R&T Challenges for Aerodynamics and Aeroacoustics

A total of 19 R&T Challenges were prioritized in the aerodynamics and aeroacoustics Area. Table A-1 shows the results. The R&T Challenges are listed in order of NASA priority. National priority scores are also shown.1 This appendix contains a description of each R&T Challenge, including milestones and an item-by-item justification for each score that appears in Table A-1.2

A1 Integrated system performance through novel propulsion–airframe integration

Flow interactions in the region of the propulsion–airframe interface during takeoff, climb, and cruise pose a complex design problem. Design compromises have a significant effect on the aircraft efficiency and on radiated noise. Research into improved techniques for propulsion–airframe integration would improve aircraft flexibility and performance, especially as aircraft speeds increase. To meet this objective, both computational fluid dynamics (CFD) and experimental tests are indispensable. Improvements in the accuracy of predictions of three-dimensional (3-D) steady and unsteady interactions between external and internal aerodynamics and aeroacoustics are required to enable design of future aeronautical systems. These interactions include the effects of steady and dynamic distortion on engine operations and the effects of hot, reacting exhaust flows on vehicle aerodynamics. They are particularly important in the design of vertical and short takeoff and landing (V/STOL),3 extremely short takeoff and landing (ESTOL), supersonic, and hypersonic airplanes. On V/STOL airplanes, exhaust jets are placed near the trailing edge of the wing where the aerodynamic stiffness of the high-speed flow increases wing lift by what is called the Jet Flap Effect (Spence, 1956). At supersonic speeds, adverse interactions between shock waves and boundary layers can increase drag and cause engine unstart. For many proposed hypersonic aircraft, the aircraft forebody is the inlet compression surface and the aircraft afterbody is the nozzle expansion surface, so that the airframe is part of the propulsion system. This is particularly the case with waveriders (Kuchemann, 1978). Propulsion–airframe integration has a significant impact on aircraft radiated noise. Improvements in test techniques and instrumentation are needed to characterize complex 3-D flow fields and acoustic radiation patterns. Key milestones include

  • Validate the predictive capablity for 3-D mean and dynamic distortion at the propulsion–airframe interface.

  • Validate the predictive capability of the impact of reacting exhaust flows on external aerodynamics.

  • Validate the predictive capability of acoustic radiation patterns from integrated propulsion–airframe configurations.


The prioritization process is described in Chapter 2.


The technical descriptions for the first 11 Challenges listed below contain slightly more detail than the technical descriptions for these Challenges as they appear in Chapter 3.


VTOL airplanes can take off and land vertically. This includes tilt-rotors, the AV-8 Harrier, and the JSF, for example. VTOL airplanes do not routinely take off or land vertically because of the range-payload penalty associated with the weight limitations of purely vertical operations. Rather, they use any available field length to develop some forward motion and wing lift during takeoff to increase the useful load (fuel plus payload). They tend to land vertically only at the end of the mission, when they are lighter, after burning fuel and/or dropping weapons.STOL airplanes use high-lift systems to take off in less distance than conventional aircraft (typically a few thousand feet). Very few STOL aircraft can safely take off on runways shorter than 3,000 ft and none on runways less than 2,000 feet. (This class does not include ultralight aircraft, kit planes, etc. that can operate out of short fields due to their small size but do not have high-lift systems).ESTOL airplanes would be able to safely take off on runways of 2,000 ft. They would have high-lift systems and thrust-to-weight ratios that are higher than conventional aircraft but not as high as VTOL aircraft. ESTOL aircraft have not yet been developed for commercial or military operations.V/STOL refers to both VTOL and STOL airplanes that convert to fixed-wing flight after takeoff; it does not include helicopters.

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