So I searched and couldn’t find this topic, but I’m sure it’s been covered, so I apologize if I’m being dense and can’t find it.
In working on a P-51, I was looking for exhaust streak photos. I found a few, but most seem clean. It looks like when exhaust streaks are present, they follow the curvature of the wing (makes sense - that’s the airflow) so they flow gently up, then back down, ending around the cockpit.
The all-metal birds tend to have discoloration (even a photo of a modern one), but the OD ones look surprisingly streak-free. Were these aircraft cleaned after missions? Do they really not get that dirty regarding exhaust staining/streaking? Is that something us modelers have done to jazz them up, but maybe doesn’t happen so much in real life?
Here are some pics to illustrate what I mean. Some are potentially publicity shots and maybe the aircraft were cleaned first, but others just aren’t.
And here are the dirty ones (much harder to find online, at least)
So what’s the deal? Were some areas of operation more prone to exhaust issues (because of fuel type/carburetor settings, etc.?), were they cleaned after each flight? Were they more common early in the war when they were used a little harder?
Or is it just random?
I’d love to know.
Thanks for reading.
-BD-
In the photos that you posted, the two dirtiest ones are both Allison engined Mustangs. That most likely has be a factor. Another factor is gonna be fuels used, as I do recall reading somewhere about fuel refining having differences in lead content and such. I seriously doubt that aircraft were cleaned after flights during busy periods of operations. Of course warbirds on the airshow circuits are usually immaculately maintained by their owners. They are big investments after all.
Very cool pics Brandon. Unless its a P-38, I’ve always gravitated towards subtle exhaust streaking. Such as the peeling on Japanese planes, I think heavy staining also detracts from the overall look. I generally add a touch more XF-1 to my cup after the post shade and thats about it. Sometimes a touch of XF-19 at the pipes to simulate the heat burns. Thats is assuming there is no photographic evidence of your particular subject that shows otherwise. The NMF Merlin powered P-51 had that dark colored metal panel aft the pipes (seen on your first pic) which hides the staining anyways… my two cents…
Joe
It depends a lot on the service situation. In peacetime, the planes are kept cleaner because ground crews have the time for it. During an intense war situation they have other, more important things to do than clean airframe. Also, during war, engines may not see as much service also- run them longer between overhauls or major service. In peacetime squadron commander’s aircraft may be especially clean and polished 
FWIW I was reading a Squadron book (Focke Wulf FW-190 in Action) and it mentioned the early variants (the A1 I believe) all had heavy exhaust streaking due to engine overheating issues. Thanks for the comments. I appreciate them. -BD-
I would add some since it looks cool! Just don’t over do it
heres a couple more pics
as mentioned a lot depends on the fuel.
Late war Luft’ aircraft used more coal based fuels which were very sooty.
US planes depend a lot on the type of engine and ground crew dedication to a clean plane.
P-51’s had light tan whitish exhaust streaks after a sortie and dont forget the oil breather streak as well