Axes of Rotation Moments Ground Effect Motion about the lateral axis or pitch is primarily  controlled by the thrust of the propeller moving the  PPC pitch up (nose up) to climb and pitch down (nose  down) at reduced throttle. Turning happens about the longitudinal axis and is the  result of the rolling motion similar to an airplane with  aileron and rudder control. To turn, pull down the wing  trailing edge on the side you want to turn to with the  steering controls. This creates drag on the correspond- ing trailing edge of the wing, thus dropping the inside  wing, and rolling the PPC into a banked turn. [Figure  2-18] There is not significant turning about the  vertical axis  because the PPC wing is designed to fly directly into  the relative wind just like an airplane. Any sideways  skidding or  yaw is automatically corrected to fly  straight with the wing design. An airplane uses the  vertical tail to fly directly into the relative wind like a  dart. The unique design of the PPC performs the same  function through the combination of wing profile/ taper, the arch or curvature from tip to tip, washout  built into the wing and/or tip stabilizer design. These  factors make the PPC track directly into the relative  wind and eliminate the need for a vertical tail surface  and rudder to make coordinated turns. Designs and  methods vary with manufacturer and wing type, but  all PPCs are designed to track directly into the rela- tive wind. Ground effect is the interference of the ground with  the airflow and turbulence patterns created by the  wing. The most apparent indication from ground ef- fect is the unexpected lift given to an aircraft as it flies  close to the ground — normally during takeoffs and  landings. To understand the pendulum effect, attach a small  weight (a pencil or paper clip works) to a 24-inch  string. Note the weight always wants to hang directly  under where you hold it. If you hold the string still  and move the weight to the side, the weight swings  and stabilizes under where you hold it. Gravity, pull - ing down on the weight to stabilize it directly un- der where it is hanging, is PPC pendulum stability.  [Figure 2-20] Wings generally want to pitch nose down or roll for- ward and follow the curvature of the airfoil creating a  negative pitching moment. This is one of the reasons  airplanes have tails. The powered parachute does not  need a tail because the airfoil is locked into a specific  position relative to the cart by the suspension lines.  [Figure 2-19]. Any pitching moment for the wing is  counteracted by the strong pendulum effect (weight  of the cart hanging directly under the center of lift).  Any swinging of the weight creates moments that  act to stabilize the swing. The wing aids to dampen  swinging. This pendulum effect is unique to the PPC  because the cart has the ability to rotate around the  PPC pendulum axis of rotation in addition to rotating  about the CG. A body that rotates freely will turn about its cen- ter of gravity. In aerodynamic terms for a PPC, the  mathematical value of a moment is the product of the  force times the distance from the CG (moment arm)  at which the force is applied.  Ground effect is usually felt when the wing is at al - titudes of less than half of the wingspan. The typical  PPC wingspan is approximately 38 feet with an aver- age wing height of about 20 feet. Therefore, ground  effect is negligible for PPCs and is typically not a factor. Figure 2-18. PPC axes of rotation.  2-9