Ed - was it due to the fuselage changing it's angle of attack to a lower drag condition or just the lower drag at that mach number due to the increased wing sweep angle. There is a big drag difference between 26 and 50 degrees sweep especially at .9 mach number. I could see 26 degrees at M=.6 or so but not at .9.

Although we never made a wing sweep airplane at McDonnell Douglas we studied them relentlessly. Lost the F-14 contract with one. Won the F-15 without one. Finally gave up on them.

If the CG of the airplane is located so that at a low sweep angle the airplane had good flying qualities (true there is a lot of electronic wizardry going on in the control system too) it would give an angle of attack of the fuse and a trim setting on the tail.

Move the wing to a high sweep and the neutral point moves aft making the airplane more stable requiring more trim from the horizontal, the CLalpha of the wing decreases so the angle of attack of the airplane goes up to make up for it and indeed the fuselage is at a different AOA.

The induce drag from lift is the same in each case, the trim drag probably increases, the wing sweep drag is significantly lower and the fuselage drag may decrease.

Looking at them I doubt the fuse drag decrease alone is enough to show the fuel flow difference you mention.

It is something to do to pass the boring hours though isn't it. You do have to wonder why the flight manual had you doing it that way.