Tom L. wrote:
On Mon, 16 Apr 2007 23:33:54 +0200, Mxsmanic
wrote:

Rip writes:

I don't know, but I'm going to find out! I can envision an aircraft with
light wing loading, like a Cessna for instance, compressing the air
locally as it creates lift. After passage of the wing, the lift created
downwash would rebound upward, kind of like skipping a stone on the
water.
Virtually no compression occurs at the speeds of a Cessna. Compression is
only an issue at high speeds. At low speeds, air behaves very much like an
imcompressible fluid.

The end result is that the downwash stays at a constant altitude,
or sinks MUCH more slowly than theory would indicate.
The downwash does not stay at a constant altitude. It sinks. It has to,
otherwise the aircraft couldn't stay in the air.

It doesn't have to continue to sink forever. It can stabilize its
position at some point.

To explain the encounter with one's own wake turbulence we need some
quantification for a particular aircraft/bank/speed:
- radii of the vortices
- "sink" rate ("sink" meaning movement away from the flight path, not
necessarily downward)
- final "sink" distance

E.g. if the vertex radius is 15 feet and sink rate 20 fpm, we hit the
wake after a 30 second turn.
If the radius is 15 feet, sink rate 100 fpm, and final distance 10
feet, we still hit it.
And so on.


Hi Tom,

The key may be to understand that while the center of vortex system
moves down, air outside the vortex _has_ to move up. (By imparting
energy to the vortex (and thereby sustaining it) lift is generated). If
you like, the "top" of the vortex can stay at the same level (or even
move up) while the center of the vortex moves down as the vortex grows
with distance behind the wing. At low angles of attack the vortex center
moves slowly down and at high angles it moves down faster (and is more
intense). I'll predict that as you fly in a circle you start to create
yet _another_ vortex with a radius equal to that of the turn. This
vortex is centered on your path of flight and will become closed if you
contine the 360 degree turn. At that point, as I see it, you may
experience the angle of bank wanting to increase -have you ever felt
that? Now add in (1) parasitic drag from the airframe (2) propwash and
(3) tail vortex and you are even more likely to encounter your own
"turbulence".

Comments?

Cheers