I would like to understand how the fuselage chines on the SR-71 family increase directional stability
From the flight manual.
Chines
The SR-71 has a blended forward wing (chine) which extends from the fuselage nose to the wing leading edge. This chined forebody is approximately 40% of the aircraft length. The chines improve directional stability with increasing angle of attack at all speeds. However, their primary purpose is to provide a substantial portion of the total lift at high supersonic speeds and eliminate a need for canard surfaces or special nose-up trimming devices.
A large rearward shift in the aerodynamic center of lift occurs when the aircraft transitions from subsonic to supersonic flight. Without chines, the center of lift would shift aft while in the transonic region and remain between 40% to 45% mean aerodynamic chord (MAC) at all speeds above Mach 1.4. A large elevon deflection would be required for trimming, and the resultant drag would be unacceptable. A similar shift of the aerodynamic center occurs at transonic speeds with chines, but the initial displacement is to a position between 35% to 40% MAC. As Mach increases, the center of lift moves forward until a position slightly aft of 25% MAC is reached at the design speed. The result is that the static stability margin is maintained at desirable levels and trim drag due to elevon position is reduced to a minimum at design speed. The SAS provides satisfactory handling qualities.
Automatic fuel tank sequencing shifts the c.g. aft to approximately 25% MAC while the fuel in tank 1 is being reduced to the right-hand shut-off level. This normally occurs during acceleration to supersonic cruise and conforms with the aft shift of the aerodynamic center.
NOTE: Because of chine effectiveness, c.g. must be moved forward of 25% if design speed is exceeded.
Additional info from a paper by Peter W. Merlin, TYBRIN Corporation, NASA Dryden Flight Research Center. Sorry if the editing is a bit disjointed.
Chines along the forebody reduced fuselage sloping, reducing RCS, while providing additional lift and stability. The two tails were canted inward, to further reduce RCS, and mounted atop the aft end of each engine nacelle. Each tail consisted of a stub fin and a rudder. The stub fin was fixed in place, extending approximately 21 inches above the nacelle surface. It was constructed of titanium alloy machined parts, plate, formed members, and sheet. The stub fin contained rudder servos and housed each rudder pivot post. One full rudder, having no fixed vertical stabilizer, was mounted on each stub fin. Each rudder extended approximately 75 inches above the stub fin. The left and right rudders were identical and interchangeable. The A-12 prototype, A-12T trainer, M-21 motherships, and YF-12A had rudders made from titanium alloy. All others incorporated rudders made largely of plastic materials. The metal rudders were built with a central structural box section and attached leading- and trailing-edge assemblies. Plastic rudders incorporated basic frame members of titanium alloy. Subordinate members, including some of the ribs, spars, and exterior surface panels were made of bonded silicone-asbestos reinforced plastic materials. The plastic rudders weighed approximately 500 pounds and the metal rudders weighed somewhat less.
Blackbird Family Tree The A-12 spawned a series of advanced airplanes based on a common airframe. All variants were known as Blackbirds and all but a few reached the prototype stage. Only the first and last variants matured into operational systems.
Johnson conceived the AF-12 as a modified A-12 airframe incorporating a fire control system coupled with the AN/ASG-18, the first U.S. coherent-pulse Doppler radar for long-range, look down/look up, and single-target attack.
The AF-12 design necessitated numerous changes to external configuration. A second crew position was added behind the cockpit to accommodate a fire control officer (FCO). Two infrared (IR) sensors, an integral part of the target tracking system, were placed on either side of the nose. The nose assembly itself was originally to be chined like that of the A-12 but was soon replaced by a radome with a circular cross-section.
The nose configuration, designed to accommodate the radar, significantly altered the Blackbird’s aerodynamics and resulted in directional stability problems. Engineers resolved the problem by adding two small ventral fins to the engine nacelles and a large, hydraulically powered folding ventral fin on the centerline of the aft fuselage. Because of its size, the fuselage fin had to be folded to one side prior to takeoff and landing.
The YF-12A and SR-71B/C trainer models had two small titanium ventral strakes mounted on the underside of each engine nacelle for additional stability, to offset aerodynamic changes caused by their forward fuselage configurations. In addition, the YF-12A also had a titanium ventral fin near the aft end. The ventral fin was so large that it had to remain folded in a stowed position during takeoff and landing. During a NASA research mission in 1975 a YF-12A lost its main ventral fin during a sideslip maneuver. The incident gave researchers an opportunity to flight-test a new material. Technicians fitted a replacement ventral fin, made of Lockalloy, on the damaged YF-12A. Lockalloy, developed by Lockheed California Company, consisted of 62 percent beryllium and 38 percent aluminum. Aircraft designers considered it a promising material for use in constructing high-temperature aircraft structures.
During September 1962, Johnson began exploring what he called a “common market” version of the A-12. A single airframe configuration, known as the R-12 Universal airplane, would serve as the basis for a reconnaissance, recon/strike, or interceptor variant, depending on customer needs. This, he believed, would greatly simplify production.
Simultaneous development of the A-12 and R-12 fueled Pentagon debate as to the need for two similar reconnaissance platforms. The Air Force used the opportunity to press its case that it should have sole jurisdiction over such a mission. This eventually doomed the A-12 to cancellation.
As Lockheed pressed ahead with the R-12, the airplane’s configuration diverged noticeably from that of the A-12. The most obvious difference was the addition of a second crew position, behind the cockpit, to accommodate the reconnaissance systems operator (RSO). The fuselage was lengthened slightly to make room for additional fuel capacity and the tail cone was extended slightly. The nose chines were broadened to improve cruise characteristics and compensate for loss of directional stability due to the change in length.
Large sections of the leading and trailing edges, vertical stabilizers, chines, and inlet spikes were made of “plastic” laminates of phenyl silane, silicone-asbestos, and fiberglass. These materials – featured primarily on the A-12 and SR-71 families – helped reduce the aircraft’s radar signature.
The presence of fuselage side-fairings, or chines, generated nearly 20 percent of the aircraft’s total lift. Acting as fixed canards they produced a favorable effect on trim drag and minimized the aft shift of the aerodynamic center of pressure as the aircraft’s speed increased from subsonic to supersonic. Additionally, vortices from the chines improved directional stability of the aircraft as angle of attack increased. The chines also provided a convenient housing for wires and plumbing on either side of the cylindrical center-body fuel tanks.
The A-12 series featured a fairly flat, sharply tapered, chined nose. The airplane was a single-sensor platform, capable of carrying a camera or radar in the Q-bay. The YF-12A had a large plastic-laminate radome with a circular cross-section to house the fire-control radar for the interceptor’s missile launch system. Fuselage chines on the YF-12A ended abruptly at the nose break. The airplane carried no reconnaissance sensors, but was fitted with missile launch bays in the forebody chines. The SR-71 used three interchangeable chined noses for the Capability Reconnaissance (CAPRE) side-looking radar, Optical Bar Camera (OBC), and Advanced Synthetic Aperture Radar System (ASARS). The CAPRE and OBC nose sections had silicone-asbestos chine panels, while the ASARS nose had a one-piece quartz/polymide radome/chine section.
The chine structure was not integral with the fuselage structure, but was attached to it as fairings. The chines were partitioned into compartments to house electronics and mission equipment. The fuselage portion was covered with titanium skin, while silicone-asbestos panels covered the chines. The chine support structure was made mostly of annealed B-120VCA. Equipment bay doors were constructed using A-110AT material with some extruded sections as stiffeners. Fuselage longerons, located at the top, bottom, and sides, consisted largely of C-120AV aged-titanium extrusions.
Although most of the plastic parts in the chines and wing edges were considered secondary structure, they were required to conform to all local aerodynamic and thermal load limits.
Except on the YF-12A, which had a different nose configuration, the Blackbirds had a pronounced chine (blended forward wing-body) extending from the leading edge of the wing to the nose. This chined forebody, accounting for approximately 40 percent of total aircraft length, improved directional stability with increasing AOA at all speeds, especially in the subsonic range. At supersonic speeds the chines also provided a substantial portion of the total lift and eliminated the need for canard surfaces or special nose-up trimming devices.
Engineers discovered an interesting effect as fuel depletion caused differential heating between the upper fuselage and the cooler lower surfaces where fuel remained. Differential expansion of upper and lower fuselage panels caused the chines to be deflected downward, marginally changing their aerodynamic characteristics.
Handling Qualities The Blackbird’s handling qualities evolved from wind-tunnel model tests that verified the lift-to-drag ratios necessary to achieve mission performance, and established the airplane’s stability and control characteristics. In order to meet performance requirements, Lockheed designers had to accept a compromise affecting the airplane’s inherent stability and control. In exchange for low drag in cruise, the engineers accepted low stability margins. If they had designed the airplane for high pitch-stability, large control deflections would have been required for trim, and the resulting trim drag would have compromised mission performance.
Low stability margins and aerodynamic damping inherent at high mission altitudes adversely affected the airplane’s dynamic response and handling qualities. Lift (from the fuselage chines) forward of the c.g. destabilized the airplane in pitch. The chines, acting as fixed canards, also adversely affected handling in sideslip maneuvers at cruise AOA. To make up for these deficiencies, Lockheed designers incorporated an automatic flight control system (AFCS) consisting of a triple-redundant SAS, autopilot, and automatic pitch trim control (Mach-trim) system.
Overall, the Blackbird’s handling characteristics were satisfactory. Although stick forces were extremely high at design cruise speed and low lift coefficients, the airplane was usually flown on autopilot under those conditions so the pilot was unaware of the high forces. The airplane had marginal lateral or directional stability under some conditions but the SAS compensated to allow safe maneuvering.
Forgot to add the other reasons for the canted fins was to reduce induced rolling moment, and for the fin to stay on the correct side of the vortices shed by the nacelles, which reduced vertical tail deflection at low speed.