Powered Parachute Flying Handbook

Chapters 2 and 3 of the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25) apply to powered parachutes and are a prerequisite to reading this book. This chapter will focus on the aerodynamic fundamentals unique to powered parachute (PPC) operations.

Aerodynamic Terms

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 airfoils produce lift in a similar manner.

Camber refers to the curvature of a wing when looking at a cross section. A wing possesses upper camber 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]

Relative wind is the direction of the airflow with respect 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.

Angle of attack is the angle between the relative wind and the wing chord line. [Figure 2-2]

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.

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 flexible 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.

Trim angle is the angle between the chord line of the wing and the horizontal plane when the PPC is in nonpowered 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 horizon, 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.

Deck angle is the angle of the cart’s lower frame (from the front wheel to the rear wheels), to the landing surface. The deck on the lower part of the conventional 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.

from above. The PPC wing comes in two wing planforms: rectangular, and elliptical. [Figure 2-5] The elliptical planform leading and trailing edges are curved to form an elliptical shape when viewed from the top or bottom. These two shapes have unique flying characteristics. Rectangular wings typically produce more drag, are lower-performance, and do not move fore and aft, relative to the cart, as quickly as elliptical wings. These characteristics are more obvious when the wing is inflating, during pitch changes, and when flying in turbulence. Rectangular wings are therefore more stable and require less effort to fly. Elliptical wings are higher-performance and more efficient due to less drag. Elliptical wings react more quickly with changing conditions and require greater pilot experience and skill during inflation, in turbulent air, and with abrupt throttle changes.

Aspect ratio is the wingspan divided by the average chord line. A PPC with a common 500-square foot rectangular wing (about a 38-foot wingspan) and with a typical mean chord line of 13 feet, would have an average aspect ratio of about 3. This relatively low aspect ratio is less efficient at producing lift. An elliptical wing with the same 500 square feet and a 45-foot wing span and an 11-foot average chord would have an aspect ratio of about 4. The PPC wing is similar to airplane wings in that the aspect ratio will differ with the specific design mission for the aircraft. Generally, rectangular wings have lower aspect ratios and lower efficiency than the higher aspect ratio and higher efficiency elliptical wings. Generally, a high aspect ratio wing, compared to a low aspect ratio wing, produces higher lift at lower angles of attack with less induced drag. [Figure 2-6].

Wing loading is a term associated with the total weight the ram-air wing must support. Wing loading is found by dividing the total weight of the aircraft, in pounds, by the total area of the wing, in square feet. Wing loading is found by dividing the weight of the aircraft, in pounds, by the total area of the wing, in square feet. For example, the wing loading would be 2.0 pounds per square foot when 1,000 pounds—a common weight for a two-seat PPC with two people — is under a 500-square foot wing. If flying with one person the aircraft weight might be 700 pounds and the wing loading would decrease to 1.4 pounds per square foot.

Gliding flight is flying in a descent with the engine at idle or shut off.

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