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This is a piece of this image from Wikipedia

enter image description here

Note the circled region. Movies often show exhaust fumes coming from those pipes when the engine is started. So I assume these are actual exhaust pipes. This design doesn't appear very often, especially in modern piston engine airplanes. Why is this the case?

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  • $\begingroup$ I think a lot of it has to do with noise. This design was popular on propeller driven fighter (military) aircraft that didn't care about noise. $\endgroup$
    – Ron Beyer
    Commented Jan 27, 2016 at 14:21
  • $\begingroup$ also, this aircraft has a V12 engine ( en.wikipedia.org/wiki/Mikulin_AM-38 ) like the Merlin in the Spitfire, so the layout makes sense. with radial engines and flat opposed engines, this type of exhaust configuration wouldn't make sense $\endgroup$
    – rbp
    Commented Jan 27, 2016 at 14:31
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    $\begingroup$ Compare to other aircraft of the same period, I think you'll find it very common indeed. $\endgroup$
    – egid
    Commented Jan 27, 2016 at 22:37

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This engine is about 10 times more powerful than a modern average piston engine. With such power you get some useful thrust from exhaust power directly. This design helps to recover it. With a small SEP, noise and keeping exhaust smoke away from the cabin probably is more important. And it's not true that it's not popular. It's popular with single engine turbines.
Source: Wikimedia (click for link)

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  • $\begingroup$ Or multi-engine turbines, for that matter. $\endgroup$
    – J W
    Commented Jan 27, 2016 at 20:31
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    $\begingroup$ I don't actually think there was much thrust to extract. The key point was not creating resistance inside the exhaust pipes themselves. $\endgroup$
    – Jan Hudec
    Commented Jan 28, 2016 at 14:27
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It was popular at that time. All the Curtiss P-40 Warhawk, North American P-51 Mustang, Hawker Hurricane, Supermarine Spitfire, Hawker Tempest, Messerschmitt Bf109 and many more had similar.

That kind of exhaust was tied to the engine, a water-cooled 2-line 12-cylinder (V-12) and later some 4-line 24-cylinder (H-24 or X-24) engines. So there was a row of cylinders on each side and each had its own exhaust pipe as short as possible to minimize its resistance.

Since then, anything with more than 6 cylinders was replaced by turbines and all the smaller engines are air-cooled, because in GA aircraft the simplicity of air-cooled engine is more useful than the slightly lower drag of water-cooled one (navy preferred air-cooled even in fighters for the same reason). And air-cooled engines generally have their exhaust pipes lead into the cooling air stream and thus hidden under the cowling.

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  • $\begingroup$ Basically anything with a V-12 had more or less that setup. Anything with a Merlin or other RR engine, all the Allison and Packard V-12s in the US, all the V-12s in Soviet and German fighters... the list goes on. Pick a V-12 engined fighter from WW2 and you'll see this exhaust style. $\endgroup$
    – egid
    Commented Jan 28, 2016 at 22:05
  • $\begingroup$ @egid, what surprises me is that there didn't seem to be any V-10, V-14, V-16 and other sizes. It was all V-12 and later some H-24. They exist in other domains, but not in aviation. $\endgroup$
    – Jan Hudec
    Commented Jan 28, 2016 at 22:33
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    $\begingroup$ Yeah, maybe related to nighttime bombing use? They used Mosquitos as pathfinders, perhaps they wanted to minimize warning at low altitude? $\endgroup$
    – egid
    Commented Jan 28, 2016 at 22:55
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    $\begingroup$ @egid, probably. After all, in a sense it was the first stealth aircraft. $\endgroup$
    – Jan Hudec
    Commented Jan 28, 2016 at 23:12
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    $\begingroup$ I believe they were called flame dampers. Exhaust flames were visible at night. $\endgroup$
    – Brinn Belyea
    Commented Jan 29, 2016 at 1:16

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