by Chief Pilot John Bastan
The atmosphere is composed of 78% Nitrogen, 21% Oxygen, and 1% other gases, such as Argon or helium. As some of these elements are heavier than others, there is a natural tendency of these elements, such as Oxygen, to settle to the surface of the earth, while the lighter elements are lifted up to high altitudes. This explains why most of the Oxygen is contained below 35,000′.
Effects of Air Density
As air becomes less dense, it reduces (1) power because the engine takes in less air, (2) thrust because the propeller is less efficient in thin air, (3) lift because the thin air exerts less force on the airfoils. In fact, density is directly proportional to pressure. The effect of increasing the temperature of a substance is to decrease its density. Thus, the density of air varies inversely as the absolute temperature varies. In the atmosphere, both temperature and pressure decrease with altitude. The higher the temperature, the greater the amount of water vapor the air can hold. Water vapor is lighter than air; consequently, moist air is lighter than dry air. Pressure, temperature, and humidity have a great influence on airplane performance because of their effect upon density.
Newton’s third law applied to a propeller
Newton’s third law states that: Whenever one body exerts a force on another, the second body always exerts on the first, a force that is equal in magnitude but opposite in direction. The recoil of a gun as it is fired is a graphic example of Newton’s third law. In an airplane, the propeller spins and forces the air backward. The air pushes the propeller (and the airplane) in the opposite direction-forward.