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Classification with Peek-a-boo for Indexing Documents on Aerodynamics: An Experiment in Retrieval

R.C.WRIGHT and C.W.J.WILSON

ABSTRACT. A new classification and peek-a-boo cards were used for the analytical indexing of documents on aerodynamics and 100 test questions were put to the system. Degree of success in retrieval is assessed, input and search times are measured, and causes of failure are examined. The combination appears to offer a promising retrieval system for complex but well-defined subjects.

Towards the end of 1956, it was proposed to prepare and (as far as possible within security limits) to distribute to industry a subject index to the reports and technical notes issued by the Aerodynamics Department of the Royal Aircraft Establishment. This index, which might later be extended to include other series of reports on aerodynamic subjects, would be based on a suitable classification system permitting conjunctive specification, and would use a clerical device of the peek-a-boo type (1). The system would employ a minimum of expensive equipment and, if successful, might be applicable to other fields requiring detailed analytical indexing of a comparatively small collection. First, however, it would be necessary to select a suitable classification of aerodynamics, do some trial indexing, and carry out tests on the system. The purpose of this paper is to describe the selected classification, the posting and searching operations, and the tests carried out.

Purpose of the experiment

Main reasons for undertaking the project were:

  1. A reliable analytical index to the 6000 reports in the series was necessary to the Establishment and would no doubt be welcomed by the aircraft industry.

  2. Mr. J.Seymour, a former Librarian of the Aerodynamics Department, had recently developed a classification for aerodynamics which appeared to

R.C.WRIGHT and C.W.J.WILSON Royal Aircraft Establishment, Farnborough, England.



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--> Classification with Peek-a-boo for Indexing Documents on Aerodynamics: An Experiment in Retrieval R.C.WRIGHT and C.W.J.WILSON ABSTRACT. A new classification and peek-a-boo cards were used for the analytical indexing of documents on aerodynamics and 100 test questions were put to the system. Degree of success in retrieval is assessed, input and search times are measured, and causes of failure are examined. The combination appears to offer a promising retrieval system for complex but well-defined subjects. Towards the end of 1956, it was proposed to prepare and (as far as possible within security limits) to distribute to industry a subject index to the reports and technical notes issued by the Aerodynamics Department of the Royal Aircraft Establishment. This index, which might later be extended to include other series of reports on aerodynamic subjects, would be based on a suitable classification system permitting conjunctive specification, and would use a clerical device of the peek-a-boo type (1). The system would employ a minimum of expensive equipment and, if successful, might be applicable to other fields requiring detailed analytical indexing of a comparatively small collection. First, however, it would be necessary to select a suitable classification of aerodynamics, do some trial indexing, and carry out tests on the system. The purpose of this paper is to describe the selected classification, the posting and searching operations, and the tests carried out. Purpose of the experiment Main reasons for undertaking the project were: A reliable analytical index to the 6000 reports in the series was necessary to the Establishment and would no doubt be welcomed by the aircraft industry. Mr. J.Seymour, a former Librarian of the Aerodynamics Department, had recently developed a classification for aerodynamics which appeared to R.C.WRIGHT and C.W.J.WILSON Royal Aircraft Establishment, Farnborough, England.

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--> have many advantages for use in a retrieval system. Experience in its practical application was desired. Tests had already been carried out with a Uniterm system for cataloguing aircraft structural data (2), and on the N.L.L. card catalogue of aerodynamic measurements (3, 4). Similar evaluation of a peek-a-boo device based on a suitable subject classification was considered desirable. It was hoped to ascertain whether an analytical indexing and retrieval project in the complex subject of aerodynamics could be designed and operated by professional librarians without specialist subject knowledge, to the satisfaction of the aerodynamicists requiring the information. PHYSICAL FORM OF THE INDEX In its final form, the index would be made up of (1) a register, i.e., a list of all documents indexed by the system, in register serial number order; (2) a manual, consisting of (a) an introduction to the system, (b) a list of subject headings in classified order, (c) an alphabetical index to the subject headings; (3) index sheets—one for each subject heading and for each designated aircraft and aerofoil; register serial numbers of documents to be indicated by the positions of punched holes. For testing purposes, however, it was decided to use only the classification schedule itself and standard 80-column Hollerith cards. Special cards or plates having a higher capacity might later be desirable, but their design, and that of suitable punching and viewing devices, could be left until the retrieval capabilities of the system had been investigated. Choice of classification After consideration of the known enumerative classifications of aerodynamics, including U.D.C., N.A.C.A., and the N.L.L. (all of which are used to some extent in the Establishment), it was decided to adopt the classification system devised by Mr. Seymour with this project in mind. After comments had been sought from subject experts, the classification was adopted with slight modifications; it appears as Table 2, together with the number of times each code was used in indexing the first 700 documents. It was expected that further modifications would follow an initial trial period of indexing. Indexing THE INDEXERS The six indexers were professional librarians employed in the Main and Departmental libraries of the Establishment. One of them (A) was librarian

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--> of the Aerodynamics Department: the remainder (B-F) had experience in aeronautical libraries but no special knowledge of aerodynamics. About half of the indexing and searching was to be done by A, so that the effect of familiarity with the subject and with the schedules might be assessed. Before starting to index, the indexers met to discuss the schedules and to resolve as far as possible differences of opinion as to the meanings or the use of particular headings. Each person then indexed a number of test pieces, and variations were discussed. It was intended to introduce further test pieces at later stages in order to see whether improved correlation came with experience of the system. PROCEDURE For test indexing the selection of documents was not limited to R.A.E. documents, some Institute of Aeronautical Sciences preprints and National Advisory Committee for Aeronautics Research Memoranda being included in order to achieve wider subject coverage, particularly of recent material. The intention was to code all those features of a document which might conceivably be used later as part of an information request. A detailed study of each document was therefore necessary, particular attention being given to the summary, conclusions and illustrations. Postings were made on a 5 by 3-inch card ruled in ten columns as follows: Thus, to indicate that a document dealt with a fighter aircraft, the code 157 taken from the schedules would be entered as 15 in column 7. All relevant codes were entered, so that the issue date 1953 would be coded as 103, 104, 105, 106, and Mach number 1.3 would be 121, 122, 123, 124. Whenever a sub-heading was used, the main heading above it was also coded. For peek-a-

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--> boo use, these codes were subsequently transferred to lists in code number order and punched on Hollerith cards. The only materials and equipment used were common stationery items and a Hollerith hand punch. The processes involved were: Examination of the document and noting appropriate codes on the 5″×3″ card illustrated above. Listing the serial numbers appropriate to each code number. Hand punching the Hollerith cards, which already have typed on them the code numbers, by which they are arranged. Times taken for these operations are discussed later. The most time consuming, and that needing the highest-paid staff, was (a). Most documents were assigned about 30 codes. At first, indexers were taking as long as 30 minutes for each document: practice, familiarity with the schedules and increasing knowledge of the subject soon reduced this to 15–20 minutes per document. It is doubtful whether an average of about 15 minutes can be much reduced, in view of the careful study of each document which is necessary to any system using highly analytical indexing. The average salary of the indexers employed worked out at 10/– ($1.40) an hour. Description of tests The test programme is in two parts: (a) When about 750 documents have been indexed. To ascertain the effectiveness of the classification, to make any necessary modifications to the schedule, and to remove causes of discrepancy between indexers. Also to assess the cost of indexing and posting and the speed of searching operations. (b) When about 3000 documents have been indexed. To ascertain the number and relevance of documents retrieved in response to a given question. Also, on the basis of search times, to ascertain the scope and form of the final index (a) for internal use and (b) for possible issue to other aeronautical libraries. Only part (a) of the programme is discussed in this paper. When 700 reports had been indexed (500 R.A.E. documents and 200 of U.S.A. origin), ten R.A.E. aerodynamicists were asked to collaborate by selecting 10 documents each from the collection and framing one question on each document. The questioners had not seen the classification schedules, and were asked to frame in their own words the sort of question that might occur in their daily work. The questions appear in Table 1. Searching was divided about equally between indexers A and B. Success was defined as the retrieval of the document on which the question had been based. Obviously this had

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--> to be limited in some way, since a search which retrieved the subject document together with about 50% of the collection could hardly be regarded as an unqualified success. Such a case could be due to faults in retrieval; it could also be due to the question having been so general that it was bound to produce a large number of answers. After discussion, an arbitrary limit of 5% of the collection (35 documents) was set as the maximum for a search to be regarded as successful. Searches producing more than this number have been classed as “partially successful” provided that the subject document is included. When a search was unsuccessful, a second search was usually made, either by the same searcher or by the other. Sometimes this consisted simply of widening the original search by discarding one or more of the factors forming the specification. In other cases completely new headings were selected to form a new search plan. Usually not more than two searches were made, but there were a few cases in which a third search was considered advisable. Analysis and discussion of results PERCENTAGE SUCCESS Of the 100 questions put, 54 were answered at the first search, a further 20 by a second search, and a further 4 by a third. In 7 cases the search was “partly” successful (subject document retrieved, but with more than 5% of the collection), and in 15 cases the search failed completely. An overall success percentage of 78 (plus a further 7% of cases in which the field was appreciably narrowed) is not unsatisfactory having regard to the circumstances in which the test was made. Many of the questions would, in normal information work, have been referred back for amplification. CAUSES OF FAILURE (a) Causes of complete failure   Question numbers   1 7 12 32 34 36 39 40 51 53 57 66 81 88 93 Misleading question         X       X   Question misinterpreted X X   Indexing omission X   X   X X X X   X X X X X X Schedule inadequacy X     X   X   (b) Causes of partial success   Question numbers   17 23 24 26 28 61 70 Question insufficiently specific     X   X     Indexing omission X     X X X X Schedule inadequacy   X  

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--> (c) Causes of earlier failure when third search successful   Question numbers   37 41 48 67 Question misinterpreted       X Indexing omission X X X   (d) Causes of earlier failure when second search successful   Question numbers   4 5 11 22 25 31 33 35 49 55 Misleading question   X   Indexing omission X   X     X   X X X Faulty search     X X   Search too specific   X   X   Schedule inadequacy   X   X     Question numbers   56 60 65 72 76 84 86 87 96 97 Indexing omission X X X   X X X X   X Search too specific X   X   Clerical error in posting   X   (e) Summary. A total of all the above single or contributory causes of complete or partial failure may be helpful in pinpointing the chief weaknesses of the system. Semantics Misleading question 3   Question insufficiently specific 2   Question misinterpreted 3 8 Input stage Indexing omission 34   Schedule inadequacy 6   Clerical error 1 41 Output stage Faulty search 2   Search too specific 4 6 Total   55 It will be noted that the largest single factor was indexing omission, and this in turn is believed to be attributable mainly to lack of subject knowledge. Next largest is the “semantics” group, failure in communication between the questioner and the searcher. The six cases of schedule inadequacy, together with other omissions noted in indexing the first 700 documents, will lead to minor revision of the classification. At this time, reconsideration will be given to the necessity for retaining some headings which have hardly been used at all, and others which have

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--> been used so often as to render them almost useless for retrieval purposes. The six cases of failure at the output stage are not considered significant: none persisted beyond a first search, and with peek-a-boo it is possible to make several searches in a few minutes. NUMBER OF CODES USED IN INDEXING Average number of codes used in indexing 700 documents 28.96 Average number of codes used in indexing the 54 documents successfully retrieved at the first search 30.59 Average number of codes used in indexing the 15 documents which could not be retrieved 30.07   Indexer A used an average of 30.50 codes for each report. Indexer B used an average of 32.94 codes for each report. Indexers C−F used an average of 29.12 codes for each report. All these averages, except the first figure of 28.96, are based only on the 100 documents on which questions were asked. ANALYSIS BY INDEXER

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--> All indexers used about the same average number of codes per document, but the success achieved varied with the indexer’s subject knowledge. Indexer A was in charge of a specialised library for aerodynamicists; B, in charge of a more general aeronautical library, had considerable experience in aviation and in library services; C to F had served in aeronautical libraries for periods varying from 5 years to only a few months. RELEVANCE OF DOCUMENTS RETRIEVED In the N.L.L. tests (3, 4) searchers were able to make a rough assessment of the relevance of each document retrieved by an examination of the detailed information printed on the N.L.L. punched cards. In the present case it would have been necessary to refer each document, or an abstract of it, to the questioner. This being a time-consuming operation, it was decided to do so only on a sample of 20 successfully answered questions. These were further restricted to cases in which 3 to 12 documents had been thrown up in the retrieval process. In 20 cases, therefore, questioners were presented with abstract cards or documents and they decided whether the documents retrieved, in addition to the ones on which their questions were based, were truly relevant in terms of their questions as set. Total documents retrieved for the 20 questions came to 146 (the number of aspect cards correlated varied from 1 to 5, but was usually between 2 and 4). It was found that 70 documents were relevant, 18 were of marginal relevance, and 58 were irrelevant. This makes a broad ratio of 3.5:1:3, which appears to be satisfactory, although it is clearly desirable to reduce the marginally relevant figure. This can probably be achieved by improved indexing. Time studies INPUT Accurate time studies were made of three main aspects of the input stage of this project. These were: times taken to index each document, times taken to post code numbers to a ledger, and times taken to transfer this information to punched Hollerith cards. Time to index documents. Indexer A, who was responsible for half of all indexing, timed himself on 17 occasions. This was done only after considerable experience of indexing difficulties and detailed knowledge of the schedules had been gained. 153 documents were indexed in 31 hours 5 minutes, which gives 13.5 minutes per document. Time to post index numbers. All papers indexed had their codes recorded on the card described in “Procedure” under “Indexing.” These codes were

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--> then transferred to a plain ledger to facilitate eventual punching. In 18 operations, codes for 474 documents were posted by 2 persons in 19 hours 16 minutes, which gives 2.4 minutes per document. Time to punch Hollerith cards. Codes for 500 documents were transferred from the ledger and punched on Hollerith cards in 19 hours 30 minutes. This was done in 12 operations by 2 persons, at an average of 2.34 minutes per document. Total times for each document subject-indexed by the system described therefore consist of 13.5 minutes for indexing, 2.4×2 minutes for posting, and 2.34×2 minutes for punching, or 22.98 minutes. These are the three time-expensive parts of the programme, but, if an allowance of 5 minutes for extracting or filing papers or associated clerical processes is made, it is reasonable to assess inclusive time for subject indexing and all allied clerical effort as an average of 28 minutes per document. Half of this is relatively expensive indexing. No great reduction in any of these times can be expected. OUTPUT The output stage of this system consists firstly in relating a question to the classification schedules, in carefully assessing the best aspect cards to be used,1 in correlating these cards and reading the holes punched all through. With practice, these were found to be rapid processes which, depending on the complexity of the question, could be performed within 2–10 minutes. The second phase consists of removing the relevant serially numbered abstract cards from the file, and offering them to the questioner, who may then wish to see some of the actual documents represented by the cards. The total output stage should not normally exceed 15–20 minutes. Conclusions The combination of subject classification and peek-a-boo appears to have many advantages as a means for the analytical indexing of documents in a well-defined subject such as aerodynamics. There would appear to be a market for the commercial development of “miniaturised” cards or plates having a capacity of 20,000 or more positions and of appropriate punching and viewing equipment. 1   Bernier (5) has found, theoretically, that “discovery of unexpected documents by use of a manipulative, correlative index is usually highly improbable if the number of terms taken at a time in searching is four or more….” It was found, as described in the paragraph on relevance of documents retrieved, that the number of peek-a-boo cards correlated usually fell within the range 2–5.

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--> Best results have been obtained by indexers with subject knowledge, and “production” indexing should be undertaken only by such indexers. The classification used was basically sound, but requires expansion in some areas and contraction in rather more, so as to achieve less variation in the number of times code-numbers are used. In many cases scope notes are required. A survey (6) of the report literature actually in use by a typical group of aerodynamicists shows that 98% of the reports were less than 10 years old and 83% less than 5 years old. This finding is likely to influence schedule revision and may necessitate re-appraisal of the scope of the present project. It also tends to minimise one criticism of peek-a-boo, i.e., that it is necessary at intervals, depending on card capacity and accession rate, to start a completely new index. REFERENCES 1. WILDHACK, W.A., STERN, J., SMITH, J., Documentation in instrumentation. American Documentation 5, 223–237 (1954). 2. CLEVERDON, C.W., THORNE, R.G., A brief experiment with the Uniterm system of co-ordinate indexing for the cataloguing of structural data (unpublished). 3. VESSEY, H.F., Test of N.L.L. card catalogue of aerodynamic measurements (unpublished). 4. VESSEY, H.F., SEYMOUR, J.R., Test of N.L.L. card catalogue of aerodynamic measurements. II (unpublished). 5. BERNIER, C.L., Correlative indexes and the blank sort. American Documentation 9, 32–41 (1958). 6. WILSON, C.W.J., Report literature used by aerodynamicists (unpublished).

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--> TABLE 1 The 100 questions and the searches based on them Question No. Question Search plan Documents retrieved Indexer Codes used Searcher Success or failure Remarks 1 Effect of incidence on pressures recorded on a Hunter aircraft. (a) 365 362 (Hunter) (a) 14 B 21 B F Peek-a-boo cards for aircraft names are not yet available, but would normally be used. In this case neither “Hunter” nor “365-Incidence” was indexed.     (b) 365 417 362 (b) 1     A F Searcher B misunderstood the question as relating to aerodynamic pressures: document related to pitot-static tube pressures. “Pitot static tubes” had been noted by indexer as a desirable new heading. 2 Aerodynamics of struts 349 264 10 C 36 A S   3 Spinning tunnel instrumentation 397 417 1 B 22 A S   4 Early methods of measuring rates of climb (relative merits of barograph and cinematograph) (a) 417 369 378 (a) 0 A 26 B F “417-Instruments” not indexed.     (b) 369 378 (b) 14     B S   5 Applications of the Ludwig-Tillman skin friction formula (a) 128,137 136,154 349,352 (a) 4 A 28 A F Indexers had noted skin friction as a necessary new indexing term.     (b) 113,128 137,352 (b) 12     A S   6 Effect of rate of approach to a stall on the stall CL 365 350 26 B 47 B S 378, if used, would reduce the number of documents to 3.

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--> Question No. Question Search plan Documents retrieved Indexer Codes used Searcher Success or failure Remarks 70 The ram efficiency of air intakes let into the sides of a wing or body. (a) 281 282, 285 13 B 42 A F “282-Located on aerofoils” not indexed.   (b) 281, 285 40     A P   71 Is the temperature recovery factor of a laminar boundary layer altered by separation of the boundary layer? 137 139 142 10 A 27 A S   72 Data required from tests employing the NACA Technique for obtaining free-flight stability data from models fitted with an all-moving tailplane which moves automatically between stops under the influence of the aerodynamic forces on it. (a) 112, 115 117, 338 406 0 A 46 A F     (b) 115, 117 406 8     A S   73 For assisted take-off, what are the advantages of a liquid fuel rocket over a solid fuel rocket? 171 271 377 3 B 32 B S   74 What are the handling qualities of the Javelin aircraft at high lift coefficients? 350 378 17 B 38 B S The 17 documents will be reduced substantially when the name Javelin (used in indexing) has been represented by a peek-a-boo card. 75 What reports from the Royal Aircraft Factory appeared during the 1914–18 war on aircraft stability theory. 100, 113 114, 117 349, 355 1 A 15 A S   76 To what extent may disturbances present in a wind tunnel invali (a) 125, 137 141, 195 12 B 30 A F  

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-->   date tests made in the tunnel to locate the position of boundary layer transition on a wing at subsonic speeds? (b) 112, 137 141, 195 392 11     A S Indexer omitted to index Mach numbers. 77 What changes in piloting technique are necessary for the safe landing of an aircraft without an undercarriage on an aircraft carrier adapted to receive it? 331 367 4 F 21 A S   78 Can Falkner’s 9-point method (for calculating the aerodynamic loading on a wing) be simplified for application to a swept wing of low aspect ratio? 113 368 195 206 212 5 E 33 B S   79 A method is required for calculating the spanwise distribution of lift on a wing of low aspect ratio at high subsonic speeds, including allowance for the non-linear variation of lift with incidence. 350 362 212 125 123 113 2 A 35 B S   80 Wanted: a good theoretical treatment of the downwash field behind a low aspect ratio wing including the behaviour of the tip vortices at supersonic speeds. 113 144 122 143 6 A 24 B S   81 Information is required on numerical methods employed in the theoretical determination of the drag of a slender body of arbitrary cross-section. 113 240 352 14 A 16 A F “240-body” not indexed. Paper was on area rule, which was indexed.

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--> Question No. Question Search plan Documents retrieved Indexer Codes used Searcher Success or failure Remarks 82 What methods have been tried in flight to improve the damping of lateral oscillation of high-speed aircraft? 112, 193 339, 355 357, 373 405 2 A 33 A S   83 Have we any experimental measurements of the influence of aero-elastic distortion on the effectiveness of trailing edge flaps on rectangular wings? 112 374 315 308 3 A 38 B S   84 What reports give design charts for the determination of the downwash angle at the tailplane for tapered wing planforms with plain flaps at low speeds. (a) 144, 126 315, 202 2 A 50 B F “202-tapered” not indexed   (b) 144, 126 315 11     B S   85 Approximate methods to suggest the requirements of an auto-control system for the control of the “long period” or “phugoid” motion of an aircraft. 278 373 9 A 16 A S   86 Experimental measurements at supersonic speeds of the flow field in the vicinity of a body of revolution at high angle of incidence. (a) 112, 122 253, 365 0 A 39 B F “365-high incidence” not indexed.   (b) 112, 122 253, 128 34     B S   87 Calculations to indicate the effect of introducing inertia (a) 109, 302 356, 358 4 C 25 A F  

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-->   weights and springs into a power-operated control system for the longitudinal control of an aircraft. (b) 278, 302 355, 358 7     A S   88 Experimental data on the effects of slipstream on tailplane effectiveness at high subsonic (a) 112, 144 236, 358 125, 123 5 A 43 B F The abstract card, issued with one of the 5 documents, refers to the subject document. Mach numbers.   (b) 112, 123 144, 236 10     A F “144-slipstreams” and “236-tail-plane” not indexed. 89 Experimental determination of the relationships between turbulent boundary layers on flat plates and cones at zero heat transfer. 112 140 137 333 11 A 35 B S   90 Experimental measurements at high subsonic speeds of the pressure distribution over the leading edge of a two-dimensional flat plate with a rounded leading edge. 112 125 333 362 4 A 31 A S   91 Wind tunnel tests at supersonic speeds on a canard configuration with ramjets. 112, 122 172, 192 392 1 A 54 A S   92 Manometers for automatic measurement of pressures in supersonic wind tunnels. 122, 362 391, 392 417 4 B 23 A S   93 The effect of small asymmetries at the nose of a cylindrical body at supersonic speeds. (a) 240, 249. 122, 255 3 C 37 B F “249-straight parallel sides,” and “255-interest at nose” not indexed.   (b) 240 249, 122 43     B F     (c) 122 240, 255 10     A F  

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--> Question No. Question Search plan Documents retrieved Indexer Codes used Searcher Success or failure Remarks 94 Theoretical methods for determining the performance of wing-tip controls. 113, 302 307, 322 3 B 35 A S   95 Any information on moving-wing guided missiles. 164, 322 195 5 A 49 B S   96 Wind tunnel tests at supersonic speeds on two-dimensional aerofoils. (a) 112 122, 195 338, 392 7 B 34 A F “392-wind tunnels” was indexed but not punched.   (b) 112, 122 195, 338 10     A S 97 Rapid recording methods for measuring signals from strain gauge wind tunnel balances. (a) 417, 413 391, 416 0 A 18 B F “413-balances” not indexed.   (b) 391 416, 417 2     B S   98 Design of a six-component strain gauge balance for use in supersonic wind tunnels. 392, 391 413, 122 5 A 23 B S   99 Flow at supersonic speeds over bodies of non-circular cross-section. 240, 254 122, 128 11 A 27 B S   100 Experimental information about the lift on wings at hypersonic speeds. 112, 120 195, 350 10 A 40 A S  

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--> TABLE 2 Classification schedules   Code No. Timesa used Bibliographic characteristics Pre 1935 100 77 “ 1945 101 149 “ 1950 102 194 “ 1955 103 500 Post 1934 104 633 “ 1944 105 555 “ 1949 106 505 “ 1954 107 192 Data/Tables/Methods; for Calculation/Reference 109 58 Bibliography/Survey 110 18 Dictionary/Directory/ Nomenclature 111 0 “Experiment”/Reports observation 112 524 “Theory”/Other 113 230 G.B. 114 493 U.S.A. 115 203 Other foreign/ International 116 3 RAE/NACA/AGARD 117 621 Firms 118 3 Other 119 77 Mach number/velocity ≥2.0 120 107 ≥1.2 121 176 ≥0.9 122 253 ≥0.6 123 280 <2.0 124 393 <1.0 125 324 <0.6 126 236 Stationary 127 12 Flow/Fluids studied 128 359 Ideal fluid 129 20 Gas 130 328 Liquid 131 18 Real fluids other than Air/Water (including Rare air/Slip flow, etc.) 132 8 Homogeneous fluid/Total immersion 133 8 Other/Free surface/ Mixed flow 134 38 In own right 135 26 In relation to solids 136 279 Boundary Layer 137 94 Flow/Fluids studied Other 138 27 Laminar/Streamline flow 139 51 Turbulent flow 140 59 Transition 141 42 Separation 142 36 Vortices 143 45 Slipstreams/Wakes/ Downwash 144 89 Jets 145 27 Blowing 146 16 Suction 147 28 Shock waves/Mach lines/ Compressibility 148 76 Circulation 149 4 Condensation/Evaporation 150 4 Convection 151 5 Dissociation 152 1 Gusts 153 16 Heating 154 41 Cooling 155 28 Acoustics/Noise/Sonic bangs 156 6 Aircraft/Aircraft components Aircraft “Type” Fighter 157 117 Bomber 158 38 Transport 159 22 Personal 160 8 Other 390 38 Shell 161 1 Bomb 162 12 Target 163 3 “Missile” 164 47 “Single” engined 167 90 Multiple engined 168 67 Jet propelled 169 146 Turbojet 170 90 Rocket 171 33 Other/Ram, pulse 172 18 Propeller driven 173 85 Turbo prop 174 21 Other 175 24 Rotary wing aircraft 176 23 Towed 177 7 Sailplane 178 7 Lighter than air 179 3 Kite 180 3 a Times used in the indexing of 700 documents.

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-->   Code No. Timesa used Aircraft/Aircraft components Aircraft “type” Parachute 181 6 Other method of propulsion 182 0 Seaplane 183 16 Amphibian 184 1 V.T.O. Type/Flying platform, etc. (not helicopters) 185 4 Composite aircraft 186 0 No wing 187 14 No body 188 5 No tailplane/No foreplane 189 50 No fin 108 20 With multiple fuselage 190 8 With fins other than single tail fin 191 15 With foreplane/Canard 192 6 Components present and/or studied Complete aircraft 193 286 Other 194 324 Wing 195 361 Monoplane 196 229 Biplane 197 16 Cruciform, Y, etc. 198 26 Other 199 1 Untapered 201 57 Tapered excluding delta, arrow 202 67 Triangular/Delta/Arrow 203 104 Other (circular, “W,” etc.) 204 6 Unswept (inc. 10° sweepback) 205 102 Swept 206 177 Forward 207 2 <40° 208 45 ≥40° 209 108 <50° 210 87 ≥50° 211 43 Aspect ratio <4 212 89 Aspect ratio ≥3.5 213 67 Aspect ratio ≥6 214 18 Leading edge sharp 215 15 Wing Leading edge ducted 216 5 Leading edge drooped 217 3 Cambered 218 25 Uncambered 219 11 ≥8% thick 220 56 <8% thick 221 42 Interest in/at/near—wings Planform 222 89 Aspect ratio 223 28 Sweep 224 37 Taper 225 19 Other/span 226 25 Section 227 63 Camber 228 25 Thickness 229 34 Other/profile 230 35 Twist 231 22 Dihedral/anhedral 232 4 Leading edge 233 42 Trailing edge 234 61 Tip 235 42 Tailplane/Foreplane 236 79 Fin 237 35 Propeller 238 32 Rotor 239 22 Body 240 139 Pointed nose 241 74 Other nose 243 24 Pointed tail 244 18 Other tail 246 54 Fineness Ratio ≥6 247 43 Fineness Ratio <6 248 15 Straight parallel sides 249 58 Other 250 42 Rotationally symmetric/ Body of revolution 253 80 Other 254 23 Interest in/at/near—Bodies Nose 255 17 Tail 256 29 Section 257 10 Sides 258 18 a Times used in the indexing of 700 documents.

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-->   Code No. Timesa used External store 259 38 Engine nacelle 260 32 Other 261 7 Wing mounted 262 26 Other 263 7 Strut mounted 264 11 Other 265 6 Canopy/Radome/Turret/ “Blister” 266 8 Bomb bay/Release gear 267 6 Guns/Armament 268 19 Escape device 269 2 Aerial/Antennas 270 2 Assisted take-off device 271 7 De-icing gear/Icing 272 4 Fuel/Fuel system 273 3 Fuel tank 274 12 Crew accommodation 275 2 Load/Passenger accommodation 276 3 Dirt excluders/Ventilation 277 5 Automatic control/ Servomechanisms/ Stabilization 278 31 Float/Planing surface 279 12 Undercarriage—Land 280 9 Internal flow 281 96 Located on aerofoils 282 26 Located on bodies, nacelles 283 34 Other/pipes 284 30 Intakes (divide as exits) 285 51 Exits 286 20 Nose/Tail 287 23 Side 288 11 With centre-body 291 21 No centre-body 292 9 Ducts 293 47 Straight sections 294 3 Bends 295 2 Contractions/ Expansions 296 20 Nozzles 297 21 Diffusers 298 18 Other 299 1 Convergence/ Divergences 300 4 Total enclosures 301 9 “Control” device 302 179 Attitude 303 71 “Control” device Drag increase 304 34 Other purpose 305 42 Boundary layer control 306 17 Wing-located 307 104 T.E. region 308 76 L.E. region 309 21 Other 310 11 Fin located 311 26 Foreplane/Tailplane located 312 34 Body located 313 13 Other location 314 5 Flap type 315 93 Plain 316 23 Split 317 18 Other 318 16 Spoilers/Vortex generators 319 15 Fences/End plates/Flow guides 320 17 Auxiliary aerofoils/Slats 321 12 All moving component/ Tip 322 16 Tabs/Balancing devices 323 20 Parachute 324 10 Thrust reversal/Jet deflection 325 10 Other objects Engine 326 74 Compressors/Turbines/ Windmills/Fans/Pumps 327 42 Cascades/Stators 328 5 Ground/Runways/Ground Facilities 329 11 Catapults/Launching devices 330 11 Ships/Watercraft 331 6 Man/Medicine 332 6 Flat Plate 333 28 Cylinder—perpendicular to flow 334 3 Particles/Droplets 335 5 Other solid 336 6 Geometry 3-Dimensional 337 274 Other 338 73 Full scale 339 173 a Times used in the indexing of 700 documents.

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-->   Code No. Timesa used Other 340 261 Rough surface/(Surface condition) 341 17 Steps 342 11 Projections 343 21 Fairings/Junctions 344 16 Distortions/Bumps/ Indentations 345 13 Recesses/Gaps 346 9 Perforated/Porous/ Slotted surface 347 35 Variable shape 348 12 Aerodynamic interest 349 569 Lift 350 193 Drag 352 232 Thrust 353 43 Moments/Stability/Control Moments 354 184 Stability 355 176 Control 356 102 Lateral 357 108 Longitudinal 358 209 Directional 359 88 Hinge 360 30 Damping 361 44 Pressure/Velocity distribution 362 192 Loads/Loading 363 83 Interference/Relative position 364 89 Stalling/High incidence 365 53 Sideforce/Sideslipping 366 34 Spinning 251 27 Landing/Take-off 367 56 Level flight 407 28 Diving/Descent/Gliding 368 34 Climbing 369 24 Turning 370 21 Hovering/Flapping 242 14 Separating/Jettisoning 371 13 Buffeting 372 15 Flutter/Oscillation 373 48 Aeroelasticity 374 23 Magnus effect 375 0 “Area rule” 376 10 Operation/Design 377 92 Load/Capacity/Performance/Range/Speed 378 79 Operation/Design Economics 381 16 Reliability/Maintenance/ Safety/Accidents 382 25 Civil operation 383 17 Military operation/ Strategy/Tactics 384 45 Ballistics interest 385 20 Structures/Materials interest 386 47 Mechanical interest 387 35 Electronics/Radar/Radio 388 24 Mathematics 389 51 Principal test equipment/ Technique Described 391 133 Wind tunnel 392 261 Continuous/Fan drive 393 45 Intermittent 394 8 Shock tube 395 4 Special purpose tunnel/Test rigs 396 23 Spinning tunnel 397 10 Water tunnel 398 4 Ballistic range 399 4 Analogy/Simulator 400 4 Ditching tank 401 2 Towing tank/Moving channel 402 12 Models mounted on air-craft 403 1 Tracks/Sleds 404 1 Free flight—Piloted aircraft 405 92 Free flight—Models/unpiloted 406 41 Components 408 24 Power/Flow production 409 15 Other 410 13 Auxiliary Apparatus/Technique 411 58 Model Design/Construction 412 49 Balances 413 18 a Times used in the indexing of 700 documents.

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-->   Code No. Timesa used Principal test Equipment/ Technique Scale Effect/ 414 31 Corrections/ Calibration Telemetry 415 8 Data Handling/ Methods/Computers 416 10 Flow visualization 252 51 Instruments 417 59 Combustion 418 17 Meteorology 419 8 Optics 290 4 Photography 165 11 Total postings in indexing 700 documents   20,270 a Times used in the indexing of 700 documents.

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