Variable-Density Tunnel - Wind Tunnel #2

Equipment used for pressurizing the Variable-Density Tunnel (VDT): The VDT tunnel is on the right; the compressors are on the left. Figure 4 in the NACA Technical Report 227 (Part 2) identifies each piece of equipment visible in this diagram. Immediately visible in the lower left corner is the Booster Compressor. In the right rear (behind the tunnel) is Primary Compressor No. 1. (Primary Compressor No. 2 is not visible.) From NACA TR 227 (Part 2):"The air is compressed in two or three stages, according to the terminal pressure in the tank. A two-stage primary compressor is used up to a terminal pressure of about seven atmospheres. For pressures above this a booster compressor is used in conjunction with the primary compressor. The booster compressor may be used also as an exhauster when it is desired to operate the tunnel at pressures below that of the atmosphere. The primary compressors are driven by 250-horsepower synchronous motors and the booster compressor by a 150-horsepower squirrel-cage induction motor." Jerome Hunsaker wrote in "Forty Years of Aeronautical Research": "In June 1921, the executive committee [of the NACA] decided to build a new kind of wind tunnel. Utilizing compressed air, it would allow for *scale effects in aerodynamic model experiments. This tunnel represented the first bold step by the NACA to provide its research personnel with the novel, often complicated, and usually expensive equipment necessary to press forward the frontiers of aeronautical science. It was designed by Dr. Max Munk, formerly of G*ttingen." Eastman Jacobs wrote in an article in a 1927 article for Aviation that: "The tunnel is inclosed (sic) within a steel shell, so that the density of the air inside may be increased by pumping air into the shell to a pressure of 300 lb. per sq. in. A 250 hp. motor, driving a propeller, circulates the air, drawing it through the five-foot test section at a velocity of about fifty miles per hour. The model is mounted in the throat of the tunnel on a balance which weighs the air forces on the model. From this data coefficients of lift, drag and pitching moment are obtained, which are equal to the coefficients which would be obtained from tests at normal atmospheric density on a similar model twenty times as large. However, if the small model were tested in the conventional type of wind tunnel, or in the variable density wind tunnel at normal density, the flow would differ from that about the large model. The coefficients, which depend on the configuration of the flow past the object, would also have different values. This change in the values of the coefficients with the equivalent size or scale of an object, is known as *scale effect.'
NASA Identifier: L405