How does an a/c fly invertedly?

[beardy]

Thank you so much for the link, it makes it all clear. Apart from the windsock!

bookworm,

Thanks for your effort I now believe you about surface pressure, I am sorry I should not have doubted. I had forgotten the question of a freighter full of budgies (does it weigh less if they are roosting or flying?) The point about closed system or elements of a system does not appear to be very important now.