Wing sweep, Л, is a function of aircraft speed to delay transonic effects. For aircraft flying at less than Mach 0.6, a wing sweep is not required. A tapered wing with a zero quarter-chord sweep has some LE sweep; the trailing-edge sweep depends on the taper ratio.
It is an essential geometrical adjustment to ensure that wing-tip effects do not create adverse conditions. A major requirement is to make the wing root stall earlier to retain aileron effectiveness at a high angle of attack (low speed) – especially during landings. A wing twist with washout would favor such behavior (and is the prevailing practice).
To ensure roll stability (see Section 12.3.3), wing dihedral and anehedral angles are used. Generally, the dihedral is associated with low-wing design and the anhedral with high-wing design; however, there are designs that are the reverse: a high wing can accommodate a dihedral. The type and extent are settled through stability analysis, which is not discussed in this book. All civil aircraft have some dihedral or anhedral angle between 1 and 5 deg. If a high wing and/or a high-wing sweep increases lateral stability more than what is required, the anhedral angle is required to reduce it to the desired level. Some low-wing Russian bombers have a high-wing sweep that necessitates an anhedral angle, when the undercarriage struts must be longer to provide the desired ground clearance.