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Looking at the rear view of the wing, anhedral is the angle the wings make angling down and dihedral is the angle the wings make angling up. [Figure 2-5]  Dihedral is the positive angle formed between the lateral axis of an airplane and a line which passes through the center of the wing. Anhedral is the similar negative angle. Wings with sweep have an “effective dihedral” characteristic that counteracts the physical anhedral to develop the required roll stability for the particular make/ model design objective. This is explained in the Pilot’s Handbook of Aeronautical Knowledge in much greater detail for further reference. Unlike airplanes which typically have significant dihedral as viewed from the front or back for roll stability, WSC wings typically have a slight amount of anhedral as shown in Figure 2-5  and effective dihedral which is a characteristic of the swept wing design. 

 Wing twist is the decrease in chord angle from the root to the tip chord, common to all WSC wings and ranging from 5° to 15°. This wing twist is also called washout as the wing decreases its angle of attack from root to tip. The term billow was originally used for the early Rogallo wings as the additional material in degrees that was added to the airframe to create the airfoil. It is still used today to defi ne the amount of twist or washout in the wing. The WSC may not have twist/washout when sitting on the ground, and must be flying and developing lift to display the proper aerodynamic twist characteristic of WSC wings. [Figure 2-6] 

The longitudinal axis is an imaginary line about which the aircraft rolls around its center of gravity (CG); it is also called the roll axis. The longitudinal axis is not necessarily a fixed line through the carriage because the roll axis changes for different fl ight configurations, but can be approximated by the middle of the propeller shaft for a properly designed WSC aircraft and is typically parallel with the fl ightpath of the aircraft as shown in Figure 2-7.  Angle of incidence is the angle formed by the root chord line of the wing and the longitudinal axis of the WSC aircraft. 

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