The birth of the airplane
It was Clément Ader who created the word airplane from the Latin avis, which means bird. This is the name he gave to the aircraft he built in 1897, the successor to the Aeolus, which was the first (on October 9, 1890) to leave the ground over a distance of about 50 meters under the sole impulse of its driving force. As a tribute to the prophetic views on the military use of aircraft expressed by Ader in several books, it was officially decided, around 1912, to call military aircraft “airplanes”. The name “aeroplane” was used for civil aviation aircraft, but it has fallen into disuse.
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Today, the word “airplane” designates both military and civil aircraft, i.e. any piloted aircraft, or aerodyne, which, among the heavier-than-air aircraft, ensures its lift by means of fixed wings or, at most, with variable geometry; their propulsion is ensured by engines. This definition therefore excludes, among aerodynes, gliders and helicopters, but not vertical take-off aircraft, because, during the take-off and landing phases, lift is provided by the propulsive force of the engines, which is then directed downwards.
The design of any aircraft is the result of a synthesis between different disciplines, among which the most important are: aerodynamics, propulsion, structure, manufacturing techniques and materials. To these basic disciplines, which were already involved in the very first aircraft, have been added techniques related to equipment, such as electronics, called avionics when applied to aviation.
This synthesis of disciplines with often contradictory requirements implies compromises that form an important part of the aircraft manufacturer’s know-how.
Military and civil aircraft are very clearly distinguished by their functions. Military aircraft must be able to carry out a mission with precision and efficiency during a conflict. This leads to a focus on performance, maneuverability, discretion (stealth) and operational safety. On the other hand, for civilian aircraft, airlines are looking for commercial profitability, after safety, i.e. the lowest possible operating costs, while offering the best possible comfort.
The classification of military aircrafts
Military aircraft are classified according to categories linked to their operational role. There are four main groups: fighters and assault aircraft, bombers, transport aircraft and training aircraft. Other groups with smaller numbers are added, such as reconnaissance aircraft, in-flight refuelling aircraft, aircraft specialising in surveillance or electronic countermeasures. Most of the aircraft in these latter groups are derivative versions of aircraft from the first groups or of civilian aircraft, after adaptation of the equipment; but few modifications are made to the airframe or the engine. Each group can be subdivided to distinguish, for example, land-based or carrier-based aircraft.
The nomenclature established by the United States clearly indicates the belonging of each aircraft to a group by the first letter or letters: F for fighter, A for attack, B for bomber, C for cargo, T for trainer. The number following these letters (F-16, C-135) is a type number. Additional letters then specify either the version (R for reconnaissance, for example) or a modification made during production.
Similar nomenclatures have been established by the British and Russians. In France, the aircraft names do not show the group to which they belong (Mirage III, Mirage 2000, Rafale, Transall), but an additional letter identifies the version (for example, the Mirage 2000 RAD: reconnaissance version for Abu Dhabi).
Unlike civilian aircraft, the operational characteristics and performance of a military aircraft are generally imposed by the client army, which establishes a program sheet defining the required specifications. A competition is opened between different manufacturers, each of which proposes an existing aircraft or a project that meets these specifications as closely as possible. This competition can be open to foreign manufacturers or restricted to nationals. Sales or purchases of military equipment abroad must be approved by the governments of the countries concerned, as they are political gestures.
Military transport aircraft as well as subsonic bombers have an architecture similar to that of commercial aircraft and, therefore, similar aerodynamic and structural characteristics. The differences lie in the constraints brought by the type of material carried and in the need to be operational in virtually all weather conditions and from rough or unprepared, possibly very short, terrain. This implies all-weather autonomous navigation equipment and, from the aerodynamic point of view, a strong hypersuspension allowing the flight at very low speeds. Furthermore, the structure and landing gear are reinforced, and generally the wing is placed in a high position with respect to the fuselage. Large doors and hatches are provided for vehicle loading and in-flight drops.
On the other hand, combat aircraft have a very specific architecture and characteristics, implied by the speed (often supersonic), acceleration and maneuverability performances that are required. The engine(s) occupy a large part of the aircraft’s total volume. They develop a thrust that tends to equal or even exceed the weight of the aircraft. The tanks also constitute an important part of the volume; while the remainder is occupied by the cockpit, the avionics (radar, computers) and the flight controls. The majority of the loads (bombs, missiles, containers) are carried attached by pylons outside the aircraft.
The main roles that can be required of combat aircraft are: air defense or interception, air superiority, penetration and ground attack or tactical support. The qualities required are, respectively, a high rate of climb, excellent maneuverability and low sensitivity to gusts and turbulence.
Performance and flight qualities
The choice of aerodynamic characteristics for a combat aircraft is more complex than for a transport aircraft, because in addition to the constraints of cruising, takeoff and landing, there are maneuvering possibilities.
Maneuverability criteria can be classified into three groups. First, those related to performance: acceleration or deceleration, turn radius, load factor, speed; second, those related to control surface efficiency: roll rate, yaw control; and third, those related to the controllability of the aircraft in extreme conditions of incidence, speed or turbulence: static and dynamic stability.
Avionics and armament
For a combat aircraft, electronic equipment and weapons are the most sophisticated and expensive components. It is estimated that avionics account for nearly 60 percent of the total cost of the aircraft, while the airframe and engine each account for only 20 percent (these proportions are respectively 20, 50 and 30 on a civilian aircraft, and the share of avionics is tending to decrease). The “fly away” price, i.e. in working order but without spares or weapons, of an aircraft like the Rafale was around 50 million euros in 2004.
The most important electronic equipment on a fighter aircraft is its radar, which is now required to have multi-function capabilities: air-to-air and air-to-ground combat, designation and tracking of multiple targets for missile firing, terrain tracking, anti-jamming. These capabilities are now possible thanks to advances in digital electronics. Similarly, heavy and cumbersome mechanically scanned antennas are being replaced by electronically scanned antennas, either passive or active, which are lighter and capable of more operating modes.
Autonomous navigation systems, with mechanical gyroscopes or gyrolasers, are a key element of the aircraft, allowing it to steer safely and precisely to a target. They can also be readjusted to compensate for the small but unavoidable drift in time by terrain recognition systems or by using signals emitted by satellites (G.P.S.-Navstar system).
Aircraft are also equipped with threat detectors (detection of electromagnetic, infrared or laser signals) and countermeasure, jamming or decoy systems. Thermal cameras, using infrared radiation, allow pilots to fly at night and better detect certain targets, such as tanks.
A combat aircraft is not a weapon in itself, but only a highly maneuverable, long-range platform capable of carrying and firing munitions. These are very varied and can range from cannon shells to nuclear-armed cruise missiles.
Except on bombers, the weapons are, for the most part, carried externally, hooked under the wing, at its extremities or under the fuselage. The internal armament is reduced to one or two cannons capable of firing between 200 and 1,000 shells of 20 to 30 mm in diameter. The external carrying points, numbering from 5 to 9, can receive a wide variety of loads thanks to appropriate pylons. External tanks, which can be jettisoned in flight once empty, are used for long-distance missions, for convoying or for in-flight refuelling of other aircraft. Various containers (or pods) for reconnaissance, electronic countermeasure or laser target designation can be carried.
The weapons themselves consist either of short- or long-range air-to-air, air-to-surface or air-to-sea missiles (from a few hundred metres for close combat to several hundred kilometers otherwise), or of gliding or guided bombs, or of rocket or grenade launchers. The maximum load that can be carried is about half the weight of the aircraft in flight order. Laser-guided bombs and certain missiles have semi-active guidance systems. In this case, it is the carrier aircraft which “illuminates” the target by directing an electromagnetic emission or a laser beam at it. The missile, or the bomb, equipped only with a detector, is then automatically directed towards the target thus designated. As long as the target is not reached, the aircraft must remain in the vicinity, which exposes it to possible counter-attacks. More sophisticated missiles have autonomous guidance systems using active radar or infrared detectors that capture the emissions produced by exhaust gases.
A new discipline appeared in the 1960s-1970s: radar stealth, also called stealth. The aim is to remain invisible for as long as possible to enemy ground or airborne radars, in order to evade the adversary’s defences, particularly its anti-aircraft missiles. The aim is to reduce the radar equivalent cross-section (R.E.S.), which is the measure of the echo emitted by the aircraft: this can range from a few square meters for untreated aircraft to only a few square millimeters for stealth aircraft. The first aircraft to use these technologies was the SR-71 Blackbird built by Lockheed, capable of flying at mach 3.5 and up to 24,000 meters altitude. The F-117A, developed from 1978, was famous in 1991, during the Gulf War, as was the B-2 bomber.
At first glance, a surveillance radar can be compared to a lighthouse that searches the sky to try to detect a target thanks to the light reflected by it. But radar has some essential differences, such as the emission of short pulses – which, by measuring the round-trip time, makes it possible to evaluate the distance to the target – or the use of coherent waves such as lasers, which make it possible to distinguish the target from the many surrounding parasites. The aircraft must be designed to reduce the reflection of these waves and their diffraction.
As in optics, reflections are maximal when the surface is perpendicular to the direction of the radar. One thus seeks to reflect and concentrate the waves in a direction different from that of the radar by inclining the surfaces; the plane then presents facets like the F-117A. Another technique consists in diffusing the waves weakly in all the directions thanks to very rounded surfaces, technique used on B-2 in the shape of flying wing. As for the diffractions, they occur on the edges. For the same reasons as previously, the rectilinear edges and inclined compared to the direction of the radars are preferable.
It is also necessary to treat the surface waves which cling to the weakly curved surfaces such as the wings or the fuselage. These waves diffract on the many small obstacles they encounter: cracks, asperities, links between materials, curvatures, etc. Thin layers of absorbing materials can reduce them, but stealth aircraft designers are mercilessly hunting down all these surface accidents.
Cavities such as engine air intakes, cockpits, antennas and radars are also sensitive areas. The electromagnetic waves penetrate there and then come out in a relatively isotropic way after multiple and complex reflections on the walls. American engineers have placed the air inlets of their stealth aircraft above the wing to take advantage of the masking effect of the wing or fuselage. But this is not enough to erase them entirely. The treatment is completed by the use of obstacles – grids or metallized surfaces – to prevent the waves from penetrating and absorbent materials intended to trap the wave. This is particularly the case for the cockpit, where the windows and domes are metallized. The air inlets of the engines are flattened and placed most often above the wings in order to mask them.
To reduce the radar signature, work on the shape of the aircraft is not enough. Materials that absorb electromagnetic radiation over as wide a frequency band as possible are applied to the surface of the aircraft and in the cavities. Two conditions are the basis for the operation of these materials. Firstly, the wave must penetrate to the heart of the material and its impedance must therefore be close to that of the air (impedance is the property of the material which causes a signal entering it to be transformed or even reflected, which is what we are trying to avoid here. This phenomenon is analogous to what happens in optics with the index of refraction). Without this adaptation of the impedance, the wave will be reflected at the surface; this is what happens on metals. Then, the energy of the wave must be absorbed in the thickness of the material where it is transformed into heat.
Impedance matching is easier to achieve with magnetic-type materials, usually composed of iron or ferrite powders of small particle size dispersed in resins, and implemented as paint or bonded coatings. These materials have the disadvantage of being quite dense and, therefore, of weighing down the aircraft. In addition, their characteristics change with temperature. Purely dielectric materials require greater thicknesses to be effective: of the order of a quarter of the wavelength considered. The adaptation of the impedance is obtained here thanks to the interference network which is established inside the structure of the material. Low density materials can be used, such as polyurethane foam or honeycomb structures, modified with resins filled with carbon black or conductive polymers.
To avoid the multiple radar reflections created by carrying weapons attached to the wings or fuselage as on a conventional fighter aircraft, the weapons are located in a hold. The hatches of the latter, like all the other moving parts of the aircraft (canopy, landing gear doors…), and the connections between panels have their front and rear edges cut in a sawtooth pattern to reduce reflections. All these edges, as well as all the facets of the external surface of the aircraft are parallel to a small number of directions – less than half a dozen per side -, thus concentrating the reflections of incident radar waves in specific directions of space, far from those of surveillance radars.
Stealth is also based on the absence of active sensors – such as radar – and radio silence in operation, thus avoiding any electromagnetic emission that could allow the detection and location of the aircraft. The latter generally operates at night. For this, it is equipped with infrared cameras, one for piloting, the other, possibly retractable, allowing the bombs to be directed towards their targets.
A particular care must be brought also to suppress the emissions in the field of the infra-red wavelengths in order to avoid the spotting by the glasses and the missiles with infra-red detectors. Thus the hot gases of the engines must be cooled by dilution and the exit of the engines masked by the wings or the drifts. Metallic or carbon particles, which absorb the infrared radiations, can also be injected in the flow of the engines, but it can only be for short durations the reserve of powder transported by the plane being limited.
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