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Travis Rozario replied the topic: Dihedral&keel surfaces bobtait book page 135
Dihedral refers to the angle of the wing from the horizonta - the yawing plane. For a dihedral low wing aircraft, you can see an obvious deflection of the wings 'upwards' from the root till the tip. This serves to increase the angle of attack of the down going wing during a level turn. This increase of angle of attack results in more lift being generated by the down going wing, thus lifting it back to normal straight and level flight. This increases lateral stability.
As stated in the text book. A aircraft with a swept back wing design, (such as the F1-11 or B1 Lancer) has high levels of lateral stability. So an anhedral aka 'negative dihedreal' wing design is employed to reduce some of that lateral stability to provide more manoeuvrability in a turn.