Apparent Wind

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Ian K said..

sausage said..

Out of interest the Americas Cup 72'ers were so efficient dead down wind that they effectively created a head wind ......apparently

No they don't go "dead down wind" they do downwind tacking. But the downwind VMG is faster than the wind.

The only device that can go "dead down wind" faster than the wind is this thing

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But the blades of the propellors don't go dead downwind, they go downwind in a spiral at , I'm guessing, about the same angle as the sail on an AC 72. It's no big deal if you think about it long enough. It's the same fundamental thing. Why the folks on the video had to build such a large machine to prove the obvious I don't know? If you stuck a water skier on a long rope behind a quick downwind tacking AC 72 he could probably swing left and right off the towline and cancel out the zig zag of the AC 72. He'd then be negotiating a course dead downwind faster than the wind. Can we consider the skier to be sailing? Of course he is.

Lets complicate things a bit now, Ian.,
At this moment vehicle is using ground to withdraw kinetic energy from air molecules,
Could the same be achieved for flying vehicle ? If the plane propelled by similar propeller could travel down wind faster that wind itself ?

OK Mal Falkner has sent me this diagram he's drawn.

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decrepit said..

Macroscien said..

>>>>>>>

The only device that can go "dead down wind" faster than the wind is this thing

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>>>>>

Lets complicate things a bit now, Ian.,
At this moment vehicle is using ground to withdraw kinetic energy from air molecules,
Could the same be achieved for flying vehicle ? If the plane propelled by similar propeller could travel down wind faster that wind itself ?

Not that easy as it sounds at first glance.
Imagine
1. that flying vehicle draw this energy from the wind as usual using propeller/ turbine blades
2. Convert this energy to electric current using permanent magnets generator
3. Using ion propulsion then propel flying vehicle to even bigger speeds (simply rocket engine propelled by propeller drawing energy from wind)

(simply instead of wheels you use alternative propulsion for flying vehicle)

To make this more confusing ( for mathematical computation) you could replace wind with light and propeller with solar cells -energy balance and end effect should be this same...

and the funny is that we are not even trying to brake and energy conservation laws or create perpetum mobile

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Macroscien said..


Lets complicate things a bit now, Ian.,
At this moment vehicle is using ground to withdraw kinetic energy from air molecules,
Could the same be achieved for flying vehicle ? If the plane propelled by similar propeller could travel down wind faster that wind itself ?

Extracting energy to power your craft basically gets down to slowing the relative speeds of the two medium you are sailing in. An AC catamaran in 18 knots of wind in a tide running out at 3 knots has 15 knots of speed differential to play with. After they've run the course you could theoretically measure the tide to be something like 3.0000001 knots and the wind to be 17.99999 knots, 14.9999 knots of differential speed left to play with. (Don't count the 9s it's a guess). Without the ground, or water to anchor on it's difficult to see how you can slow down the wind and extract energy from it.

Albatross do extract energy from just the wind however by dipping up and down into different layers of the wind. Between the higher speed upper layers and the lower speed lower layers. After they've flown by the wind profile is immeasurably reduced. But that's a pretty subtle way of extracting energy, with the drag involved I doubt they could go downwind faster than the wind.

These guys are flying down wind faster than the wind, but not for any distance, ( look at it like a ping pong ball bouncing between two bats )

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Ian K said..

Macroscien said..


Lets complicate things a bit now, Ian.,
At this moment vehicle is using ground to withdraw kinetic energy from air molecules,
Could the same be achieved for flying vehicle ? If the plane propelled by similar propeller could travel down wind faster that wind itself ?

Extracting energy to power your craft basically gets down to slowing the relative speeds of the two medium you are sailing in. An AC catamaran in 18 knots of wind in a tide running out at 3 knots has 15 knots of speed differential to play with. After they've run the course you could theoretically measure the tide to be something like 3.0000001 knots and the wind to be 17.99999 knots, 14.9999 knots of differential speed left to play with. (Don't count the 9s it's a guess). Without the ground, or water to anchor on it's difficult to see how you can slow down the wind and extract energy from it.

Albatross do extract energy from just the wind however by dipping up and down into different layers of the wind. Between the higher speed upper layers and the lower speed lower layers. After they've flown by the wind profile is immeasurably reduced. But that's a pretty subtle way of extracting energy, with the drag involved I doubt they could go downwind faster than the wind.

These guys are flying down wind faster than the wind, but not for any distance, ( look at it like a ping pong ball bouncing between two bats )

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Fascinating feat of engineering in such simple but speed effective glider.
Obviously doesn't works in flat terrain tough.
Here we use updraft energy of the wind and then gravitational force down.
No ground anchor or contact needed.
But for our consideration of free flying plane faster then wind we don't have any anchors in the air ( beside maybe magnetic field which is too week to be considered).
What we could do is albatross trick. The plane has two parts , each position in different atmosphere layer, which usually has different wind speed.
Part are connected with super strong tether . Now we could use this speed differential in winds at different highs as an air anchor to extract this energy.
Then our plane hanging completely in the air could travel indefinitely on flat path with ever increasing speed ( till reach equilibrium with air resistance eventually) . Still interesting that plane will be travelling faster that any of this two wind layers , even faster than this two combine.

From there we just one step to create plane travelling across the glob at flat path at any direction completely on free wind energy. Now up to smart engineers to test different models if that will be set of two gliders connected by the string or set or two rotor blades swirling in two different wind speeds. I guess that travelling at speeds up to 500 miles per hour on free wind energy is quite appealing.

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Ian K said..

Yes, or you could use this super strong 36 gauge wire to clip onto a runner on a railway track. Kite trains, plenty of wind up high.

Exactly. This runner could be the best to test what is upper speed limit for the ground wind powered vehicle.
We know what is max speed for now for the:

water - about 72 knots

land yacht , or ice yacht- about twice as much.

But we are still constrained by the stability - tipping forces from finding out what is maximum practically achievable.
Most of current design loose increasing amount or energy on keeping device stable , close to the ground, to counterweight tipping forces. Railway track could allow to eliminate those almost completely with all force now converted into propulsion.

There is simple illustration of the down the wind faster principle

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decrepit said..

I don't think so Macro, the wheels are powering the prop, don't think you'd get this thing to work without wheels.

Just to clarify, That is the prop powering wheels , not vice versa
(then wheels push vehicle forward)

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Ian K said..

(I should be out sailing, still recouping energy from yesterday's downwinder, did we tell you about our downwinder?)

Lets imagine traditional sailing boats dead down the wind. What usually they do? Release spinnaker in hope that wind will be still faster then their boat.

Now imagine that instead of the huge stationery bag hanging of the boat you attach kite spiraling in tight circle down the wind pulling that boat.

We know the kite will be flying at very high speed at this spiral trajectory. I wonder if linear component down the wind could be also faster then wind itself .

It could be specifically useful for very light vessels because hard to imagine the most efficient kite could compete with sailing area of the huge spinnaker. So advantage of kite lay in shear speed not pulling force.

There is one more difference when comparing spinnaker to the rotating kite tough. Spinnaker could be 100 m2 and kite 20 m2 but what really matter for kite is sweeping area during this circular flight - depending on the radius of the sweep - and here actually could lay another advantage of the kite sail, or flying wing we could say.

On this same principle we do calculate wind turbine power output by measuring rotor diameter not the area of single blades.What is even more surprising that the most efficient will be 2 blades and increasing amount to 3 or more lower efficiency of wind turbine rotor. What count is the total area swept by rotating blades and tip of blades could travel with speeds close to sound barrier since the wind speed could be just moderate 20-30 knots.

Simply if we could design rotating sail for this boat if should prove much more effective then ordinary sail down the wind.