Tom L. writes:

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

It will sink indefinitely unless some other force acts to stop it. In theory,
it will sink until it reaches the ground.

In other words, in theory, an aircraft lays down a swath of downwash from the
time it leaves ground effect on the runway until the time it lands in ground
effect again. The entire swatch theoretically touches down on the ground
eventually.

E.g. if the vertex radius is 15 feet and sink rate 20 fpm, we hit the
wake after a 30 second turn.

Twenty feet per minute is too slow. The downwash will move at at least a few
knots, and even three knots is 300 fpm.

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