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.

So what are the right numbers?
We know from experience that a right combination of those numbers
exists in reality.

- Tom