Benjamin A. Gorman
Now that the history has been established, there is a need to turn to science to explain how the Wrights were able to accomplish their feat of flying. To understand what makes an airplane fly requires some knowledge of aerodynamics, a branch of fluid dynamics. For an eighth grader in a social studies course, the approach may be what a student needs to know in simple terms. First an understanding of the forces that act upon an airplane in flight. They are (1) lift—the upward acting force, (2) weight—the downward acting force, (3) thrust—the forward acting force, and (4) drag—the backward acting force. Lift and thrust are the two forces that airplane designers must consider so as to overcome the forces of weight (gravity) and drag (air resistance). Lift is created by the way airplane wings are designed so that the pressure of the air over the wings is less than the pressure under them. The difference in pressure—less above the wing and greater below—causes the wing to lift. Thrust is provided by a motor-driven propeller or jet engine that pulls the plane forward.
The Swiss scientist, Daniel Bernoulli in the l8th century, discovered the fact that the pressure of a fluid decreases at points where the speed of the fluid increases. He found that the high speed of flow is associated with low pressure and low speed with high pressure. The application of this to the wing of an airplane will result in lift. The wing or airfoil is designed to increase the velocity of the airflow above the surface, thereby decreasing pressure above the airfoil. The air must go faster over the curved edge of the top than under the bottom. Therefore the air pressure is lower above the wing where it moves faster. Lift is more complicated than explained here. It also includes the factor of air circulation which is needed to enhance the flow speed and lower the pressure above the wing. (Note student activity one). Other interacting features of fluid dynamics are involved as well. But for the purpose of this unit, lift is presented in simple terms only.
A propeller is a shaped structure or airfoil that provides lift when air is forced over its leading edge. The propeller works by creating greater air pressure on one side of its surface than the other. As its twisted blades cut through the air, they pull the plane along, because the pressure behind them is greater than the pressure in front.
If an airplane wing is tilted upwards, it will produce greater lift than if it were level. A wing is always mounted on the fuselage at a very slight angle so that it will produce lift when the plane is moving. As the plane flies and changes the angle at which the wing meets the air, the plane will go up or down. This angle is called the angle of attack; the greater this angle is up to a certain point, the more lift and drag there will be. When the angle of attack becomes about 18-20 degrees, the air cannot flow smoothly over the wings’ upper surface. It has to flow straight over the top surface and this causes a churning of air behind the wing as it tries to follow the surface. There is a sudden increase in pressure on the upper wing surface. This immediately cuts down lift and increases drag and the plane is said to “stall”. When this occurs, the plane does not have enough airspeed or lift to hold it up and it may slide side ways or backward into a spin. Lift and drag are also affected by several other factors: the size of the wing, the shape of the airfoil, the speed of the plane, and the density of the air itself.
* * *
The Pilot
. . . .Flight check.
The science of flying and the Wrights’ technical adaptations produced an aircraft that worked. The continued experimentation and the application of aerodynamics have resulted in the modern air travel that we can experience today. One aspect of that first flight at Kitty Hawk, that marked it as a true first flight, was that Orville Wright was in control of the Flyer.
Early pilots flew by their reactions to what they could see from their open cockpit and what their feelings of weight or weightlessness told them about whether the plane was climbing or descending. Most flying today is done by a combination of reference to instruments and visual observations. The instruments give information about altitude, speed, direction, attitude and how the engine is operating. Students may be asked what information a pilot would need to know in order to get them thinking about being “in the driver’s seat.” In a small private plane, the instrument panel is not much more complicated than the dashboard of a car. Using the instruments, a pilot could fly entirely without looking out the windshield except for takeoff and landings assuming proper radio communication.
The altimeter is one of the most important instruments; it measures the height of the aircraft above a given level. The altimeter is a barometer that measures the air pressure and converts the readings into feet of altitude. The pilot must know that the plane is flying high enough to clear the highest terrain or obstruction along the way. To reduce the potential of a midair collision, the pilot must be sure to fly the correct altitudes in accordance with air traffic rules especially on flights at more than 3, 000 feet. The altimeter has a knob to adjust the instrument to take into account changes in barometric pressure at different points of the flight as reported by weather stations.
An air speed indicator tells how fast the plane is traveling in relation to the air around the plane. It works by measuring air pressure. But the pressure it measures is the impact the plane has on the air. In other words, how hard the air is hitting the instrument as it moves through the air or the difference between pilot, or impact pressure, and static pressure. The impact is calibrated to give the airspeed in miles per hour, or knots, or both. The indicator is marked according to a standard color-coded system for safe operation. For example, air speed needs to be controlled for normal cruising, maneuvering, landing or stalling.
The magnetic compass may be familiar with its cardinal headings and graduations for every five degrees. Remember that it will not point to True North Pole but to the Magnetic North Pole some 1,300 miles away. Thus the variation must be accounted for and can only be read when the plane is flying straight and level. The gyro compass uses a gyroscope to keep the course and is adjusted to correspond with the magnetic compass indicator after a short period of level flight.
The turn and bank indicator was one of the first modern instruments used for controlling an aircraft without visual reference to the ground or horizon. It tells the pilot when he is turning and how well he is executing the turn, whether he has too much or too little bank for the rate of turn, Coordination and balance in straight and level flight can be checked. This indicator is actually two instruments, a ball and a turn needle. The ball part is simply an agate or steel ball which is free to move inside a curved, sealed glass tube filled with kerosene. The lowest point of the glass tube is in the center. In straight and level flight, gravity keeps the ball there, centered between two wires. In a turn the ball will be kept centered by centrifugal force if the aircraft is neither skidding or slipping. If the rate of turn is too great for the angle of bank (skidding), the ball will fall to the high or outside of the turn. If the rate of turn is too slow for the angle of bank ( slipping), the ball will move to the low side or inside of the turn. The turn needle indicates the rate at which the aircraft is turning about its vertical axis.
Besides the flight instruments, other indicators show the aircraft’ s engine is operating; they include the tachometer, the oil pressure gauge, and the oil temperature gauge. These instruments are basic to any airplane; the larger or more sophisticated the craft, the more instruments, each revealing position and condition of the plane.
But even before the pilot leaves the runway, he must have a fundamental knowledge of the atmosphere and weather behavior. Consideration of air density is important. Air has weight and substance; it tends to resist anything passing through it. But the air is constantly changing. Sometimes it is dense and heavy; at high altitudes, it is thin and light. A pilot must be aware of how and why the air changes so as to expect its effects on the lift of a plane. The density of the air is affected by altitude, temperature and humidity. The higher you travel the less pressure you feel. At 18,000 feet, the pressure is about one half of what it is on the earth’s surface.
Temperature and humidity affect air density and make a difference in flying. When air is heated, it expands and has less density. Because this is true, a pilot needs a longer runway to take off from on a hot day than a cool day. Humidity is the amount of water vapor in the air. Water vapor weighs less than perfectly dry air, and thus, the air is denser on a very dry day when the humidity is high. A pilot will have to make a much longer run to take off on a warm, humid day than he will on a cool, dry one because the air will be less dense. At an airport high above sea level, a longer takeoff will be necessary because the density will be lower. For example, a takeoff in a small private plane from Denver in Colorado, about 5,000 feet above sea level, will require 2,000 feet of runway compared to 1,000 feet at one near sea level. The factors of altitude, temperature and humidity are interrelated; changes in one will affect the others. A pilot must know the effects of the factor to determine how the plane will fly in the air on a given day. These scientific considerations may be difficult for students to grasp but they do point out the range of knowledge that a pilot must have to fly safely and according to schedule.
Unlike Orville Wright, who was able to shift his weight and operate some wires to control his Flyer, the modern pilot has the benefit of years of technological improvements and refinements and hours of training. Pilots control the aircraft by increasing or decreasing speed and by movable surfaces on five of the plane’ s airfoils: the two wings, the small sail wings and the fin. When these surfaces are moved, the air flow is changed and this changes the altitude and direction of the plane. On the wings, the movable surfaces are called ailerons; on the tail wings, (horizontal stabilizers) they are called elevators. They are connected to the wheel or stick in the cockpit.
The control stick can be moved in all directions; as it moves, it changes the position of the ailerons and elevators. Moving the stick to the right causes the left aileron to go down and the right one up; this rolls the plane to the right because the ailerons change the curvature of the wing surface. If the stick is pressed forward, the elevators move downward increasing lift and forces the tail upward, making the plane’s nose drop. When the stick is pulled back, the elevators turn upward, decreasing the tail’s assembly’s lift. Thus the plane’s nose is forced up.
Footpedals control the rudder which is the vertical stabilizer on the fin. The throttle controls the engine which increases or decreases the thrust. The control surfaces work together in maneuvering the aircraft. For example, in order to climb, the pilot would not just pull back the stick to lower the tail and raise the nose because the drag would rapidly increase and the wing would lose lift. So when the stick is pulled back, the pilot opens the throttle to gain more speed. This keeps the lift power high and helps the plane to climb.
Every pilot must also develop the ability to navigate with precision. To navigate successfully, a pilot must know the plane’s position at all times no matter the weather or darkness and be able to calculate flight time and fuel consumption. Using charts, instruments and preparation, the pilot maps the flight course so that he knows at all times where he is, what direction he should be flying, and how long it will take him to get to the destination.
Pilots must be acquainted with the aircraft’s airworthiness: its structural strength, stress factors, lifting capacity, and flight characteristics. These are important for safe aircraft and engine operation. Considerations of weight loads and balance conditions are added responsibilities fo
r
the pilot. While flying, pilots must have good radio communications especially with control towers at airports and know the language of the airwaves. The list of duties and requirements of a pilot does not end here; their skill and knowledge does not eliminate the need for good judgement.
Now it’s time to take her up.
* * *
