Originally Posted by Mad (Flt) Scientist
The usual oscillatory modes for a conventional aircraft are:
Longitudinal (2)
* Short Period Pitch Oscillation : characterised by a (duh) short period, basically a pitching motion of oscillatory nature with little change to the flightpath. Usually quite well damped.
* Phugoid : a much longer period oscillatory motion, essentially an energy-interchange mode between potential and kinetic energy as the aircraft climbs/slows then decends/accelerates. Usually at near-constant angle-of-attack. Often poorly damped, as it is damped in part by drag, which is usually minimized in design for economic reasons.
Lateral-Directional (3)
* Dutch Roll : a combined rolling and yawing motion, oscillatory in nature, driven mainly by cross-coupling between the aerodynamic forces in roll and yaw (a sideslipped aircraft wishes to roll as well as yaw; a rolling aircraft reacts in yaw as well as in bank). Often poorly damped. Many aircraft have 'yaw dampers' fitted which control rudder inputs to augment damping. Ironically, it's often easier for a human to damp with roll controls, due to lag/timing issues.
* Spiral Mode : a first order mode, invariably unstable, that will if unattended cause a slowly increasing divergence in roll and yaw.
* Roll Mode : another first order mode, closely related to the roll damping of the aircraft. Usually stable.
Those 5 are the ones generally worried about most. (Unless you're dealing with active control/FBW, in which case all bets are off....)
YouŽd expect the spiral mode to be the worst, because it is the one with negative stability, and therefore the mode that will fly the craft into ground...
Originally Posted by Mad (Flt) Scientist
What do deesigners do?
An important question! If I ever find out .... ok, sorry.
Well, the first thing to note is that the stability and control design aspects are rarely primary for a design, and certainly not for a transport category aircraft. Unlike, say, a fighter or aerobatic machine, the degree to which an airliner 'flies nice' is secondary to the economics.
Hm... wouldnŽt there be important economics points about stability?
Pitch stability defines the CoG range - move CoG too far to the rear, and the tailplane AoA equals that of main wing, so that the pitch stability vanishes; move CoG too far forward and the tailplane has inverted stall and the plane falls nose over.
How do you add CoG range to a plane? I suspect it would require enlarging the tailplane - with economic penalty in form of trim drag.