OK.. make a home-brew wind-tunnel... and we can settle if a wing HAS to produce a downwash to create lift. The crudest of wind tunnels will work well enough.

Here's the wind tunnel plan: get a refrigerator box and a box fan. Cut the side of the rifrigerator box to insert a window of some sort. (you have to be able to see whats happening..) The box fan is set to PULL air out of the refrigerator box.
(The crude wind tunnel will only give about 5 to 7 mph equiv... so you aren't going to do a lot of tests with it...)

Build a rig to hold a flat bottom wing at ZERO AOA.

Build another little stand to hold a cigarette appx 4 inches in front of the LE. (need a smoke trail to observe...)

Light the cigarette and turn on the fan. If you have it all set up correctly... the airflow will pull the smoke right along the upper srface of the wing... and you wll see it droop below the bottom of the wing as it goes off the TE.

Now how the heck did that happen if there is no downwash?

Try again at -2 deg AOA. You STILL get the downwash. (+2 to -2 deg AOA is about all this setup is good to test. You'll stall the wing with much more than that. Airflow is too slow.)