Patent Publication Number: US-2011048847-A1

Title: Noise attenuation device for reducing noise attenuation in a jet engine test cell

Description:
This application is a continuation-in-part application of U.S. patent application Ser. No. 12/552,771, filed Sep. 2, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the testing of jet engines. More particularly, the present invention relates to testing a jet engine in a jet engine test cell while substantially attenuating the noise caused by testing the jet engine. 
     2. Description of the Prior Art 
     Noise from jet engine testing can cause auditory health effect including hearing loss and deafness, as well as non-auditory health effects such hypertension and nervous disorders. This type of noise also disturbs sleep of individuals in proximity to the test site; affects the performance of children in school and decreases the value of real estate surrounding the test site. In addition, noise from jet engine testing is one of the most common sources of tensions between surrounding communities and miliary air bases, and the military needs to aggressively pursue any and all available means to reduce its impact. 
     Typical sound levels for noise varies from 60 dBA for normal conversation to 70 dBA for vacuum cleaner to 130 dBA or more for a jet engine at 100 feet. OSHA (Occupational Safety and Health Administration) regulations require that engineering controls be used or that personal protective equipment be provided for a worker exposed to sound levels greater than 85 dBA for more than 8 hours. 
     As military aircraft engines become more powerful and noisier, as aircraft operations expand, and as land areas proximate to military operational bases are developed for commercial and residential use, jet engine testing noise and other issues create substantial disagreement and tension between the military and local officials. 
     Thus, there is an urgent need to significantly reduce jet engine noise by up to 20 dBA during static testing of a jet engine in a jet engine test cell. There is also a need to provide the military with a new state of the art device for testing jet engines in a jet engine test cell which is relatively inexpensive and also brings jet engine ground noise limits within acceptable limits to surrounding communities. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes some of the difficulties of the past, including those mentioned above, in that it comprises a relatively simple design and is a highly effective device for testing jet engines in a jet engine test cell. The noise attenuation device comprising the present invention reduces the noise level of the jet exhaust from the engine under static testing to a level acceptable for surrounding environments. 
     The noise attenuation device of the present invention comprises an empty carbon steel pipe of predetermined length and diameter placed within a jet engine test cell which is a building like structure used to test jet engines. The jet engine under test is also positioned within the test cell in proximity to the front end portion of the carbon steel pipe. The carbon steel pipe is aligned axially with the direction of exhaust flow from the exhaust port of the jet engine being tested. The carbon steel pipe includes flow restrictors downstream from the entrance of front end of the carbon steel pipe. The function of a flow restrictor within the carbon steel pipe is to gradually slow the jet engine exhaust as the jet engine exhaust travels through the carbon steel pipe. Gradually slowing jet engine exhaust flow reduces frictional flow losses and allow recovery of pressure head from the jet engine exhaust flow. Recovering pressure head assist in maintaining high augmentation flow rates through the noise attenuation device and the jet engine test cell. 
     Located at the rear end of the carbon steel pipe is an exhaust blockage structure which functions as a flow restrictor. The blockage structure used in the noise attenuation device may be either a full blockage cone or a partial blockage cone with openings. In addition, the exhaust blockage structure may be a grill end piece attached to the rear end of the carbon steel pipe. 
     The cross sectional flow area in the rear portion of the carbon steel pipe increases for a jet engine exhaust-cool air mixture passing through the rear portion of the carbon steel pipe to more than double. A substantial portion of the flow of the jet engine exhaust-cool air mixture is then forced through openings within perforated side walls located in the rear end portion of the carbon-steel pipe. 
     Cold air enters the front end of the carbon-steel pipe at a temperature of approximately 70 degrees Fahrenheit and has a mass of approximately three times the mass of the hot jet engine exhaust. By adding three times the mass of cold air to that of the hot jet engine exhaust, the hot jet exhaust becomes intimately mixed with the cold air forming the jet engine exhaust-cool air mixture which flows through the carbon steel pipe to the outlet end of the carbon steel pipe. When the jet engine exhaust-cool air mixture from a jet engine operating at after-burner conditions reaches the outlet end of the carbon steel pipe its temperature is reduced from 3800° F. to less than 1200° F., the design limit required to protect the noise attenuation device. 
     Adding the cold air mass to the jet engine exhaust reduces the average velocity of the resulting mixture stream by a factor of about four. Reducing the velocity of the jet engine exhaust flow reduces the intensity of turbulent fluctuations and the acoustic power produced by the jet engine exhaust plume. 
     An annular region through which the exhaust-cool air mixtures flows is formed between the side plates and the inner wall of an augmenter within the jet engine test cell. The gas flow pattern of the exhaust-cool air mixture is maintained in a forward direction within the annular region by air flow inducted into the entrance or front end of the noise attenuation device and the augmenter of the jet engine test cell. 
     The annular region formed between the side plates and the inner wall of the augmenter provides greater flow area, slowing the velocity of the jet engine exhaust-cool air mixture and reducing the power of noise produced by the jet engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view in perspective of the noise attenuation device for testing jet engines comprising a preferred embodiment of the present invention; 
         FIG. 2  is an end view of the noise attenuation device of  FIG. 1  without a flow deflector; 
         FIG. 3  is an end view of the noise attenuation device of  FIG. 1  with a flow deflector; 
         FIGS. 4A and 4B  depict the gas flow pattern of the jet engine exhaust plume within the noise attenuation device of  FIG. 1 ; and 
         FIG. 5  is a schematic diagram which illustrates the positioning of the noise attenuation device of  FIG. 1  within a jet engine test cell which is used to conduct static testing of a jet engine; 
         FIG. 6  is an enlarged view of the noise attenuation device positioned with augmenter of the jet engine test cell; and 
         FIG. 7  is an illustration of the grill end piece attached to the rear end of the carbon steel pipe to limit jet engine exhaust flow through the rear end of the carbon steel pipe when the noise attenuation device is within the jet engine test cell. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to  FIGS. 1 ,  4 A and  4 B, the full scale cylindrical-shaped noise attenuation device  30 , which is 51 feet in length, has a diameter of 6 to 14 feet, weighs approximately 25 tons, and is fabricated from empty sections of carbon-steel pipe  32 . The carbon-steel tube  32  has an inside diameter of approximately 6 feet and is formed of ½ inch thick carbon-steel plate. 
     Noise attenuation device  30  incorporates fluid dynamic control elements to catch, slow and deflect the jet engine exhaust plume  35  from a jet engine  34 . The carbon-steel tube  32  is aligned axially with the direction of jet engine exhaust flow from the exhaust port of a jet engine  34  under going a static performance test. 
     The slowing of the jet engine exhaust plume  35  from jet engine  34  operates to reduce the intensity of otherwise persistent noise-generating turbulent eddies from the jet, thereby reducing the noise produced by the static testing of the jet engine  34 . 
     As is best seen in  FIG. 1 , the noise attenuation device is mounted on a test support structure  60 . The test support structure  60  has a rectangular shaped base  62  in the forward portion of the structure  60  and another rectangular shaped base  64  in the rear portion of the structure  60 . Supporting two sections of the noise attenuation device  30  joined to each other by a connection flange  79 , base  62  of test support structure  60  includes a pair of vertically orientated support cradles  66  and  68  which are located near the front and rear end of base  62 . Base  64  of test support structure  60  also includes a pair of vertically orientated support cradles  70  and  72  which are located near the front and rear end of base  64 . The two sections of the empty carbon-steel pipe  32  of noise attenuation device  30  rest on the cradles  66 ,  68 ,  70  and  72  of test support structure  60  in the manner illustrated in  FIG. 1 . Angled support members  75 , angled at approximately forty five degrees, are provided for each of the cradles  66 ,  68 ,  70  and  72  to insure stability of the cradles  66 ,  68 ,  70  and  72  during the static testing of a jet engine. Each cradle  66 ,  68 ,  70  and  72  also has a pair of clamping devices  74  which secure carbon-steel pipe  32  to each cradle  66 ,  68 ,  70  and  72  of the test structure  60 . The centerline of the carbon-steel tube  32  is positioned approximately 5′-2″ above the ground when the carbon-steel tube  32  is portioned on the support structure  60  in the manner shown in  FIG. 1 . 
     Referring again to  FIGS. 1 ,  4 A and  4 B,  FIGS. 4A and 4B  shows the gas flow field through the noise attenuation device  30 . The technical objective of the noise attenuation device  30  (as shown in  FIGS. 4A and 4B ) is to ensure a safe and acceptable flow of the jet exhaust plume  35  through the interior of the noise attenuation device  30  and to reach acceptable noise reduction levels. To meet this technical objective, the noise attenuation device  30  is designed to: (1) slow jet exhaust by mixing jet engine exhaust with entrained air (momentum exchange); (2) cool exhaust at 3800° F. with a 3:1 ratio of entrained air (thermal mixing); and (3) convert velocity head to pressure head for changing flow directions within the noise attenuation device  30 , shroud  52  and flow deflector  54 . 
     The exhaust of the jet engine  34  under test is positioned near the front end portion  40  of the carbon-steel pipe  32  of device  30 . As seen in  FIG. 4A , the front end portion  40  of carbon-steel pipe  32  is angled outward to provide for adequate air flow of cold air from the atmosphere. It should be noted the diameter of the angled front end portion  40  of device  30  is 7 feet 10⅞ inches at its maximum width. 
     The cold air  80  entering the carbon-steel pipe  32  (represented by arrows  80 ) is at a temperature of approximately 70 degrees Fahrenheit and has a mass of approximately three times the mass of the hot jet engine exhaust (represented by arrow  82 ). By adding three times the mass of cold air  80  to that of the hot jet exhaust which reaches 3800° F., the hot jet exhaust becomes intimately mixed (as represented by arrows  84 ) with the cold air forming a jet engine exhaust-cool air mixture  84 . 
     The length and diameter of the noise attenuation device  30  are sized to accommodate the gas mixing and cooling process within the carbon-steel pipe  32  (approximately 51 feet in length and a pipe radius of approximately of 3 feet 9 5/16 inches). When the hot jet engine exhaust-cool air mixture  84  reaches the outlet end  41  of pipe  32 , its temperature is reduced to less than 1200° F. which is the design limit set to protect the noise attenuation device  30  structural elements from over-heating during the short time periods of engine operation at after-burner power. 
     To ensure adequate slowing of the exhaust plume  35  from the jet engine  24  being tested, the outlet end  36  of pipe  32  of the noise attenuation device  30  is blocked or partially blocked by a blockage cone  38 . The blockage cone  38  used in the noise attenuation device  30  may be either a full blockage cone or a partial blockage cone with openings. The blockage cone  38  illustrated in  FIG. 4B  includes a plurality of openings  42  which allow a portion of the jet exhaust-cool air mixture to exit the outlet end  36  of the noise attenuation device  30  through partial blockage cone  38  (in the manner illustrated by arrows  44 ). Depending upon the type of engine currently being tested, the blockage cone  38  will either include or exclude the openings  42 . For example, when a Pratt and Whitney J-52 turbojet engine is being tested a full blockage cone is used for the test. Similarly, when a General Electric F-404 engine is being tested a partial blockage cone of the type illustrated in  FIG. 4B  is used for the test. 
     Referring to  FIG. 4B ,  FIG. 4B  shows that the flow of the exhaust plume  35  does not have a clear shot at proceeding through the noise attenuation device  30  without significant slowing of the jet engine exhaust. The cross sectional flow area for the jet engine exhaust is more than doubled as a substantial portion of the flow of the jet engine exhaust-cool air mixture  84  is forced through openings  46  ( FIG. 4B ) within perforated side plates/walls  48  located in the rear portion of the carbon-steel pipe  32 . Arrows  86  indicate the flow pattern of the jet engine exhaust through the openings  46  within side walls  48  into an annular gas flow region  50  surrounding the rear end portion of carbon-steel pipe  32 . 
     As seen in  FIGS. 4A and 4B , the side plates  48  and shroud  52  which surrounds the rear end portion of the carbon-steel pipe  32  forms an annular region through which exhaust flows. Also as seen in  FIGS. 4A and 4B , the gas flow pattern is in both a forward direction and a rearward direction which is indicated by arrows  88  and  90 . A flow deflector  54  attached to the carbon-steel pipe  32  is positioned at the front end of shroud  52 . The flow deflector  54  forces jet engine exhaust exiting the forward end of shroud  52  to reverse direction and flow in the same direction as jet engine exhaust exiting the rear end of shroud  52 . 
     Thus, it can be seen that a portion of the jet engine exhaust is forced to change flow direction twice before exiting noise attenuation device  30  (in the manner indicted by arrows  90 ). Changing flow direction requires pressure differentials and the required pressure head is obtained from that of the high velocity jet engine exhaust. 
     With respect to the noise attenuation device  30 , pressure is recovered by a diffuser effect. The pressure head recovered is used to turn and maintain the flow of engine exhaust through the perforated side walls  48  and turn the engine exhaust back again to an axial direction in the annular region  50  of noise attenuation device  30 . This slows the flow of the exhaust, reducing the power of noise producing flow turbulence and functions as a shroud for the major noise producing regions of the jet engine  34 . 
     In the preferred embodiment, the perforated side walls  48  are located approximately 35 feet from the front end portion  40  of the carbon-steel pipe  32 . 
     Intensity of noise produced by a jet is highly dependent on the velocity of the jet. At the high velocities characteristic of military jet engine exhausts (2000-4500 ft/second), the conversion of kinetic flow energy to acoustic noise dramatically increases with velocity. Noise attenuation device  30  reduces velocity by capturing the exhaust plume  35  in a tunnel formed within the interior of carbon steel pipe  32  and mixing the jet engine exhaust  82  in a confined region (interior of the carbon steel pipe  32 ) with approximately three times the mass of cold air as the quantity of jet engine exhaust  82  flow. Adding the cold air mass (represented by arrow  80 ) to the jet engine exhaust  82  reduces the average velocity of the resulting mixture stream (represented by arrows  84 ) by a factor of about four. By reducing the velocity of the jet engine exhaust flow the intensity of turbulent fluctuations is reduced and the acoustic power emitted by the jet engine  34  is also dramatically reduced. 
     Referring to  FIGS. 2 and 3 , noise attenuation device  30  is depicted in  FIG. 2  with the shroud  52  which surrounds the rear end portion of the carbon-steel pipe  32 . Shroud  52  is attached to the carbon-steel pipe  32  by four equally spaced ½″ thick support plates/fins  56 . The support fins  56  are attached to the outer surface of the carbon steel pipe  32  and the inner surface of the shroud  52  by welds (not shown). Utilizing the support fins  56  in the manner illustrated in  FIG. 2  allows for substantially unrestricted jet engine exhaust flow within the annular gas flow region  50  (as shown in  FIG. 42 ). 
       FIG. 3  depicts the noise attenuation device  30  with the shroud  52  which surrounds the rear end portion of the carbon-steel pipe  32  and the flow deflector  54  which is attached to the carbon-steel pipe  32  and is positioned at the front end of the shroud  52 . Also shown in  FIGS. 2 and 3  in phantom is one of the cradles  72  and its associated clamps  74 . 
     Field testing of the noise attenuation device  30  and the flow deflector  54  with the Lockheed F-404 engine were completed for engine power levels up to and including after-burner, proving the adequacy and operation of the flow deflector  54  for use in noise attenuation during the static testing of a jet engine. Noise measurements were recorded at five engine power levels. These measurements showed increasing noise reductions with increasing engine power in both near field and far field locations reaching total noise reductions greater than 20 dBA at after burner conditions for both locations. 
     Near field measurements were made at approximately ninety feet from the noise attenuation device and the jet engine under test. Far field measurements were taken at approximately two miles from the noise attenuation device and the jet engine under test. The insertion loss for the near field measurements reached a maximum of 20.6 dBA at maximum after-burner. Testing without noise attenuation device  30  resulted in a noise level of 138 dBA while testing with noise attenuation device  30  resulted in a noise level of 117.4 dBA, with the difference being 20.6 dBA at maximum after-burner. The insertion loss for the far field measurements reached a maximum of 34.3 dBA at maximum after-burner. Testing without noise attenuation device  30  resulted in a noise level of 74.1 dBA while testing with noise attenuation device  30  resulted in a noise level of 39.8 dBA, with the difference being 34.3 dBA at maximum after-burner. 
     Noise attenuation device  30  was constructed of carbon steel with an upper service temperature of approximately 1200° F. while noise attenuation device  30  is designed to be subjected to an operating environment for testing the General Electric F-414 jet engine at after burner conditions of 3900° F. Noise attenuation device  30  is not exposed to these extreme temperature conditions for long periods of time, but exposure times of even a few minutes at after burner temperature conditions would cause rapid degradation of the carbon steel pipe  32 . To address this concern, sufficient air (approximately 3:1 mass ratio) s needed to be entrained into the noise attenuation device  30  to provide an jet engine exhaust gas/cool air mixture which, when fully mixed, will have an average temperature that does not exceed 1200° F. 
     Further, the noise attenuation device structure to prevent structural resonance problems caused by the intense vibrational testing of the jet engine. The design of the noise attenuation device structure includes (a) longitudinal segments of the main body of noise attenuation device  30  being broken into uneven lengths to reduce and eliminate full or partial length longitudinal resonant body frequencies, and (b) numerous longitudinal reinforcing bracing bars  76  and circumferential reinforcing bracing bars  78  were added to the structure to reduce and eliminate resonant frequencies that can occur in isolated regions of the structure. 
     As previously discussed, the carbon-steel pipe  32  has a front end section and a back end section which are attached to each other by a front and back joining flange  79  which is shown in  FIG. 1 . 
     Referring to  FIGS. 5 and 6 , another embodiment of the present invention for testing a jet engine  91  in a jet engine test cell  92  is depicted in  FIG. 5 . The jet engine  91  under test is positioned within the test cell in proximity to the front end portion  40  of the carbon steel pipe  32 . The carbon steel pipe  32  is aligned axially with the direction of exhaust flow from the exhaust port  94  of the jet engine  91  being tested. The carbon steel pipe  32  includes flow restrictors downstream (e.g. grill end piece  120 ,  FIG. 7 ) from the entrance of front end  40  of the carbon steel pipe  32 . The function of a flow restrictor, such as grill end piece  120 , within the carbon steel pipe  32  is to cause slowing of the jet engine exhaust as the jet engine exhaust travels through the carbon steel pipe  32 . Slowing the jet engine exhaust flow reduces frictional flow losses and allows recovery of pressure head from the jet engine exhaust flow. Recovering pressure head assist in maintaining high augmentation flow rates through the noise attenuation device and the jet engine test cell  92 . 
     The jet engine  91  under going static testing within the test cell  92  rests on a portable jet engine holding rack  96  that has wheels  97  and a pair of cradles  99 . The cradle  99  is locked into place to prevent forward movement of the jet engine  91  caused by the thrust generated by the jet engine  91 . The jet engine  91  rests upon the cradles  99  during static testing of the jet engine within the jet engine test cell  92 . The jet engine  91  being tested is located above an oil catch  98  which traps oil and other contaminants from the jet engine  91  being tested. 
     As shown in  FIG. 5  the carbon steel pipe  32  of noise attenuation device  30  is positioned within an augmenter  100  which is located in the detuner room  102  of jet engine test cell  92 . The augmenter  100  of test cell  92  has a diameter of approximately 15 feet 6 inches at its front end and an overall length of approximately 82 feet which is sufficiently large to allow positioning of the carbon steel tube  32  of noise attenuation device  30  within the interior of augmenter  100 . 
     Cold air enters the front end  40  of the carbon-steel pipe  32  at a temperature of approximately 70 degrees Fahrenheit and has a mass of approximately three times the mass of the hot jet engine exhaust  104  from the exhaust port  94  of the jet engine  91  under going static testing. By adding three times the mass of cold air to that of the hot jet engine exhaust  104 , the hot jet exhaust  104  becomes intimately mixed with the cold air  103  forming the jet engine exhaust-cool air mixture  110  which flows through the carbon steel pipe to the outlet end  41  of the carbon steel pipe  32 . When the jet engine exhaust-cool air mixture  110  from a jet engine  91  operating at after-burner conditions reaches the outlet end of the carbon steel pipe its temperature is reduced from 3800° F. (jet engine exhaust  104 ) to less than 1200° F. (exhaust-cool air mixture  110 ), the design limit required to protect the noise attenuation device. 
     The effective cross sectional flow area in the rear end portion  41  of the carbon steel pipe  32  increases several times for the jet engine exhaust-cool air mixture  110  ( FIG. 6 ) as flow passes through the rear end portion  41  of the carbon steel pipe  32  and expands into an adjoining augmenter space  112 . A majority of the flow of the jet engine exhaust-cool air mixture  110  passes through openings  46  within perforated side plates/walls  48  located in the rear end portion  41  of the carbon-steel pipe  32 . 
     It should be noted that as shown in  FIG. 6  the reference numeral  106  represents the jet engine hot exhaust from the exhaust port  94  of the jet engine  91  undergoing static testing. The reference numeral  103  identifies the cold air mass entering the front end  40  of the carbon steel pipe  32  and reference numeral  103  also represents cold air entering the annular space  112  enclosed by the carbon steel pipe  32  and augmenter  100  of jet engine test cell  92 . 
     Reference numeral  110  represents the jet engine exhaust-cool air mixture flowing through the carbon steel pipe  32 . The reference numerals  106  and  108  represent the jet engine exhaust temperatures and velocity within the interior of the carbon steel pipe  32  as the cold air mass  103  mixes with the jet engine exhaust  104 . The jet engine exhaust-cool air mixture  110  then exits the carbon steel pipe  32  through openings  46  and the grill end piece  120  located at the outlet end  41  of carbon steel pipe  32 . 
     Adding the cold air mass to the jet engine exhaust  104  reduces the average velocity of the resulting mixture stream  110  by a factor of about four. Reducing the velocity of the jet engine exhaust  104  flow reduces the intensity of turbulent fluctuations and the acoustic power produced by the jet engine exhaust plume. 
     An annular region  112  through which the jet engine exhaust-cool air mixtures flows is formed between the side plates  48  and the inner wall  101  of augmenter  100  located within the jet engine test cell  92 . The gas flow pattern of the exhaust-cool air mixture  110  is maintained in a forward direction within the annular region  112  by air flow inducted into the entrance or front end  40  of the noise attenuation device  30  and the jet engine exhaust-cool air mixture  110  inducted into the augmenter  100  of the jet engine test cell  92 . 
     The annular region  112  formed between the side plates  48  and the inner wall  101  of the augmenter  100  provides greater flow area, further slowing the velocity of the jet engine exhaust-cool air mixture  110  and reducing the power of noise produced by the jet engine. 
     When the carbon-steel pipe  32  of noise attenuation device  30  is positioned within the augmenter  100  of jet engine test cell  92 , neither the shroud  52  or flow deflector  54  are used as components of noise attenuation device  30 . 
     As shown in  FIG. 5 , the augmenter  100  of jet engine test cell  92  has acoustic lined panels  130  positioned around the rear portion of augmenter  100 . The use of acoustic lined panels  103  is optional and depends upon the user&#39;s requirements to further reduce jet engine noise generated by a jet engine  91  under going static testing within the jet engine test cell  92 . 
     The acoustic line panels  130  operate as noise attenuation devices o reduce noise levels from the jet engine  91  under going testing within the jet engine test cell  92 . The jet engine test cell  92  also has a plurality of turning vanes  132  which direct the jet engine exhaust-cool air mixture  110  upward through a jet engine exhaust stack  134  located at the end of the jet engine test cell  92 . The jet engine exhaust stack  134  also has acoustic lining insulation to further reduce noise generated by the jet engine exhaust. 
     Referring to  FIG. 7 , there is shown a grill end piece  120  attached to the rear end  41  of the carbon steel pipe  32 . The grill end piece  120  is designed to limit jet engine exhaust flow through the rear end  41  of the carbon steel pipe  32  when the noise attenuation device  32  is positioned within the jet engine test cell  92 . The grill end piece  120 , for example, may include a plurality of equally spaced apart horizontal grill members  122 , a plurality of equally spaced apart vertical grill members  124  and a ring structure which is attached to the rear end  41  of carbon steel pipe  32 . The ring structure  126  has the horizontal grill members  122  and vertical grill members  124  attached thereto and also provides support for the horizontal grill members  122  and vertical grill members  124 . Further, the horizontal grill members  122  and vertical grill members  124  are interlaced with each other in the manner shown in  FIG. 7 . 
     From the foregoing, it may readily be seen that the present invention comprises a new unique and exceedingly useful noise attenuation device for use in testing a jet engine which constitutes a considerable improvement over the known prior art for outdoor testing of jet engines on test stands and in test cells. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.