The Federal Aviation Administration (FAA) shares the Environmental Protection Agency's (EPA) concerns about lead emissions from small aircraft. Owners and operators of more than 200,000 piston-engine aircraft operating in the United States rely on aviation gasoline (avgas) to power their aircraft.
Symmetry (noun). a due proportion of the several parts of a body to each other; adaptation of the form or dimensions of the several parts of a thing to each other; the union and conformity of the members of a work to the whole. Symmetry (noun).
Rhythm (from Greek á¿¥ÏÎ¸Î¼ÏÏ, rhythmos, any regular recurring motion, symmetry  (Liddell and Scott 1996)) generally means a  movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions (Anon. 1971, 2537).