

Flying at No Mechanical Energy Cost: The Secret of Wandering Albatrosses - nkurz
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0041449

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pg
Could an aircraft do it?

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marvin
Wow, a chance to reply to pg! Yes, it could and has been done. Glider pilot
Helmut Reichmann describes the technique for how to do this in a glider in his
book, "Cross-Country Soaring". He has also performed the maneuver a couple of
times.

You have to be flying in an area where there is strong wind shear, which means
a sharp difference in wind speed with altitude. You could detect such wind
shear when climbing or descending through it, either with GPS or by watching
the ground and seeing how much you drift in relation to it. Cloud movements
are another option. You also need a very maneuverable and aerodynamically
efficient plane, so you won't lose a lot of energy from the sharp banking
maneuvers required.

So assume that the wind speed increases sharply with altitude, maybe around 20
knots over 100 meters of altitude. You start out flying the same direction as
the wind and then sharply dive 100-200 meters down into the space where the
wind is weaker. This turns part of your altitude into kinetic energy, so your
ground speed increases by about 100-150 kph. But due to the change in wind
speed, your airspeed velocity has changed less than it would in dead air. So
your total energy has increased. You can then make a sharp (>120 degree) turn
in the direction you want to go, so you face partly into the wind again. You
then sharply pull up, gaining about 100 meters of altitude and losing some of
your velocity. The relative wind speed has increased with altitude, so your
kinetic energy loss in relation to the air from pulling up is less than it
would be in dead air. Overall, you have gained velocity, moved the aircraft
and maintained your altitude "for free".

This process can be repeated, and you can keep doing it (albeit with some
nausea, unless you're used to these sharp maneuvers) as long as there is
sufficient wind shear. It will be easier to move perpendicular to the wind
direction than directly with or against the wind, but the energy (altitude,
velocity) you gain from this maneuver could be used to glide in any direction.

I don't want to make any grand claims that this technique can be used for
anything practical (i.e. passenger transport). Glider pilots don't use it in
competitions, because there are lots of techniques for moving around without
an engine that are a lot better and easier to exploit. (You can gain altitude
in thermals, ridge lift or mountain waves - and translated to horsepowers, a
thermal carrying a 500kg glider upwards by 3m/s is a very powerful engine).
But the technique is very cool as an intellectual curiosity.

[http://www.cumulus-
soaring.com/books/CrossCountrySoaring/Cro...](http://www.cumulus-
soaring.com/books/CrossCountrySoaring/Cross-CountrySoaring.htm)

[Edit: Actually, it's interesting that the authors mention possible
applications for this to robotic aircraft. I'm sure you could make a robotic
glider that used the meteorological principles that glider pilots use to move
around without engines. The "Albatross" technique would only be a small part
of this - glider pilots have extensively studied techniques for moving around
without an engine, and there are lots of them. Glider pilots manage >100kph
average velocities over >500km journeys on days with good weather, and robotic
aircraft could in principle do the same].

~~~
incomethax
So it's essentially like sailing, except in a glider/aircraft.

That's pretty cool. Are there any directions that you would essentially end up
"in irons" like with sailing?

~~~
marvin
There isn't a direct glider analogy to being "in irons" (you can always dive
to gain airspeed, assuming you have any altitude to lose of course - not
having any altitude would mean you are guaranteed to crash, since gliding is
basically flying slightly downhill all the time).

But there are plenty of ways to get in trouble when gliding, most notably
flying into an area where there aren't any sources of lift. This means you
can't gain any more altitude, and you'll gradually lose your remaining
altitude and probably have to land in a field or something.

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Gravityloss
Wasn't this known already much earlier?

There has been an RC glider community doing dynamic soaring probably for
decades, it's easy to find videos on youtube.

Basically it's just extracting energy from the wind velocity difference.

Of course you could use it at much larger scales, like a huge UAV dynamically
soaring high up if the winds are in different directions at different
altitudes.

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nilsbunger
Rc gliders do it behind a ridge, taking advantage of big wind speed
differences caused by the ridge. It's pretty fantastic to see in a video or in
real life.

But I'm not sure whether it translates to generalized flying at high altitude
- is there enough windshear without a close-by hill for this to work? Can it
work in any direction?

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jbuzbee
I think my jaw dropped the first time I came across videos of RC gliders doing
400+ mph. No motor. 400 mph. Here's one clocked at 399. Others on YouTube show
over 450

<http://www.youtube.com/watch?v=Oix6sHKzOLU>

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SudarshanP
Wow at last we can have our flying cars that don't need fuel... Just kidding.

Can a minimally powered aircraft on mars or venus do this without spending a
dime on fuel or panels for flight? That would be a cool technology
demonstration getting the whole world shocked. And we could throw all kinds of
exotic material like carbon fiber and thin strong foils and kites coz getting
these beasts there would be way more costlier than building them. I see very
interesting short term future possibilities for this. Maybe even an X-Prize
for this would help.

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cristianpascu
How can this be an "evolutionary adaptation" is beyond my understanding. How
can a genetic mutation lead to such complex flight strategy? Anyone can clue
me out? Or is it just another irresponsable "just-so story"? [1]

[1]
[http://www.newyorker.com/arts/critics/books/2012/09/17/12091...](http://www.newyorker.com/arts/critics/books/2012/09/17/120917crbo_books_gottlieb?currentPage=all&mobify=0)

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andreasvc
It can be an instinct; an innate behavior that is not learned, such as a bear
preparing for hibernation, or certain birds migrating in fixed patterns. When
animals display these specific behaviors even in the absence of instruction
from their parents or other members of their species, then it must be genetic,
even though it's probably not known how that would work specifically.

The accusation of a "just-so story" is made against a claim of how an
adaptation came about. But that's not happening here, they just state that it
is an evolutionary adaptation, without a theory of how it was acquired.

~~~
cristianpascu
I see "instincts" as being a cover term rather than an explanation. It's
another way of saying animals just know how to do stuff.

~~~
andreasvc
You'll just have to accept that there can be a solid scientific concept
without an explanation. Gravity is another example.

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SeanDav
Never ceases to amaze me that so much scientific innovation can still be
derived from understanding mother nature.

From optimum pathing with slime molds to effortless gliding by Albatrosses and
so much in between and still to be discovered.

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sopooneo
Wouldn't this technically be gliding rather than flying? I had thought that by
definition it was only flight if you could produce enough thrust to maintain
velocity and altitude in still air.

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mythealias
Flight is general term for "unassisted" motion through air. By unassisted I
mean no support structure from surface.

So even a ball through up in air is technically in flight.

While gliding implies flight with no thrust and is the primary mode for
Albatross flight, they still produce trust by flapping their wings.

