Page 23 - January 2006

Fig. 2. Primary sonic boom carpet swept over the ground.
the boat and splashes into the nearby shoreline after traveling some distance. The sonic boom is an analogous conically- shaped wake created in three dimensions by the supersonic aircraft.
One can easily see that because the sonic boom noise is made continuously along the flight path, millions of U.S. res- idents will regularly hear sonic boom noise on a daily basis if supersonic civil aircraft become popular and fly overland. The good thing to know about this picture is that while con- ventional sonic booms are known be obtrusive to the public, the only sonic booms being considered for overland flights are sonic booms which have been tailored to minimize annoyance.
Aircraft manufacturers believe they can build small supersonic jets which have a “low-boom” sonic boom signa- ture which will barely be noticeable. What are some advan- tages of such “low-boom” small supersonic jets?
Current and future regulations and the advantages of overland flight
One distinct advantage of “low-boom” designs is that overland flight would be possible. The law in the United States since 1973 assumes that any and all sonic boom noise
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is unacceptable. The Code of Federal Regulations,
14CFR91.817, currently prohibits civil aircraft from exceed- ing Mach 1. It also prohibits any sonic boom from reaching the ground. This regulation was developed and enacted in a technical environment where boom shaping was considered either impossible or too technically risky. Similar, but not
5 identical, international laws are also effective worldwide .
However, if “low-boom” flight were deemed acceptable,
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the door would be open to overland flight . For example, for
a cross-US round trip between New York, New York (NY) and Los Angeles, California (LA) one would depart NY at 8:00 a.m. fly to LA in 2.5 hrs. One could then conduct a six hour business meeting and then fly back to NY in 2.5 hours. No overnight layover would be required.
Similarly, a trans-Atlantic round trip between London, England and New York with a 3.5 hour flight time could be completed in one day with no overnight stay needed. One can clearly see the time savings by not staying overnight
between departure and return flights for each round trip scenario.
On November 13, 2003 the Federal Aviation Administration held a Civil Supersonic Aircraft Technical Workshop7 to gather information that might be used to reassess 14CFR91.817. Unfortunately, this workshop was held at the same time as the Fall 2003 ASA meeting in Austin, TX, so there was little input from or recognition by ASA members. There were a number of presentations at this workshop by industry indicating the tremendous strides that have been made toward building aircraft with substantially
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reduced sonic boom . Many of these same companies have
urged the FAA to revise their regulations to allow overland supersonic flight so long as those flights are quiet enough to be acceptable to the public.
Boom minimization
In the late 1960s and early 1970s, Seebass and George9 were the first to put forward workable ideas on how aircraft could be designed to produce minimal sonic booms. The basic idea was to carefully control the cross-sectional area and lift of the aircraft, as functions of distance from the air- craft nose. These ideas were expanded upon by Mack and Darden10 in the 1980s. Linear theory was used, at best an approximation with the nonlinear flows present around a supersonic body.
However, over the past 30 years, progress has been made in the areas of computer power, computational fluid dynam- ics (CFD), and optimization. Computers have increased many orders of magnitude in speed and memory capacity since the 1970s, and algorithms have improved substantially. Industry now knows much more than they did during the time of Concorde, and they believe they can design aircraft that will have substantially quieter sonic booms via aircraft cross-sectional area and lift shaping. One can now include all fluid dynamic nonlinear effects to predict an aircraft’s near- field supersonic flow pattern. The updated computer algo- rithms for “low boom” design have also been validated by numerous laboratory tests.
This technical advance, coupled with multivariate opti- mization procedures, now allows aircraft designers to design
Lowering the Boom 21