Aerodynamic Terms Longitudinal axis  is an imaginary line about which  the aircraft rolls; it is also called the roll axis. The  longitudinal axis is not a fixed line through the cart  because the angle of incidence changes in turbulence  and with loading changes.  Chapters 2 and 3 of the  Pilot’s Handbook of Aeronau- tical Knowledge  (FAA-H-8083-25) apply to powered  parachutes and are a prerequisite to reading this book.  This chapter will focus on the aerodynamic fundamen- tals unique to powered parachute (PPC) operations. Angle of incidence  is the angle formed by the chord  line of the wing and the longitudinal axis of the PPC  cart. The cart longitudinal axis is not the same as the  aerodynamic longitudinal axis defined in the previous  paragraph. [Figure 2-2] Unlike an airplane, the angle  of incidence can change in flight because of the flex - ible line attachment between the wing and the cart.  Angle of incidence can change for different types of  flight configurations and PPC designs; this is covered  in detail in the “Moments” section. Airfoil is the term used for surfaces on a powered  parachute that produce lift, typically the wing itself.  Although many different airfoil designs exist, all air- foils produce lift in a similar manner. Camber  refers to the curvature of a wing when look- ing at a cross section. A wing possesses  upper cam- ber  on its top surface and  lower camber  on its bottom  surface.  Leading edge  describes the forward edge of  the airfoil. The rear edge of the airfoil is called the  trailing edge . The  chord line  is an imaginary straight  line drawn from the leading edge to the trailing edge.  [Figure 2-1]  Trim angle  is the angle between the chord line of the  wing and the horizontal plane when the PPC is in non- powered gliding flight. [Figure 2-3] The PPC wing is  designed at a slight angle, with the chord line inclined  downward to the horizontal plane to maintain the  manufacturer-designed angle of attack during gliding,  level and climbing flight. This “trim angle” is built  into the powered parachute by the manufacturer and  cannot be adjusted by the pilot moving the controls. Pitch angle  is the angle the PPC wing chord makes  with the horizontal plane. Pitch angle is what you can  see. Many pilots confuse the pitch angle, which you  can easily see and feel, with the angle of attack which  may not be as perceptible. [Figure 2-4] For example,  the pitch angle in an engine-out glide could be minus  8 degrees, in level flight 10 degrees above the hori - zon, and in a climb it could be 28 degrees above the  horizon. These are significantly different angles you  easily see. Pitch angles are covered in greater detail  in Chapter 6. Relative wind  is the direction of the airflow with re - spect to the wing; it is usually parallel to and opposite  the PPC  flight path . Relative wind may be affected  by movement of the PPC through the air, as well as by  all forms of unstable, disturbed air such as wind shear,  thermals, turbulence, and mountain rotors. When a  PPC is flying through undisturbed air, the relative  wind is parallel to and opposite the flight path.  Deck angle is the angle of the cart’s lower frame  (from the front wheel to the rear wheels), to the land- ing surface. The deck on the lower part of the conven- tional cart frame can be used to visualize deck angle.  An imaginary line between the front and back wheel  axles can also be used on unconventional carts. Angle of attack  is the angle between the relative wind  and the wing chord line. [Figure 2-2] Leading edge Chord line Lower camber Upper camber Trailing edge Relativ wind F light path Figure 2-1. Aerodynamic terms of an airfoil. 2-1