Helicopter aerodynamics is the branch of physics dealing with the forces and pressures exerted by air in motion. The atmosphere, the mass of air, which completely envelops the earth, is composed of varying and nonvarying constituents. The nonvarying constituents include oxygen (21%) and nitrogen (78%). The varying constituents include CO2, argon, hydrogen, helium, neon, krypton, and water vapor, which will vary from negligible amounts to approximately 4% by volume (100% relative humidity). Air is a fluid and is affected by changes in temperature, pressure, and humidity
