Subsonic Ultra Green Aircraft Research: Phase III – Mach 0.75 Transonic Truss-Braced Wing Design

This design report summarizes the Transonic Truss-Braced Wing (TTBW) work accomplished by the Boeing Subsonic Ultra-Green Aircraft Research (SUGAR) team during the time period of July 2014 through October 2016 under SUGAR Phase III.

In Phase II, aerodynamic estimates were derived from conceptual methods that predict drag based on a database of designed shapes. An empirical database for TTBW strut-wing intersections is not known to exist and this study is oriented toward gaining the prerequisite data for lower-order design space exploration by exercising higher-order tools and ultimately wind tunnel test. The detailed design exercise conducted during Phase III utilized modern Navier-Stokes-based computational fluid dynamics tools and determined vehicle cruise drag to be within 1% of the Phase II conceptual estimate, however, some disagreements exist on a component-by-component basis. Through the use of these high-fidelity methods, uncertainty in the predicted fuel consumption of the truss-braced wing configuration has been greatly reduced.

The main strut was found to account for approximately 10% of the total airplane drag, with interference effects between the wing and strut making up about 1% of the airplane drag. Aerodynamic operability requirements were met at the cruise Mach number, but some uncertainty remains regarding buffet margin at the maximum operating Mach number. A key source of this uncertainty is a lack of confidence in the applicability of buffet prediction methods that were empirically-derived using data from cantilever wings. In addition, exploration of active technology that can be used to mitigate buffet at Mach numbers higher than cruise without impacting dispatch reliability have not been studied. Further investigation into this issue should be considered.

The current TTBW configuration, in conjunction with technology insertion outlined in previous phases of study, has the potential to reduce fuel consumption by 57% as compared to a consistently sized cantilever configuration with technology levels representative of the 2008 single-aisle fleet. A final truss-braced wing geometry, which is appropriate for a high-speed wind tunnel test, has been generated. A 4.5% scale wind tunnel model has been constructed and tested in the NASA Ames Research Center’s Unitary Plan Wind Tunnel (UPWT) 11-Foot Transonic Wind Tunnel (11-Foot TWT).

Test data shows that drag rise data collected compares well with CFD prediction indicating that interference effects are minimal and that the truss system is not changing the overall cruise speed of the configuration. The stability and control data indicates the configuration compares well with pretest predictions in all areas except spoiler effectiveness at dive Mach number. Here spoilers indicate reversal at low deflections, a phenomenon the test team has experienced in prior configurations that should clear at higher deflections. Test data could not be generated to verify this due to model load limitations. The drag buildup data shows mixed results with some increments matching and some that do not. The root cause for this has been determined to be an unacceptably high level of surface roughness that is unable to be closed via post-test analysis. This also caused the overall drag levels of the wind tunnel test data to be offset from the test predictions by 30 counts at the design lift coefficient and Mach number. It is recommended the model be stripped of paint, polished, and a second tunnel entry be made.

The test team employed several methods of data collection including PSP, IR, and MDM data. These techniques were important for test due to the limited surface pressure data available from the physical pressure taps. In the future, surface roughness caused by using these techniques should be carefully considered during the test planning phase. Recommendations for testing using these techniques have been developed.