NAVWEPS 00-BOT-80  STABILITY ,AND CONTROL  slipstream creates a normal force at the plane  of the propeller similar to a wing creating lift  by deflecting an airstream. As this normal  force will increase  with an increase  in airplane  angle of attack, the effect will be destabilizing  when the propeller is ahead of the cg. The  magnitude of the unstable contribution de-  pends on the distance from the c.g. to the  propeller and is largest at high power and low  dynamic pressure. The normal force created  thrust line is below the c.g., thrust produces a  positive or noseup moment and the effect is de-  stabilizing.  On the other hand, if the thrust  line is ,located above the c.g., a negative  moment is created and the effect is stabilizing.  A propeller or inlet duct located ahead of  the c.g. contributes a destabilizing effect. As  shown in figure 4.11, a rotating propeller in-  clined to the windstream causes a deflection  of the airflow. The momentum change of the  261 