# Principles of Flight

Principles of Flight

Structure of Atmosphere:

The atmosphere is composed of:

78% Nitrogen

21% Oxygen

01% Other Gases

Some of these elements are heavier then others, there is a tendency of the heavier gases such as Oxygen, to settle to the surface of the earth. This explains why most oxygen is contained below 35000 feet altitude.

At sea level air pressure is 14.7 Lb/sq in.(29.92 In Mercury) For this reason the weight of the atmosphere at 18,000 feet is only ½ what it is at sea level.

Atmospheric Pressure

The density of air has significant effects on the airplane performance

As air becomes less dense it reduces:

Power: because the engine takes in less air

Thrust: because the propeller is less efficient in thin air

Lift: because thin air exerts less force on the airfoils.

In fact , density is directly proportional to pressure.

Effect of Temperature on Density

Density of air varies inversely proportional to the absolute temperature. (This is only true at constant pressure)

Effect of Humidity on Density

Water vapor is lighter than air, consequently, moist air is lighter dry air.

The higher the temperature, the greater amount of water vapor it could hold.

Pressure, temperature and humidity have a great influence on the airplane performance, because their effect on density.

Newton’s Laws of Motion and Force

Sir Isaac Newton 17th Century Philosopher.

Newton’s First Law

A body in rest stays in rest, and a body in motion tends to stay at the same speed and direction.

Newton’s Second Law

When a constant force acts upon a body, its resulting acceleration is inversely proportional to the mass of the body and directly proportional to the force applied.

Newton’s Third Law

Whenever a body exerts a force on another, the second body exerts on the first, a force of equal magnitude but in opposite direction ( Action/Reaction Law)

Magnus Effect

This low-pressure area produce an upward force known as the “ Magnus Effect” This mechanically induced circulation illustrates the relationship between circulation and lift.

Bernoulli’s Principle of Pressure

Mr. Daniel Bernoulli was a Swiss mathematician. Explains how the pressure of a fluid (liquid or gas) varies with the speed of motion.

Airfoil Design

The end, which faces forward in flight, is called the leading edge, and is rounded; while the other end, the trailing edge, is quite narrow and tapered. A reference line often used in discussing the airfoil is the chord line, a straight line drawn through the profile connecting the extremities of the leading and trailing edges. Another reference line, drawn from the leading edge to the trailing edge, is the “mean camber line.” This mean line is equidistant at all points from the upper and lower contours.

a positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing.

The balance of the lift needed to support the airplane comes from the flow of air above the wing.

The weight, speed, and purpose of each airplane dictate the shape of its airfoil.

Pressure Distribution

In general, at high angles of attack the center of pressure moves forward, while at low angles of attack the center of pressure moves aft.

The balance of an airplane in flight depends, therefore, on the relative position of the center of gravity (CG) and the center of pressure (CP) of the airfoil.