the propeller, which produces thrust.

I'm going to need to see the free body diagram of that. I'm kinda with Greg, I understand the upwind condition better than the downwind, but I didn't watch the downwind vid.



#16 gruntguru

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Posted 06 June 2021 - 21:29

I'm going to need to see the free body diagram of that. I'm kinda with Greg, I understand the upwind condition better than the downwind, but I didn't watch the downwind vid.

 

Energy flow is reversed for the downwind vs upwind cases.

Running downwind, the wheels harvest energy (retarding the vehicle) and use it to drive the fan (propelling the vehicle), making it a kind of motorised "super-spinnaker".

Running upwind the fan harvests energy (retarding the vehicle) and uses it to drive the wheels (propelling the vehicle).



#17 Kelpiecross

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Posted 07 June 2021 - 06:16

  I think the first move with this DWFTTW  conundrum  is to accept that it is a genuine effect and just not  a crappy hoax  - and I think that it is pretty clear that the effect does occur.   Accepting that it does happen it must be able to be explained.  My (slightly vague)  solution is that it is a matter of relativity.   

  No matter how fast, slow or even stationary the cart travels (or doesn't travel) and in any direction   where the driven wheel touches the ground this spot is stationary   -   ergo    the wind "sees"   the contact spot on the surface as stationary -  so relative to this spot the wind speed is always the same no matter the movement of the cart..   Ergo (again)  the propeller (turbine?)  having a direct mechanical relationship to the wheel  also  always "sees" the wind as unchanging.   The propeller always "sees" the wind as if the cart was stationary.   Thus  the wind speed or direction  is irrelevant.   As my old Mate  Einy would say "It's all relative Matey".          

 

  On a more philosophical note - I do wonder if it is a similar effect that we always "see" the speed or light as unchanging no matter how fast or what direction we are travelling.  We are the propeller/stationary spot on the road.    



#18 GreenMachine

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Posted 07 June 2021 - 06:45

      On a more philosophical note - I do wonder if it is a similar effect that we always "see" the speed or light as unchanging no matter how fast or what direction we are travelling.  We are the propeller/stationary spot on the road.    

 

We don't.  Google 'red/blue shift'.



#19 Kelpiecross

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Posted 08 June 2021 - 12:43

We don't.  Google 'red/blue shift'.

 

  Red/Blue Shift - doesn't matter  c  remains constant. 



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#20 Kelpiecross

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Posted 08 June 2021 - 13:04

 

Energy flow is reversed for the downwind vs upwind cases.

Running downwind, the wheels harvest energy (retarding the vehicle) and use it to drive the fan (propelling the vehicle), making it a kind of motorised "super-spinnaker".

Running upwind the fan harvests energy (retarding the vehicle) and uses it to drive the wheels (propelling the vehicle).

 

 

  If the wind suddenly dropped while the the vehicle was moving,  the inertia of the vehicle/wheels etc.  driving the  the propeller   would actually turn the propeller into a fan  - but to propel the vehicle back the way it came.  

 

   This (I think ) is the whole crux of this puzzle  -  the vehicle is moving forward  - but the pitch on the propeller is opposite to that  that would be needed to propel the vehicle as if the propeller was motor-driven. 

 

   If the pitch is opposite to what it normally would be  the air is actually  stationary relative to the propeller blade surface  -  so the following  wind also "sees"   the blade surface  as stationary  and thus is  quite happy to turn the propeller in the required direction no  matter what the speed of the vehicle downwind.        



#21 gruntguru

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Posted 08 June 2021 - 21:30

Another way to look at this:

 

The propellor is simply a means of connecting the vehicle to the air above the vehicle. A perfect propellor would provide zero slip. Now replace the perfect propellor with a "ceiling" - another road surface above the vehicle, moving horizontally relative to the actual road. If we add some wheels and tyres pressing on this moving ceiling we can contrive a gearing mechanism that will generate any vehicle speed we want - and in either  any direction.



#22 Kelpiecross

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Posted 09 June 2021 - 04:59

Another way to look at this:

 

The propellor is simply a means of connecting the vehicle to the air above the vehicle. A perfect propellor would provide zero slip. Now replace the perfect propellor with a "ceiling" - another road surface above the vehicle, moving horizontally relative to the actual road. If we add some wheels and tyres pressing on this moving ceiling we can contrive a gearing mechanism that will generate any vehicle speed we want - and in either  any direction.

 

 

  Eh?



#23 Canuck

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Posted 09 June 2021 - 14:05

  Eh?

Hey hey - easy on the Canuckistani interjection.



#24 gruntguru

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Posted 09 June 2021 - 21:36

  Eh?

Its a car.

It has four wheels pressing down on the road.

It has one or more wheels pressing up on an overhead road (which is moving horizontally relative to the lower road).

There is a transmission connecting the upper wheels(s) to the lower ones.


Edited by gruntguru, 09 June 2021 - 21:37.


#25 GreenMachine

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Posted 09 June 2021 - 23:08

Its a car.

It has four wheels pressing down on the road.

It has one or more wheels pressing up on an overhead road (which is moving horizontally relative to the lower road).

There is a transmission connecting the upper wheels(s) to the lower ones.

 

Try to keep the big words to a minimum, ok?



#26 gruntguru

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Posted 10 June 2021 - 02:08

K.



#27 gruntguru

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Posted 10 June 2021 - 03:04

The effect of gear ratio is fascinating.

A ratio of 1 (upper and lower wheels rotate same direction at same speed) gives an invalid result (infinite vehicle speed).

Less than 1 (upper wheel rotates slower than lower) drives the vehicle upwind

Greater than 1 drives the vehicle downwind.

Approaching 1 increases vehicle speed eg a ratio of 1.1 produces a downwind speed of 10x wind speed.  A ratio of 0.9 produces an upwind speed of 10x wind speed.


Edited by gruntguru, 10 June 2021 - 03:05.


#28 Kelpiecross

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Posted 10 June 2021 - 06:27

  I don't see that what you are describing  has much (or any)  relevance to the DWFTTW  puzzle.   I think (if I understand correctly)  these gear ratio effects you are talking about  can be seen more often in epicyclic gear trains.  

 

Hey hey - easy on the Canuckistani interjection.

 

 It's not Canuckstan -  the correct name is Canadia.     



#29 gruntguru

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Posted Yesterday, 01:48

The gearing is equally relevant to a DWFTTW or UWFTTW machine. The ratio of wheels/propeller is a factor in determining what speed is possible.