Military Aircraft Drag

Although military aircraft topics are not discussed here, and instead are found in the Web at www. cambridge. org/Kundu, this important topic of military aircraft drag estimation is kept here.

Military aircraft drag estimation requires additional considerations to account for the weapon system because few are carried inside the aircraft mould lines (e. g., guns, ammunition, and bombs inside the fuselage bomb bay, if any); most are exter­nal stores (e. g., missiles, bombs, drop-tanks, and flares and chaff launchers). With­out external carriages, military aircraft are considered at typical configuration (the pylons are not removed – part of a typical configuration). Internal guns without their consumables is considered a typical configuration; with armaments, the aircraft is considered to be in a loaded configuration. In addition, most combat aircraft have a supersonic-speed capability, which requires additional supersonic-wave drag.

Rather than drag due to passenger doors and windows as in a civil aircraft, mil­itary aircraft have additional excrescence drag (e. g., gun ports, extra blisters and antennas, and pylons) that requires a drag increment. To account for these addi­tional excrescences, [3] suggests an increment of the clean flat-plate equivalent drag, f, by 28.4%.

Streamlined external-store drag is shown in Table 9.8 based on the frontal max­imum cross-sectional area.

Table 9.8. External-store drag

External store

CDn (Based on frontal area)

Drop tanks

0.10 to 0.20

Bombs (length/diameter < 6)

0.10 to 0.25

Bombs/missiles (length/diameter > 6)

0.25 to 0.35

Bombs and missiles flush with the aircraft contour line have minor interference drag and may be ignored at this stage. Pylons and bomb racks create interference, and Equation 9.17 is used to estimate interference on both sides (i. e., the aircraft and the store). These values are highly simplified at the expense of unspecified inac­curacy; readers should be aware that these simplified values are not far from reality (see [1], [4], and [5] for more details).

Military aircraft engines are buried into the fuselage and do not have nacelles and associated pylons. Intake represents the air-inhalation duct. Skin friction drag and other associated 3D effects are integral to fuselage drag, but their intakes must accommodate large variations of intake air-mass flow. Military aircraft intakes operate supersonically; their power plants are very low bypass turbofans (i. e., on the order of less than 3.0 – earlier designs did not have any bypass). For speed capa­bilities higher than Mach 1.9, most intakes and exhaust nozzles have an adjustable mechanism to match the flow demand in order to extract the best results. In gen­eral, the adjustment aims to keep the Vi„take/Vm ratio more than 0.8 over opera­tional flight conditions, thereby practically eliminating spillage drag (see Figure 9.7). Supersonic flight is associated with shock-wave drag.

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