|Quoting 2H4 (Reply 2):|
I wonder if the switch from the -100s oval duct to the -200s circular duct had anything to do with this
Sure that change has something to do with the surge tendency at high pitch angle.
Anyway, I would assume that the most important change is that the bottom of the -200 intake is lifted quite a few inches higher above the fuselage - more out of the turbulent boundary layer.
Since the intake was changed from oval to circular, then there are probably much more important changes in the internal shape of the S-duct. To produce an efficient S-duct and make it work well at high subsonic speed is a very difficult task. It is far from just a bent tube. That's the reason why Douglas "gave up" on that and accepted the weight penalty of placing their #2 engine high on the DC-10.
Never mind how well an S-duct is designed, it will always mean a slight efficiency penalty compared to a straight intake. So maybe Douglas was right assuming that the weight penalty of the extra structure for the high engine was minor compared to the S-duct engine efficiency penalty.
Anyway there were people 35 years ago who insisted that Douglas with the high engine made a shortcut in order not to be too late on the market compared to the head on competitor Lockheed L-1011 Tristar. Saving the time and engineering resources to design and test a good S-duct. Time would show that it was hardly needed since the Tristar suffered massive EIS delays caused by the RB
-211 engine development troubles and the RR