The present invention relates generally to reducing open aircraft cavity acoustic resonance, and more particularly to an apparatus for expelling high frequency pulses of pressurized gas to reduce acoustic resonance within an open weapons bay of an aircraft in flight.
This application is somewhat related to my copending and commonly assigned patent applications xe2x80x9cJET NOISE SUPPRESSORxe2x80x9d, Ser. No. 09/973,176, xe2x80x9cHIGH FREQUENCY PULSED FUEL INJECTORxe2x80x9d, Ser. No. 09/973,161 and filed on even date herewith. The contents of these even filing date applications are hereby incorporated by reference herein.
As is well known, military aircraft sometimes carry weapons to be discharged during flight. The weapons are often carried within one or more cavities, commonly known as weapons bays. The weapons bays usually include a pair of doors, which are opened only when release of the weapon stores is anticipated. During the remainder of the aircraft flight, the weapons bay doors are closed, presenting a smooth surface to enhance aerodynamic performance as well as other characteristics, such as stealthiness, for example.
When the weapons bay doors are opened in flight, a thin region called a shear layer is created wherein the airflow abruptly transitions from a low speed flow inside the cavity to a high speed flow outside the cavity. This shear layer is characterized by instability which causes the shear layer to form tight, circular rotating pockets of flow commonly referred to as vortices. These vortices impinge on the rear wall of the cavity causing high levels of resonance and high acoustic levels inside the weapons bay. This acoustic resonance can be strong enough to damage the aircraft or its systems, and, therefore, is quite undesirable.
Attempts have been made in the past to reduce acoustic resonance occurring within an open aircraft weapons bay. For example, U.S. Pat. No. 5,699,981 to McGrath et al. describes a system incorporating a cylindrical member disposed on the surface of the aircraft near the leading edge of the cavity. The cylindrical member is projected into the airflow of aircraft in flight to create vortices to reduce acoustic resonance. Similarly, U.S. Pat. No. 5,340,054 to Smith et al. describes an apparatus for acoustic reduction using a series of perturbation pins placed on the surface of the aircraft for generating vortices to disrupt the shear layer, for the purpose of reducing acoustic resonances within an open cavity. While somewhat effective, these prior art systems are not without the need for improvement. For example, these patents describe systems requiring the attachment of structures onto the surface of the aircraft, increasing drag as well as significantly interrupting the desirable smooth aircraft surface. Moreover, their effectiveness decreases dramatically at aircraft speeds up to and exceeding Mach 1.
A system recently described by Parekh, D. E. et al., Innovative Jet Flow Control: Mixing Enhancement Experiments, AIAA Paper No. 96-0308, American Institute of Aeronautics and Astronautics, AIAA, Aerospace Sciences Meeting, 34th, Reno, Nev., Jan. 15-18, 1996, includes vibrating wedges driven by piezoelectric actuators. This system, while providing the proper high frequency range of operation, is unsuited for application to the weapons bay problem because it is incapable of providing sufficient amplitude.
Another recent system for providing the desirable high frequency suppression operation is described in my U.S. Pat. No. 6,296,202 entitled Aircraft Weapons Bay Acoustic Suppression Apparatus, assigned to the assignee of the present invention. This acoustic suppression device utilizes an oscillatable spoiler plate which is extended into the airstream. At least one shaker is placed in operative engagement with the spoiler plate in order to oscillate the spoiler plate at high frequencies and high amplitudes to seed the shear layer with frequencies which directly complete with the natural frequency of the shear layer vortices.
This system represents a distinct improvement over the prior art because it provides high amplitude oscillation in addition to the high frequency operation. This desirable high amplitude operation is not obtainable with the prior art systems such as Parekh""s described above.
While my system represents a significant advancement over the earlier acoustic suppression systems, further improvement and refinement is desirable. More specifically, in certain situations, electromechanical devices of this type might weigh too much, or might consume too much electrical power. Also, it is well known that mechanical devices with moving parts are subject to failure and maintenance requirements. In these situations a light weight device including an injector having no moving parts might be desirable.
A need exists therefore for an improved aircraft weapons bay acoustic suppression apparatus. Such an apparatus would provide improved high frequency acoustic resonance reduction, enhancing aircraft operation as well as aircraft longevity.
Accordingly, it is a primary object of the present invention to provide an aircraft weapons bay high frequency acoustic suppression apparatus overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide an aircraft weapons bay high frequency acoustic suppression apparatus providing acoustic resonance reduction over a wide range of aircraft operating conditions.
It is yet another object of the present invention to provide an aircraft weapons bay high frequency acoustic suppression apparatus which can be utilized on a wide variety of aircraft.
It is still another object of the present invention to provide an aircraft weapons bay high frequency acoustic suppression apparatus that includes an injector unit for injecting high frequency pulses of pressurized gas into the airstream.
Still another object of the present invention is to provide an aircraft weapons bay high frequency acoustic suppression apparatus including an injector unit having no moving parts.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, an aircraft weapons bay acoustic suppression apparatus is provided to dramatically reduce acoustic resonance within an open weapons bay of an aircraft in flight.
As is known in the art, opening the weapons bay doors in flight gives rise to the creation of a thin region called a shear layer where the airflow abruptly transitions from a low speed flow inside the cavity to a high speed flow outside the cavity. This shear layer is characterized by instability which causes the shear layer to form tight, circular rotating pockets of fluid flow commonly referred to as vortices. These vortices impinge on the rear wall of the cavity and correspondingly generate an acoustic wave which propagates in the opposite way, upstream. The acoustic wave thus generated interacts with the shear layer to influence the size and spacing of the vortices. If the frequency and phase of the acoustic wave coincides with that of the shear layer instabilities, the vortices generated by this xe2x80x9cforcedxe2x80x9d shear layer can become a whole number (1, 2, 3, etc.) with respect to the cavity, generating high levels of undesirable acoustic resonance. The effects of this can often be strong enough to damage the aircraft or its systems, and, as can be appreciated, are quite undesirable.
In accordance with the teachings of the present invention, the aircraft weapons bay high frequency acoustic suppression apparatus includes a spoiler received within the weapons bay. The spoiler is mounted such that it can be retractably extended into the airstream of the aircraft in flight. An injector unit for injecting high frequency pulses of pressurized air into the airstream is received within the spoiler. The injector unit includes a resonance tube and a nozzle having an outlet directed externally from the aircraft, into the airstream.
Resonance tubes are well known in the art. In general, resonance tubes are fluidic devices that receive a pressurized fluid input and provide a pulsating fluid output. The nature of the output, such as pulse frequency and amplitude is variable and depends on the upon the dimensions of the resonance tube.
The resonance tube outlet is placed in fluid communication with a nozzle. During operation of the aircraft weapons bay high frequency acoustic suppression apparatus, the spoiler is extended into the airstream of the aircraft in flight. Compressed air is directed independently into the resonance tube and into the nozzle via inlet valves. The resonance tube provides a pulsating output which is directed into the nozzle. The pulsating output of the resonance tube perturbs the flow of air in the nozzle, effectively breaking it up into discrete slugs or pulses which exit the nozzle and are directed into the airstream.
This high frequency discharge of high pressure gas into the airstream effectively seeds the shear layer with frequencies which directly compete with the natural frequency of the shear layer vortices even at very high aircraft speeds, something heretofore not possible with the acoustic suppression methods of the prior art.
Advantageously and according to an important aspect of the present invention, the injector has no moving parts, enhancing reliability as well as reducing costs. The spoiler can be made according to designs and configurations known to those skilled in this art, reducing cost as well as simplifying retrofit into existing aircraft.