Merged DDWFTTW in a “Real” Wind

[Michael C]

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I'm hoping to get firm answer from my rigorous test protocol on that, independently of accepting any of those I've seen put forward.
After all, using a propellor "backwards" as this one does, isn't something that's been investigated seriously, as the need to power something this way is probably never going to show up.

The prop only operates "backwards" under wind speed: under wind speed the air is flowing through the prop from back to front. When we're designing a vehicle to run at its best at well over wind speed, we're not so interested in the details of how it performs under wind speed.

It's clear that when starting from a stationary position, the fixed-gear ddwfttw cart is operating inefficiently. It's just like a bicycle built for high speed that only has one gear ratio: it's slow to start and may even need a push. To make a downwind cart that has good acceleration right from the start but still gets to above wind speed, you'd need to use a variable pitch prop that can go from negative to positive pitch. But the simplest fixed-gear version will give the best performance when running faster than the wind at the speed it's designed for.

 

[EHocking]

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I'm just "trusting" in the stability of the 3-wheeler configurations, which go straight when pushed in no-wind conditions.
Working with the miniscule forces I have to, I don't wanna get too complex.
Too many things to make move with as little friction as possible.

Nothing too complex and no friction added to the system.

Just think of an upright attached to the front wheel's axle and at the top of the upright, a non-swivelling weathervane that is parallel to the wheel.



Prevailing wind will keep the swivelling, steering wheel pointed in the direction of the wind, and hopefully the cart steering DDW.

Just the same principle that keeps a a windmill's vanes facing the wind (OK, but skewed 90^o).

 

[A.T.]

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I have lots of time to do nothing and entertain myself?

An interesting challenge would be to build a low-wind DDWFTTW cart. Big blade area and ultralight. A guy posting as tobermoryphil build this one from balsa:

If it efficient enough, it could run in a miniature wind tunnel to observe the entire acceleration under controlled conditions.

 

[humber]

Nothing too complex and no friction added to the system.

Just think of an upright attached to the front wheel's axle and at the top of the upright, a non-swivelling weathervane that is parallel to the wheel.

[qimg]http://www.internationalskeptics.com/forums/imagehosting/thum_26614d9ccdced3070.gif[/qimg]

Prevailing wind will keep the swivelling, steering wheel pointed in the direction of the wind, and hopefully the cart steering DDW.

Just the same principle that keeps a a windmill's vanes facing the wind (OK, but skewed 90^o).

No, vanes have a preferred position in the wind field. The direction where work is minimised. That means pointing into the apparent wind.
Automatic steering is "complicated" because they have mechanisms to compensate for that, or work at some offset to the apparent wind.

 

[Dr. Keith]

No, vanes have a preferred position in the wind field. The direction where work is minimised. That means pointing into the apparent wind.
Automatic steering is "complicated" because they have mechanisms to compensate for that, or work at some offset to the apparent wind.

What the hell was that! 50 pages in and he starts to sound like he has some idea of what he's talking about.

I'll go take a leak and see how he gets from here to absurdity.

 

[humber]

What the hell was that! 50 pages in and he starts to sound like he has some idea of what he's talking about.

I'll go take a leak and see how he gets from here to absurdity.

Wind vanes and automatic steering have different tasks. The latter have linkages and dampers etc.
A stationary vane points into the wind. If moved from that position by hand, it will return to that previous stable position. Same if moving, but into the apparent wind. If the movement is large, then the vane will oscillate until the energy gained in the balancing mass is dissipated. That depends on the damping effect of the wind, which is non-linear, while the constant acceleration of the chase vehicle as it follows the cart, keeps pumping energy into it. That is one reason why the NALSA sampling rate of 5Hz is way too low - >350Hz is used in vane calibration and verification - so won't read all of the vane's motion, but only points every 0.2s so missing a lot. Aliasing is the term for that effect.

 

[EHocking]

No, vanes have a preferred position in the wind field. The direction where work is minimised. That means pointing into the apparent wind.

How can you not understand how a weather vane or a windmill orintates itself in the wind?

A weather vane points into the wind, because the part with the largest suface area is blown to the downwind position, i.e., arrow head into the wind, "fletching" downwind.

A wind powered water pump's blade rotor is kept facing the wind by means of a "rudder" vane, or rotor, that is at 90^o to the rotor disc. The rudder always points downwind, so keeping the rotor disc facing the wind.
This example of a windmill anemometer (aerovane) should be sufficient to demonstrate this fact.

Just as in the steering wheel set up in the diagram above.

ETA: Think of it as a rigid flag that, instead of swivelling on the flag pole, swivels the pole. The flag will always fly downwind.

 

[A.T.]

Nothing too complex and no friction added to the system.

Just think of an upright attached to the front wheel's axle and at the top of the upright, a non-swivelling weathervane that is parallel to the wheel.

[qimg]http://www.internationalskeptics.com/forums/imagehosting/thum_26614d9ccdced3070.gif[/qimg]

Prevailing wind will keep the swivelling, steering wheel pointed in the direction of the wind, and hopefully the cart steering DDW.

Not bad. To get past wind speed DDW without the wheel turning by 180°, one could restrict the wheel steering to < 45° and allow the vane to flip over the top, to the back.

 

[humber]

How can you not understand how a weather vane or a windmill orintates itself in the wind?

A weather vane points into the wind, because the part with the largest suface area is blown to the downwind position, i.e., arrow head into the wind, "fletching" downwind.

It points in to the true wind when not in motion, and then the apparent wind when it is. That is why it is necessary to calculate the true wind speed from the apparent angle and windspeed. That is what NALSA and everyone else does. The mass on the the other end means it's a second-order system, so capable of oscillation.

As for the remainder, all wind turbines have such an arrangement so they point into the wind. If not, the generated power falls as the cube of the cosine of the angle away from that.

The cart's rotor is a turbine to the tailwind, so its power should fall if presented away from ddw.
That is what I have measured, and the cart works better when not ddw, because the opposing turbine force of the prop is lower.

The cart is in motion, so a vane on it, will point into the apparent wind and not the true wind.

 

[EHocking]

Not bad. To get past wind speed DDW without the wheel turning by 180°, one could restrict the wheel steering to < 45° and allow the vane to flip over the top, to the back.

At the moment, not an issue with I Ratant's carts.

 

[Dr. Keith]

Not bad. To get past wind speed DDW without the wheel turning by 180°, one could restrict the wheel steering to < 45° and allow the vane to flip over the top, to the back.

That was my first thought, but do you think the cart would survive the instability between 0.8X and 1.2X wind speed?

 

[EHocking]

It points in to the true wind when not in motion, and then the apparent wind when it is. That is why it is necessary to calculate the true wind speed from the apparent angle and windspeed. That is what NALSA and everyone else does. The mass on the the other end means it's a second-order system, so capable of oscillation.

All of which has nothing to do with subject of a potential system to orient I Ratant's cart's front wheel downwind.

As for the remainder, all wind turbines have such an arrangement so they point into the wind.

Just as I said - and so can be used to orientate the front wheel so the cart is steered DDW.

<snip irrelevance>The cart is in motion, so a vane on it, will point into the apparent wind and not the true wind.

And since at the moment, none of I Ratant's carts are able to travel faster than the wind, the apparent wind direction = true wind direction, so the "vane" will always point downwind, thus keeping the front wheel steering the cart DDW.

 

[humber]

That was my first thought, but do you think the cart would survive the instability between 0.8X and 1.2X wind speed?

That it could travel >Ws ddw is the assumption being tested, but the vane won't work. To keep the vehicle stable, the wheel will need a castor, so the vane must generate enough force to overcome that, when the apparent wind is at its lowest as it crosses WS

 

[humber]

All of which has nothing to do with subject of a potential system to orient I Ratant's cart's front wheel downwind.Just as I said - and so can be used to orientate the front wheel so the cart is steered DDW.And since at the moment, none of I Ratant's carts are able to travel faster than the wind, the apparent wind direction = true wind direction, so the "vane" will always point downwind, thus keeping the front wheel steering the cart DDW.

Into the apparent wind.

 

[EHocking]

Into the apparent wind.

To reiterate.

...since at the moment, none of I Ratant's carts are able to travel faster than the wind, the apparent wind direction = true wind direction...

 

[humber]

To reiterate.

Assumes its conclusion.

 

[EHocking]

Assumes its conclusion.

If you are travelling at 10mph in a southerly directly in a 15mph northerly wind a windmill anemometer held in your hand will indicate a 5mph northerly wind.

If I Ratant's cart is moving less than the wind speed a vane on the front of the cart will point downwind, i.e., if the cart is moving <windspeed, the apparent wind direction at the cart = true wind direction.

 

[humber]

If you are travelling at 10mph in a southerly directly in a 15mph northerly wind a windmill anemometer held in your hand will indicate a 5mph northerly wind.

If I Ratant's cart is moving less than the wind speed a vane on the front of the cart will point downwind, i.e., if the cart is moving <windspeed, the apparent wind direction at the cart = true wind direction.

No. The vane ALWAYS points to the apparent wind, if the true wind changes and comes from the rear left, it will tend to point to that. If the cart should turn in that direction, that will reduce the force from the true wind, and the vane will return to the apparent wind. It will oscillate. Wobble.
Another factor is the caster. They work by slightly raising the wheel against gravity, so "self-centering", and the will also oppose the vane and wobble for that reason too. It will follow the apparent wind and wobble.

 

[I Ratant]

An interesting challenge would be to build a low-wind DDWFTTW cart. Big blade area and ultralight. A guy posting as tobermoryphil build this one from balsa:

[qimg]http://www.ybw.com/forums/attachment.php?attachmentid=4431&d=1269872699[/qimg]

If it efficient enough, it could run in a miniature wind tunnel to observe the entire acceleration under controlled conditions.

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I've seen an upwind driving cart.... couple of them, it's a college challenge type thing, where the cart has to move upwind towards a fan. And IMF, uphill also.
And one here where it definitely looks "collegiate", with a beer can on the wheels axle driving an incredibly convoluted prop, and going towards the fan.
May go downwind very well also... but..

 

[phunk]

If you are travelling at 10mph in a southerly directly in a 15mph northerly wind a windmill anemometer held in your hand will indicate a 5mph northerly wind.

What if you are travelling 10mph south in a 15mph southwest wind?