Forward swept keels below the nose of supersonic aircraft, multiple keels, thermal keels and even linear ramjets integrated into various keel shapes all show potential for reducing sonic boom and merit further study, says a University of Alabama, Virginia Polytechnic Institute and Weidlinger Associates team.
"As a means of increasing fineness ratio, a keel can replace a nose extension better than twice its length," says Alabama researcher Rodney Bowersox. The DARPA effort, he adds, was aimed at further maturing the technology by cutting the size of solid keels or the power needed by thermal keels.
Building on previous studies of the "phantom spike" or "keel" concept, the latest research applied the idea to a smaller class of vehicle and, for the first time, included studies of a forward-swept design.
It was also the first to employ computational design methods for keel configurations, and was extended to include double use of a keel both as a linear ramjet and as a air-heater.
The size reduction goals, which were attained by replacing the single solid keel with a dual keel splayed at 20º, produced "very encouraging results" despite generating stronger shocks.
These were still "well within QSP design goals", says Bowersox. Total area reduction using the dual keel design was a factor of four.
Tests of a thermal keel were also conducted in Virginia Polytechnic's 230mm (9in) x 230mm supersonic windtunnel at simulated speeds of Mach 2.4, a total temperature of 27ºC (80ºF) and a pressure of 3.7bar (54lb/in2).
Ethylene was injected from holes drilled in the base of the keel and ignited by a plasma torch. "Results show the concept is viable, and is an effective design to reduce sound on the ground," says Bowersox.
Further experiments will look at a linear ramjet integrated in the keel. This would not only extend the effective keel length, but could also generate 100% of the thrust required for sustained supersonic cruise, he adds.
Source: Flight International
