NAVWEPS OO-BOT-BO  STABILITY AND CONTROL  not vary with C, since all changes in lift would  take place at the c.g.  In this case, the wing  contribution to stability would be neutral.  When the c.g. is located behind the a.c. the  wing contribution i,s unstable and the curve  of C, versus CL for the wing alone would have  a positive slope.  CONTRIBUTION OF THE COMPONENT  SURFACES. The net pitching moment about  the lateral axis is due to the contribution of  each of the component surfaces  acting in their  appropriate flow fields. By study of the con-  tribution of each component the effect of each  component on the static stability may be ap-  preciated. It is necessary to recall that the  pitching moment coefficient is defined as:  Since the wing is the predominating aero-  dynamic surface of an airplane, any change in  the wing contribution may produce a sig-  nificant change in the airplane stability.  This  fact would be most apparent in the case  of the  flying wing or tailless airplane where the wing  contribution determines the airplane stability.  In order for the wing to achieve stability, the  c.g. must be ahead of the a.c.  Also, the wing  must have a positive pitching moment about  the aerodynamic center to achieve trim at  positive lift coefficients. The first chart of  figure 4.7 illustrates that the wing which is  stable will trim at a negative lift coefficient if  the C,,, is negative.  If the stable wing has a  positive C,,, it will then trim at a useful posi-  tive CL. The only means available to achieve  trim at a positive CL with a wing which has a  negative C,,, is an unstable c.g. position aft of  the ax.  As a result, the tailless aircraft  cannot utilize high lift devices which incur  any significant changes in C,,,.  Thus, any pitching moment coefficient-re-  gardless of source-has the common denomi-  nator of dynamic pressure,  q, wing area, S, and  wing mean aerodynamic chord, MAC.  This  common denominator is applied to the pitch-  ing moments contributed by the fuselage and  nacelles, horizontal tail,  and power effects  as well as pitching moments contributed by  the wing.  WING.  The contribution of the wing to  stability depends primarily on the location  of the aerodynamic center with respect to the  airplane center of gravity.  Generally, the  aerodynamic center-or a.c.-is defined as the  point on the wing mean aerodynamic chord  where the wing pitching moment coefficient  does not vary with lift coefficient.  All changes  in lift coefficient effectively take place at the  wing aerodynamic center. Thus, if the wing  experiences  some change in lift coefficient, the  pitching moment created will  be a direct  function of the relative location of the a.c. and  c.g.  WhiIe the trim lift coefficient may be altered  by a change in c.g. position, the resulting  change in stability is undesirable and is unsat-  isfactory as a primary means of control.  The  variation of trim CL by deflection of control  surfaces is usually more effective and is less  inviting  of disaster.  The early attempts at  manned flight led to this conclusion.  When the aircraft is operating in subsonic  flight, the a.c. of the wing remains fixed at the  25 percent chord station.  When the aircraft  is flown in supersonic flight, the ax. of the  wing will approach the 50 percent chord sta-  tion.  Such a large variation in the location  of the a.c. can produce large changes in the  wing contribution and greatly alter the air-  plane longitudinal stability.  The second  chart  Since stability is evidenced by the develop-  ment of restoring moments, the c.g. must be  forward of the a.c. for the wing to contribute  to positive static longitudinal stability.  As  shown in figure 4.6, a change in lift aft of the  c,g. produces a stable restoring moment de-  pendent npon the lever arm between the a.c.  and c.g.  In this case, the wing contribution  would be stable and the curve of CM versus CL  for the wing alone would have a negative slope.  If the c.g. were located at the a.c., C, would  252  M  ‘“=qS(MAC) 