Stealth and Threat Aviodance

Lockheed SR-71 Blackbird

The SR-71 was the first operational aircraft designed around a stealthy shape and materials. There were a number of features in the SR-71 that were designed to reduce its radar signature. The first studies in radar stealth technology seemed to indicate that a shape with flattened, tapering sides would avoid reflecting most radar energy toward the radar beams' place of origin. To this end, the radar engineers suggested adding Chines to the design and canting the vertical control surfaces inward. The aircraft also used special radar-absorbing materials which were incorporated into saw tooth shaped sections of the skin of the aircraft, as well as cesium-based fuel additives to reduce the exhaust plumes' visibility on radar. Despite these efforts, the SR-71 was still easily detected on radar while traveling at speed due to its large exhaust stream and air heated by the body (large thermal gradients in the atmosphere are detectable with radar). The SR-71's radar cross section (RCS) of almost 10 square meters was much greater than the later F-117's RCS, which is similar to that of a small ball bearing.

The overall effectiveness of these designs is still debated; Ben Rich's team could show that the radar return was, in fact, reduced, but Kelly Johnson later conceded that Russian radar technology was advancing faster than the "anti-radar" technology Lockheed was using to counter it. The SR-71 made its debut years before Pyotr Ya. Ufimtsev's ground-breaking research made possible today's stealth technologies, and, despite Lockheed's best efforts, the SR-71 was still easy to track by radar and had a huge infrared signature when cruising at Mach 3.2 or more. It was visible on radar since air traffic control tracked it when not using its transponder, and missiles were often fired at the aircraft.

Although equipped with defensive electronic countermeasures, the SR-71's greatest protection was its high top speed, which made it almost invulnerable to the attack technologies of the time. Over the course of its service life, no SR-71 was shot down, despite many attempts to do so. It flew too fast and too high for surface-to-air missile systems to track and shoot down, and was much faster than the Soviet Union's fastest aircraft of the time, the MiG-25, which had a top speed of Mach 3.2 at high altitude, however the engines would burn up at that speed. All the SR-71 pilot had to do was to accelerate.
Chines Head-on view of an A-12 (precursor to the SR-71) on the deck of the Intrepid Sea-Air-Space Museum, illustrating the chines.
One of the Blackbird's interesting features was its chines, sharp edges leading aft on either side of the nose and along the sides of the fuselage.

The Blackbird was originally not going to have chines. At its A-3 design stage, the fuselage had a circular or vertical oval cross section. Dr. Frank Rodgers, of the Scientific Engineering Institute (a CIA front company), had discovered that a section of a sphere—round on the bottom and flat on top—had a greatly reduced radar reflection. He adapted this to a cylindrical fuselage by 'stretching' the sides out and leaving the bottom round. After the advisory panel provisionally selected Convair's FISH design over the A-3 on the basis of RCS, Lockheed adopted chines for its A-4 through A-6 designs, and used them in redesigning the A-11 into the A-12.
The aerodynamicists discovered that the chines generated powerful vortices around themselves, generating much additional lift near the front of the aircraft, leading to surprising improvements in aerodynamic performance. The angle of incidence of the delta wings could then be reduced, allowing for greater stability and less high-speed drag, and more weight (fuel) could be carried, allowing for greater range. Landing speeds were also reduced, since the chines' vortices created turbulent flow over the wings at high angles of attack, making it harder for the wings to stall. The Blackbird can, consequently, make high-alpha turns to the point where the Blackbird's unique engine air inlets stop ingesting enough air, which can cause the engines to flame out. Blackbird pilots were thus warned not to pull more than 3 g, so that angles of attack stay low enough for the engines to get enough air. The chines act like the leading edge extensions that increase the agility of modern fighters such as the F-5, F-16, F/A-18, MiG-29 and Su-27. The addition of chines also allowed designers to drop the planned canard foreplanes. Early design models of what became the Blackbird featured canards.
When the Blackbird was being designed, no other airplane had featured chines, so Lockheed's engineers had to solve problems related to the differences in stability and balance caused by these unusual surfaces. Their solutions have since been extensively used. Chines remain an important design feature of many of the newest stealth UAVs, such as the Dark Star, Bird of Prey, X-45 and X-47, since they allow for tail-less stability as well as for stealth.