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## Axes of Rotation 

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 

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. 

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. 

## Moments 

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. 

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. 

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] 

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