Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии
How do aeronautical engineers study aircraft and design new ones?
As the use of the scientific method became increasingly important, it also became clear to aircraft designers that testing their hypotheses with human subjects was too risky. Wind tunnels were the first tool of aeronautics to be developed. In the very early 1900s designers built models of their aircraft and placed them in tunnels through which air could be blown to simulate flight. While wind tunnels did provide valuable information and were certainly safer than human flight, there were many questions that were left unsolved simply because the interactions of all the forces on an aircraft were too complex for the analysis methods of the day.
The advent of the computer changed everything. Now massive quantities of data could be gathered from wind tunnel tests and analyzed quickly and efficiently using the computer. In addition, new tools were developed.
Next came flight simulators which enabled a pilot to fly without ever leaving the ground. Flight simulator cockpits were designed to be exact duplicates of real aircraft cockpits. Motion systems were added and have evolved to the point where it is very hard to tell the difference between an airplane ride and a simulator ride.
As computers became more sophisticated, they became able to handle vast amounts of data. Aeronautical researchers began simulating airflow in a computer. Computational Fluid Dynamics was born. As advances in computer graphics have been made, it is now possible to sit at a desk and watch a computer-generated airplane fly - complete with the ability to visualize airflow and pressures as well as fly the airplane from takeoff to landing.
However, even with our increased ability to use computers, simulators and wind tunnels, the final and most definitive test of an aircraft is whether or not a pilot can fly it. Flight test, in which a human climbs into the cockpit and flies the aircraft, was originally the first tool of aeronautics but now remains the final and most important test that an aircraft must undergo. Vast improvements have been made in the safety of flight test and the ability of ground engineers and pilots to predict and avoid hazardous situations. All the tests using the other tools of aeronautics result in an aircraft being far more flight-worthy by the time it reaches flight test than it has in the past.
Microwaves for Radar
World War II provided the impetus to harness microwave energy as a means of detecting enemy planes. Early radars were mounted on the Cliffs of Dover to bounce their microwave signals off Nazi bombers that threatened England. The word radar itself is an acronym for RAdio Detection And Ranging.
Radars grew more sophisticated. Special-purpose systems were developed to detect airplanes, to scan the horizon for enemy ships, to paint finely detailed electronic pictures of harbors to guide ships, and to measure the speeds of targets.
These were installed on land and aboard warships. Radar—especially shipboard scales toward an Allied victory in World War II.
Today, few mariners can recall what it was like before radar. It is such an important aid that it was embraced universally as soon as hostilities ended. Now, virtually every commercial vessel in the world has one, and most larger vessels have two radars: one for use on the open sea and one, operating at a higher frequency, to “paint” a more finely detailed picture, for use near shore.
Microwaves are beamed across the skies to fix the positions of aircraft in flight, an essential aid to control the movement of aircraft from city to city across the nation. These radars have also been linked to computers to tell air traffic controllers the altitude of planes in the area and to label them on their screens.
A new kind of radar, phased array, is now being used to search the skies thousands of miles out over the Atlantic and Pacific oceans. Although these advanced radars use microwave energy just as ordinary radars do, they do not depend upon a rotating antenna. Instead, a fixed antenna array, comprising thousands of elements like those of a fly’s eye, looks everywhere. It has been said that these radars roll their eyes instead of turning their heads
Weight, or gravity, is the force which always acts downward, toward the center of the earth. It is the total sum of the masses of all its components and contents multiplied by the strength of the gravity, commonly referred to as the number of g’s. The weight may be considered to act as a single force, representing all its components and contents, through a single point called the center of gravity.
Weight is the most reliable force, which always acts in the same direction and gradually decreases as airplane fuel is used. The center of gravity shifts as the weight is redistributed. Although the terms “mass” and “weight” are often confused with each other, it is important to distinguish between them. Mass is a property of a body itself and measures a body’s quantity of matter. Weight, in contrast, is a force representing the force of gravity acting on a body. It is also loosely called gravity. To illustrate the difference, one could describe an object that is taken to the Moon, where the force of gravity is weaker, about one-sixth that on Earth. On the Moon, the object will weigh only about one-sixth as much as it did on Earth. The mass of the object will be the same on the Moon or anywhere else. In other words, it will continue to have the same amount of matter.
When an object moves relative to a fluid, either a gas or a liquid, the fluid exerts a frictional force on the object. This force which is referred to as a drag force, is due to the viscosity, or stickiness, of the fluid and also, at high speeds, to the turbulence behind and around the object. To characterize the motion of an object at different speeds relative to the fluid and to understand the associated drag, it is useful to understand Reynolds numbers. The Reynolds number depends on the properties, such as length and velocity, of the fluid and the object relative to the fluid. In case of an airplane, which flies through air, the Reynolds number for air is smaller than that for water because of the lower density of the air. For example, an object of one millimeter long moving with a speed of 1 millimeter per second through water has the same Reynolds number as an object 2 millimeters long moving at a rate of 7 millimeters per second in the air. The drag manifests itself differently for different Reynolds numbers associated to it.
When the Reynolds number is less than 1, as in the case of fairly small objects, such as raindrops, the viscous force is directly proportional to the speed of the object. For large Reynolds numbers, usually above a value between about 1 and 10, there will be turbulence behind the body, known as wake, and hence, the drag force will be larger and it increases as the square of the velocity instead of its linear dependence on the velocity. When the Reynolds number approaches a value of around 1,000,000, the drag force increases abruptly. For above this value, turbulence exits in the layer of fluid lying next to the body all along its sides.
Airplane wings and other airfoils are designed to deflect the air so that, although streamline flow is largely maintained, the streamlines are crowded together above the wing. Just as the flow lines are crowded together in a pipe constriction where the velocity is high, so the crowded streamlines above the wing indicate that the airspeed is greater than below the wing. Hence, according to Bernoulli’s principle which states that velocity increases as pressure decreases, the air pressure above the wing is less than that below the wing, and there is a net upward force, which is called dynamic lift, or lift.
In fact, Bernoulli’s principle is only one aspect of the lift on a wing. Wings are usually tilted slightly upward so that air striking the bottom surface is deflected downward. The change in momentum, a product of mass and velocity, of the rebounding air molecules results in an additional upward force on the wing. As the air passes over the wing, it is bent down. The bending of the air is the action; the reaction is the lift on the wing. To generate sufficient lift, a wing must divert air down. To increase the lift, either or both the diverted air and downward velocity must be incremented.
A force pushing an airplane, or any object, forward is called thrust. The thrust is produced by the engines of the airplane or by the flapping of a bird’s wings. The engines push fast-moving air out behind the plane, by either propeller or jet. The fast-moving air causes the plane to move forward, countering drag. Since the Wright brothers first flew in 1903, aeronautical engineers have created a multitude of airplane types, every one of which has dealt with the same four forces of weight, drag, lift, and thrust. All people have to deal with the challenges of stability with respect to these forces. Flying faster than the speed of sound has its own special demands, but the underlying forces of weight, drag, lift, and thrust remain the same.
In some sense, it is easier to fly in space, which is devoid of air, than it is to fly in air. However, spaceflight has its own special challenges. In space, one must deal with only two forces, weight and thrust. Thrust provides the force to lift a rocket into space. Once in orbit, a spacecraft no longer needs propulsion. Short bursts from smaller rockets are used to maneuver the spacecraft. To change its orientation, a spacecraft applies torque, a twisting force, by firing small rockets called thrusters or by spinning internal reaction wheels.
Imagine a track meet. The runners all line up at the starting line. At this point, their velocity is 0—they aren’t moving. Then, the starting gun goes off, and the runners push off. They begin to increase their speed.
We say that they accelerate. To most people, acceleration means simply “speeding up.” In science, though, the word has a different meaning. It is the rate at which velocity changes. Remember that velocity involves the direction in which an object moves as well as its speed. So accelerating the object may involve changing its speed or changing its direction (or both).
Последнее изменение этой страницы: 2016-04-11; Просмотров: 52; Нарушение авторского права страницы