Powered Parachute Flying Handbook

Chapter 2 - Aerodynamics of Flight


Drag is the resistance to forward motion through the air. Drag opposes thrust. Aerodynamic drag comes in two forms:

1. Induced drag: a result of the wing producing lift;

2. Parasite drag: resistance to the airflow from the cart, its occupants, suspension lines from the wing, interference drag from objects in the airstream, and skin friction drag of the wing.

Induced drag is the result of lift, and its amount varies as discussed above for lift. Induced drag creates organized circular vortices off the wing tips that generally track down and out from each wingtip. [Figure 2- 12] This is true for all aircraft that use wings including PPC, weight-shift control and fixed wing aircraft. The bigger and heavier the aircraft, the greater and more powerful the wingtip vortices will be. This organized swirling turbulence is an important factor to understand for flight safety. Refer to Section 7-3 of the Aeronautical Information Manual (AIM) or Chapter 12 of the Pilot’s Handbook of Aeronautical Knowledge (FAA-H- 8083-25) for additional discussion.

Parasite drag is caused by the friction of air moving over the structure. Just as with lift, parasite drag increases as the surface area of the aircraft increases and dramatically increases as airspeed increases, at the square of the velocity. Therefore, doubling the airspeed will quadruple your parasite drag. [Figure 2-13]

The PPC has relatively slow speeds, but plenty of items (area) for the wind to strike including wing, lines, pilot, cart, engine, wheels, and tubes. Parasitic drag can be reduced by streamlining the items but since the PPC flies at relatively slow airspeeds, the extra weight, cost, and complexity of streamlining the PPC is generally not incorporated into the design.

Total Drag is the combination of parasite and induced drag. Total Drag = Parasitic Drag + Induced Drag

To help explain the force of drag, the mathematical equation D = Cd · q · S is used. In this equation drag (D) is the product of drag coefficient (Cd), dynamic pressure (q) determined by the velocity squared times the air density factor, and surface area (S) of the cart and the ram-air wing (S). The drag coefficient is the ratio of drag pressure to dynamic pressure.

Induced and parasitic drag have opposite effects as angle of attack decreases and speed increases. Note the total drag. It is high at the slowest air speeds at high angles of attack near the stall, decreases to the lowest at the most efficient airspeed, and then progressively increases as the speed increases. The PPC wing is typically designed to fly at a speed generally above lowest overall total drag. Too slow, and the wing would be near its critical angle of attack. Too fast, and the power to maintain level flight or climb would be excessive. The manufacturer determines the speed range of the wing based on the weight range, and the resultant location on the total drag diagram. [Figure 2-14]

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