Aeronautics and astronautics – Miscellaneous
Reexamination Certificate
2002-03-22
2004-03-02
Swiatek, Robert P. (Department: 3644)
Aeronautics and astronautics
Miscellaneous
C244S119000, C181S211000
Reexamination Certificate
active
06698684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to aircraft fuselage design. More particularly, it relates to aircraft fuselage configurations that control the build-up, magnitude, and directional characteristics of pressure waves generated by an aircraft flying at supersonic speed so as to reduce the sonic boom at ground level.
2. The Prior Art and Background
A conventional aircraft in flight produces pressure waves in the medium it flies through. These pressure waves propagate at the speed of sound. When the aircraft flies at subsonic speed, these pressure waves propagate in all directions around the aircraft, including ahead of the aircraft. When the aircraft flies at supersonic speed, these pressure waves cannot propagate ahead of the aircraft because the aircraft is traveling faster than the
propagation speed of the waves. Instead, the pressure waves generated by such a conventional aircraft coalesce into two shock waves, one formed by the nose of the aircraft and the other formed by the tail of the aircraft. These shock waves are characterized by an abrupt pressure increase across the wave. With respect to the shock wave formed by the nose (the “bow shock”), the pressure increases abruptly from about ambient to above ambient. The pressure decreases from above ambient to below ambient in the region between the bow shock and the shock wave formed by the tail (the “tail shock”). The pressure then increases abruptly from below ambient to about ambient across the tail shock.
In the limit, weak shock waves propagate in the form of a Mach cone having a shape defined by the Mach angle &mgr;. The Mach angle &mgr; is a function of the Mach number M, which is defined as the ratio of the speed of an object to the speed of sound. The Mach angle &mgr; can be determined using the equation:
sin
(
μ
)
=
1
M
,
or
μ
=
sin
-
1
(
1
M
)
The shape of the Mach cone produced by an aircraft in supersonic flight can be represented by rotating a line drawn at an angle &mgr; to the aircraft's direction of travel about a line representing the aircraft's direction of travel, so that the tip of the Mach cone points in the direction of travel.
These shock waves can propagate great distances away from the aircraft and eventually reach the ground, where they can produce significant acoustic disturbances called sonic booms. Sonic booms are so named because of the sounds created by the abrupt pressure changes when the shock waves pass a reference point on the ground. The acoustic signature of a sonic boom commonly is characterized as an N-wave because the pressure changes associated with the acoustic signature resemble the letter “N” when plotted as a function of aircraft length. That is, an N-wave is characterized by the abrupt pressure rise associated with the bow shock, commonly referred to as “peak overpressure,” followed by a substantially linear decrease in pressure to below ambient pressure, followed by the abrupt rise to ambient pressure associated with the tail shock. Sonic booms can cause objectionable sounds and vibrations. For these reasons, supersonic flight over populated areas has long been limited by regulation.
In order for supersonic flight over land to be acceptable, the pressure disturbances that cause the sonic boom's acoustic signature must be controlled so that the effects of the abrupt pressure changes caused by the shock waves are minimized at ground level. Many attempts have been made to modify the design of supersonic aircraft in order to adjust the sonic boom signature. These modifications have included changes to wing design, as described in U.S. Pat. No. 5,934,607, issued to Rising, et al., for a “Shock Suppression Supersonic Aircraft.” Another approach involves incorporating air passages through the fuselage or wings of supersonic aircraft, such as the structures described in U.S. Pat. No. 4,114,836, issued to Graham, et al., for an “Airplane Configuration Design for the Simultaneous Reduction of Drag and Sonic Boom”; U.S. Pat. No. 3,794,274, issued to Eknes, for an “Aircraft Structure to Reduce Sonic Boom Intensity”; and U.S. Pat. No. 3,776,489, issued to Wen, et al., for a “Sonic Boom Eliminator.” Further attempts at reducing the sonic boom caused by supersonic aircraft include the addition to the aircraft of structure arranged to disrupt the air flow patterns as the aircraft travels at supersonic speed. Examples include the structure described in U.S. Pat. No. 3,709,446, issued to Espy, for a “Sonic Boom Reduction” and U.S. Pat. No. 3,647,160, issued to Alperin, for a “Method and Apparatus for Reducing Sonic Booms.”
Another attempt to control the sonic boom in a supersonic aircraft uses a blunt nose to increase the air pressure immediately adjacent to the nose of the aircraft, thus disrupting the normal formation of the pressure wave that causes the acoustic signature. This disruption results in the reduction of abrupt pressure changes in the acoustic wave that strikes the ground. A blunt nose, however, creates a significant amount of drag on the aircraft, drastically decreasing its efficiency. U.S. Pat. No. 5,740,984, issued to Morgenstern, for a “Low Sonic Boom Shock Control/Alleviation Surfaces” describes a mechanical device on the nose of the airplane which can be moved between a first position effecting a blunt nose when sonic boom reduction is desired and a second position effecting a streamlined nose when sonic boom reduction is not required, thereby removing (in the streamlined configuration) the drag penalty inherent in a blunt nose design. U.S. Pat. No. 4,650,139, issued to Taylor et al., discloses a blunt-nosed spike which can be extended from a space vehicle's fuselage.
U.S. Pat. Nos. 5,358,156, 5,676,333, and 5,251,846, all issued to Rethorst and all entitled “Supersonic Aircraft Shock Wave Energy Recovery System” (collectively “the Rethorst patents”), describe a supersonic aircraft having a spike extending from the front of the aircraft and a forward ring on the fuselage for eliminating a sonic boom. The spike is said to direct the bow shock onto the manifold ring which recovers the shock energy and converts it to useful work. The spike is said to be extendable, but it does not include a complex surface contour, and it is not disclosed to include a number of telescopically collapsible sections. Instead, it is disclosed as a single cylindrical member which tapers to a point at its leading end.
U.S. Pat. No. 3,643,901, issued to Patapis, discloses a ducted spike for attachment to a blunt body operating at supersonic speed for the purpose of receiving and diffusing oncoming air to reduce pressure drag on and erosion of the blunt body.
U.S. Pat. No. 3,425,650, issued to Silva, discloses an apparatus which can be extended on a boom from the front of an aircraft to deflect air outwardly therefrom.
U.S. Pat. No. 3,655,147, issued to Preuss, covers a device attached to the lower forebody of an aircraft for the purpose of reflecting pressure disturbances caused by the aircraft's flight in directions away from the ground.
Although some of the foregoing references are directed to sonic boom mitigation, none of them address the sonic boom signature shaping techniques of the present invention.
SUMMARY OF THE INVENTION
The present invention provides an improvement in aircraft design which is directed to mitigating the effects of sonic booms at ground level. An aircraft according to the present invention includes a spike which extends from the aircraft's nose in a direction substantially parallel to the aircraft's length to effectively lengthen the aircraft. A longer aircraft generally is expected to produce a sonic boom of lesser amplitude at ground level than a shorter aircraft of similar weight because the pressure disturbance is distributed over a greater length. Therefore, a sonic boom created by an aircraft according to the present invention is expected to be of lesser intensity than a sonic boom created by a conventional supersonic aircraft of similar weight and otherwise simila
Hancock, Jr. Jimmy L.
Henne Preston A.
Howe Donald C.
Wolz Robert R.
Gulfstream Aerospace Corporation
Howrey Simon Arnold & White , LLP
Swiatek Robert P.
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