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Guys can u please explain to me how that saw tooth reduces the RCS..?
Found this article bout stealth features of F 117. hope solves your query... Don curse if it doesn... Thanx...
Stealth techniques today concentrate upon the reduction of the radar cross section and infrared emissions of an airframe, as these parameters are critical to the performance of radar and infrared fire control and guidance systems.
How this is achieved becomes more evident upon closer examination of a specific design.
The unique geometry of the F-117A reflects the state of the art in RCS modelling techniques in the late 1970s, in stark contrast to the more refined B-2A geometry. The faceting technique derives from the use of the method of geometrical optics (see [1] page 114) which essentially says that an impinging ray (beam) is reflected at an angle equal to the incident angle relative to the normal to the reflecting surface (ie shine a torch beam at a mirror and see the effect). For this to be true though the wavelength must be much smaller than the dimensions of the reflecting flat surface and hence it is clear that the F-117A is designed to defeat high to mid band microwave radars.
By breaking the area of the airframe into flat facets, the designers sought to reflect impinging radar beams away from the radar. This is also the reason why the external geometry has no curved edges. Straight edges reflect principally in directions given by the above rule, therefore by arranging all areas to be flat and all edges to be straight, the designers could ensure that most impinging microwave energy is reflected away from the aircraft at angles which are determined by the instantaneous orientation of the airframe relative to the searching radar. As the frontal RCS is of greatest importance tactically, the edges and surfaces of the airframe about the frontal aspect are all arranged at shallow angles with respect to an impinging wave.
The result is not only a weak radar return but also a continuously scintillating one, scintillation will cause problems in many target tracking systems. In this fashion by clever shaping the RCS of the airframe was dramatically decreased. This alone was however inadequate as other detail contributors to the aircraft's RCS would have dominated the return. Hence the cockpit canopy windows were coated with an electrically conductive layer and the inlets were covered by a fine mesh grill, with holes smaller than the wavelength of the victim radars. Potentially good reflectors such as the engine fan faces and cockpit interior are thus hidden away.
Electrical discontinuities associated with panel edges and control surfaces at angles close to normal to frontal aspect beams could also make a measurable contribution to frontal RCS, therefore the canopy edges, weapon bay, undercarriage doors and FLIR bay have serrated edges. The angles used in the detail features are again shallow with respect to frontal aspect beams.
Shaping has thus been the principal RCS reduction technique used in the F-117A design. In addition, radar absorbent materials were used for some panels and radar absorbent coating over the area of the aircraft. The RCS of the aircraft has been estimated in the range of 0.001-0.01 square metres, which is incidently between 1% to 10 % of the RCS of a typical chicken [1] (subsequently released information indicates it to be closer to 0.001-0.0001 m2).
The aerodynamic penalties incurred by airframe shaping to minimise RCS have been considerable. Sharp edges and flat surfaces create vortices and thus severely disturb laminar flow causing parasitic drag. The large sweepback angle and low aspect ratio results in a shallower lift-curve slope which forces a higher nose attitude in landing configuration, this is confirmed by the high position of the canopy which in turn incurs an additional drag penalty. Another consequence of this effect is limited lift on takeoff requiring taller undercarriage to facilitate the required AoA on rotation. Highly swept wings are also poor performers at low speed, producing considerable lift induced drag, the F-117A will almost certainly have a narrow range of optimal high subsonic operating speeds where the parasitic and lift induced drag terms appropriately balance.
