IIRC there was a Tupolev by someone years ago which modelled icing of the windshield, and I blv the L-39 by Lotus does the same for the whole canopy
As does the superb Eterndard IV model by RFN ...
Clever - never thought much of these kinds of effects, but they could be interesting ... could have other implications - oil on surface from a damaged engine, as mentioned with the RA Spitfire.
Thinking further outside the box - and my idea comes from the Virtavia F-105, which has strike/post strike models (but no releasable weapons). On many recent models that do have releasable weapons, perhaps there could be a way that with weapons loaded, model is clean (clean as in not dirty, aircraft has been cleaned before launch); once weapons are fired, (ie after mission, gun muzzle residue, oil, maybe even some light flak damage).
Obviously, the logic is a bit weak - it certainly doesn't follow that every mission where weapons are launched would result in getting dirty or damaged - but if you're really after that kind of effect, that's one way to trigger it. You could trigger it to fuel state (30% = damage), but that's just as weak, because if you air refueled, you'd get "clean" again. Similarly, some 60s era aircraft launched with partial tank loads than air-refueld after taking off (early S.1 Buccs, as an example), so then you'd launch "dirty/damaged" and then get clean again.
On the civilian firebomber side of things, you could mirror the above process so that after releasing your retardant, extra smoke, dirt, weathered/blistered paint appears.
A subtler , more authentic effect might be when when the touchdown_dirt effects get triggered, that you get some dirt on your aircraft ... for STOL and bush aircraft, might be a bit less contrived.
Sounds like a mountain of work, and likely more applicable to a combat-type sim than FSX ... but then again, it was not that long ago that I recall thinking that external pilot animations were hokey and contrived ... and now they're de rigeur. Just keeping an open mind ...
DL
In the summer months, you just can't avoid it...sometimes (if you let is build up) it gets so thick, it's difficult to clean even with a power washer! I would love to see the bugs build up on the leading edges, spars, wings, gear, flaps, antennas, vertical stab...etc.
