Lesson 26 Flashcards#
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1. Name the five families of RF signature contributors on an aircraft.
Cavities (inlets, exhausts, cockpit), radome and antennas, wing/body speculars and junctions, edges and surface waves, and gaps/seams/panels/stores.
2. What are the four design verdicts a contributor can be assigned?
Shape (redirect the energy), mask (hide the cavity), treat (tame the radiator with materials/FSS), and manage (maintain the floor — seams, fasteners, coatings, stores).
3. In the shape verdict, what physical quantity is attacked, and where does the energy go?
Shaping attacks directivity \(D\): you cannot delete the energy, only choose where to send it. Slope and blend every surface so no flat plate or right angle faces the threat, sending the echo somewhere other than back home.
4. Which verdict applies to inlets, exhausts, and the cockpit, and why?
Mask — they are cavities by function and cannot be sloped away. You hide the cavity (serpentine ducts, shielded/top inlets, mesh screens, conductive canopy coat) so the wave never reaches the multi-bounce interior.
5. Why is the engine inlet one of the loudest contributors on a legacy aircraft?
The duct plus compressor face forms a cavity: energy bounces multiple times inside and returns strongly toward the radar — the corner trap taken to the extreme. Fixes hide the compressor face: serpentine (S-)ducts, top-mounted/shielded inlets, and mesh screens with holes far smaller than a wavelength.
6. Why must the radome and antennas be treated rather than shaped or deleted?
An aperture built to radiate efficiently is, by reciprocity, an efficient scatterer, and the aircraft must still radiate and receive. So it gets compromises: FSS radomes transparent only in the ship's own band, flush/conformal apertures, and EMCON.
7. What is the verdict and fix for a vertical tail meeting the fuselage at 90 degrees?
Shape — cant the tails outward or delete them (flying wing). A 90-degree dihedral is a corner reflector that retroreflects across a wide angle.
8. What does the "manage" verdict cover, and why does it need continuous maintenance?
Gaps, seams, panels, fasteners, coatings, and stores. Shaping only sets a floor; a proud fastener, misaligned panel, or chipped coating becomes a new loudest scatterer that can own the nose-sector RCS — so LO fleets check and restore signatures continuously and carry stores internally.
9. What is planform alignment?
Grouping every edge and seam into a few shared orientations so their diffraction spikes stack into a small number of narrow butterfly lobes pointed away from the nose sector.
10. State the edge specular lobe peak model used in the demo.
\(\sigma_{\text{peak}} = 10\log_{10}(1.25\,L^2)\) dBsm, so peak grows as \(L^2\) (longer edge = louder flash). Each edge fires a lobe at its specular normal and at the mirror direction (normal + 180°); the lobe falls off as a Gaussian in dB, \(\sigma(d)=\text{peak}-0.5(d/w)^2\) with width \(w\approx 1^\circ\).
11. In the planform-alignment demo, how does aligning the edges change the compass and the nose sector?
It cuts the loud fraction of the compass (above \(-5\) dBsm) by about two-thirds and drops the nose-sector max to about \(-24\) dBsm — essentially the \(-25\) dBsm shaping floor. The surviving butterfly lobes get taller because the energy is concentrated, not deleted.
12. Why do flying wings (B-2, B-21) represent planform alignment taken to its limit?
They have few edges, few lobes, and no tail-body corners — every edge on the planform shares its angles, so almost all energy is concentrated into a handful of known lobes off the mission axis.
13. Why did early stealth aircraft use facets while later ones use continuous curvature?
1970s prediction codes could only compute flat-panel returns (F-117: all facets); later codes handled continuous curvature (B-2 onward), which kills the many small facet spikes and spreads specular energy smoothly.
14. State the RCS budget equation and what it implies.
\(\sigma_{\text{tot}} \approx \sum_i \sigma_i\) — contributors add in linear space, so the sum has a tyrant: the loudest scatterer owns the signature. Budget every contributor under the line and fix loudest-first.
15. State the fourth-root law and what a −20 dB RCS reduction buys.
\(R_{\max} \propto \sigma_{\text{tot}}^{1/4}\): a \(-20\) dB median-RCS cut gives \(0.32\,R\) (about a third of the reach) and shrinks the defended ring area (which falls as \(\sigma^{1/2}\)) to about 10 percent of its original value.