text: of oscillation which coincide with 
  the pilot- 
  control system response lag. Also, the high 4 
  the aerodynamic 
  flight condition provides 
  capability for failing 
  loads during the 
  flight 
  oscillation. 
  
  and pitch inertia and each inertia is a measure 
  of the resistance to rolling, yawing, or pitching 
  acceleration of the airplane. The long,slender, 
  high-density fuselage with short, thin wings 
  produces a roll inertia which is quite small in 
  comparison to 
  inertia. 
  These characteristics are typical of the modern 
  airplane configuration. The more conventional 
  low speed airplane may have a wingspan 
  greater than the fuselage length. This type of 
  configuration produces a relatively 
  large roll 
  inertia. A comparison of these configurations 
  is shown in figure 4.34. 
  
  If a pilot induced oscillation is encountered 
  the pilot must rely on the inherent dynamic 
  the airplane and immediately 
  stability of 
  release the controls. 
  If the unstable excitation 
  is continued, dangerous oscillation amplitudes 
  will develop in a very short time. 
  
  ROLL COUPLING 
  
  Inertia coupling can be illustrated by con- 
  sidering the mass of the airplane to be con- 
  centrated in two elements, one representing the 
  mass ahead of the c.g. and one representing the 
  mass behind the c.g. There are two principal 
  axis systems to consider: (1) the aerodynamic, 
  or wind axis is through the c.g. in the relative 
  the inertia axis is 
  wind direction, and (2) 
  through the c.g. in the direction of the two 
  element masses. This axis system is illus- 
  trated in figure 4.34. 