Aeronautical innovators at NASA’s Langley Research Center in Virginia are one step closer to confidently crafting a viable commercial airliner that can fly faster than the speed of sound, yet produce a sonic boom that is quiet enough not to bother anyone on the ground below. Wind tunnel tests of scale model airplanes verified that new approaches to designing such aircraft would work as hoped for when aided by improved computer tools, which were used for the first time together in each step of the design process.
Nuisance noise generated by a commercial supersonic jet's sonic booms during cruise, and by its powerful engines at takeoff and landing, has kept the speedy aircraft from entering service in the United States – except for Europe's Concorde, which was limited to trans-Atlantic flights only.
Using the computer tools, teams led by Boeing and Lockheed Martin, and funded through a NASA Research Announcement, came up with designs for two small supersonic airliners that would carry between 30 and 80 passengers and potentially enter service in the 2025 timeframe.
The computer tools show that one way to reduce the perceived loudness of a supersonic jet's sonic boom is to change the aircraft shape, in part, by lengthening the aircraft’s fuselage, making it much more slender. Theoretically, the noise issue could be solved by a really, really long aircraft body. But while an 800-foot-long airliner may lead to publicly acceptable sonic booms, an aircraft that size still must fit at its gate, make turns while taxiing to the runway without hitting anything and generally not require an expensive redesign of the nation's airports.
To help reach their goals, the engineers relied on earlier studies that revealed how an aircraft’s overall configuration could modify the shape of the supersonic shockwaves coming off the airplane so that the atmosphere then reduces the sharpness of the wave. By the time the shockwave reached the ground the shock would be removed, resulting in a nearly inaudible sonic boom.
Two other design considerations are important. The first reduces the size of the proposed commercial airliner so it carries fewer passengers and is lighter. The second slows the cruising speed. While the Concorde cruised at twice the speed of sound, or Mach 2.0, this airliner would cruise at a slightly slower Mach 1.6 to Mach 1.8.
The measurement NASA researchers are using to base their work on is called perceived decibel level, or PLdB. PLdB is a different flavor of decibels than the measurement (dBA) often quoted when discussing how loud, for example, a rock concert is compared to a kitchen blender or library reading room. Concorde's sonic boom noise level was 105 PLdB. The PLdB that researchers believe will be acceptable for unrestricted supersonic flight over land is 75, but NASA wants to eventually beat that and reach 70 PLdB.