A Method for Reducing Sonic Boom Strength by Tailoring the Shape of the Propulsive Streamtube

Over the past decade, Gulfstream Aerospace Corporation has published on several methods for tailoring the shape of the nacelle for reducing its profile and contribution to the sonic boom signature of an aircraft traveling at supersonic speed. Through computational and experimental campaigns, these design methods have been shown effective at reducing the strength of the nacelle’s compression features. However, even heavily attenuated shocks can remain unacceptably strong for aircraft constrained by an ultra-quiet sonic boom design requirement. This particular challenge was the catalyst for Gulfstream’s recent development of a concept for removing all remaining significant shock features created by the nacelle, an outcome impossible using traditional design methods. The technique uses purpose-tailored extensions of both the inlet compression and nozzle expansion surfaces to precisely control the shape of the propulsive streamtube fore and aft of the nacelle. Careful concurrent matching of the nacelle geometry permits tangential lofting of the streamtube onto and from the nacelle surface. The method is predictable, adaptable, and extremely effective at nearly eliminating discrete compression and expansion features generated by the propulsion system at the design operating point while being robust against off-design operating conditions. Along with sonic boom strength, the nacelle’s profile drag is reduced by as much as 70 percent. Computational fluid dynamics analyses have proven the concept to be an important component of a portfolio of ultra-low sonic boom design technologies. Included in this paper is a general discussion of the benefits of the shaped streamtube methodology applied to an inlet-nacelle-nozzle system sized for a 15,000 lbf thrust class turbofan engine operating at a local Mach number of about 1.65. The inlet in this example employs axisymmetric relaxed isentropic compression, while the nozzle features an axisymmetric expansion plug and dualannular high-flow bypass exhaust. Selected CFD-based solutions are included, and challenges associated with the use of a shaped streamtube propulsion system are discussed.