Stretch up on your toes with your hands in the air, and your fingertips might just brush the cool metal belly of an aircraft. Gravelly Point near National Airport is exhilarating — windy, noisy and visually thrilling. Watch the planes approach along the gentle curve of the Potomac River and pass over your head with landing gear extended. Then listen carefully for the sound the air makes. A cross between flapping sails and an inhalation, the wind zips up the space left by jet engines.