Challenge of Stealth Aerodynamics

The invention of aircraft that are almost invisible to ground or surface-to-air – missile radars promises to be an effective defensive measure for reconnaissance and attack airplanes. This development has taken six paths so far, the first three of which are a distinct challenge to stability and control designers:

Faceted airframes replace the smooth aerodynamic shapes that produce at­tached flows and linear aerodynamics. Radar returns from faceted shapes, such as the Lockheed F-117A, are absent except for the instants when a facet faces the radar transmitter.

Parallel-line planforms have the same sweep angle on wing leading and trailing edges and on surface tips and sharp edges. Parallel-line planforms concentrate radar returns into narrow zones that are easily missed by search radars. This is the Northrop B-2 stealth method, augmented by special materials and buried engines.

Suppressed vertical tails are either shielded from radar by wing structure or eliminated altogether. The Lockheed F-22 has shielded vertical tails, the B-2 none at all.

Blended aerodynamics eliminate internal corners such as wing-fuselage inter­sections. Internal corners can act as radar corner reflectors. The Rockwell B-1 uses this technique to reduce its radar signature.

Buried engines and exhausts hide compressor fan blades and hot exhaust pipes from radar and infrared seekers.

Radar-absorbent materials are used, generally nonmetallic. This is a highly classified subject.

The challenges of faceted airframes, parallel-line planforms, and suppressed vertical tails to stability and control engineers are illustrated by current stealth airplanes.