ALBERT

Description: A team was formed to tackle the sonic boom softening issues of the current Boeing HSCT design. The team consisted of personnel from NASA Ames, NASA Langley, and Boeing company. The work described in this paper was done when the first author was at NASA Ames Research Center. This paper presents the sonic boom softening work on two Boeing High Speed Civil Transport (HSCT) baseline configurations, Reference-H and Boeing-1122. This presentation can be divided into two parts: parametric studies and sonic boom minimization by CFD optimization routines.

Description: The topics covered include the following: low sonic boom design perspective; design approach for reduced sonic boom; target sonic boom waveforms; airplane design for reduced sonic boom loudness; design procedure for low sonic boom; wing design and nacelle lift effects; area distributions and F-functions due to volume; fuselage area distributions; low sonic boom design, configuration 3B; sonic boom characteristics; sizing, performance, and noise characteristics; a summary of phase 3 configurations; impact of sonic boom design constraints; and wing loading considerations.

Description: A brief resume of sonic boom minimization methods is given to provide a background for a new, empirical modification of the Seebass and George minimum-nose-shock sonic boom F-function and signature. The new 'hybrid' F-function has all the inherent flexibility of application found with the Darden-modified Seebass and George F-function. In addition, it has enhanced this flexibility and applicability with neglegible increase in nose and/or tail shock strength. A description of this 'hybrid' F-function and signature is provided, and the benefits of using them to design high-performance, low-boom aircraft are discussed.

Description: The versatility of High Speed Civil Transports (HSCT) will be operationally limited by regulations that prohibit overland supersonic flight. This limitation gives impetus to the study of aerodynamic designs for reduced sonic boom. An HSCT design with an 'acceptable' sonic boom can allow routine overland supersonic cruise that would provide increased productivity and economic viability. During this four-year NASA-sponsored study, several configurations were designed for reduced sonic boom. An iterative technique was used in which the standard linear supersonic and Whitham sonic boom methods are extended. For the most severe sonic boom constraint of 72 dBA sonic boom loudness and 0.75 lb/sq ft shock strength at the ground, an economic benefit for operating at Mach 1.7 overland was not realized because of a decrease in the ratio of payload to takeoff gross weight. Additional design work is required to develop the best compromise between the low-boom requirements and optimum cruise performance.

Description: The objective of this study was to provide low sonic boom concepts, geometry, and analysis to support wind tunnel model designs. Within guidelines provided by NASA, two High Speed Civil Transport (HSCT) configurations were defined with reduced sonic boom that have low drag, high payload, and good performance. To provide information for assessing the feasibility of reduced sonic boom operation, the two designs were analyzed in terms of their sonic boom characteristics, as well as aerodynamics, weight and balance, and performance characteristics. Low drag and high payload were achieved, but both of the blended arrow-wing configurations have deficiencies in high lift capability, fuel volume, wing loading, balance, and takeoff gross weight. Further refinement of the designs is needed to better determine the commercial viability of low boom operation. To help in assessing low boom design technology, the two configurations were defined as wind tunnel models with altered aft-bodies for the wind tunnel sting mounting system.

Description: Several aspects of designing for reduced sonic boom were investigated to assess the adequacy of the conventional modified linear theory. For a simple test case of a nacelle with a small forecowl angle (2 degrees) mounted below a flat plate, the linear theory compared favorably for a case with simulated nacelle lift and for a computational fluid dynamics (CFD) analysis. In a second study, several methods of analyzing the area distribution due to volume were examined. And finally, in a preliminary study, the effect of forebody shape on the rise time of the bow shock was investigated, indicating a significant increase (several msec) can be obtained by proper forebody shaping.

Description: Sonic boom wave form parameters as related to loudness were investigated analytically. The parameters studied include rise time, duration, maximum overpressure and initial overpressure. The design criteria of a 72 dBA for corridors and 65 dBA for unconstrained flight were chosen based on a review of human response testing. The 72 dBA criterion suggests that 1.0 psf shock waves may be acceptable. On that basis, acceptable low sonic boom wave forms were explored with respect to cruise conditions, aerodynamic lifting length requirements and configuration design at M 1.5 and M 2.4. An M 2.4 baseline arrow wing configuration was studied as a possible vehicle for M 1.5 cruise overland. Modifications made to approach the low boom wave form included a slightly longer forebody, staggered nacelles, a lifting arrow wing horizontal tail, and carefully tailored lift and volume elements. The same wave form criteria applied for M 2.4 cruise results in a low boom configuration that has significant weight, length and balance penalties. Further detailed design work is required to reach the target wave form and resultant loudness level for overland cruise at M 1.5. These results so far suggest that a properly designed M 2.4 overwater configuration may be capable of M 1.5 overland operation with sonic boom noise characteristcs that meet the criterion.

Description: The HSR sonic boom technology program includes a goal of reducing the objectionable aspects of sonic boom. Earlier HSCT sonic boom studies considered achieving significant sonic boom reduction by the use of arrow-wing planforms and detailed shaping of the airplane to produce shaped waveforms (non N-waves) at the ground. While these design efforts were largely successful, the added risk and cost of the airplanes were judged to be unacceptable. The objective of the current work is to explore smaller configuration refinements that could lead to reduced sonic boom impact, within design and operational constraints. A somewhat modest target of 10% reduction in sonic boom maximum overpressure was selected to minimize the effect on the configuration performance. This work was a joint NASA/Industry effort, utilizing the respective strengths of team members at Boeing, NASA Langley, and NASA Ames. The approach used was to first explore a wide range of modifications and airplane characteristics for their effects on sonic boom and drag, using classical Modified Linear Theory (MLT) methods. CFD methods were then used to verify promising, modifications and to analyze modifications for which the MLT methods were not appropriate. The tea m produced a list of configuration changes with their effects on sonic boom and, in some cases, an estimate of the drag penalty. The most promising modifications were applied to produce a boom-softened derivative of the baseline Boeing High Speed Civil Transport (HSCT) configuration. This boom-softened configuration was analyzed in detail for the reduce sonic boom impact and also for the effect of the configuration modifications on drag, weight, and overall performance relative to the baseline.

Description: Low sonic boom studies have continued during the last year with the goal of exploring the ability of practical airplane designs to achieve significantly reduced sonic boom-loudness with reasonable performance penalties. At the 1993 Sonic Boom Workshop, improvements to the low-boom design methods were described and early results of two low-boom configurations -935 and -936) were presented. Now that the low boom design methods are reasonably mature, recent design activities have broadened somewhat to explore refinements to the -935 and -936 designs. In this paper the results are reported of a detailed systems study and performance sizing of the -935 (Hybrid sonic boom waveform) and the -936 (Flat-top waveform). This analysis included a second design cycle for reduced cruise drag and balance considerations. Another design study was of a small-wing version of the -935. Finally, some preliminary results of the recent LARC UPWT test of the -935 configuration are given, along with a proposed alternative method for extrapolating wind tunnel pressure signatures to the ground. The various configurations studied is also summarized. The topics covered by this paper are as follows: Systems study results of the Baseline -939 and low boom configurations -935 and -936, Small wing derivative of the -935, Wind tunnel test results of the -935, Test-derived F-function and propagation to the ground, and Future considerations (boom-softened baseline, overwater issues, and operations).