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Open-Jet Wind TunnelThis wind tunnel was designed and constructed many years ago at Virginia Tech by members of the Aeronautical Engineering faculty. Presently it used primarily for undergraduate instruction.Facility DescriptionThe student 3-Foot Open Test Section Instructional Tunnel is an open-throat return type, with the test section shown in Figure 1. A plan view showing some of the construction details is given in Figure 2. The tunnel consists of a tube of circular section with diameter varying from 39 inches to 72 inches. The main body of the tunnel is constructed of two thicknesses of 1/4-inch waterproof plywood nailed on the inside of circular forms placed at intervals of 16 inches. The plywood construction was chosen after a study had been made of several alternatives such as steel, concrete, and wood. The cost of the concrete was prohibitive and the use of steel was eliminated because of difficulty of construction and the probability of its being noisy because of a sound-board effect. The inside walls were given one coat of shellac which was rubbed down to a smooth finish followed by four coats of spar varnish, each of which was rubbed down. The outside was finished in a similar manner except that each coat was not rubbed.
The exit cone is of unusual design. A wooden form was first built which was larger than the final section but approximating it in shape. Wire netting was nailed over this form and cement plaster applied to it. By means of a sweep which was the shape of the desired section, the plastered interior was worked down to the correct contour. After the plaster had set it was more accurately finished by hand and finally varnished. In the corners of the tunnel are four sets of straightening vanes. These vanes are equally spaced and their purpose is to turn the air around the corners of the tunnel and to maintain a uniform air velocity across the sections of the tunnel. The numbers in successive corners are 16, 17, 18, and 20, respectively, starting from the throat and following the direction of air flow. During the construction of these vanes, special care was taken to ensure a smooth finish and contouring of the vanes to provide the maximum turning efficiency. The vanes are mounted in frames separate from the tunnel itself, which frames can be removed for any adjustments that may be required. Power is furnished by a 35 H.P., D.C. motor (General Electric, 230 volts), which is supplied by a motor-generator combination (Westinghouse 60 KW D.C. Generator, 125 volts @1750 rpm, and a Westinghouse D.C. motor, 3 phase, 60 cycle, 100 H.P., 220/440 volts @ 1495 rpm). In order to obtain the desired speed control, separate excitation is furnished by a smaller motor-generator set (General Electric 10 K.W., 80 amp generator, and 220 V, 60 cycle, 38 amp motor). The propeller is a four-bladed Hartzell propeller which was specially made according to specifications furnished the company. The propeller drive shaft is a solid steel shaft mounted on three S.K.F. ball bearing assemblies. One bearing is inside the tunnel and is supported by two struts which go through the tunnel to the foundation and a third strut to the top of the tunnel. This method prevents any movement which might cause the propeller to strike the wall. The clearance between the wall and the propeller is 3/16 inch. The driven pully on the shaft is carried between the two outside bearings. The end bearing is used to carry the propeller thrust as well as part of the radial load from the belt and pulley, the other bearings being free to move axially. The speed range of the tunnel is from 0 to 150 miles per hour, but for most routine work a speed of 100 miles per hour is the maximum used. A vibration point occurs at 36 miles per hour. Air speed is measured by two methods. A pitot-static tube was used first to explore the test section to determine the quality of the air flow. Leads from the static and dynamic tubes are connected to an inclined differential manometer graduated to read to 0.01 inch of water. The calibration of the tube and manometer includes a correction for compressibility of air. Readings of the manometer are taken directly and referred to a curve which gives the corrected air velocity in feet per second. Also, a "setting" manometer is present that indicates dynamic head as determined from static ports located in the tunnel. This "setting" manometer readings must be corrected to obtain the true dynamic head. InstrumentationPressures are measured in the tunnel using a traditional manometer board. Forces and moments are measured using a strain gauge balance system borrowed from other facilities as required. |
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