I find this puzzling. In aviation there is something known as the ground effect. The wing pushes the air down, thereby lifting the plane. If the plane is near the earth, the air has less room to go, so it starts “pushing back”, lifting the airplane even more than normal. This makes life difficult for novice pilots, they line up nicely to arrive near the beginning of a runway, gliding down at a gentle angle, and suddenly the plane refuses to go down and they get long, overshooting the runway.
Now wings of a Formula 1 car work along similar lines, just upside down, so by the same argument I would actually expect that the rear wing should work better, not worse, if it has a roof above. The “air has no place to go” argument mentioned above would actually mean that the rear wing produces a markedly stronger downforce.
One logical conclusion is that any argument that explains the loss of downforce for an F1 car in a tunnel would have to fail for airplanes in order to be correct. This makes me wonder: The difference between these two cases might be very subtle, perhaps too subtle to be explainable in layman terms.
I’ve been thinking how to explore differences in configuration between an F1 car and an airplane and here is one idea I had: Perhaps it is not the flow from the rear wing, but the flow front the front wing that is at the root. It goes up, gets reflected down, hits the rear of the car and plays havoc with the flow there, so the rear wing, instead of getting nice smooth laminar flow, gets a load of dirty air and its effectiveness get compromised so much that it loses more downforce than it gains by the “ground effect” from above. Of course I might be totally wrong, I can see a few places where this story is suspect. Perhaps somebody can come up with a better one.
Speaking of tunnels, will we ever live to see somebody driving an F1 car on the roof of a tunnel? It should not be so hard, just find a very long tunnel with a circular crossection, you can enter it driving on the ground and then sort of corkscrew through it. I’m surprised Red Bull did not do it yet. If I ever win a lottery, I’m going to Marussia the first thing in the morning :-).