1、SAE AIR*02 66 8357340 0009062 7 w - 44 - /o AIR 902 Issued 5-15 66 Revised this is not the actual slant distance when the acoustic test point is to the side of the flight path. e. Occasionally acoustic measurements of regularly scheduled commercial flights are taken. very difficult to coordinate tes
2、t efforts with disinterested commercial operators. It is Any system to be used by the ground observer to determine minimum aircraft distance should satisfy the following requirements: a. Have a Vange of 100 to 2000 ft with an accuracy of at least 12%, which is equivalent to a maximum of 1 dB variati
3、on inmeasured sound pressure levels. b. Be capable of making a determination on a rapidly moving target. Additionally, a desirable system should: c. Provide Ifon the spot“ information, so that necessary corrections can be given to the pilot for subsequent passes. Be operable over the usual range of
4、conditions prevailing during acoustic tests. d, e. Be self-contained, portable, and simple to operate. A photographic technique using a standard camera answers all but one of these requirements, that of providing “on-the-spotft information. When this is necessary, a quick-development camera system m
5、ay be used. 3. PROCEDURE 3.1 Calibration - The photographic technique for measurement of distance requires accurate knowl- edge of only one of the optical properties of the camera, its focal length. This is determined from an on-center picture of some object whose size and distance have been accurat
6、ely measured and whose orientation is such that its measured dimension is perpendicular to the line of sight. (For simplicity the calibration distance should be great enough to require a focus setting of infinity. ) Focal length is then calculated from the simple proportion, COPYRIGHT SAE Internatio
7、nal (Society of Automotive Engineers, Inc)Licensed by Information Handling Services SAE AIR*702 66 8357340 0009066 4 -5 - 3. 3 3. where f = focallength, inches r = range, or distance from camera lens to object eh = length of image on photograph n = length of object normal to line of sight. Technique
8、 - As the airplane flies over the observers position, the cameraman eeps it centered in his viewfinder. At the estimated moment of nearest approach he takes the picture. if the flight path is nearly overhead the cameraman will be required to make a rather awkward maneuver, in which case it is well t
9、hat he start from a semi-reclining position, to avoid ending in one. Limits on how near or far the airplane may be from the observer depend on airplane size and camera type. in general, the airplane should be no closer than twice its body length or wing span, otherwise the image is difficult to keep
10、 framed; and it should not be so far that the image becomes too small for accurate measurement. For an ordinary camera this would limit the range to about 20 body lengths; but if a longer range must be measured, a calibrated telephoto system may be employed. Calculation - Distance to the airplane at
11、 the time of the photograph is determined from: fe n “n r= - where 8 cases the segment will be fuselage length or wing span. is the length of a segment of the airplane normal to the line of sight. in most n As an example, if the fuselage length, eo, is 150 ft; length of its image, en, is 2 in. ; foc
12、al length of the camera is 5 in. ; and if the fuselage was perpendicular to the line of sight at the moment the picture was taken, so that 4 = 8 then O 5x150 2 r =- = 375 ft. 4. ACCURACY - Four possible sources of experimental error related to the camera technique are: 4.1 Measurement of Image Size
13、- Experience has shown that errors made during a test in measuring image size rarely exceed 5%; they are usually the result of haste or an interference of some por- tion of the airplane with the image segment to be measured. Such errors affect calculated distance, but they must exceed 6% to cause as
14、 much as 1/2 dB error in sound pressure level (SPL) normalized by inverse-square law. They are generally eliminated in subsequent laboratory examination of the image, during final processing. COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesI
15、 SAE AIR*702 bb m 8357390 00090b7 b m -6- 4.2 Orientation of Airplane - An error results from the airplane being turned so that the segment to be measured is not perpendicular to the line of sight (Figure i), if the fuselage is being used as the measurement standard, this occurs whenever the picture
16、 is taken too soon or too late, whether during an overhead pass or a pass to one side. Data from the overhead pass may be salvaged by switching to wing-span measurement, since the wing span is perpendicular to the line of sight throughout such a pass, Fuselage data from either an overhead or a side
17、pass can be corrected if a reasonable estimate can be made of the orientation angle e - the angle by which the fuselages actual position differs from normality to the line of sight. The correction formula for range based on fusel- age measurement is feo cose r= “n where r is the slant range at the t
18、ime the photograph is made, Correcting this to minimum slant range (or effective altitude), fe- cose COSC u = r cosq = min Ln r where cp is the angle between.the line of sight and the normal to the flight path. On the presumption that the fuselage axis is oriented along the flight path (a practical
19、necessity), the two angles are equal and the formula becomes 2 f c. cos e O - r “n min It is unlikely that 8 would ever be as large as 20 deg, for which the range correction would be only 12% or about 1 dB in SPL by inverse square law. It is evident, therefore, that only negligible errors in field d
20、ata will result from failure to take small values of 8 into account. There is generally no point in attempting a correction for wing-span nonynormality, since this di- mension is used only for an overhead pass. In the event the photograph is taken before or after nearest approach, it may be desired
21、to correct from actual range to minimum range. The formula is = r cos? = U 4 n r min where, as before, ? is the angle between the line of sight and the normal to the flight path. in this case the tolerance angle for producing an error no larger than 1 dB is 27 deg. In none of the above cases need ac
22、count be taken of the possible inclination of the fuselage to the flight path, since even at low speed, with modern high-lift devices the angle rarely exceeds a few degrees. COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesSAE AIR*702 66 9 83
23、573LiO 0007068 8 - 4.3 Off-Center Image - The camera should be pointed so that its axis coincides with the line of sight. if it is not aimed properly (Figure 2), the image of the airplane will be off-center d inches in the photograph; this image will therefore be slightly too long, due to the increa
24、se in distance from the lens to the film. A range formula embodying a correction for this effect can be derived in the following form (neglecting second order effects) : ed ed where ed is the length of the off-center image, in inches, and d is the distance from film center to center of the image, or
25、 image displacement, also in inches. The size-of this correction is limited by the film length; assuming a 5 in. focal length and a 3 x 4 in. film, the correction at most could be (1 + 3) = 1.16 = 1.077 which would produce an SPL change of only O. 6 dB. Normally the displacement should never exceed
26、1 in. , giving a range correction of 1.02, or less than O. 2 dB. 4.4 Focal-Plane Shutter Distortion - if the camera has a focal-plane shutter, and if the image is moving across the focal plane at the moment the shutter is actuated, distortion may result: the image may be lengthened or shortened from
27、 its-true value, depending on whether the shutter moves with or in opposition to the image motion. However, under even the most extreme conditions the error in length could hardly exceed lo%, with a corresponding SPL error of less than one dB. in practice the error is likely to be zero, since ordina
28、ry photographic technique calls for fftrackingft a moving object so that its image in the camera is stationary. From the size of the errors and their possible sources, it may be concluded that with ordinary care in the taking of the pictures no corrections whatever should be necessary. This has been
29、 confirmed by experience in the field. 5. CONCLUSION - The photogrzphic technique provides a practical means for determining observer- to-aircraft minimum distances; however, consideration should be given to other systems if they are applicable to the problem and meet the specified requirements. PRE
30、PARED BY SAE COMMITTEE A-21, AIRCRAFT EXTERIOR NOISE MEASUREMENT COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling Servicesr io f r - r COB p min ( Illustration based on simple i lens theory) / PHOTOORAPHIC IMAGE n FIGURE 1 CAMERA TEmQUE HIR RANGE DETEIIMINATION e COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling Services- SAE AIR*90E! bb m 8357340 0009070 b m -9- OBJECP k? R m pc O / f COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling Services
