NAVWEPS OO-SOT-80  STABILITY AND CONTROL  of oscillation which coincide with the pilot-  control system response  lag.  Also, the high 4  flight  condition provides the aerodynamic  capability for failing flight loads during the  oscillation.  If a pilot induced oscillation is encountered  the pilot must rely on the inherent dynamic  stability  of the airplane and immediately  release  the controls. If the unstable excitation  is continued, dangerous oscillation amplitudes  will develop in a very short time.  ROLL COUPLING  The appearance  of “inertia coupling” prob-  lems in modern airplanes was the natural result  of the progressive change in aerodynamic and  inertia characteristics to meet the demands of  high speed flight.  Inertia coupling problems  were unexpected only when dynamic stability  analyses did not adequately account for the  rapid changes in aerodynamic and inertia  characteristics  of airplane configurations. The  The term of “intertia coupling”  is somewhat  misleading because the complete problem is  one of aerodynamic as well as inertia coupling.  “Coupling”  results when some disturbance  about one airplane axis causes a disturbance  about another axis. An example of uncoupled  motion is the disturbance provided an airplane  when subjected to an elevator deflection. The  resulting motion is restricted to pitching  motion without disturbance in yaw or roll.  An example of, coupled motion could be the  disturbance provided an airplane when sub-  jected to rudder deflection. The ensuing mo-  tion can be some combination of yawing and  rolling motion.  Hence, the rolling motion is  coupled with the yawing motion to define the  resulting motion.  This sort of interaction  results from aerodynamic characteristics  and is  termed “aerodynamic coupling.”  A separate  type of coupling results from the  inertia characteristics  of the airplane conligura-  tion.  The inertia characteristics of the com-  plete airplane can be divided into the roll, yaw,  315  When the airplane is rotated about the  inertia axis no inertia coupling will exist but  aerodynamic coupling will  be present.  Part  C of figure 4.34 shows the airplane after rolling  90” about the inertia axis. The inclination  which was initially the angle of attack (a) is  now the angle of sideslip (-6).  Also the  original zero sideslip has now become zero  angle of attack. The sideslip induced by this  90° displacement will  affect the roll rate  If the airplane shown in figure 4.34 were in  some flight condition where the inertia axis  and the aerodynamic axis are alined, no inertia  coupling would result from rolling motion.  However, if the inertia axis is inclined to the  aerodynamic axis, rotation about the aero-  dynamic axis will create centrifugal forces and  cause a pitching moment. In this case, a  rolling motion of the aircraft induces a pitch-  ing moment through the action of inertia  forces. This is “inertia  coupling”  and is  illustrated by part B of figure 4.34.  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  wind direction, and (2) the inertia axis is  through the c.g. in the direction of the two  element masses. This axis system is illus-  trated in figure 4.34.  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 the pitch and yaw 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. 