88 replies to this topic

[MatsNorway]

This is so dumb... or?

 

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

[Canuck]

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.

[GreenMachine]

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.

[gruntguru]

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.

[Greg Locock]

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.

[Canuck]

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

[desmo]

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. 

http://www.nalsa.org...cordAttempt.pdf

 

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.

[gruntguru]

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.

[Greg Locock]

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

[gruntguru]

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.

[Greg Locock]

They used a different transmission setup.

[Fat Boy]

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.

[gruntguru]

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.

[Wuzak]

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.

[Fat Boy]

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.

[gruntguru]

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).

[Kelpiecross]

  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.    

[GreenMachine]

      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'.

[Kelpiecross]

We don't.  Google 'red/blue shift'.

 

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

[Kelpiecross]

 

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.        

[gruntguru]

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.

[Kelpiecross]

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?

[Canuck]

  Eh?

Hey hey - easy on the Canuckistani interjection.

[gruntguru]

  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.

[GreenMachine]

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?

[gruntguru]

K.

[gruntguru]

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.

[Kelpiecross]

  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.     

[gruntguru]

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.

[desmo]

Vice has done a rather lightweight pop-sci piece on the controversy- https://www.vice.com...laws-of-physics

 

UCLA Physics prof Alexander Kusenko has wagered $10,000 that a DWFTTW craft is basically thermodynamically impossible, which adds a hint of narrative spice. Kusenko argues that the anomalous-looking data are the result of transient wind speeds and momentum.

[gruntguru]

Oh dear -  he is going to wind up being rather embarassed. DWFTTW cannot be disproved using thermodynamics. Constant velocity motion at any speed requires zero energy input.

[gruntguru]

Details of the wager can be found here. https://sites.google...sics-wager/home

There is also a slideshow from Kusenko which suggests other phenomena which could explain the speeds achieved but no details of the logic which leads to his conclusions - very disappointing.

 

A statement from desmo's link ("Wind-powered vessels can exceed wind speeds when they are oriented at an angle to the wind direction, they argue, but not when a vessel is exactly parallel to the downwind force.") has prompted another thought. Althoght the DWFTTW vehicle is travelling directly downwind, there is part of it that is travelling crosswind - the propeller blades - and this is why it works.

[Fat Boy]

Oh dear -  he is going to wind up being rather embarassed. DWFTTW cannot be disproved using thermodynamics. Constant velocity motion at any speed requires zero energy input.

As I've said previously, I don't really see the case for their DWFTTW claims, but I'm perfectly happy let someone else burn the brain cells and have the argument. Having said this, your final sentence is incorrect (or, at the least makes very large assumptions) and has little to do with the one preceding it.

[gruntguru]

It should be obvious that fixed-sail craft can travel FTTW when travelling cross-wind and that the phenomonon can be explained as a "slippery wedge". A watermelon seed can be propelled much faster than the fingers that are squeezing it.

 

Similarly the DWFTTW vehicle is "wedged" by the relative motion between the wind and the ground plane. Did you read and understand the analogy I posted earlier - replacing the wind with a second ground plane (ceiling) moving relative to the first?

Edited by gruntguru, 21 June 2021 - 22:17.

[Fat Boy]

Like I said, I'm not getting into the discussion, per say. I'm simply commenting on your sentence which implies there is no thermodynamic explanation for the machine/scenario combination. There is.

[Fat Boy]

Vice has done a rather lightweight pop-sci piece on the controversy- https://www.vice.com...laws-of-physics

 

 

How very Vice of them.

[gruntguru]

Apologies, I picked the wrong post - I deleted the incorrect reference from my reply.

Edited by gruntguru, 21 June 2021 - 22:24.

[Kelpiecross]

 Assuming  that the vehicle moves one prop revolution (one pitch) to one revolution of the wheel it is geared to:   if the vehicle is moving forward with the prop in a normal forward pitch -  then when towed in still air,  the air flows over the prop blades at a certain rate as it winds it way through the air.  However if the prop has the same pitch but in the reverse direction (as it does in wind-driven mode)  -  when the vehicle is towed forward the prop is busily winding its way in the opposite direction leading to a zero airflow over the prop blades. 

  So - any following wind "sees"  the  prop as stationary initially no matter what speed the vehicle is travelling.     

[Kelpiecross]

It should be obvious that fixed-sail craft can travel FTTW when travelling cross-wind and that the phenomonon can be explained as a "slippery wedge". A watermelon seed can be propelled much faster than the fingers that are squeezing it.

 

Similarly the DWFTTW vehicle is "wedged" by the relative motion between the wind and the ground plane. Did you read and understand the analogy I posted earlier - replacing the wind with a second ground plane (ceiling) moving relative to the first?

 

 This explanation  may make sense to you but not to me (or probably anyone else).   

[gruntguru]

Have you seen something on this forum to suggest other members have similar thought processes to yourself?

I must have missed it.

[gruntguru]

 Assuming  that the vehicle moves one prop revolution (one pitch) to one revolution of the wheel it is geared to:   if the vehicle is moving forward with the prop in a normal forward pitch -  then when towed in still air,  the air flows over the prop blades at a certain rate as it winds it way through the air.  However if the prop has the same pitch but in the reverse direction (as it does in wind-driven mode)  -  when the vehicle is towed forward the prop is busily winding its way in the opposite direction leading to a zero airflow over the prop blades. 

  So - any following wind "sees"  the  prop as stationary initially no matter what speed the vehicle is travelling.     

Nice proof. Did you notice that when the wind starts to blow there MUST be some "slip" as it moves over the prop ie the vehicle won't work with a perfect (zero-slip) prop - the gearing must be changed (or some slip permitted).

 

The zero-slip prop is the same as my "moving ceiling" analogy. As the gearing approaches 1:1 the vehicle speed approaches infinity and at 1:1 there is no solution. When the ratio exceeds 1:1 the velocity ratio turns negative and the vehicle switches from DWFTTW to UWFTTW.

 

Edited by gruntguru, 23 June 2021 - 21:49.

[Kelpiecross]

Have you seen something on this forum to suggest other members have similar thought processes to yourself?

I must have missed it.

 

 Other members are possibly  too polite to say that your brilliant "moving ceiling"  theory is crap.  I am not too polite.      

[Kelpiecross]

Nice proof. Did you notice that when the wind starts to blow there MUST be some "slip" as it moves over the prop ie the vehicle won't work with a perfect (zero-slip) prop - the gearing must be changed (or some slip permitted).

 

The zero-slip prop is the same as my "moving ceiling" analogy. As the gearing approaches 1:1 the vehicle speed approaches infinity and at 1:1 there is no solution. When the ratio exceeds 1:1 the velocity ratio turns negative and the vehicle switches from DWFTTW to UWFTTW.

 

 

  This is known as trying to blind people with science - makes no sense at all.  

[BRG]

I saw the clip on YT and didn't understand the 'science'. Then this thread came along with many helpful expositions of what was happening.  I still don't understand. But maybe that is just me.

[Greg Locock]

No. When it came up originally I got my head around it, this time, no.

[gruntguru]

I like KC's approach in post #28. Once I worked out what he was trying to say. Let me try.

If you gear the prop and wheels at 1:1 ratio and tow the thing in still air, the prop does nothing - it just screws its way through the air with neither thrust nor drag. Now add a gentle following wind - the prop looks like a stationary "sail" even though the vehicle is moving faster than the wind.

 

This 1:1 ratio produces (potentially) infinite speed (see chart below). No matter how fast the vehicle is travelling the "sail" always appears stationary to the wind.

 

The chart shows "ratio of vehicle speed to wind speed" vs "gear ratio" asssuming a "zero slip" prop. Ratios >1:1 reverse the thrust and cause the vehicle to travel upwind.

 

Edited by gruntguru, 27 June 2021 - 03:31.

[Kelpiecross]

If you tow the thing in wind-driven mode, the reverse pitch winds its way backwards as fast as the vehicle is going forwards -  thus it never appears to move. 

 The prop doesn't act  as a "sail" - it acts as a turbine.

 If you had a stationary turbine next to the  stationary vehicle - then you release both -  if both are equally loaded (the stationary turbine electrically; the vehicle by inertial acceleration forces, drag etc.)  then the prop will start to spin up equally in both cases - causing electrical power to flow in the stationary turbine's case and the car to accelerate away into the distance - but both will have the the same prop RPM.         

[gruntguru]

The car "accelerating away into the distance" will create a higher apparent wind so its turbine will have higher rpm.

 

(I assume your entire post is about UWFTTW. Is that what you mean by "wind-driven mode"?)

[Kelpiecross]

The car "accelerating away into the distance" will create a higher apparent wind so its turbine will have higher rpm.

 

(I assume your entire post is about UWFTTW. Is that what you mean by "wind-driven mode"?)

 

 No - downwind faster than the wind.  Wind -driven mode as opposed to towed mode.  I think the apparent wind (like the speed of light)  remains the same.  .  

[gruntguru]

In that case, your statement 

 

 

The prop doesn't act  as a "sail" - it acts as a turbine.

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)