The Wind Powered Sail-less Boat

I should be ashamed of myself for lobbing such a piece of red meat as DDWFTTW into a pool of alligators like this forum just to watch the action! ;-)

The attached file is rather off topic, but it is an elegant analysis of another form of wind-powered travel that can go down-wind 40% faster than the wind. Enjoy.

 

Albatrosses can go downwind faster than the wind!

An Albatross, or a glider, or any aircraft can certainly travel downwind faster than the wind. It needs to move through the airmass in order to stay aloft.

It's speed over the ground directly downwind = its flying speed + the windspeed.

I don't think anyone could dispute that.

But you never know on here!....:?:

 

Thank you Speer for that beautiful attachment.

I had long known of the capacity for energy extraction across a sheared flow like that but had never come across an in depth analysis of it.

I had also observed the phenomenon being used by seabirds- not only albatrosses use the technique, although they must undoubtedly be the masters of it.

The technique is actually even more powerful than as described in the analysis due to ground effect. This allows the bird to drastically increase its L/D during a portion of the upwind part of the cycle. This applies in flat water as well.

Naturally, when there is wind there is also waves and (this is alluded to in the presentation) this allows the bird to time its cycles with the upwind portion of the waves and further enjoy the lift off of them.

It is interesting to note that the upwind portion of the wind is not necessarily the backs of the waves. In the waning part of a weather system the wind decreases to a value inferior of that of the celerity of the wave trains created previously by the stronger winds.

There is yet another phenomena which i saw used in a calm (when there is no wind shear) and that is simply flying down the faces of old swells. By adjusting flight speed to coincide with the celerity of the swell the bird remains in the lift area and "surfs" down it, at least until that swell reaches the end of its group and decays.

The upshot of all this is that there are many ways to extract energy from a velocity gradient and the speed of the object is limited by its own efficiency, unbounded by the actual velocities, or velocity differences within the energy giving flow.

 

I think what you're describing is "slope lift", which is not what the albatross uses. It uses the horizontal wind shear -- the vertical velocities near the waves are relatively insignificant.
The most spectacular example of Dynamic Soaring today is done by RC sailplanes. This is done on the downwind side of a hill, by circling through the shear layer which comes off the hill crest, much like an albatross circles in the surface boundary layer shear. The current DS speed record is 371 mph, in roughly a 40 mph wind. Here's one video as an example:
http://www.youtube.com/watch?v=Vi0hrjqU15I

 

That's right, which is why in the presentation Barnes specifies waveless seas, so as to analyze dynamic soaring independently from slope lift effects.

In reality the bird soars dynamically, but also takes advantage of the slope lift where and when possible. So the bird really has a range of options from which to maximize it's energy efficiency.

 

it would be so easy to get sucked into this again
thank god I dont know Jack about birds

as for why can the rc go so fast
think circular flight pattern on an oblique plane in a steady updraft
normally angular momentum is defined as
J=mvr
( dam I hope I remembered that right )
but this case is not simple angular momentum as there are two drivers in the system
gravity and wind
not sure I can but maybe I can dream up a defining equation for the amazing speed that rc is gaining in these conditions
I think the energy in a flywheel is defined as on half its moment of inertia times its angular velocity squared ( squared part is the kicker )
basically
E=1/2AvI2
ok doesnt write very well on this computer
now two things about that number
angular velocity is measured about a radian with a fixed center of rotation
and we dont have a fixed center of rotation
so there are multiple centripetal forces at work
both I think would result in the squaring of the velocities they impart
( not sure thats a fair way to handle it but it feels right )
( feel free to help if you know the proper calculation )
dam I havnt tried to work this king of thing out in a while
ok so
there is a oblong instead of circular flight pattern but Im going to work it out like it was a flywheel with an imaginary increase in the length of some of the radians with out an increase in time for those radians because they center of rotation is altering relative to ground not relative to wind
( if that makes any sence to anyone else feel free to applaud )
because the wind is pushing this thing at whatever wind speed is faster with each loop
so Im going to try to add for the wind boost at perpendicular to wind
and emagine its neutral on the dead down wind and up wind runs
into and upwind there is no relative gain but the velocity caried out of each turn should carry along just fine
also there is a gravity gain with each loop 9.91N I think for the drop in altitude considering mass and that the uphill climb can be attributed to wind direction and lift
so thats 9.91N
measured in
drum roll people
NEWTONS
ok gotta go flip laundry
brb

 

ok
dam I got sucked in again
complex orbital mechanics was one of my favorites
the short answer is that oblong pattern is helping out a lot
the oblique nature of the pattern is adding some
the wind itself is the main driver and is driving him faster with each circuit competed
the gravitational force is adding some
so
hella fast
hows that for a technical explanation

guess laundry brought me back to reality eh
love B

dam I just remembered
its called hyperbolic excess velocity and there is some formula that some one is about to rub my nose in that covers the concept

Hella fast

 

Ummm.... Actually that sailplane is flying in an area of sink, not updraft.

 

did any one notice the smog in the background of that flicker
wow

 

In RC Dynamic Soaring, updrafts and gravity are both are irrelevant. There is no updraft on the downwind side of the hill where DS is done, and the airloads are vastly greater than the gravity forces. The plane is driven entirely by the horizontal velocity difference across the shear layer.

One can make a crude force/power analogy of DS and conventional sailing:
* The wind shear layer is like the air/water interface, but smeared vertically somewhat.
* When the glider is above the shear layer its wings act like sails.
* When the glider is below the shear layer its wings act like the keel.
* Both the DS glider and the sailboat extract power from the velocity difference across the shear layer or air/water interface.
One difference is that the "sail" and "keel" driving forces on the DS glider are intermittent, while on a sailboat they are steady.

One could imagine a flying fish doing "dynamic soaring" in a strong wind storm. The fish leaps into the air against the wind, spreads its fins/wings and does a 180 deg turn, dives back into the water with fins mostly folded, does a 180 deg turn underwater, leaps back into the air, etc. If its drag is low enough, the fish could do this by inertia alone, without any muscle power.

 

Is gravity really irrelevant? As a first thought I would think gravity is very necessary, since it used to gain speed. Is it not important that there is gravity and thus you are able to gain potential energy while going against the wind?

I think there always has to be a connection between the two different velocities/fluids. In DDFTTW it is the transmission between the propeller and turbine and in soaring it is the gravity. I may be wrong about soaring, since I haven't yet understood it well enough.


Joakim

 

The high and low points of the orbit are about 50m apart vertically. That translates to a change of 500 J/kg in potential energy. When the glider is going at 160 m/s (360 mph), its kinetic energy is 12800 J/kg. So the potential energy changes are just a few percent of the kinetic energy, i.e. negligible. Another way to look at it: When a 160 m/s glider drops down the 50 meters without drag, its new speed from the extra energy is only 163 m/s --- not a significant change.

The only requirement is that there is a zero net force vector, in order to produce a steady state operation. In DS, the net force vector is zero when averaged over the orbit, since the orbit center stays in place, and doesn't accelerate away.

 

I gotto wonder about that
if you look at any attempted circular pattern in a cross wind up draft
or call it what you may you would get some slipping relative to ground
thus my thought there would be an excess hyperbolic velocity
also since the velocities of the radian is squared it actually would add significantly
I think
I didnt crunch any numbers
but
seems like it aught to be
B

could be why there plane broke

 

That's a nice analogy you've offered and should help people imagine what is going on. The plane is almost "bouncing" between being a keel and being a sail.

However, to be slightly pedantic, there will always be sink to leeward of a mount like that. The amount of sink will depend naturally on the specific topography,flow separation etc.. The sink is adverse to soaring so the pilot must be fairly proficient at getting into DS mode or he'll lose the model down the slope.
But yes, once at speed the sink becomes a relatively unimportant factor.