Why do the engine nacelles on the P2V Neptune extend so far back? I was studying a picture of a P2V Neptune seen from below, and what struck me was how far back the engine nacelles reach. Unfortunately, I can't find that original photo but this one shows what I'm talking about:

Image source
Unlike all the other piston-powered planes I've seen, the engine nacelles actually extend past (what I presume to be) the flap hinge line, ending behind the trailing edge of the wing. There is a noticeable gap in the nacelle to allow the rear part to rotate with the flap. Since the relatively flat radial engines don't even reach back to the leading edge, and the landing gear are situated far forward from the back of the nacelle, I don't see what advantage these elongated nacelles give. I doubt they help reduce drag, as they add surface area and don't end smoothly.
That shape was adapted after wind-tunnel tests showed the original less-tapered nacelle creating downforce, which meant the rest of the wing had to deal with more upward bending (lift distribution more toward the tips). To solve this problem, the beaver-tail-shaped end was used – reducing the flow separation that caused the downforce. The thrust-axis was also tilted 3 degrees down to reduce the overall bending moment (like how the weight of the fuel does that for big jets, here it's a tiny bit of downward thrust).
Nacelle-extension isn't unique to the P2V; you can find it on the B-25 bomber for example, although not as flat.




Wassall, J. B. &quot;The Development of the Lockheed P2Y Neptune: A Paper by a Senior Engineer of the Lockheed Aircraft Corporation Describing the Evolution of a New Aeroplane.&quot; Aircraft Engineering and Aerospace Technology (1950). https://doi.org/10.1108/eb031906