Quiet Supersonic Transport (QSST) - Background
Supersonic flight over the United States and other countries is a challenging environmental issue for a viable supersonic commercial aircraft. Current FAA regulations prohibit civil flights at Mach numbers greater than one without case-by-case exceptions approved by the Administrator. Many other countries have similar restrictions.
Previous research has shown that the highly impulsive nature of the "N-wave" sonic-boom signatures of all existing supersonic aircraft is the primary cause of negative response and regulatory limitations on supersonic travel. Conclusions of NASA research further indicate the exceptional difficulty of designing an aircraft with an "N-wave" signature of sufficiently low amplitude for general public acceptance. However, the research also found that a "shaped" signature was less objectionable and that a reasonably achievable amplitude wave could meet Committee on Hearing and Bioacoustics of the National Research Council (CHABA) guideline for acceptable noise impact to the general public, depending on frequency of exposure.
A sonic boom occurs due to pressure waves that occur when an aircraft moves at supersonic speeds. During subsonic flight, air displaced by a passing plane flows around the plane in the manner water flows around an object in a stream. However, for a plane flying at supersonic speeds, the air cannot easily flow around the plane and is instead compressed, generating a pressure pulse through the atmosphere. The pressure pulse intensity decreases as a consequence of movement from the airplane, and changes shape into an N-shaped wave within which pressure raises sharply, gradually declines, then rapidly returns to ambient atmospheric pressure. A wall of compressed air that moves at airplane speed spreads from the wave and, in passing over ground, is heard and felt as a sonic boom. The rapid changes in pressure at the beginning and end of the N-wave produce the signature double bang of the sonic boom.
Research has recently shown that boom intensity can be reduced by altering aircraft shape, size, and weight. For example, small airplanes create a smaller amplitude boom due to a lower amount of air displacement. Similarly, a lighter aircraft produces a smaller boom since an airplane rests on a column of compressed air and a lighter plane generates a lower pressure column. An aircraft that is long in proportion to weight spreads the N-wave across a greater distance, resulting in a lower peak pressure. Furthermore, wings that are spread along the body and not concentrated in the center as in a conventional aircraft produces a pressure pulse that is similarly spread, resulting in a smaller sonic boom.
Shaping of a sonic boom refers to a technique of altering source pressure disturbance such that a non-N-wave shape is imposed on the ground. Shaping sonic boom can reduce loudness by 15-20 dB or higher with no added energy beyond that to sustain flight. Shaping to minimize loudness is based on insight regarding changes in aircraft pressure disturbances during propagation to the ground.
The N-wave shape results because the front of a supersonic aircraft generates an increase in ambient pressure while the rear generates a decrease in pressure. Variation in propagation speed stretches the disturbance during propagation to the ground. Shaped boom techniques typically attempt to prevent coalescing of the pressure disturbance by adding a large compression at the aircraft nose and an expansion at the tail with pressure in between constrained between the compression and expansion. The shaped boom stretches the ends of the signature faster than the in-between pressures, creating a non-N-wave sonic boom at the ground.
Boom reduction makes a supersonic aircraft less objectionable by minimizing the loudness of a sonic boom. Audible frequencies in a sonic boom occur in the rapid pressure changes, or shocks, at the beginning and end of the typical N-waveform. More quiet shocks have decreased pressure amplitudes and increased pressure change time durations.
Although sonic boom reduction is an important design criterion for a supersonic aircraft, other considerations always impact design decisions. For example, a useful aircraft will have an appropriate capacity for holding passengers and/or cargo and be a suitable configuration for safe operation. Some design aspects include integration of landing gear and airframe.
Shaped sonic boom signatures are achieved by tailoring the volume and lift distribution of an aircraft. In addition to low sonic boom capabilities, a commercially viable supersonic aircraft must have low drag to achieve range, fuel efficiency, and payload goals. An embodiment of a flow diagram provide capabilities to design aircraft with low drag (high performance) and reduced sonic boom capabilities. Part of the theoretical background is based on the George-Seebass-Darden theory, which requires the pressure disturbance caused by a low boom aircraft to follow an inversely calculated equivalent area distribution goal to result in the lowest shock strength at the ground. When the equivalent area due to geometric area and lift sum to the equivalent area distribution goal, a minimized ground sonic boom is expected.
The method for configuring an aircraft for low sonic boom supersonic flight conditions includes scaling an equivalent area distribution goal curve to approximate an ideal equivalent area distribution goal curve. A design constraint requiring the equivalent area distribution curve of the aircraft to be at the goal curve can be relaxed to allow the equivalent area distribution curve to be at or below the equivalent area distribution goal curve. According to other embodiments, an aircraft includes a wing configured to generate a first area of expanded airflow and a first area of compressed airflow following the area of expanded airflow. The areas of expansion and compression are configured to be at or below an equivalent area distribution goal that minimizes sonic boom disturbance by redistributing areas of lift on the wing. The reduced lift generated by the area of expansion is balanced by the additional lift generated by the area of compression.
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