Challenges and Opportunities for Subsonic Transport X-Plane Acoustic Flight Research

Aircraft system noise aspects of experimental aircraft acoustic flight research are analyzed. Experimental aircraft are seen as a key development step toward the introduction of a full scale low noise subsonic transport in the future, especially when considering an unconventional aircraft configuration integrating a range of advanced noise reduction technologies. Possible design scenarios for an experimental aircraft are considered where the scale of the experimental aircraft relative to the future, full scale aircraft is likely a major cost driver. Aircraft system noise predictions are presented for a NASA modeled Mid- Fuselage Nacelle subsonic transport concept. The predictions are made for the total airframe system noise at 100, 50, 25, and 12.5% scale of the full scale, future version of the concept, both without and then with a set of noise reduction technologies. The noise reduction technologies include the dual use fairing of the Krueger flap, the continuous mold line for the trailing edge high lift flap, and the pod gear concept for the main gear. The predictions are treated as simulations of flight test measurements of an experimental aircraft that are then processed to full scale as flight data would be. The analysis shows that the combined impact of frequency shift, atmospheric absorption, and background noise cutoff is to establish a realistic upper limit on useful frequency from the experimental aircraft noise. The implications for instrumentation requirements are also noted for high frequency, as well as for the challenge of identifying sources that are reduced significantly by the proposed noise reduction technologies. For the experimental acoustic flight research to be most useful for the objectives of improving the prediction of the future full scale aircraft, it is indicated that the scale should be above 75%. As the demonstrator scale approaches 50%, the limitations become more severe for direct impact to the prediction of the full scale future concept.