How sensitive to disruption / efficient are modern airliner wings?

Factual Questions
1

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?

2

Paging our resident pilots, who tend to hang out in our Aviation omnibus thread:

@LSLGuy
@Johnny_L.A
@Richard_Pearse

3

Along maybe the first 25% of the top surface from front towards back, the answer is “a lot”. Surface roughness there is a killer of lift.

Farther back, surface roughness only really affects drag. Likewise along the bottom.

The real problem is very small scale roughness, not big. Coating the entire front span of a wing in sandpaper might well be fatal. Having paint peeling off here and there exposing different layers is no big deal.

I have seen lots of really scabrous paint on the underside of wings. No problem.

Something flimsy like a kite would simply be shredded by the airflow if somehow it was tied to a hoist point.

4

A post was merged into an existing topic: Spammer ponam45 posts

5

Got it. If I follow my understanding right (I’m an anthropology major, forgive me) that’s because the boundary layer separating at the leading edge is what cause the wing to stall? Is the definition of a stall? Presumably this is both why the electrical deicing system on the 787 doesn’t cover the entire wing? And, I guess, why the extent of the chord coverage on the ATR was an issue? (Or was seen as one, at least—maybe that was overblown?)

Is this just (or “more”) true for relatively long chords relative to the overall size of the airfoil? Does impact from dust and other stuff kicked up from the ground not immediately mar the leading edge of a helicopter’s main rotor, for example, or do they behave so differently that the comparison is irrelevant?

…I guess, thinking about it, that should’ve been obvious from photos of warbirds in WW2 coming back with holes punched in them, that it would have to cover a significant area and in the right place for the airplane to become unflyable. I wasn’t certain if the necessities of efficiency have made modern airplane sless tolerant to that kind of thing, somehow.

(and thanks, all!)

6

The comparison is relevant. Any given dent from any given impact only matters so much. If you collect enough small dents you’d eventually have a problem. But it needs to be pretty extensive.

Many jetliners now don’t deice the full span of the wing. Not necessary. Which suggests they can lose at least some of the leading edge profile without significant issue.

Efficiency is sure nice. But each individual airplane develops bangs and dents and warpage over its service life. And each individual engine loses efficiency over its service life too. In airline service each plane’s consumption is tracked and as any given airplane slowly becomes less efficient, a small bias is added to its individual fuel planning to account for those losses over factory new spec. Once that number gets big enough to pay for taking the airplane out of service to find and address the causes of its inefficiency that’ll get scheduled.

The sharkskin idea, as well as boundary layer ingestion, are two great laboratory ideas that never seem to make it into production. They’d deliver a very welcome couple - few percent additional efficiency. But both are highly dependent on the wing remaining absolutely pristine; a minor layer of bugs could wipe out the extra gains. The belief is that in actual revenue service those surfaces would not be able to be affordably maintained pristine enough to get the benefit. They’d just add costs to acquisition and to maintenance.

7

I remember back when I was doing private pilot training, there was a bit mentioning that some small aircraft relied more on “laminar flow”, a very smooth wing with mininal disruption to the airflow, flush rivets and a glossy paint job. One safety note said that this was particularly critical cause of an accident involving a Grumman Tiger aircraft; the pilot took off in the dewey morning from a freshly mowed grass airstrip and crashed not long after takeoff because grass clippings stuck to the wings.

8

thx for triggering a thought I carry along for quite some time:

would a golfball (dimple) desgin of a wing or airfoil in general be something desireable (we all have seen the vids what happens if you take the dimples out of a golf ball) … I understand the dimples help, keep the airflow clinging to the object (wing, airfoil, sportscar).

Then or course, reason kicks in … if it were, it would long be standard … the same is true for formula1/nascar/supersportscars …

any thoughts or perspective on that (was it ever done?), w/out creating a new thread?