PASSIVE AERODYNAMIC SONIC BOOM SUPPRESSION FOR SUPERSONIC AIRCRAFT

Abstract: Sonic boom suppression apparatus for an aircraft (100) including a nose portion (102) having convex upper surfaces and a flat underside, slotted portions on the fuselage (106) or wings (104) of the aircraft, shock cancellation surfaces incorporated in the engine portion (108) of the aircraft, and area/lift distribution tailoring, all of which are preferably used concomitantly.

Description: PASSIVE AERODYNAMIC SONIC BOOM SUPPRESSION FOR SUPERSONIC AIRCRAFT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to high-speed aircraft and, more particularly, to long-range supersonic cruise aircraft having sonic boom shock suppression capabilities. 2. Description of the Prior Art Various techniques for reducing sonic boom by suppressing shock waves formed on or around high-speed aircraft have been tried for years. United States Patent No. 5,289,995 to Greene teaches the incorporation of an internal passageway, an articulated intake structure, and an articulated exhaust structure, which is connected to the intake structure by the passageway. The cross section is essentially the aircraft's fuselage and engine inlet combined, which helps cancel shocks form the fuselage and engine(s). United States Patent No. 4,582,276 to Gerhardt teaches incorporating a U-shaped aerodynamic structure into the design of a supersonic aircraft with the U- shaped structure enabling cancellation of shock waves by expansion waves generated by the apices of the triangularly-shaped legs of the U-shaped structure. United States Patent No. 5,740,984 to Morgenstern teaches altering the direction and shape of the shock wave by using a control device, located at or near the nose section of the aircraft, that has a control surface that can be moved between a retracted position, wherein drag on the aircraft is reduced and an extended position, wherein air pressure at the nose section is increased to alter the shape of the shock wave created by the aircraft and, hence, the sonic boom that impacts the ground. United States Patent No. 4,114,836 to Graham et al. discloses an aircraft having plural passageways, each having its longitudinal axis substantially parallel to the flight path of the aircraft. The passageways extend through the wings, the fuselage, the horizontal elevators, and the rudder for the purpose of permitting air to pass through these areas, thereby reducing drag and sonic boom. United States Patent No. 4,436,261 to Koleff discloses a sonic boom concentrator for a V/STOL aircraft in the form of a fin having curved sidewalls and truncated ends. United States Patent No. 5,518,204 to Tracy discloses a supersonic flight aircraft having a longitudinal body and a laterally extending wing, in which laminar airflow conditions are maintained over the leading edge and adjacent the surface of the wing. As a rule, the foregoing techniques can be categorized as shock cancellation, area/lift distribution tailoring, shock directionality alteration, and slotted edges. Each technique enables sonic boom control and/or wave drag reduction. However, each concept independently has limitations that preclude the development of a feasible "shock free" vehicle. In addition to the published patents, the issue of reducing drag and sonic boom associated with supersonic aircraft has been the subject of research at certain universities. One such program by George and Seebass involved the theory to tailoring the area and lift distribution verses length to minimize the sonic boom shock strength at the ground. Another such program involves the development of a two-dimensional shock canceling model proposed by Busemann in 1935, as illustrated in Figure 1. Finally, George examined redirecting boom laterally and showed how to calculate a directionally altered boom reduction underneath a vehicle that persists into the far field. The foregoing techniques can be categorized as Artificially Blunted Leading Edge (ABLE), area/lift distribution tailoring, shock cancellation and shock directionality alteration. Each technique enables sonic boom control and/or wave drag reduction. However, each concept independently has limitations that preclude the development of a feasible "Low Sonic Boom" vehicle. SUMMARY OF THE INVENTION It is, therefore, an aspect of the present invention to provide a novel "Low Sonic Boom" high-speed aircraft utilizing various body portion orientations, such as lifting bodies, flying wings, blended wings along with other such orientations as illustrated in Figures 4, 11a, lib, 12a, 12b, 12c, and 12d by incorporating many of the synergistic passive shock reduction and cancellation devices or techniques, while overcoming many of the disadvantages and drawbacks associated with such devices or techniques. Another aspect of the present invention is to suppress sonic boom in a high-speed aircraft capable of supersonic flight by incorporating shock cancellation surface configurations around engine nacelles, as illustrated in Figures 8 and 9, shaping the aircraft fuselage to provide shock directionality, incorporating slotted edges in the wing, body, or canard of the aircraft, and designing the area and lift distribution to tailor the ground shock strength and to reduce wave drag of the aircraft. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram illustrating structure that facilitates shock cancellation; Figure 2 is a diagram showing far-field sonic boom signature with and without passive aerodynamic sonic boom suppression; Figure 3 illustrates Georges & Seebass simplified assumption; Figure 4 illustrates a vehicle developed using George & Seebass technique; Figures 5a, 5b, and 5c illustrate various leading edge configurations and resultant shock formations; Figures 6a and 6b illustrate multiple Artificially Blunted Leading Edge (ABLE) nose configurations designed for a representative low-boom tailored aircraft; Figures 7a and 7b illustrate the application of Shock Cancellation to the engine nacelle integration; Figure 8 shows how shock cancellation allows tailoring of area/lift distribution; Figure 9 illustrates computational results of the application of shock cancellation; Figure 10 illustrates the dihedral effect on shock directionality; Figures 11a and lib illustrate application of fuselage shaping to shock directionality; and Figures 12a, 12b, 12c, and 12d depict an aircraft incorporating structures, surfaces, and configurations in accordance with the teachings of the present invention including an Artificially Blunted Nose, Area/Lift tailoring, vehicle shaping for "Shock Directionality," and nacelle shrouds for "Shock Cancellation. " DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Variations, however, may be readily apparent to those skilled in the art, since only the generic principles of the present invention have been defined herein specifically to provide teachings for an aircraft configuration that encompasses many long, sought-after features. The present invention uses a combination of synergistically-combined techniques to achieve reduction of the sonic boom of supersonic airplanes, and, in particular, to achieve a reduction of sonic boom by 12-25dBA while reducing the drag of the vehicle and maintaining a "practical design." Typically, supersonic commercial flights over land have been prohibited due to the highly-annoying perceived loudness of sonic booms. Such perceived loudness is a function of the magnitude of the shock strength (change in pressure). A typical N-wave sonic boom consists of a rapid pressure jump or shock at the beginning, followed by a gradual expansion, and finally, a rapid recompression shock at the end. These two shocks become amplified in magnitude due to coalescence with other waves emanating from the vehicle, usually the wing and inlet/nacelle (Figure 2). Only the shocks are audible (not gradual pressure changes), creating the two shock "boom- boom" sounds characteristically associated with sonic booms. It has been determined, therefore, that to reduce the loudness of sonic booms, the size of the shocks needs to be reduced. George and Seebass developed the theory for tailoring the area and lift distribution versus length to minimize the shock strength at the ground given parameters of aircraft weight, flight altitude and Mach number. To minimize the shock strength, the sum of the area and lift must exactly follow the George and Seebass distribution. Darden implemented the theory in a computer program to automatically calculate these distributions. These provide the starting point for passive sonic boom reduction used herein. However, three characteristics of George and Seebass shock- minimized distributions result in impractical airplane designs because the distributions require: 1) blunter noses than minimum drag shapes - leading to reduced performance, 2) smooth distributions through the nacelle region - not possible with existing engine designs, 3) a one-dimensional simplifying assumption - so the distributions are only calculated directly under the vehicle; and nonplanar and azimuthally-varying effects are not considered (Figure 3). Low sonic boom vehicle shapes have been developed by application of this tailoring theory. Once such concept is shown in Figure 4. By the theory, this structure would appear to exhibit very low sonic boom and suggested that significant benefits had been achieved from area/lift distribution tailoring. It has been later concluded that the addition of nacelles created strong shocks underneath, and the lower sweep on the outboard wing created strong shocks at azimuths off to the sides. Additional techniques are necessary to suppress all shock sources of a realistic vehicle. To achieve shock-minimized signatures with practical shapes, three other technologies have been combined, either individually or in combination, with area/lift distribution tailoring: 1) artificially blunted leading edges (ABLE), 2) shock cancellation, 3) nonplanar shock directionality. Area/Lift Tailoring with ABLE ABLE implementation to achieve shock reduction is based on earlier studies that show midfield and far field overpressures are minimized with a blunt leading edge (not Figure 5a). Sharp leading edges (Figure 5b) generate strong compressions along the vehicle that coalesce to produce a strong far field overpressure. A blunt edge or vehicle nose creates a strong bow shock, but subsequent compressions are weak and less likely to coalesce. In addition, the dissipation with altitude of strong initial shocks is greater. High wave drag of blunt edges or noses has traditionally precluded practical applications. This has been described as the "low boom, high drag" paradox. To address this phenomenon, "slotted edge" technology has been developed, which significantly reduces the drag associated with blunt nose/blunt edge designs, as well as far field overpressure of leading edge shocks. One embodiment of this, which is shown in Figure 5c, takes the form of a "slot-in- wing" structure. For a significantly small channel, the choked, internal flow induces a strong normal shock in front of the nose. The resultant effective blunt body produces an external flow structure similar to a conventional blunted geometry, but it is significantly lower combined pressure and friction drag. CFD has shown up to 40 percent reduction in drag relative to an equivalent blunt airfoil. There are many ways of incorporating this "slotted edge" technique on a "Low Boom" supersonic aircraft. One method is the incorporation of the "slotted edge" along the leading edge of the wing of the aircraft. A second method is a blunted nose with slots on the fuselage. This places a normal shock inlet at the nose to generate the strong shock, usually due to bluntness, with less drag because the flow does not stagnate, instead flowing through the ABLE channel as described in Dr. Ruffin's patent application. We use the ABLE nose to satisfy the bluntness required of a tailored distribution - with lower drag levels. One embodiment of the concept would be Figure 5c as a flying wing configuration. Possible ABLE applications to the forebody of a tailored vehicle are shown in Figures 6a and 6b (and achieved a tailored low boom successfully in NASA Ames' ballistic range). The bluntness required of a tailored distribution is fundamental to preventing shock coalescence. Shock Cancellation Referring now to Figure 1, there is shown a two-dimensional, shock-free environment corresponding to a model developed by Busemann in 1935, comprising a biplanar structure exhibiting parallel surfaces. Busemann' s study helped explain how a shock-free environment of parallel surfaces leads to theoretically zero wave drag (due to volume) by shock cancellation. With the Busemann biplane at zero incidence, as shown in Figure 1, shocks produced at the leading edges are canceled by expansions at the shoulders of the opposite surfaces. The expansion waves are then canceled by shocks at the trailing edges. The result is low wave drag, no sonic boom, and no lift. As the incidence angle is increased in order to produce lift, an undesirable shock is produced off the lower surface. This makes the Buseman biplanar configuration impractical for a lift-producing, "Low Boom," supersonic aircraft. However, the Busemann structure can be incorporated on the aircraft to cancel shocks off "non-lift producing" structure, such as the engine nacelles. The incorporation of this concept in a nacelle shroud and reflexed wing is shown in Figures 7a and 7b. The reflexing of the wing beyond the minimum drag amount to further reduce the nacelle shock can be of further benefit for area/lift tailoring. Figure 8 shows how shock can cancellation allows the tailored area/lift distribution to be met. Figure 9 shows how even a short shroud can greatly reduce shock strength and move the shock location further aft, which also helps in tailoring area/lift. Nonplanar Shock Directionality To aid in tailoring an area/lift distribution, both the inlet/nacelle and wing shocks need to be reduced. As proposed by George, shock directionality can be used to reduce the wing shock due to lift. Once way to reduce the lift shock is to use nonplanar shock directionality through integration of the dihedral effect. With the fairly high-sweep angle of the wing common to supersonic aircraft, increasing the dihedral angle has the effect of lengthening the vehicle signature and directing the shock to the sides, both of which reduce the sonic boom signature on the ground. Figure 10 illustrates the dihedral effect and its resulting signature length stretching and proportional strength reduction, which combines to make area/lift tailoring easier for further reductions. Another way to take advantage of nonplanar shock directionality for lift shock reduction is to reduce the midbody fuselage volume above the wing beyond that needed for wave drag reduction without reducing it as much under the wing, as shown in Figure 11a. This generates an expansion that lowers the pressure above the wing everywhere the expansion fan covers, increasing lift without changing the pressure under the wing. With this structure, the angle of attack needed to generate the same lift is lower, which results in a reduction in pressure below the wing relative to without the structure. This reduces the lift shock making area/lift tailoring easier. Figure lib also shows that by carrying lower pressure above the wing relative to below the wing (also known as lift) to the back end of the vehicle, the smaller expansion spike necessary to prevent aft shock coalescence in area/lift tailoring can be generated behind the trailing edge of the wing. The lower pressure on the upper surface of the wing should have its camber designed to generate this spike (shown in Figure 2) when it spills off the back end of the wing. The camber design for the spike can be done in conjunction with the midbody induced expansion described above. This method of generating the spike maximizes the length that can be used for area/lift tailoring, which allows achievement of the lowest ground shock strengths theoretically possible with tailoring. Figures 12a - 12d show an aircraft 100 constructed in accordance with the teachings of the present invention that incorporates various ones, if not all, of the foregoing techniques. The nose section 102 is seen to include an 103 ABLE blunted nose with slots for drag reduction. The wing 104, wing dihedral 105, and midbody 106 are shaped for shock directionality decreasing the lift shock strength, which makes the tailored area/lift distribution easier to achieve. The lower pressure above the wing at the far aft trailing edge spills off to generate the expansion spike needed to prevent aft shock coalescence. Shock cancellation surfaces are incorporated around the engine nacelles 108 and reflexing of the wing above the nacelles. The aircraft further incorporates area and lift distribution tailoring for additional wave drag reduction and sonic boom reduction. Those skilled in the art will appreciate that various adoptions and modifications of the invention as described above can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims: In the Claims 1. An aircraft embodying sonic boom suppression capabilities, comprising: a body portion including a nose portion, a fuselage, a wing portion, and an engine portion, a tailored area/lift distribution, and at least one other structural means, associated with said fuselage, wing portion, or engine portion, for suppressing sonic boom wave forms. 2. The aircraft of claim 1, wherein said at least one other structural means is chosen form the group consisting of shock cancellation surfaces in engine nacelles and slotted leading edges at said wing portion. 3. The aircraft of claim 2, wherein said at least one other structural means comprises all of said structural means in said group. 4. The aircraft of claim 2, wherein said body portion comprises a lifting body. 5. The aircraft of claim 2, wherein said body portion comprises a blended wing. 6. The aircraft of claim 2, wherein said body portion comprises a flying wing. 7. The aircraft of claim 4, 5, or 6, wherein said aircraft includes an engine shroud shock cancellation surface and a slotted leading edge at said nose portion. 8. The aircraft of claim 7, wherein said engine portion has a leading edge. 9. The aircraft of claim 8, wherein said engine shroud cancellation surface lies behind said leading edge of said engine portion. 10. The aircraft of claim 9, wherein said aircraft includes a wing reflex integrated with said tailored area/lift distribution to assist in achieving a tailored reduced shock strength at the ground. - ll ll. The aircraft of claim 10, wherein said aircraft further includes a wing dihedral with at least one other structural means chosen from the group consisting of slotted leading edges at said nose or wing portion, shock cancellation surfaces and nonplanar shock directionality structures. 12. The aircraft of claim 11, wherein said aircraft further comprises a midbody structure which reduces in volume above said wing portion more than below said portion which is integrated with said tailored area/lift distribution to achieve a tailored reduced shock strength at the ground. 13. The aircraft of claim 12, wherein said aircraft has lower pressure carried above said wing portion relative to below said wing portion (lift) far aft on said aircraft in the exact amount to generate an expansion spike in area/lift tailoring needed to prevent aft shock coalescence behind said wing portion.

Inventors: HARTMANN TOM, MORGENSTERN JOHN M, HARTMANN, TOM, MORGENSTERN, JOHN, M.
Applicants: LOCKHEED CORP, LOCKHEED MARTIN CORPORATION
Application year: 2000
Main classification: B64C30
IPC codes: B64C23, B64C30