The miracle of flight exists because man has the technology to oppose natural forces that keep all objects on the ground. Four forces affect an aircraft — two assist flight (thrust and lift), and two resist flight (gravity and drag). The important thing to note here is that when an aircraft is flying straight and level, all four of these forces are balanced, or in equilibrium.
Thrust is created by the engines. As propeller blades push air through the engine (or as jet fuel is combusted to accomplish the same end), the aircraft moves forward. As the wings cut through the air in front of the aircraft, lift is created. This is the force that pushes an aircraft up into the air.
Lift occurs because air flows both over and under the surface of the wing. The wing is designed so that the top surface is "longer" than the bottom surface in any given crosssection. In other words, the distance between points A to B is greater along the top of the wing than under it. The air moving over the wing must travel from A to B in the same amount of time. Therefore, the air is moving faster along the top of the wing.
This creates a difference in air pressure above and below—a phenomenon called the Bernoulli effect. The pressure pushing up is greater than the downward pressure, and lift is created. If you're banking, lift occurs in a slightly sideways direction. If you're inverted, lift actually pulls you downward toward the ground. Note that lift occurs perpendicular to a line drawn parallel to the centerline of the wing and occurs at a slightly backward angle.
Several factors determine how much lift is created. First, consider the angle at which the wing hits the air. This is called the angle of attack, which is independent of the aircraft's flight path vector. The steeper this angle, the more lift occurs. At angles steeper than 30° or so, however, airflow is disrupted, and an aircraft stall occurs. During a stall, no lift is created. The aircraft falls into a dive and can recover lift only after gaining airspeed.
Drag opposes thrust. Although it mainly occurs because of air resistance as air flows around the wing, several different types of drag exist. Drag is mainly created by simple skin friction as air molecules "stick" to the wing's surface. Smoother surfaces incur less drag, while bulky structures create additional drag.
Some drag has nothing to do with air resistance and is actually a secondary result of lift. Because lift angles backward slightly, it is has both an upward, vertical force and a horizontal, rearward force. The rearward component is drag. Another type of drag is induced at speeds near Mach 1, when a pressure differential starts building up between the front and rear surface of the airfoil. The pressure in front of the wing is greater than the pressure behind the wing, which creates a net force that opposes thrust. In WW II aircraft, this last type of drag occurred only during prolonged dives.
Gravity is actually a force of acceleration on an object. The Earth exerts this natural force on all objects. Being a constant force, it always acts in the same direction: downward. Thrust creates lift to counteract gravity. In order for an aircraft to take off, enough lift must be created to overcome the force of gravity pushing down on the aircraft.
Related to gravity are G-forces—artificially created forces that are measured in units equivalent to the force of gravity.
A "G" is a measurement of force that is equal to the force of gravity pushing down on a stationary object on the earth's surface. Gravitational force actually refers to an object's weight (Force equals Mass times Acceleration, or F = ma.). An aircraft flying level at low altitudes experiences 1G. Extra G-forces in any direction can be artificially created by sudden changes in velocity or in the direction of motion. Good examples are a takeoff, a tight turn in an aircraft at moderate to high speed or a loop maneuver.
G-forces can be either positive or negative. Positive Gs make you feel heavier because they act in a relative downward direction. They push you back into your seat and primarily occur during sharp turns or steep climbs. Negative Gs make you feel lighter because they're pulling in a relative upward direction. When you're in a steep dive, they pull you out of your seat. The direction of G-forces is always relative to the position of the aircraft—if you're flying upside-down, upward Gs actually pull in a downward direction.
Apparent weight refers to how heavy something seems considering the current direction and magnitude of G-forces acting on it. In level flight, 1G is acting on the aircraft and the pilot—both weigh the same as they do when stationary. If the pilot makes a steep climb, the positive G-force temporarily acts on both the pilot and the aircraft, making them in essence heavier throughout the climb. Any sudden increase or decrease in acceleration brings about a change in apparent weight of an object.
Physical Effects of G-Forces
Human bodies can withstand approximately 9 or 10 positive Gs or 2 to 3 three negative Gs for several seconds at a time. Exceeding positive G limits for longer than that causes blood to collect in the lower part of the body and torso. The brain and retinas receive less blood, and therefore less oxygen. Eventually, vision turns gray, followed by tunnel vision and pilot blackout. Excessive negative Gs have a similar effect, except that blood pools in the brain and upper torso. This causes the small capillaries in the eyes to swell, creating a redout effect.