Aircraft Classification and Their Operational Environment
An aircraft can be classified based on its role, use, mission, power plants, and so forth, as shown in Chart 4.1. Here, the first level of classification is based on operational role (i. e., civil or military discussion on military aircraft is given on Web site) – and this chapter is divided into these two classes. In the second level, the classification is based on the generic mission role, which also would indicate size. The third level proceeds with classification based on the type of power plant used and so on. The examples worked out in this book are the types that cover a wide range of aircraft design, which provides an adequate selection for an aircraft design course.
Figure 4.2 indicates the speed-altitude regimes for the type of power plant used. Currently, low-speed-low-altitude aircraft are small and invariably powered by piston engines of no more than 500 horsepower (HP) per engine (turboprop engines start to compete with piston engines above 400 HP). World War II had the Spitfire aircraft powered by Rolls Royce Merlin piston engines (later by Griffon piston engines) that exceeded 1,000 HP; these are nearly extinct, surviving only in museum collections. Moreover, aviation gasoline (AVGAS) for piston engines is expensive and in short supply.
The next level in speed-altitude is by turboprops operating at shorter ranges (i. e., civil aircraft application) and not critical to time due to a slower speed (i. e., propeller limitation). Turboprop fuel economy is best in the gas turbine family of engines. Subsonic cargo aircraft and military transport aircraft may be more economical to run using turboprops because the question of time is less critical, unlike passenger operations that is more time critical with regard to reaching their destinations.
The next level is turbofans operating at higher subsonic speeds. Turbofans (i. e., bypass turbojets) begin to compete with turboprops at ranges of more than 1,000 nm due to time saved as a consequence of higher flight speed. Fuel is not the only factor contributing to cost – time is also money. A combat aircraft power plant
Chart 4.1. Aircraft classification
Figure 4.2. Engine selections for speed – altitude capabilities
uses lower bypass turbofans; in earlier days, there were straight-through (i. e., no bypass) turbojets. Engines are discussed in more detail in Chapter 10.
Figure 4.3a illustrates the thrust-to-weight ratio of various types of engines. Figure 4.3b illustrates the specific fuel consumption (sfc) at sea-level static takeoff thrust (TSLS) rating in an ISA day for various classes of current engines. At cruise speed, the sfc would be higher.
Design lessons learned so far on the current trend are summarized as follows: 
Figure 4.4. Range versus passengers
of passenger movement. Lower acquisition costs, lower operational costs, and improved safety and environmental issues would act as design drivers. The SST would attempt an entry and HST operations still could be several decades away.
• Military aircraft design: Very agile aircraft incorporating extensive microprocessor-based control and systems management operating below Mach 2.5, high altitude (> 60,000 ft), and BVR capabilities would be the performance demand. The issue of survivability is paramount – if required, aircraft could be operated unmanned. The military version of hypersonic combat aircraft could arrive sooner, paving the way to advance civil aircraft operations. Armament- and missile-development activities would continue at a high level and would act as one of the drivers for vehicle design.