Exhaust Stains

[Gary]

Ok here's a question from a guy who hasn't got a clue but is trying to attach logic to something.

Exhaust stains on an airplane models with inline engines tend to always point downward from the exhaust port. Yet there is all this prop wash, so much so that real life pilots had to account for the wash and tourque with opposing rudder. (is that right? I've read this in dozens of books about taking off and before the aircraft is properly trimmed)

My question is this. With all the propwash corkscrewing around the fuselage how come the exhaust stains are mirror imaged on either side? Shouldn't one side be at least slightly higher than the other?

I like asking these kinds of questions of you guys because no one here makes you feel too stupid about such a question.

[sotoolslinger]

I can't explain it scientifically but I weather aircraft by looking at real AC or pics of real ones and I have not seen evidence of this. If it is present you can not see it in scale.

[sequoiaranger]

>I like asking these kinds of questions of you guys because no one here makes you feel too stupid about such a question.<

There really isn't a "stupid question". If you WONDER about something, that is to your credit already.

>Exhaust stains on an airplane models with inline engines tend to always point downward from the exhaust port.<

If an exhaust port points slightly downward, the exhaust is thrust slightly downward, too, but not by much! Within a millisecond the exhaust will begin being blown in the opposite direction the plane is heading. Exhaust is just a gas, and has so little mass that it gets deflected easily. At idle, with a slowly-turning prop, MAYBE there is a slight extension of the exhaust "stain" in the direction of the exhaust manifold port, but we're talking maybe an extra inch at most. If you see a stain "down" a few inches from the port, it is mainly because the port itself deflects the wind down a bit, and the exhaust follows. With exhaust ports that are near inline with the wing, you will sometimes see exhaust stains that follow the airfoil and curve up and over the wing.

>My question is this. With all the propwash corkscrewing around the fuselage how come the exhaust stains are mirror imaged on either side? Shouldn't one side be at least slightly higher than the other?<

Though logic might think so, the fact is that the "corkscrewing" effect is minimal, and in relation to the wind thrust once the plane gets going, minuscule. The vector of wind forces is STRAIGHT BACK along the line of flight.

In my own opinion, exhaust stains are frequently overdone on models. Even "dirty" engines spew few particulates that would stick to the side of a fast-moving airplane. One can find examples of massive "stains", but I think usually the "stain" is a HEAT effect on the paint, not the "dirt" in the exhaust. And exhaust stains should be "soft" and subtle, not streaks, like oil. So I wouldn't recommend using PAINT to try to duplicate exhaust stains. I use charcoal pastels rubbed on fine sandpaper, then the powder dabbed on behind the exhausts with a brush, fading quickly with distance.

As always, detailed examination of actual photographs gives the best clues. Unfortunately for modelers, warbirds still around are usually meticulously maintained and will not show typical combat wear, so you can't go by them.

[Daryl J.]

Had the digital camera been along, there would have been a photograph taken of the Pilatus PC-12 I was on yesterday.   The (heavy!!!!!) stains involved almost the whole sides of the aircraft and about 1/2 the wingspan!    It was pretty cool looking.    Some of it of course was mud from all this terrible PacNW weather but it seemed to give a foundation for the exhaust stains to stick.    The pattern did not follow the exhaust stacks at all.



HTH,
Daryl J.

[PR19_Kit]

There's quite a few photos of Merlin engined Spitfires, usually desert schemed versions, where the exhaust stains on both sides of the aircraft go up after leaving the ports. Then they follow what I'd always assumed was the airflow over the wing root, up and back and eventually down again before fading out completely.

I've also noted other types where the exhaust stains follow a similar path, but at this time on Christmas night I'm darned if I'm going to look up references........

[kitnut617]

I think it all depends on the airflow, this pic of a Lanc clearly shows the staining on the sides of the nacelles 'below' the wing but then it changes direction and goes over the wing.

http://www.flygplan.info/images/avro_lancaster800.jpg

[Scooterman]

Here's some Spad pics I took this fall at the Alliance airshow.  Not heavy like you're use to seeing but if you look hard enough you can see the stains.  Also note the the heavy areas are not stains but the paint actually burnt off.

[Weaver]

On jet aircraft, a lot of the very conspicuous stains you see are actually from APU exhausts which are running when the aircraft is stationary. The dirty fins on Tornados are a classic example.

[PR19_Kit]

On jet aircraft, a lot of the very conspicuous stains you see are actually from APU exhausts which are running when the aircraft is stationary. The dirty fins on Tornados are a classic example.

Hm, I don't think so......

Tonka dirty fins are mostly caused by the upper thrust reversers blowing the exhaust over the fins.

They don't have brake chutes and reverse is de riguer for Tonkas every time they land. I've seen this effect with my own eyes numerous times at Lossiemouth, where my daughter was the Caravan Controller for three tours and I got to sit in there with her on many occasions.