Why do the engine nacelles on the P2V Neptune extend so far back?
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