So the obvious question is, if they knew why they put such a large tailplane on the thing in the first place, what kind of logic prevailed to remove it for the MD-11- a fact made worse by the MD-11s fuselage stretch and consequent need for more elevator authority due to the change in the moment arm calculus..
Slats One, I flew the '10 for 12 yrs, including 6 as a trainer. Douglas did not want to build de-icing equipment into the DC10 tail assembly, so they oversized it in case icing trials showed a tendency toward heavy ice buildup - in the event it didn't. (You have to remember that it was designed at the end of the '60's)
When McDD designed the MD11 they kept the DC10 wing and put winglets on. The horizontal stab was reduced in size as it was clear that the DC10's stab was indeed too large and led to issues such as weight, drag & structural as outlined above. Launch customers said the a/c would not achieve its payload/range targets so they "tweaked" the wing with aerodynamic tricks. I believe LSAS was added during testing as a result of initial flight tests, but am not 100% sure.
The result was a
reasonable product but one that required a deft hand on the stick