i-m NATIONAL ADVISORy COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED February 19^3 as Restricted Bulletin THE BELT METHOD FOE MEASURING PRESSURE DISTRIBUTION By Blake W. Corson, Jr. Langley Memorial Aeronautical Laboratory Langley Field, Va. UNIVERSITY OF FLORIDA DOCUMENTS DEPARTMENT 120 MARSTON SCIENCE LIBRARY P.O. BOX 11 7011 NESVILLE.FL 32611-7011 USA .<,--.•, i. 'it NACA WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution. L - 2hk s ~t A NATIONAL ADVISORY COMMITTEE F OR AEROHAUTICS THE BELT METHOD FOR MEASURING PRESSURE DISTRIBUTION By Blake W. Corson, Jr. SUMMARY The measurement of pressure distr i "fcn.it ion may be accomplished rapidly for any nnater of locations deemed necessary in model or full-scale investigations by use of the "belt" method. Seasonable accuracy may be ob- tained by careful use of this method. INTRODUCTION In wind-tunnel investigation of engine cowlings it is desirable to obtain a clear picture of pressure dis- tribution everywhere in the vicinity of the engine instal- lation. The iiiro ort anc e of pressures within the cowling which bear directly on engine cooling is obvious. Of equal interest are the pressures over various regions of the cowling exterior in that these affect the internal pressures available for engine cooling, indicate drag qualities associated with external flow, and determine the critical compressibility speed. Full-scale engine nacelle installations recently- tested were not provided with surface pressure orifices. If such provisions had been made, the time and expense for installing a satisfactory number of 'ores sure orifices would have been excessive and the time required for con- necting and disconnecting their leads whenever the cowling had to be removed would have made their use impractical. If external pressure distributions were to be measured, it would have to be by a quickly applicable method. Since pressure measurements were desired at a large number of points, and inasmuch as surface pressure orifices were missing, these measurements could have been made by the use of a "mouse." However, the time lost during the shutdowns that would have been necessary for the relocation of the mouse made this method prohibitive also. \ DESCRIPTION CF !1 BILT" LET HOD Tha construction of e mouse suggested the use of a belt of small-diamet er copper tuoos, scented together, each of which is provided with a single orifice at a point where the pressure measurement is to be Dade. All of the tubes are sailed at oris end and connected to manometer leacs at the other. Such a belt may readily be olaced at any location that the measurement of a pressure distribu- tion i 3 desired. Since the application of a belt to the surfs ce of a body in effect changes the shape of the body, it is real- ized that the pressure distribution over the body near the belt will be altered by its presence. On the other hand, bhe use of the belt method permits a means for obtaining a pressure distribution over a large area during a single run. Alteration to the nod el is inconsequential. The belt may be easily moved from one region to anotner on the model and may be discarded entirely upon completion of pressure-distribution measurements, leaving no impedimenta on the model to interfere with subsequent tests. DESCRIPTION OP TEST In order to ascertain the feasibility and reliabil- ity of this method for measuring pressure distribution, a brief test was made to determine the extent of agreement between a pressure distribution measured by the belt method and one measured by the use of surface orifices. These tests were conducted in the hACA propeller-research tunnel using a body of revolution as the mo^el on which the pressure distributions were measured. A photograi of the bell installed on the model is shown in figure 1. Figure 2 is a sketch showing the con- struction of the belt. The model used as a subject had 17 surface orifices in a plane on its upper surface- The pressure belt was rade with 19 tubes, but the extreme tube at each edge was not used. A single orifice was made in each tube. Orifices were located at distances aft from the leading edge to correspond approximately to the surface orifice locations on the body. Conper tubing of 0. 040-inch diameter was used for the belt, which when completed was about 6/ 4 inch wide. The maximum diameter of the model was approximately 21 inches. The leading edge of the belt was screwed to the interior surface of the lip, the belt was then vt-s-t to conform snugly with the surface of the model and was secured at the trailing edge with Scotch tape. The belt was located approximately 6 inches to one side of the plane of the surface orifices. The leads from the pressure belt and the surface' orifices were connected to the same multi-ole manometer and e simultaneous reading made at an airspeed of approx- imately 100 miles per hour. DISCUS3I01J OP RESULTS The results of the tests ere shown in figure 3. Pressures were plotted directly in inches of alcohol i the plot showing variation of pressure with distances fro:'- the leading edge of the body. It can "be seen that there is £ood agreement between the pressure distribution measured with the pressure belt and that obtained with the surface orifices. It is believed that for many inves- tigations the slight loss of accuracy incurred by the use of a belt for measuring pressure is more than offset by the simplicity of its structure and application. Several factors should be considered in the fabrica- tion and use of such a belt. The maximum permissible diameter of the copper + uc;s used for making the belt de- pends on the diameter and shape of the body on which the belt is to be used. So tests have jet been made to estab- lirh this relation. The curvature of the surface probably affects the reliability cf pressure measurements by the belt method. In the subject tests the air flew at the body surface was parallel to tie tubes comprising the belt. It is for this syp6 of application that the belt method if intended. Error would very likely be incurred by oblique flow over the belt. No attempt i 3 Lade here to describe refinements of fabrication or to pcint out the possible uses for the oelt method. The test described demonstrates that the belt method used with discretion will b ive a measurement of pressure distribution closelj approximating that obtained from the use of surface orifices. Langley Memorial Aeronautical Laboratory, National Advisory Committee for Aeronautics, Langley Field, Va. X Digitized by the Internet Archive in 2011 with funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/beltmethodformeaOOIang NACA Fig-. 1 O .2 -M 3 r— X) 0) X» t +• u- (0 »•-« tJ c (U •f-« S- -** 3 <0 i— • co -»-■ u 0) J- C p- •— < I ho C ,_( 01 S-i (0 Vj CD E \ NACA 8. »g H. S N. i. 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