NAVWEPS DD-807-80  STABILITY AND CONTROL  nacelles deserves consideration in several in-  stances. Body upwash and variation of local  Mach number can influence the wing lift while  lift carryover and downwash can effect the fu-  selage and nacelles forces and moments.  of figure 4.7 illustrates the change of wing  contribution possible between subsonic and  supersonic flight.  The large increase in static  stability in supersonic flight can incur high  trim drag or require great control effectiveness  to prevent reduction in maneuverability.  HORIZONTAL TAIL.  The horizontal tail  usually provides the greatest stabilizing influ-  ence of all the components of the airplane.  To  appreciate the contribution of the horizontal  tail to stability, inspect figure 4.9. If the air-  plane is given a change in angle of attack, a  change in tail lift will occur at the aerody-  namic center of the tail.  An increase in lift  at the horizontal tail produces a negative  moment about the airplane c.g. and tends to  return the airplane to the trim condition.  While the contribution of the horizontal tail  to stability is large, the -magnitude of the  contribution is dependent upon the change in  tail lift and the lever arm of the surface.  It is  obvious that the horizontal tail will produce a  stabilizing effect only when the surface is aft  of the c.g.  For this reason it would be inap-  propriate to refer to the forward surface of a  canard (tail&St)  configuration as a horizontal  “stabilizer.”  In a logical sense,  the horizontal  “stabilizer”  must be aft of the c.g. and-  generally speaking-the farther aft, the greater  the contribution to stability.  FUSELAGE AND NACELLES.  In most  cases, the contribution of the fuselage and  nacelles is destabilizing.  A symmetrical body  of revolution in the flow field of a perfect fluid  develops an unstable pitching moment when  given an angle of attack.  In fact, an increase  in angle of attack produces an increase in the  unstable pitching moment without the devel-  opment of lift.  Figure 4.8 illustrates the pres-  sure distribution which creates this unstable  moment on the body of revolution.  In the  actual case of real subsonic flow essentially  the same effect is produced. An increase in  angle of attack causes an increase in the  unstable pitching moment but a negligible  increase in lift.  An additional factor for consideration is the  influence of the induced flow field of the wing.  As illustrated in figure 4.8, the upwash ahead  of the wing increases  the destabilizing influence  from the portions of the fuselage and nacelles  ahead of the wing.  The downwash behind  the wing reduces the destabilizing influence  from the portions of the fuselage and nacelles  aft of the wing.  Hence, the location of the  fuselage and nacelles relative to the wing is  important in determining the contribution to  stability.  Many factors influence the change in tail  lift which occurs with a change in airplane  angle of attack.  The area of the horizontal  tail has the obvious effect that a large surface  would generate a large change in lift.  In a  similar manner, the change in tail lift would  depend on the slope of the lift curve for the  horizontal tail.  Thus, aspect ratio, taper,  sweepback, and Mach number would deter-  mine the sensitivity of the surface to changes  in angle of attack. It should be appreciated  that the flow at the horizontal tail is not of  the same flow direction or dynamic pressure  as  the free stream.  Due to the wing wake, fuse-  lage boundary layer, and power effects, the q  at the horizontal tail may be greatfy different  from the 4 of the free stream. In most in-  The body of revolution in supersonic flow  can develop lift of a magnitude which cannot  be neglected. When the body of revolution in  supersonic flow is given an angle of attack, a  pressure  distribution typical of figure 4.8 is the  result.  Since the center of pressure is well  forward, the body contributes a destabilizing  influence.  AS is usual with supersonic con-  figurations, the fuselage and nacelles may be  quite large in comparison with the wing area  and the contribution to stability may be large.  Interaction between the wing and fuselage and  256 