Introduction to Aeronautics
First of all - what is aero?
"Aero" is a Greek prefix signifying air. Air is made up of a mixture of gasses, and thus is itself a gas. However, in all the reading about aeronautics you have done air is referred to as a fluid. For instance, air obeys the laws of fluid dynamics. The technical definition of a fluid states that a fluid is any substance that flows. Obviously water flows, but so does air and so do powders! So, technically speaking, air and powders are fluids. Most important to our study of aeronautics is the fact that air obeys the physical laws of fluids.
What is aeronautics?
Aeronautics is typically defined as the art or science of flight, or the science of operating aircraft. This includes a branch of aeronautics called aerodynamics. Aerodynamics deals with the motion of air and the way it interacts with objects in motion, such as an aircraft. Both of these branches are a part of the tree of physical science. Aviation, however, refers to the operation of heavier-than-air craft.
How did aeronautics begin?
The theoretical basis for these branches stems from the work of Sir Isaac Newton in the 1600s. Newton developed laws that defined the effects of forces acting on objects in motion or at rest. He also developed the concept of viscosity, or fluid friction, which is the resistance of air or any other fluid to flow. Daniel Bernoulli, in the 1700s, developed the principle that the speed of a fluid is directly related to pressure. That is, the faster the flow of a fluid, the lower the pressure that is exerted on the surface it is flowing over. For example, if air is flowing faster over the top of a surface than under a surface, the pressure on the top of the surface will be less than that underneath. Understanding of these concepts was necessary to the development of flight. Without understanding the aerodynamic principles of flight, humans would simply be mimicking the actions of birds. It was demonstrated through many spectacular yet often disastrous attempts that pure imitation would not enable humans to fly.
How did aeronautics evolve past the imitation of birds?
The science of aeronautics really began to evolve in the late 18th and early 19th centuries. Philosophers and early scientists began to look closely at physical phenomena such as gravity and motion. As paths of communication were established between distant cultures, the understanding of flight began to coalesce. With their wealth of understanding of kites, rockets and fireworks, the Asian cultures defined and harnessed propulsion. The Europeans with their penchant for analysis, definition and precision, began to piece together the concept of force. This growth in knowledge and communication continued throughout the 19th century. By the very late 19th and early 20th centuries, this knowledge had evolved to the point where people sought to put it to practical use. As space is the frontier of today, flight was a frontier of that time.
Along with factual knowledge, the method of discovery as well as trial and error evolved into the scientific method. The scientific method became a widely accepted process to question, analyze, test and verify results. Concepts and ideas that were subjected to the scientific method received general acceptance and were used as bases for generating new ideas.
The classification and definition of forces involved with flight were developed. We know them today as lift, drag, weight and thrust. Scientists began to understand how they worked together to enable an object heavier than air to fly. Once these concepts were well understood, it was only a matter of time before humans figured out how to not only fly, but to control their flight. Balloons, which by this time were old news, enabled people to fly but aeronauts remained at the mercy of the wind to determine where they went. With the invention of the airplane people could fly when, how and where they wanted. Another frontier had been conquered. Within a few short years, airplane designers refined the shape of wings and overall construction to improve airplane performance and safety. Further improvements in airplane design allowed flight to become accessible to everyone.
What is an airplane?
What is the difference between aircraft and airplane? Aircraft is the more general term, and refers to any heavier-than-air craft that is supported by its own buoyancy or by the action of air on its structures. An airplane is a heavier-than-air craft that is propelled by an engine and uses fixed aerodynamic surfaces (i.e. wings) to generate lift. So, every airplane is an aircraft, but not every aircraft is an airplane! Gliders are aircraft that are not airplanes. The Space Shuttle is definitely an aircraft, but it is not an airplane. It does not carry engines for propulsion. Helicopters are also aircraft that are not airplanes because their aerodynamic surfaces are not fixed - they rotate.
Why are there so many different types of airplanes?
The characteristic that most readily identifies the type, performance and purpose of an airplane is the shape of its wings. There are four basic wing types: straight wings, sweep wings (forward-sweep/sweepback), delta wings and the swing-wing (or variable sweep wing). Each shape allows for premium performance at different altitudes and at different speeds.
Another important discriminator between airplanes is speed. Airplanes fly at subsonic, transonic, supersonic and hypersonic speeds. These speed classifications are called the "regimes" of flight. The suffix -sonic refers to the speed of sound, which is dependent on altitude and atmospheric conditions (nominally 340 meters per second). "Mach" is a term used to specify how many times the speed of sound an aircraft is traveling. Mach 1 is one times the speed of sound. Mach 2 is twice the speed of sound, and so on. Mach numbers less than 1 are speeds less than the speed of sound.
Subsonic refers to all speeds less than Mach 1.
Transonic refers to all speeds from approximately Mach .9 to Mach 1.5 - that is, the speeds at which an aircraft is going through the speed of sound or "breaking the sound barrier".
Supersonic refers to all speeds greater than the speed of sound, which is the same as saying all speeds above Mach 1.
Hypersonic refers to all speeds greater than Mach 5. Note that an aircraft flying at hypersonic speeds can also be said to be flying at supersonic speeds.
Every modern aircraft that is built today is built for a specific purpose. Airplanes are designed for different altitudes, different speeds, different weight-carrying capacities, and different performance. Jet fighters are relatively lightweight, highly maneuverable and very fast.
They are designed to carry a relatively small amount of weight, including fuel, which necessitates refueling on long flights. Passenger airplanes are larger, carry more weight, and can fly longer distances. However, they are less maneuverable and slower than jet fighters. Other aircraft like the SR-71, are designed to fly at very high altitudes and high speeds for very long periods of time. Every aircraft fills a particular niche in the gigantic matrix that is modern aviation.
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.