Air inlet in jet fighter

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Air Inlets

The tremendous amount of power that can be extracted from a single, modern turbopropulsion system has already been discussed. To generate this power with maximum efficiency, the large quantities of propulsion-system air must be delivered to the engine face with minimum aerodynamic loss, turbulence, and flow distortion. High efficiency must be maintained for different engine-operating conditions, different aircraft speeds and altitudes, and for a wide spectrum of angles of attack and sideslip. As one example, a jet transport must inhale air efficiently in the near static condition at the beginning of takeoff roll, in [240] the relatively low-speed, high-power climb condition, and in high-speed, high-altitude cruise flight. At all flight conditions, the propulsion-system air must be decelerated to a low-speed, high-pressure state at the engine compressor face. The detail design of the air intake and internal flow system determines the efficiency with which the air is delivered to the propulsion system. In this case, the efficiency is defined as the ratio, expressed in percent, of the average total pressure of the air entering the engine to that of the free-stream air. The total pressure is the sum of the static, or ambient, pressure of the air and the impact pressure associated with its motion. Modern jet transports may cruise with values of the pressure recovery, that is efficiency, of 97 to 98 percent. Supersonic aircraft with well-designed, practical inlet and internal flow systems may have pressure recoveries of 85 percent or more for Mach numbers in the 2.0 to 2.5 range.

The demanding requirements for high inlet and internal-flow-system efficiency stimulated a large amount of research, development, and engineering effort in the years following the end of World War II. Fortunately, this effort could be based on a solid foundation of earlier work on such things as cowlings and radiator scoops for piston engines. Inlet activity intensified as aircraft penetrated the transonic and supersonic speed ranges, and the field of inlet and internal flow system design soon became a well-recognized engineering specialty. Especially in modern fighters that may have thrust-to-weight ratios in the order of 1, the inlet and its integration with the airframe exert a powerful influence on the overall aircraft design. The aim in engine-airframe integration is to minimize airplane drag, weight, and complexity and to maximize propulsion-system efficiency while, at the same time, ensuring that the aircraft mission requirements have not been compromised. A detailed discussion of the many facets of inlet design is beyond the scope of the present discussion; however, a few examples of inlets that have been used on civil and military aircraft are illustrated and described in the following paragraphs.

Already shown in the discussion of aircraft noise is an inlet typical of those currently employed on modern subsonic transport and strategic bomber aircraft. The splitter rings in the inlet shown in figure 10.8 are part of an experimental installation, which, as mentioned, are not used on production aircraft. The open nose inlet shown is simple, is light in weight, and when used with a pod-mounted engine, has a short, low-loss duct connecting the engine to the inlet. High-pressure recoveries that are relatively insensitive to normal variations in angle of attack and sideslip are possible with this type of inlet.

In contrast to the pod-type mounting found on so many multiengine transport aircraft, most fighters have one or, at the most, two engines situated inside the fuselage. A variety of inlet locations and designs have been employed to supply air to the propulsion system on these aircraft. Each of these arrangements have both advantages and disadvantages. Four typical installations employed on fighter aircraft are illustrated in figure 10.9. These do not by any means constitute all the successful configurations that have been employed on such aircraft over the years. Most installations, however, are variants of those shown.

[243] Figure 10.9 - Four inlet locations used on jet-powered fighter aircraft. [NASA]

The simple nose inlet employed on the North American F-86 fighter is illustrated in figure 10.9(a). As indicated previously, this type of installation enjoys good characteristics through a wide range of angle of attack and sideslip and, when located in the front of the fuselage as contrasted with a pod, is free from aerodynamic interference effects-such as flow separation-from other parts of the aircraft. The long internal duct leading from the inlet to the engine, however, tends to have relatively high pressure losses. In addition, interference between the duct and the pilot's cockpit may be encountered. In some designs, the duct passes under the cockpit; in others, it is split and passes around the cockpit on either side of the pilot. Perhaps the largest drawback of the nose inlet, however, is that neither guns nor radar can be mounted in the front of the fuselage. A nose inlet has not been used on a new fighter in the United States since the early 1950's.

[242] The chin inlet employed on the F-8 airplane shown in figure 10.9(b) has many of the advantages of the simple nose inlet but leaves space in the front of the fuselage for radar or guns and has a somewhat shorter internal duct. Care should be taken in such a design to ensure that at no important flight condition does separated or unsteady flow enter the inlet from the nose of the aircraft. The proximity of the inlet to the ground introduces a possible risk of foreign object ingestion, and, obviously, the nose wheel must be located behind the inlet. The chin inlet, however, is a good choice for some applications and is employed on one new contemporary aircraft (the General Dynamics F-16).

Shown in figure 10.9(c) is the wing-root inlet installation employed on the McDonnell F-101 fighter. Inlets located in this manner offer several advantages. Among these are short, light, internal flow ducts, avoidance of fuselage boundary-layer air ingestion, and freedom to mount guns and radar in the nose of the aircraft. Further, no interference between the cockpit and internal ducting is encountered in this arrangement. The short, curved internal ducts, however, require careful design to avoid flow separation and associated losses, and the inletwing integration must be accomplished in such a way that neither the function of the wing nor the inlet is compromised. Wing-root inlets were used on a number of aircraft in the first decade of the jet fighter, but such inlets are not suitable for modern fighters of high thrust-to-weight ratio because of the large-size inlets required by these aircraft and the difficulty of integrating them with the wing.

Side-mounted inlets as used on the Grumman F11F are illustrated in figure 10.9(d). Used on both single- and twin-engine fighters, the side-mounted inlet arrangement probably offers the best compromise of all the conflicting aerodynamic, structural, weight, and space requirements, and it is used on many modern combat aircraft. Great flexibility in inlet size, shape, vertical position, and fore and aft location is offered by the side-mounted installation. Although the F I IF is a design of the 1950's, side-mounted inlets are used on many fighters of the 1970's and 1980's, as described in chapter 11. Before leaving the discussion of figure 10.9(d), it should be noted that the boundary-layer diverter plates are located so as to prevent ingestion by the inlets of the fuselage boundary-layer air. Such boundary-layer diverters are a feature, really a complication, of all fuselage-mounted inlets.

The inlets just described are of the fixed-geometry type; that is, they do not change shape or size as the aircraft speed varies. Fixed-geometry inlets are suitable for aircraft designed to operate at subsonic...

[244] and low supersonic speeds. For flight at Mach numbers much beyond 1.6, however, variable-geometry features must be incorporated in the inlet if acceptably high inlet pressure recoveries together with low external drag are to be achieved. This complication is dictated by the physical laws governing the flow of air at supersonic speeds. The nature of supersonic flows is not discussed here, but two variable-geometry inlets are illustrated in figure 10.10. Shown at the top in figure 10.10(a) is the D-type side inlet used on the McDonnell Douglas F-4 fighter. Evident in the photograph are the large fixed diverter plates that also serve to begin compression of the entering flow. The adjustable ramps provide further compression along with the desired variation of throat area with Mach number. The angle of the ramps varies automatically in a prescribed manner as the Mach number changes.

The quarter-round inlet equipped with a translating centerbody or spike, as used on the General Dynamics F-111 airplane, is illustrated in figure 10. 1 0(b). The inlet is seen to be bounded on the top by the wing and on one side by the fuselage. An installation of this type is often referred to as an "armpit" inlet. The spike automatically translates fore and aft as the Mach number changes. Although not evident in the photograph, the throat area of the Inlet also varies with Mach number. This is accomplished by expansion and contraction of the rear part of the spike. The diverter for bypassing the fuselage boundary air is also shown in the photograph. The cover over the inlet is to prevent foreign objects from entering the propulsion system while the aircraft is parked on the ground, and, of course, is removed before flight.

The design of inlet systems for supersonic aircraft is a highly complex matter involving engineering trade-offs between efficiency, complexity, weight, and cost. Some of the factors involved in supersonic inlet design are discussed in references 157 and 179, and the problems of engine-airframe integration on supersonic aircraft are summarized well in reference 180. The highly important problem of selecting and integrating the variable-geometry nozzle of afterburning engines is beyond the scope of the present discussion but is also included in the material presented in reference 180.