Milt,
You will not find downwash directly beneath a moving aircraft.
You will find it a little way behind, shaped like a long teardrop.
Proportional but not equal to the aircrafts weight.
About a third of potential downwash is lost as wingtip vorticies.
Which is unfortunate because complete span downwash would be useful in some applications (pun intended)
If you have the chance you will see this "footprint" when the aircraft is flown close to some visible media, such as smoke, water or a crop such as wheat.
Half a wingspan or less close, that is, or you will see nothing.
