On 6/6/2010 7:39 AM, Gary Osoba wrote:

Yes. Your wing is a machine, and the work it performs imparts a
downward flow to the air it moves through. When that downward force is
aligned in a direction that opposes the movement of the air, it gains
energy. The air movement can be from the side, from above, or below-
the most efficient case since this vector opposes gravity. The
transfer of energy from air motion can be increased by manipulating
the inertial field of the glider, and there is an optimal g loading or
unloading for each case. Although physicists define such inertial
forces as "psuedo", the wing does not know this and must develop twice
the lift to sustain 2g flight as 1g flight, three times the lift for
3g flight,etc. The power transferred from the air to the wing
increases linearly with g force increases, while the the losses
associated with the increased g loadings are fractional and therefore
nonlinear, yielding excess power. This excess power can be carried by
the glider into a differential airmass with relative sink by a coupled
acceleration and a portion of it can be transferred to this airmass.
The case of 0g accelerations (freefall) is special in that
theoretically the wing doesn't produce induced drag. Theoretically
only, because the lift distribution will never be perfect- especially
in the unsteady flows which punctuate a soaring environment. In
practice, I have found 0g to be the best target for accelerations
since most of our wing sections are not designed to fly efficiently
upside down and everything is happening so quickly you lose less if
you guess wrong on the strength of the relative downdraft.

Much of this is counterintuitive. For example, here's something
presented in a 2001 lecture on the subject. It is stated as
exclusionary to emphasize how flight through a discontinuous
atmosphere can up-end long held conventions.

"For any body of mass moving through or in contact with a medium that
is not uniform, the most efficient path(s) for a given power input
will never be defined by a straight line or a constant speed." -
Osoba's Theorem of Dynamic Locomotion

The concise statement of this is "...never be defined by a constant
velocity..." since velocity incorporates both speed and direction but
most pilots don't understand the term that way.

Best Regards,

Gary Osoba

Can someone explain that first part? Is it really obvious? Seems
critical to the theory and I don't get it. Seems like when the wing
imparts a downward force on the air and displaces it, work is done on
the air. While the forces should be equal and opposite, the work is not.
In fact, energy is conserved. So the energy came out of the wing and
into the air. The wing doesn't know the air is moving relative to the
earth or anything else. And the air doesn't know its moving either. Its
wafting along at a nice steady pace (convenient inertial reference
frame) when the wing comes along and shoves it. The harder you push and
more air you displace, the more energy is transferred out of the wing
and into the air. Where does the energy into the wing come from?

Its not because the wing accelerates up due to the increased lift force.
The lift force generated by the wing is normal to its path through the
local air. Always. So that force curves the flight path. And by
definition, no work is done by a force normal to the displacement. But
the increased lift does increase the drag force, which works opposite
the direction of motion (negative work, which transfers energy to the
air). How does aggressive vertical maneuvering help?

Seems like dribbling a flat basketball. The bounce is kind of lossy.

But plenty of smart people see that it works, so I'm missing something?

I suppose dynamic soaring on the edge of a thermal might work. Looping
at the edge, diving in the core, pulling up vertical in the sink would
increase airspeed on each side of the cycle. But that requires pulling
in sink and pushing (pulling the top of the loop is more efficient) in
the lift. Hard to believe that is more efficient than normal thermalling
at adding energy. And its the opposite of this theory. But the
horizontal version works spectacularly well for the RC dynamic soaring
guys (record is well over 400 mph!) although they do not use the
turnaround high g turns to gain energy and they also do not pull at the
gradient, but go directly for the airspeed increase on both sides of the
cycle.

I'm skeptical. There are plenty of good reasons to pull hard once in
awhile. But its a necessary evil used only when it really pays off to
put the glider exactly where you want it right now.

-Dave Leonard

Looking forward to the Parowan experiment next week. I'll be the control
case, cruising sedately along like grandma on her way to church on Sunday...