One of the more complex and challenging areas in aerospace technology is the prediction of paths of aircraft components after the release of items such as external stores, canopies, crew modules, or vehicles dropped from mother ships. Aerodynamic interference phenomena between vehicles can cause major safety-of-flight issues, resulting in catastrophic impact of the components with the airplane, and unexpected pressures and shock waves can dramatically change the expected trajectory of stores. Conventional wind
81 tunnel tests used to obtain aerodynamic inputs for calculations of separation trajectories must cover a range of test parameters, and the requirement for dynamic aerodynamic information further complicates the task. Measurement of aerodynamic pressures, forces, and moments on vehicles in proximity in wind tun-nels is a challenging technical procedure. The
use of dynamically scaled free-flight models can quickly provide a qualitative indication of sepa-ration dynamics, thereby providing guidance for wind tunnel test planning and early identification of potentially critical flight conditions.
Time sequence of a canopy ejection test in the Langley 300 mph 7- by 10-Foot Wind Tunnel. The staff of the facility conducted extensive sepa-ration studies of aircraft stores and components using a net to catch components released from models under test.
Separation testing for military aircraft compo-nents using dynamic models at Langley evolved into a specialty at the Langley 300 mph 7- by 10-Foot Tunnel, where subsonic separation studies included assessments of the trajectories taken by released cockpit capsules, stores, and canopies. In addition, bomb releases were simulated for several bomb-bay configurations, and the trajectories of model rockets fired from the wingtips of models were also evalu-ated. As requests for specific separation studies mounted, the staff rapidly accumulated expertise in testing techniques for separation clearance.75
Free-flight drop model of the X-15 research aircraft undergoes separa-tion testing beneath a B-52 model in the Langley 300 mph Low-Speed 7- by 10-Foot Tunnel. The model was mounted under the left wing of the bomber for these tests for visibility.
One of the more important separation studies conducted in the Langley tunnel was an assess-ment of the launch dynamics of the X-15/B-52 combination for launches of the X-15. Before the X-15, launches of research aircraft from carrier aircraft had only been made from the fuselage centerline location of the mother ship, and in view of the asymmetrical location of the X-15 under the right wing of the B-52, concern arose as to the aerodynamic loads encountered during separation and the safety of the launching pro-cedure. Separation studies were therefore con-ducted in the Langley 300 mph 7- by 10-Foot Tunnel and the Langley High-Speed 7- by 10-Foot Tunnel.76 Detailed measurements of the aerodynamic loads on the X-15 in proximity to the B-52 under its right wing were made dur-ing conventional force tests in the High-Speed
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
82 MODELING FLIGHT
Tunnel, while the trajectory of a dynamically scaled X-15 model was observed during a separate investiga-tion in the Low-Speed Tunnel. The test set up for the low-speed drop tests used a dynamically scaled X-15 model under the left wing of the B-52 model to accommodate viewing stations in the tunnel. The model used conventional Froude-number scaling procedures so that the model and full-scale aircraft translational accelerations were equal; therefore, the effects of Mach number could not be determined. Initial trim set-tings for the X-15 were determined to avoid contact with the B-52, and the drop tests showed that the result-ing trajectory motions provided adequate clearance for all conditions investigated.
During successful subsonic separation events, a bomb or external store is released, and gravity typically pulls it away safely. At supersonic speeds, however, aerodynamic forces are appreciably higher relative to the store weight, shock waves may cause unexpected pressures that severely influence the store trajectory or bomb guidance system, and aerodynamic interference effects may cause catastrophic collisions after launch. Under some conditions, bombs released from within a fuselage bomb bay at supersonic speeds have encountered adverse flow fields, to the extent that the bombs have reentered the bomb bay. In the early 1950s, the NACA advisory committees strongly recommended that focused efforts be initiated by the Agency in store separation, especially for supersonic flight conditions. Langley responded with an investi-gation of supersonic bomb releases at a Mach number of 1.62 in the 9-Inch Supersonic Tunnel.77 The dynamic model test examined the effect of various store locations on a swept wing and several bomb-bay configurations. Results of the study were among the first to illustrate the adverse interference effects of supersonic releases.
Researchers within Langley’s Pilotless Aircraft Research Division (PARD) used their Preflight Jet facility at Wallops to conduct research on supersonic separation characteristics for several high-priority military pro-grams.78 The preflight facility was designed to check out ramjet engines before rocket launches, consisting of a blow-down–type tunnel powered by compressed air exhausted through a supersonic nozzle. The main jet was square in cross section and was constructed in two sizes: 12 inches and 27 inches. Test Mach num-ber capability was from 1.4 to 2.25. With an open throat and no danger to a downstream facility drive system, the facility proved ideal for dynamic studies of bombs or stores after supersonic releases. PARD devised appropriate scaling laws for the simulation of Mach number and conducted extensive separation tests.79
One of the more crucial tests conducted in the Wallops Preflight Jet facility was support for the develop-ment of the Republic F-105 fighter bomber, which was specifically designed with forcible ejection of bombs from within the bomb bay to avoid the issues associated with external releases at supersonic speeds. For the test program, a half-fuselage model (with bomb bay) was mounted to the top of the nozzle, and the ejection sequence included extension of folding fins on the store after release. A piston and rod assembly forcefully ejected the store from the open bomb bay, and high-speed photography documented the motion and trajectory of the store. The F-105 program expanded to include numerous specific and generic bomb and store shapes requiring almost 2 years of tests in the facility. The ejection tests in the Wallops facility were supplemented by force and moment measurements in the Langley 4-Foot Supersonic Tunnel, where the aerodynamic loads acting on the store were measured for various positions of the store relative to the aircraft. Numerous generic and specific aircraft separation studies in the Preflight Jet facility from 1954 to 1959 included F-105 pilot escape, F-104 wing drop-tank separations, F-106 store releases from an internal bomb bay, and B-58 pod drops.
83 The national challenge in advancing the state of the art in store separation dynamics was ultimately taken over by the research and development establishments within the Air Force and Navy, and NASA’s role was minimized after the 1960s.