Originally posted by Sentinel Chicken
There is a 30% reduction in the size of the MD-11 horizontal tailplane compared the tailplane of the DC-10-30. I could be wrong on the reasoning for the size difference, but I seem to recall it has something to do with the longer moment arm from the center of gravity (or is it center of lift?) to the tailplane thanks to the longer fuselage of the MD-11.
The relation of the horizontal stabilizer to the CG is one factor in the reason that the stabilizer of the MD-11 is smaller than that of the DC-10. Basically, the further the two are from each other, the more authority the horizontal stabilizer will have. You can see how the opposite is true on the 747SP which has a noticeable larger horizontal and vertical stabilizer. But that is only half the story. As I understand it, the MD-11 stores fuel in it's horizontal stabilizer and is able to transfer it forward and aft in order trim the aircraft in flight and obtain the optimal CG location for flight. Because the MD-11 operates in a much tighter CG range, it does not need as large of a horizontal stabilizer to counteract large nose down pitching moments. As the others have mentioned, by reducing the size of the horizontal stabilizer you reduce drag making the aircraft more efficient. Also by moving the CG aft by pumping fuel into the tail of the aircraft, you will also make the aircraft more fuel efficient (but at the price of stability).