The area of propulsion as it relates to aeronautics is primarily the science and engineering of methods of propelling or moving aircraft through the air. Since all forms of flight from birds, aircraft to rockets all use some method of propulsion of one sort or another. Today most propulsion for aircraft comes from moving air or gasses rapidly towards the rear of the aircraft causing an equal but opposite reaction resulting in the aircraft moving forward. The engines which generate the force causing the air to move are either piston engined propeller driven, turbine powered (prop-jet) propeller driven, turbo-jet or turbo-fan type of jet engine, or rocket engines or motors.
The engineers and scientists working on aircraft propulsion need to understand the airflow over propellers and blades and vanes within the engine, airflow through ducts, passages, and tubing, and how best to utilize the expanded gasses and airflow to generate the desired force (thrust) so that it can be used effectively to propel the aircraft. In addition, these engineers have to be able to minimize the amount of noise generated from the engine.
Within the engine the engineers and scientists working on aircraft propulsion need to understand how to utilize the combustion of fuel to best generate the desired thrust while using the least amount of fuel possible. In addition, today the concerns of these engineers is also to minimize the pollution developed within the engine.
Engineers and scientists in the field of aircraft propulsion have to
deal with a wide variety of topics which all relate to this field.
For example; Aerodynamics, the study of airflow - as it relates to
airflow into the engine. Thermodynamics, the study of heat and energy
systems using heat. - as it relates to the burning of fuels for
generating the hot gasses used for propulsion in most engines.
The area of Structures as it relates to aeronautics is primarily the science and engineering of methods of making the frame or structure of an aircraft support its aerodynamic shape. The aircraft structure is what lies beneath the skin of the aircraft, it keeps the aircraft's skin in place, just as our bones do so for our bodies.
A typical aircraft has a skin supported by an underlying structure which holds the shape of the skin, to make sure the aircraft retains the aerodynamics designed into it. The type structure of an aircraft differs depending on whether it is the shape of the wings, fuselage or empennage, etc. that is being supported. Many different designs for the underlying structure are in use today. However, they all do the same basic things, that is provide an efficient means of safely supporting the aerodynamic shape of the aircraft while making it a usable structure for the purpose of the aircraft. That is. the shape and structure of a B747 provides for the maximization of passenger and cargo space. Whereas, the shape and structure of the B2 Bomber provides for maximization of the "stealthiness" of the aircraft while allowing it to carry the ordnance and crew that it requires.
Modern structures in aircraft today make use of metals (aluminum, steel, magnesium) and composites (carbon fiber, kevlar, fiberglass in a resin matrix) as the primary materials. However, in the past wood, plywood, steel tubing, and cloth formed the primary aircraft structure. Generally, the strength to weight ratio drives the selection of the material and design of structure used. However, other considerations such as aircraft heating can force the use of more exotic materials (titanium) or more heat resistant materials (stainless steel honeycomb, ceramics) in highly specialized aircraft such as the Space Shuttle, SR-71, or XB-70.
The area of Performance as it relates to aeronautics is primarily the science and engineering of methods of making the performance of the aircraft best match its intended function. That is, once an aircraft is built what are the best methods of flying the aircraft so that it can accomplish its mission safely and efficiently.
Factors such as fuel burn, take off speed, cruise speed and altitude, maximum speed, maximum altitude, landing distance, landing speed, maximum take off weight, maximum landing weight, etc. have to be defined and made available in such a manner that a pilot can efficiently and safely fly the aircraft. Virtually every aspect of flight of an aircraft is the concern of Performance Engineers, for any regime where the performance data is in error can cause an accident or failure of the aircraft to complete its mission.
The performance of an aircraft is designed into it, since its mission determines its basic shape, intended speed and range. Once the first aircraft is designed and built it is tested (certification process) to determine its actual performance capabilities under a wide variety of conditions. This data is then put into an order in which the pilot can understand how to fly the aircraft efficiently and what to do in an emergency. Although great effort is made to develop accurate performance data before the aircraft is put into service, the performance data is refined throughout the life of the aircraft.
Essentially, Performance data is the "how to use" information of an aircraft.
The area of Design as it relates to aeronautics is primarily the science and engineering of methods of designing the aircraft. This field combines the work of all of the other fields to produce a usable, safe, economical and efficient aircraft. While the Aerodynamicists determine the basic shape of the aircraft and the Structural Engineers determine the structure to be used, and the Controls engineers determine how to control it, and the Propulsion engineers determine how to power it, the Design engineers determine how the work of the various disciplines fit together to provide the levels of performance which the Performance engineers need to convey to the pilot.
The discipline of aircraft designers pulls together the work done by the others and then determines how well this design fits the requirements for the aircraft. If the design fall short of its requirements, then various items are modified to try and bring the aircraft closer to the required parameters. This "iterative" process can go on may times and can be very costly. It can entail significant changes to the engines to increase power or decrease fuel burn, for example. It can force the Aerodynamicists to change the shape of the aircraft to reduce drag or increase lift. It can force the controls engineers to make the aircraft respond faster or slower depending on the problem.
The aircraft designers are, in effect the architect of the aircraft. They make the various disciplines work together to complement each others work and improve the aircraft to a point where it can be used to accomplish its intended mission.
The area of Controls as it relates to aeronautics is primarily the science and engineering of methods of controlling an aircraft in flight and on the ground. This field also covers many of the seemingly mundane functions of the aircraft such as the operation of the air conditioning and toilets. On the other hand this field covers the operation of the flight control surfaces (ailerons, elevators and rudders), the autopilot, radio and navigation instrumentation, operation of the engines, landing gear, etc.
Controls on old aircraft generally consisted of cables which moved the control surfaces and throttled the engines. As the aircraft have become more sophisticated, the meaning of "controls" has also changed significantly. Today control and avionics systems are largely computerized, however, the same principles of flight which governed the original aircraft still apply. That is, how do you turn the aircraft, climb, speed up, and land safely? This all is within the discipline of Controls Engineers. As the aircraft have become larger, faster, and more computerized the precision and complexity of the controls have increased accordingly. As a result the aircraft flying today are the most complex machines ever built. The Space Shuttle probably leads the way as the most complex machine ever build of any type (its a spacecraft while in orbit, the rest of the time its an aircraft).
Today's Controls Engineers must understand the physical process they are trying to control, such as the influence of moving the rudder on the movement of the aircraft, and how its movement changes with speed. They must also understand how the equipment they are using to control the rudder works and what its limits are. This all needs to go into the design of the aircraft and needs to provide an predictable method of control to make the aircraft usable. In today's sophisticated aircraft most things are controlled by the computers which control the flight of the aircraft, on the Boeing 767 even the toilets are controlled by the computers.
The area of Aerodynamics as it relates to aeronautics is primarily the science and engineering of methods of designing the shape of the aircraft so that it enables and enhances flight through the air. Although aerodynamics is well known for determining the shape of an aircraft, it plays a key influence in all of the other disciplines discussed here.
Aerodynamics, simply put, is the science which deals with the flow of air over the aircraft fuselage and wings. It determines the amount of lift the wings of the aircraft can generate, how controllable or stable the aircraft is, how much energy is required to propel the aircraft, how much payload or cargo the aircraft can carry, and how far and fast it can go. This is because the key to flight is lift, the key to sustained flight is lift and drag. In the design of an aircraft the requirements for propulsion, development of the aircraft structure, how controllable the aircraft is, and how fast, far, and high (performance) are all dependent on the aerodynamics of the aircraft.
Since Aerodynamics involves the flow of air over the wings it relies on the principles of fluid flows which govern the flow of all liquids and gasses. In particular "Bernoulli's Principle" which relates the relative velocities and areas to pressure provides the basic principle of how lift is generated. Other factors relate to the movement of a body through air. Namely how much drag is created, how uniform the drag and lift are (stability), how lift and drag can be altered to control the aircraft, and finally how these factors influence shape of the aircraft.
As the aircraft increases in speed and nears the speed of sound (Mach 1) the principles of airflow become more complex. At low speeds, significantly below the speed of sound (sub-sonic), the air acts essentially as an "incompressible" fluid, like water. While all liquids have some compressibility it is very small. This incompressibility produces the airflow behavior we are familiar with and which influences the vast of majority of aircraft flying today. However, as the aircraft approaches and then exceeds the speed of sound (supersonic) the air begins to behave as a "compressible" fluid. It can be thought of as taking on spring like characteristics. This is seen (or heard) in the form of a shock wave (sonic boom). When looking at an aircraft designed to fly at supersonic speeds it has a sleek and pointed appearance, whereas, a sub-sonic aircraft tend to be more blunt and rounded. Therefore, the speed at which the aircraft is designed to fly also, as a result of aerodynamics, has a significant influence on the shape and design of the aircraft.
All of the above disciplines determine why and how aircraft fly. They also determine why a B747, a Military Supersonic Fighter, and the Space Shuttle look and perform so differently. Yet they are all based on the same principles which allow a bird to fly.