After the invention of the airplane, designers and engineers created new aircraft for a variety of uses. Airplanes became a part of daily life. Aircraft were used regularly to ship cargo and to transport people. Over time, the speed of aircraft has increased. Aircraft are grouped based upon how fast they can fly. These groups are the speed regimes (pronounced ``ra-jeems'') of flight. There are five basic speed regimes. These include the earliest aircraft to the most modern.
Low Speed Flight, 0-100 MPH
The early development of human flight includes air vehicles in this speed range. The first air vehicles included kites, balloons, and gliders. These unpowered aircraft were very slow. The development of relatively lightweight engines paved the way for early airships and winged aircraft, but the materials, knowledge and technology available limited these aircraft to low speeds.
These vehicles had to be very lightweight because of the limited power of the available engines. To build lightweight structures, designers used external bracing. That, and the open fuselage designs of the day, resulted in vehicles with high drag. So the limited thrust available was overcome by the drag produced even at speeds as low as 50 mph. The most famous example is the 1903 Wright Flyer.
Modern vehicles in this category include kites, balloons, hang gliders, ultra-light hobby aircraft, and airships. As in the early days, these aircraft are limited by the power available from small, light engines and by lightweight structures. These aircraft are generally faster than their predecessors because of stronger, light-weight materials (nylon and aluminum), improved knowledge of aircraft design, and improved engines (with a higher ratio of power to weight).
Medium Speed Flight, 100-350 MPH
In order to build faster aircraft, several areas or technologies had to be improved. First, the drag had to be reduced substantially. This was accomplished largely by developing enclosed, streamlined fuselages and stronger wings that did not require external bracing. This meant the structure had to be made stronger, but hopefully not heavier. Thus, materials and structures were developed with a higher ratio of strength to weight. Next, the thrust had to be greatly increased without making engines too heavy. Thus, the engine's ratio of power to weight increased. All of these areas improved in a leapfrog manner. For example, a new engine might be developed with 50 percent more power (good!) that weighed 25 percent more (not so good!) Note that this engine has a higher power-to-weight ratio, but it also weighs more. To carry this extra weight, the fuselage would have to be stronger and heavier. The resulting aircraft might not really be faster until it could be designed to be lighter. Finally, knowledge of aerodynamics improved through the use of new tools such as wind tunnels and a lot of basic research.
The vehicles that are found in this regime are limited by the source of the thrust and, to a lesser extent, the drag. The engines are almost all propeller types, and the wings are almost all straight and fairly thick. Vehicles in this category are propeller craft like Fokker, Junkers, Cessna and Beechcraft. The need for higher-performance aircraft during WWII accelerated the development of aircraft in this speed regime. At the same time that speed was being improved, so was the carrying capacity of aircraft.
High Speed Flight, 350-760 MPH
In order to go faster, aircraft needed engines with even higher performance (power/weight). The development of the jet engine was the first key to faster aircraft. As aircraft exceeded 500 mph, the drag increased rapidly, and a new aircraft design was needed to reduce the drag. The development of the thin, sweepback wing provided the second key to high speed flight. As before, improvements in aerodynamic knowledge and technologies such as materials contributed to the evolution of modern, high-speed aircraft.
The resulting vehicles were limited by the power of their engines and the amount of drag they made. Generally, the more swept the wings, the faster they could go. They were all sleek-looking shapes that minimized drag as much as possible. Aircraft were approaching the speed of sound, and drag was increasing dramatically. The speed of sound is about 760 mph at sea level and about 650 mph at 35,000 feet, and is called Mach 1 (for one times the speed of sound). At these speeds, the air has difficulty "flowing" around the airplane, with the result that shock waves form and drag increases dramatically. Also, the aircraft was difficult to control at these speeds. This largely limited aircraft to flying below the speed of sound, called subsonic flight. There are no airplanes that spend significant time flying between 700 and 800 mph because of the high drag and the control issues.
Modern airplanes that fly slower than the speed of sound (fast, but still subsonic) all peak out in speed well short of the speed of sound (below Mach <0.9). Higher speed flight would be desirable, but today the best tradeoff between speed and economy for trans-porting a large number of passengers or cargo is near Mach 0.8. Vehicles in this class are the commercial Boeing 700 series, the Boeing B-47 Strato Jet, Vickers Viscount, the Lear Jet, and many military aircraft.
The actual speed of sound depends on the compressibility and density of the air as well as its temperature. This means that an airplane flying at the speed of sound at ground level under normal atmospheric conditions will actually be flying at a speed of 760 mph. However, while flying the at the speed of sound at 37,000 feet at normal stratosphere temperature, the airplane would actually be flying about 660 mph. If an airplane is flying slower than the speed of sound we say it is moving at subsonic speed. If it is flying at the speed of sound it is traveling at sonic speed. If it is flying faster than the speed of sound it is traveling at supersonic speed. The speed of sound is measured in Mach numbers. If an airplane is traveling at Mach 1 it is moving at sonic speed or at the speed of sound. If it is traveling at Mach 2 it is moving at two times the speed of sound. The word "Mach" comes from the Austrian scientist Ernst Mach who studied airflow and the speed at which sound travels.
Supersonic Flight, 760-3,500 MPH (Mach 1 to Mach 5)
With the desire to fly faster, primarily for military aircraft, aeronautics technologies were developed to fly efficiently above Mach 1. Efficiency is a relative term - the aircraft are still very expensive to operate and most are for military use. To date, only one aircraft, the Concorde, provides commercial transportation above Mach 1. Efforts are underway to develop new technologies so that a more cost-effective supersonic airplane can be built in the future. Supersonic aircraft have special high-performance jet engines that can make a lot of thrust, very thin wings that have lots of sweep, and use novel materials to provide strength.
Early fuselages tended to be shaped like a wasp's body (thin at the waist). The thinning of the fuselage helps reduce the drag that the airplane makes when flying near the speed of sound. It is relatively easier to fly above Mach 1 than near Mach 1. Hefty engines are needed to provide the thrust necessary to push the airplane through the air at such high speeds. The wings are super thin and swept to slice through the air while making as little disturbance as possible. The most modern supersonic aircraft spend so little time near Mach 1 and have such powerful engines, that they are not shaped as much like a wasp's body. Still, these aircraft have sleek overall shapes that are carefully designed to minimize supersonic drag.
It is interesting that airplanes designed to fly supersonically do not fly very well at sub-sonic speeds. The same features that let them fly fast do not work well when flying slowly. In fact, flight at the lowest speeds - takeoff and landing - is an extra challenge when designing these aircraft. Vehicles in this category include the commercial airliner Concorde, F-15 Eagle and the SR-71.
Hypersonic Flight, 3,500-7,000 MPH (Mach 5 to Mach 10)
With the advent of rocketry, the first hypersonic vehicles were developed. Although they are not airplanes, rockets travel at these speeds as they accelerate into Earth orbit. Also, re-entry capsules such as those in the Apollo program travel at these speeds as they descend from orbit. Once again, new technologies and new vehicle shapes had to be developed for hypersonic flight. In particular, new materials had to be developed to handle the intense heating caused by atmospheric friction.
The best known examples of hypersonic flight vehicles are the rocket-powered X-15 and the space shuttle which flies through all of the speed regimes when it re-enters the Earth's atmosphere. The space shuttle is coasting from a very high speed and high altitude when it flies hypersonically. It is decelerating the entire time. There are no aircraft today that can cruise at these speeds. Research programs are underway to develop new engines that can operate at these speeds so that we can develop aircraft to cruise in this speed regime. Two such experimental aircraft tested include the X-33 and the X-34. It is a tremendous challenge to design an airplane shape and an engine that can take off subsonically, accelerate through supersonic speeds, and cruise efficiently hypersonically.
What's faster than hypersonic? Hypersonic flight occurs at very high altitudes where the air is very thin. This helps reduce the drag and the heating due to friction. This thin air and high speed is part of what makes it so difficult to design an engine for these aircraft. To fly faster than hypersonic speed requires even thinner air at higher altitude. At these speeds and altitudes a vehicle is essentially outside the atmosphere and would more correctly be called a spacecraft. The space shuttle is both a spacecraft and aircraft.
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