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Aviation History
1923
1923 - 0348.PDF
thisraffect the results ? The effect will certainly be different . in the different forms of channel, and intercomparisons are required for their interpretation. Many such questions may be asked. The various forms of tunnel have been described in numerous publications ; it will suffice to mention them here :— (1) The N.P.L. type. A continuous rectangular tube with devices for securing a uniform flow and diffusing the air steadily into the room. (2) The Eiffel type. This has a long diverging cone at the exit of the working part of the tunnel, in which the velocity of the air falls gradually as the fan or blower is approached and the air is discharged into the room at a lower speed than in the N.P.L. type. Thus the power required to produce a given speed in the working section is much less. The inlet and outlet cones are attached to a central chamber of much greater section than the tunnel itself. In this chamber, which can be made airtight, the measurements are effected. (3) The R.A.E. type. The working section resembles that of the N.P.L., but the short expansion cone is replaced by a long diverging cone similar to that of the Eiffel type. The room in which the channel is placed is separated into two sections by a honeycomb wall which acts as a diffuser. Experiments at the N.P.L. on a model of a large channel had shown this to be practicable. (4) The Gottingen type. This is a so-called " racecourse " tunnel. The air is never discharged into the room, but circulates round, making four right-angle bends in each circuit. By the use of curved blades at the corners, a dis- tribution of flow has been secured which is practically uniform over the working section. (5) To these must now be added the compressed air channel at Langley Field described by Prof. Ames. Thus, some three years since, Air-Commodore Brooke Popham, then Director of Research, wrote to the Advisory Committee of Aeronautics :— " It has been suggested that it would be desirable for com- parative trials of exactly similar models to be carried out in representative wind channels in England, France, and America, the object being to ascertain from the comparison of the results whether the method of carrying out aerodynamic research in these countries are identical, and whether any improvements in method could be made. I shall be glad of the opinion of the Advisory Committee on this matter, and if they agree that it is desirable, for suggestions as to the nature of the model and of the exact tests which should be carried out." At the time the Advisory Committee for Aeronautics was in process of becoming the Aeronautical Research Committee, and the new body took the matter into consideration at an early meeting. The suggestion was approved, and the following proposals for tests were made :— (1) Determination of lift, drag, and centre of pressure of a standard aerofoil at various angles of incidence. (2) Tests of a complete model aeroplane, including the complete determination of forces and moments, and of the more important stability derivatives. (3) Resistance measurement at zero angle of yaw of a very good streamline airship model. Letters were written, asking for an expression of opinion on the question, to :— Colonel Crocco, Institute Sperimentale Aeronautico, Rome ; M. G. Eiffel, Laboratoire Aerodynamique, Paris ; M. le Directeur, Institute Aerotechnique de l'Universite de Paris, St. Cyr ; Mr. William Knight, Paris, at that time representing in Europe the American National Advisory Committee for . - Aeronautics. The French and Italian authorities replied, approving tile proposed plan and making certain valuable suggestions as to the form of the models, the material to be employed in their construction, etc. M. Toussaint (St. Cyr) added a note suggesting a definition of the conditions of turbulence during the test. Mr. Knight acknowledged the letter, and stated that it was being transmitted to the U.S. Advisory Committee at Wash- ington for their consideration, and in due course a reply was received promising the co-operation of the Committee. The replies were discussed by the Aerodynamics Sub- Committee, and as the result it was agreed to construct two airship models differing only as to the length of the parallel portion of the body, and to postpone the construction of the complete model ; this item has since been allowed to lapse, partly because of the difficulty of transporting the delicate model, partly because the main object of the enquiry-—the intercomparison of the results obtained in different channels. JUNE 28, 1923 and the determination of the corrections involved—could be achieved better by the use of a simpler model. For the aerofoil tests it was decided to use a 6 in. by 36 in. R.A.F. 15 aerofoil which had been cut by the Royal Aircraft Establishment while the design of the airship model was put in hand at the N.P.L. Details as to these various proposals were sent to the participating laboratories in September, 1920. Subsequent applications to co-operate were received from the Director of the Rijks-Studiendienst voor de Lucktvaart, Holland, the Associate Air Research Committee of Canada, and the Imperial Navy, Japan. These requests were in all cases approved ; while, though it is fully recognised that from the scientific standpoint the co-operation of the laboratories at Gottingen and Vienna is very desirable, political considerations have up to the present put serious difficulties in the way of arranging this. The aerofoil tests at the N.P.L., where each of the five channels was utilised as opportunities were found in the intervals of other researches, occupied a year. The duplex channel was not employed. The aerofoil was sent to the R.A.E. in September, 1921, and a report on the British tests was drawn up and presented to the Aeronautical Research Committee in May, 1922. The model was then forwarded to France, and the results from the French Laboratories were received in November, 1922. These results have been com- pared with the British, and the Secretary of the Research Committee has visited Paris and discussed them with the representatives of the French Laboratories. Beyond stating that the agreement is quite satisfactory, I do not propose to discuss them here, for it has been rightly agreed to reserve public discussion until all the results are to hand and can be treated together. It was intended as the next step to send the model, after careful measurement at the N.P.L., to Rome, but on communicating with the Director of the Instituto Sperimentale Aeronautico, he wrote that they were equipping the laboratory afresh, and asked for delay. The next sug- gestion was to send the model to Ottawa, in the hope that it might go hence to Washington after the American authorities had completed their tests on the airship models. The Canadian Committee, however, wrote that they were putting up a new channel and could not undertake the tests at once. A similar reply has been received from Amsterdam, and in consequence arrangements are being made to send it to the United States, and a letter has been addressed to the Japanese Naval ifttache, asking if the Japanese Laboratory could receive it later in the year. It had been hoped to complete-the Europen tests before sending the model overseas. The two model airships were constructed in the workshop of the Metrology Department of the N.P.L., and were tested in a 7 ft. channel at the Laboratory. On the completion of this they were sent to the R.A.E. and tested in a 4 ft. channel, and on their return, after re-measurement, they were sent to the National Advisory Committee for Aeronautics at Wash- ington. Information has been received that the tests in the Washing- ton Navy Yard and the McCook field tunnels are finished, and it has been agreed to extend the work by including the channels of the Bureau of Standards and of the Massachusetts Institute for Technology, provided it is possible to have the work com- pleted within three months. And now I have completed the task I undertook, to give an account of the steps taken to secure a comparison between the results obtained in various wind channels and the methods of research applied in various countries. It is incomplete— of this no one is more fully aware than myself—but the work necessarily proceeds slowly ; the results are valueless unless every precaution required for accuracy has been taken. When they have been received and compared, they should prove of great value, and their discussion may well form a» important item in the next International Air Conference. [In an appendix to his paper. Sir Richard Glazebrook described the different types of balances in use in this country. As the ordinary type is already well known to readers of FLIGHT, we omit this section of the appendix, and in the following quote but the part dealing with the comparatively new types of wire suspension.—ED.] Models supported on Wires : for Measurement of Drag only.— This method was probably first used by Prandtl, of Gottingen, who supported stream-line bodies in the wind tunnel by means of four wires, and transmitted the drag by means ofj an inclined wire passing through the side of the wind tunnel to a balance outside. The method has also been employed in other laboratories, the drag being sometimes determined by observ- ing the displacement of the model as a pendulum under the wind forces. At the N.P.L. the wire suspension is used, in conjunction with the existing aerodynamic balance, for drag measurement on stream-line bodies such, as airship forms. It
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