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Chapter 1 Forces Acting on an Airplane in Flight
How Does a Heavier-Than-Air Flying Machine Defy the Law of Gravity?
A heavier-than-air craft flies by obeying a different law; Newton’s third law of motion. An airplane flies by creating a downward force on a mass of air that is equal to its own weight. In return, this mass of air produces an upward force on the airplane and supports it.
Newton’s third law of motion states that every action (or force) gives rise to a reaction (or opposing force) of equal strength but of opposite direction.
The wing of an airplane has a very special cross sectional shape called an airfoil section. When this airfoil moves through the air the relative wind strikes it at an angle called the angle of attack.
The air in the relative wind strikes the leading edge of the airfoil and some flows over the top and some across the bottom. The air flowing over the top finds the surface dropping away from it, and, in the same way you speed up as you run down a hill, the air speeds up. According to Bernoulli’s principle, when the air speeds up, its pressure drops and the low pressure above the wing pulls the air down to the surface and as it leaves the wing it is deflected downward.
Figure 1. The angle of attack is the acute angle between the chord line of an airfoil and the relative wind.
The air flowing below the wing finds the surface rising into its path. This slows down the air and its pressure increases. As the air leaves the airfoil it is deflected downward.
When the weight of the air deflected downward equals the weight of the airplane, the air supports the airplane.
Five things affect the amount of air deflected downward:
- Shape of the airfoil
- Angle of attack
- Area of the airfoil
- Density of the air
- Speed of the air
The shape and area of the airfoil are physical characteristics of the airplane. The density of the air is determined by the outside air temperature. The altitude, the speed of the air, and the angle of attack are controlled by the pilot.
Figure 2. The shape of the airfoil causes the air through which it is passing to be deflected downward.
Five Forces
In straight and level flight at a constant airspeed and altitude five forces are in balance on an airplane:
- Thrust, acting forward, is caused by the propeller moving air rearward.
- Lift, acting perpendicular to the relative wind, is caused by the wing deflecting air downward.
- Weight of the aircraft, caused by gravity, acts toward the center of the Earth.
- Drag, acting in the direction opposite to thrust, is caused by the resistance of the air as the aircraft moves through it.
- Tail load is a downward aerodynamic force produced by the horizontal tail deflecting air upward. The amount of tail load is determined by the airspeed and it is used for longitudinal stability.
Figure 3. In straight and level, unaccelerated flight, the five forces are balanced.
Chapter 2 Axes of an Airplane
An airplane moves in three dimensions and is controlled by rotating it about one or more of its three axes. See Figure 4.
Figure 4. The three axes of an airplane
- Longitudinal (roll) axis—Extends through the aircraft from nose to tail passing through the center of gravity. The airplane is rotated about its longitudinal axis by changing the amount of lift produced by the left or right wing. This is done by deflecting the ailerons. The ailerons are connected in such a way that as the aileron on one wing moves downward increasing the lift, the one on the opposite wing moves upward decreasing the lift, and the airplane rolls.
- Lateral (pitch) axis—Extends across the airplane from wing tip to wing tip, passing through the center of gravity. The airplane is rotated about its lateral axis by varying the tail load. This is done by deflecting the movable horizontal tail surface. When the trailing edge of the surface moves upward the downward tail load increases and the nose rises. When the trailing edge moves downward the tail load decreases and the nose drops.
- Vertical (yaw) axis—Extends through the airplane vertically, passing through the center of gravity. An airplane is yawed (not turned) by changing the horizontal aerodynamic force on the vertical tail by deflecting the movable vertical tail surface.
Control of an Airplane
An airplane is controlled by varying the forces that rotate it about one or more of its three axes.
Pitch Control
The movable horizontal tail surfaces (elevators, stabilator, or ruddervators) vary the tail load. When the control column or stick is moved aft, the trailing edge of the surface rises, deflecting air upward. This increases the downward tail load and causes the airplane to rotate nose-up about its lateral axis. When the column is moved forward, the trailing edge moves downward, deflecting air downward, canceling the downward tail load and producing an upward force on the tail. The tail moves upward and the airplane rotates nose-down about its lateral axis.
An airplane is free to rotate about its three axes so it does not always maintain a fixed relationship with the Earth. Because of this, the terms “up” and “down” do not necessarily mean the same thing they mean when we are earthbound. In normal flight, moving the control column back causes the nose to ...
