T. Runaway,
"First of all, the air (at high volume and velocity) is bouncing off walls, creating drag."
Air does not "bounce of walls". It's a fluid, remember? "The air slides along the walls" would be a more appropriate description. You'll lose a bit of energy to friction but not very much.
Consider how much longer pipes are used every day without much resistance at all. How much harder is it blowing through a pice of rubber house if you bend it into a U (without deforming it) than if you keep it straight? <img src="wink.gif" border="0">
Take a look at cut-through diagrams of jet engines. Many have the air going back and forth. There are engines with the air flowing backwards through the combustion chamber, turning at each end. You've seen those small turboprops with the exhausts right behind the propellers, right? Those take in air, turn it around, send it forward through the engine and turn it around again before letting it out. All with very little loss of efficiency.
"Second, would the shape of the duct not generate lift and thus an up-load on the tail, which would require an even greater down load (and more drag)?"
That's a negative. In the end, no air is accelerated downwards since it is all being let out through the engine in the same direction it came in from.
"I have often wondered if the DC-10/MD-11 layout with a straight duct was more fuel efficient."
You probably lose a bit of pressure in the inlet. On the other hand, the DC-10 probably has a significantly heavier tail structure to carry around the world which also adds to fuel costs. What's to prefer? I don't know and I don't think there's much of a difference. Won't lose sleep over that one anyway - even though it could be fun to read a more detailed analysis. <img src="smile.gif" border="0">
"The DC always seemed to cruise more level than the L10, at least from the perspective in the cabin."
Ramp rumours has it that the DC10 should have a different angle of incidence... it's a fact that I spent lots of morning hours shoveling cargo in and out of the bulk cargo bay in the tail to get the W&B right. :/
Cheers,. . /ft
[ 24 January 2002: Message edited by: ft ]</p>