The outboard engine nacelle of a B-52 appears to have a change of shape too extreme for just blending to the center-line of the engine pod. Why is the B-52 outboard engine nacelle shaped the way it is?
The B-52 was build in different versions (A-H) and the engines and their installation differ between these version1.
The image in the question most likely shows the engine configuration of a B-52H.
The following drawings suggest that this also changed the inlet design2.
There seem to be different engine-designations depending on the application. For the B-52H, JT3Ds (TF33-P-3) were used3.
Pictures of the TF33-P-3 inlet show they are symmetrical. (While the question is focused at the outboard engine-inlet, this question can only be answered taking the other engine and the placement relative to the wing into account as well).
At the symmetry-plane the TF33-P-3 inlet-section extends further than the rest of the inlet-lip.
I would guess that there are multiple reasons and considerations which led to this design. Three of which will most likely have been:
- Basic aerodynamic considerations like stagnation-point and pressure distribution will prefer a sharp/point nose over a flat area. Especially since the air needs to be diverted from the engine-pylon.
- The cruise speed of the B-52H is Mach 0.914 which means the nose could also be some kind of anti-shock-body reducing wave-drag by applying the area-rule.
- The nose between the two engines prevents aerodynamic interaction of the two engines, for example during cross-wind operation.
The negative aspects of placing two jet-engines close to each other were also discussed in another SE-question with respect to commercial airliners.
A lot of pictures of this intake make it quite hard to determine what is the exact geometry. But the one below is by far the best - thank you @ymb1.
Before, it was hard to tell exactly what's happening. In some pictures, it looked like the inboard engine was slightly further forward of the outboard engine.
But that is now shown to be incorrect. The above picture shows where the two intakes touch, the duct protrudes further forward, with a sort of "nose". This can be clearly seen from the parts of the inlet lit by the sun, and the bump in the shadow of the engine on the ground.
I would strongly expect this is to keep the airflow smooth, and reduce aerodynamic interference between the two inlets, and avoid turbulence that would cause inlet pressure distortion. This is, to avoid a difference in total pressure for each engine, between the side that is adjacent to the other engine, and the side that isn't. Uneven total pressure, or inlet distortion, can cause compressor stalls.
That looks like an optical illusion due to the angle to me. A photo on this site shows a more frontal view of the outboard engines, with just the equal centre division between the two engines appearing. Close-up underneath.
Why does the B-52 outboard engine nacelle have a sharp change in shape?
Not just the outboard, but all 8 engines on a B-52 have the same inlet design.
The original engines had a smaller fan diameter and symmetrical round inlets.
I suspect going to a larger fan diameter created flow problems which the engineers solved by modifying the inlets.
This inlet design was already used on the Convair B-36 D, which had twin turbojets added on the outer wing to give it a higher top speed when penetrating enemy airspace. During cruise, only the six piston engines would run and the jet engine inlets were plugged to reduce their aerodynamic drag. This plugged state is shown in the picture below (picture source):
Since the B-36 was a purely subsonic design, any intention of area ruling in this design can be confidently ruled out (pun intended).
Seems the Convair engineers knew something that the Boeing engineers didn't. But in Seattle they caught up eventually, and the larger airflow of the P&W TF-33 turbofans made them use the same trick from the Boeing B-52 H on. All versions before used the J57 turbojet and the simple B-47 style nacelle shape (B-52 F below, picture source):
The reason is the distorted airflow from spillage if one engine of a pair has to be shut down. By pulling the divider forward in the center of the nacelle, any spillage will occur away from the running engine, ensuring symmetric intake flow there.