I had a pre-dawn flight on an Airbus today, and found staring at the top of the wing, and the hoist points you can see here: https://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Airbus_A319_left_wing_during_the_landing._S7_Airlines%2C_Moscow-Pavlodar._May_2009.JPG/2560px-Airbus_A319_left_wing_during_the_landing._S7_Airlines%2C_Moscow-Pavlodar._May_2009.JPG
In my sleep-deprived haze, for some reason I started imagining something—like a child’s kite or a balloon—getting snagged and pulled onto the wing’s upper surface. …Also, the trailing edge of the wing looked pretty dirty. It got me wondering how sensitive and/or precise modern airfoils actually are.
I know that icing can be dangerous, and there are lots of examples of engine separation being catastrophic. But, particularly in the latter case, it’s not clear to me how much of that is from the wing surface being altered by itself and how much is from damage to internal structures / control surfaces / hydraulics, etc.
The A-319 has a wing area of 124m². If a 1m² kite (toy) got its string caught in the hoist point and pulled against the wing surface and stuck there, would that do anything meaningful? What if it was a plastic garbage bag or something else a little more 3-dimensional? Say, a flying creature hooking its claws in to hitch a ride instead of sliding off like a pigeon?
Trying to think of how to phrase this more formally: how much would the surface of a modern airliner’s wing need to be perturbed in order to be consequential? “Consequential” meaning that the plane would behave unexpectedly—stall uncharacteristically, say, or the pilot would notice having to adjust the trim. Does it have to be enough that the airflow separates completely? In the “object gets plastered to wing” scenario, is the amount of wing area impacted most significant, or its location along the chord, or the fraction of spanwise flow, or something else entirely?
Periodically you hear about surfaces designed to mimic sharkskin, and I know that efficiency is very important for airlines now. But obviously the wings aren’t e.g. stippled like golf balls, or resurfaced before every flight, so bare, mostly smooth metal with whatever tiny abrasions you get from dust and other particles must be good enough without engineering for microscopic tolerances, but how far does that go? Are there rules about painting wings?