Manticore
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DESIGN FOR AIR COMBAT
True supercruiser concept
Experience has shown that even when a combat encounter occurs at supersonic speed, the {59} reduction in specific excess power as a result of manoeuvring causes a rapid slowing to subsonic speed if the engagement is pursued. NASA research during the 1970s on wing planforms and sections for a second-generation supersonic transport appeared to show how to maintain both speed and manoeuvrability. It was to this end that in the late 1970s the USAF launched the Supersonic Cruise and Manoeuvring Programme (SCAMP). As part of this effort General Dynamics and NASA investigated over 150 different configurations in the course of 3,600hr of wind-tunnel testing before selecting the cranked-arrow (or compound-sweep delta) planform.
Fig 46 General Dynamics F-16 design evolution.
Then, in mid-1980, the USAF defined a more immediate requirement for an improved ground attack aircraft to succeed the F-4 and F-111. General Dynamics shifted its sights to this goal and changed certain details of its cranked-arrow wing to meet the new requirement. The changes affected the camber, twist and trailing-edge reflex, optimising the wing for supersonic speed at low level. Remarkably, the new wing was not part of a completely new aircraft but was married to the existing F-16. As can be seen in Fig 46, the transformation was profound. The new 60m2 wing was mated to the basic F-16 structure by means of two fuselage plugs, one 0.91 m long and inserted ahead of the undercarriage, and a 0.67 m section aft. To provide for ground clearance on rotation the longer rear fuselage was angled up by 3° and the ventral fins deleted. The upsweep puts the thrust line below the CG, helping to improve rotation on takeoff. This, together with the very advanced wing, allowed the F-16XL (subsequently known as the F-16E) to be rotated at speeds down to 195km/hr, leading to a field-length requirement only two-thirds that of the F-16A. Flight testing of the F-16E showed it to have a lift/drag ratio between 10 and 45% better than that of the basic F-16A, and it could roll and pitch faster in any configuration. It could also pull an AFCS-limited 9g over twice the Mach-number range. While the F-16E offers no L/D improvement in subsonic manoeuvres, it does retain the subsonic cruise efficiency of the F-16 planform. Where it scores is in supersonic cruise performance: at Mach 2.2 its L/D is over 9. This is due to the improved fineness ratio arising from the
{60}
The General Dynamics F-16E's cranked-arrow wing was developed over 3,600hr of wind tunnel testing in the late 1970s. The high degree of sweep on the inboard leading edge means that flaps were fitted only outboard of the kink. The absence of a forward cockpit canopy frame is unique to the F-16 (General Dynamics)
increased fuselage length and even better wing/body blending. Even though the wing area is more than double that of the standard F-16, the skin friction drag is only 22% more, due partly to the deletion of the horizontal tail.
The cranked-arrow planform of the F-16E comprises a sharply swept (70°) leading-edge inboard section lying within the shock cone of the nose and, at 63% semi-span, a 50° outboard section of thin profile and sharp leading edge. This is designed to obtain the low wave drag associated with highly swept or thin wings without the aerodynamic penalties of sweep or structural problems of thin sections. However, the F-16E takes the delta planform {61} much further in that the experience gained with the leading-edge strakes of the basin F-16 enabled General Dynamics to maximise the vortex-lift benefit. The two swept panels of the cranked-arrow planform produce vortex systems which mutually interfere. At low angles of attack the leading-edge vortex from the inboard wing passes over the root chord of the outboard wing panel. In addition, vortex lift is available at the tip as a result of the action of the outboard leading-edge vortex. At high AOA the single primary vortex system acts over the whole of the outer panel. Thus augmented vortex lift occurs at supersonic speed, while at lower speeds the benefits of the primary vortex counter the high induced drag which plagued earlier delta planforms.
Fig 47 F-16E flying qualities compared with those of the F-16A. Lateral/directional stability is improved; external loads do not adversely affect flying qualities: there are no limitations due to buffet, wing rock, nose slice, deep-stall trim points, or spin tendency; there are no limits on angle of attack, minimum speed and bank angle; and the full range of manoeuvres can be performed while carrying the maximum load of air-to-ground stores.13
The improvements in stability and control with and without stores were such that no limitations due to buffet, wing rock or nose slice, nor spin tendency were encountered during the flight test programme. Angle-of-attack excursions resulted when the airspeed dropped to zero but the aircraft always recovered without any pilot input. 360° rolls at maximum g/maximum AOA similarly failed to cause any departure from controlled flight (Fig 47).
The major difference between the application of vortex flow to transonic fighters (e.g. F-16, F-18) and to the supercruise fighter (e.g. F-16E) is the extent of vortex lift available. The supercruise fighter, having more of the wing highly swept, develops more of this lift. Only a small fraction of the increased lift comes from the potential (or attached-flow) lift; the rest is due to the vortex lift acting over the increased wing area. This extra lift increases instantaneous turn rate, now regarded as more important than sustained turn rate, which largely governed the original F-16 design. Newer gunsights and missiles like the AIM-9L Sidewinder reduce the need to hold the target in the sight for weapon aiming. General Dynamics relinquished a small amount of sustained manoeuvrability in order to double the 9g envelope and move it into the high supersonic regime.
It should not however be forgotten that the F-16E was initially developed for ground attack. In this respect the new wing increased internal fuel capacity by 82%, which eliminated for most missions the weight and drag of external tanks. This gave it a 45% increase in combat radius with twice the weapon load of the F-16A and a more than 120% increase with the same weapon load. The aircraft is equipped with 17 store stations with 29 hard-points. In the event the F-15E, a development of the two-seat F-15C was chosen as the dual-role fighter for the USAF, though it remains possible that development of the F-16E will continue.
