IEEE Spectrum July, 2013 - 54

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the same at the beginning and the end of the trajectory. We also
found that the closer the bird got to the water's surface, the lower the total energy; right near the water, there was no gain at all.
Our work therefore proved that the waves and related turbulence play no significant role in the bird's flight. We discovered
that total energy tops out after the bird makes its upper turn, so
this has to be the part of the cycle in which it extracts the most
energy from the wind.
On top of this energy analysis we looked at the aerodynamic
forces acting on the bird, which were quite complex. The essence is as follows: The local shear wind lets the bird change
the direction of the lift it generates as a kind of propulsive force
during the upper curve of the cycle. At that moment, the strong
wind acts like a kind of outboard engine. Afterward, the bird
dives close to the water and then turns back up, in the process
incurring a braking effect. The bird still has a "propulsion profit"
over the whole cycle that just manages to overcome drag. As
long as it keeps up that pattern of dips, swoops, and turns, it
can keep on flying-flying for free.
In short, the insights offered by our GPS measurements largely confirmed the findings of our optimization-based analysis.
We now know a lot more about the fascinating flight of these
beautiful animals than we did before. Of course, there is always
room for refinement.
For one thing, we weren't able to measure the birds' air speed
directly because any appropriate sensor, such as a pitot tube,
would stick out of the plumage quite a bit-probably enough
to affect flight and surely enough to annoy the birds. Instead
we had to rely on rather coarse remote wind information provided by satellite measurements (QuickSCAT ocean surface
wind vectors).
In the future, we'd also like to measure attitude-the angles
of roll, pitch, and yaw. The rapid development of inertial sensors made with microelectromechanical systems should allow
us to do that the next time around.
This project is more than a sideline in natural history. The
open sea isn't the only place that offers a shear wind field suitable for dynamic soaring, and wandering albatrosses aren't the
only pilots that rely on it to conserve energy. Such boundary
layers also exist within the high-altitude jet streams that encircle Earth; even though these layers are much thicker than
the one the albatross flies in, the physics are the same. That
means that UAVs with the proper aerodynamics and programming might one day exploit them to stay aloft for weeks or
months on end, allowing them to serve as surveillance nodes
or radio relay links.
And indeed, flight optimizations we've done in related
research show that aircraft having the aerodynamics of today's gliders or sailplanes could perform sustained dynamic
soaring in these atmospheric layers by accordingly scaled
maneuvers. "Surfing the jet stream" is no mere fantasy; it is a
reasonable strategy for aeronautical engineers to pursue. n
post your coMMents online at
http://spectrum.ieee.org/albatross0713

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2013