Originally Posted by
Cubdriver
I do not know a lot about MD11's and it would seem obvious no airplane should have any neutral or unstable characteristics in any axis (roll pitch or yaw), but a statically stable airplane can lose a lot in terms of controllability if it is overly stable. Fly-by-wire controls and artificial stability systems permit optimizing stability to better suit a set of requirements. Transport aircraft like the MD-11 tend to be fairly maneuverable and they handle well enough in statically stable configurations, but fighters do not and it is common to make them statically unstable in one or more axes. I wouldn't assume the FAA refuses to certify designs that are statically unstable without any augmentation, only that they are safe with augmented controls.
Cubdriver has it right, the MD-11 and a few other designs-like the Airbus 340 use tail fuel management to configure for optimum CG during cruise. This reduces drag by limiting the need for stab trim at cruise. An aft CG tends to limit stabilizer drag. This is true for most fixed wing aircraft.
During descent the fuel is drained from the tail and the aircraft enters a normal mid MAC CG condition. Tail or stabilizer size has nothing to do with it.
The aircraft is never unstable or even neutral in stability.
Handling in pitch and yaw is quite precise in all flight regimes, roll can be somewhat truckish(over stable) at slower speeds.
The smaller stab/ elevator is a result of a longer moment arm when compared to its DC-10 siblings.
The MD-11 does have a fairly high wing loading, and this results in both high approach speeds and higher than normal(for most aircraft) rates of descent on a typical 3 degree glide path. Flare and glidepath control is a little more critical. Nothing unstable, it just takes a bit more attention than other designs.