Are we talking about a problem in control or stability?
An increase in fuselage length increases the weight of the aircraft proportionally. With an unchanged wing area this will increase the wing loading. The relationship between wing loading and dynamic pressure is ½ dv^2, so a higher wing loading will give a higher stall speed which will necessitate higher landing speeds. Lower ceilings and cruise speeds are also present when increasing the fuselage length without changing the wing. The higher the weight per unit length of wingspan, the faster the downwash, and the less efficient the horizontal stabilizer if it is affected by wing downwash (i.e. not a T-tail).
The larger the aircraft in terms of mass and inertia the larger the tail control and stabilizing surfaces required. Increased inertia makes the aircraft more sluggish in response to control inputs. If upset from its original state, greater control authority is needed to damp and oppose the motion. Just as it takes more of a force to stop a spinning car tire (tyre if you prefer) then a bicycle tire. F=mv^2/r
If we add appropriate plugs in the fuselage, without changing the control surfaces we will require larger deflections of those surfaces to initiate a change. This is as above, to overcome the increase in mass and inertia. As the change takes effect, a larger damping force will be required to bring the aircraft back to equilibrium. From the pilots perspective it will seem sluggish in control and the short period pitch oscillation (SPPO) will not be as highly damped.
[Edited: This is the second edit, changed format and key concepts, deleted phugoid and eigenvalue comments, thanks all for their tolerance]
Last edited by '%MAC'; 14th Apr 2002 at 01:27.