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WIND TUNNELS OF NASA

Chapter 3 - Through the Barnstorming Days to World War II

The First Big High Speed Tunnel

[24] Even while the 11 and 24 inch HSTs were carrying out their first investigations of the mysterious Mach 1 regime, it was obvious they had two serious shortcomings. First, the rapid blowdown of the VDT restricted tests to less than a minute. Second, the tunnels were so small that model sizes were limited to dimensions of only a few inches. NACA needed a high speed tunnel big enough to test sizeable models of complete aircraft on a continuous basis. Consequently, in 1933, Manly J. Hood and Russell G. Robinson, both at Langley, formed a design group for the purpose of unshackling high speed aerodynamic research from the VDT restrictions of short run duration and small size.

drawing

(Top) The origin of shock waves. (Bottom) Shock waves can {e seen only rarely under natural conditions. In the controlled conditions existing in a wind tanner, shock waves can be made sharply visible by means of a schlieren optical system. First, a small but intense source of light outside the test section illuminates the test section through a parabolic lens. A second lens collects the light rays on the other side of the test section and focuses them on a sharp edge (called a knife edge). Any air disturbances in the wind tunnel will deflect the light rays up or down, depending on the changes in air density. Triangles A and B show how high and low pressure disturbances, respectively, will bend the rays. Light rays bent downward will be inter by the knife edge and create a dark area on the screen; those bent upward from light areas. The net effect is that changes in air density produced by shock waves and other flow disturbances show up as dark and light areas on the screen. (Right) A schlieren photograph of shock waves around a model in a supersonic tunnel.

schlieren photograph

The resultant tunnel, completed in March 1936, was 8 feet in diameter. Driven continuously by an immense 8000 horsepower electric motor, airspeeds of 575 mph (Mach 0.75) were attained. Later, in February 1945, airspeeds were increased to Mach 1 by replacing the 8000 horsepower motor with one developing 16 000 horsepower.

The engineers designing the new tunnel immediately encountered two problems that had not been serious in low speed tunnels. The first problem involved an effect discovered in 1738 by the Swiss mathematician Daniel Bernoulli. Bernoulli observed that as the velocity of flow in a duct is increased by constricting the cross sectional area, the static pressure of the fluid drops. In wind tunnel design, this means that the air pressure in the chamber containing the high velocity test section will be lower than in the rest of the tunnel. Thus, for the new tunnel, the test chamber had to withstand a powerful, inwardly directed pressure.

[25] Ordinarily, Langley engineers would have solved this problem by simply building a welded steel pressure vessel around the test section. But these were Depression days and to help put unskilled people to work it was decided to build the whole tunnel of reinforced concrete. An igloo like structure around the test section had walls that were 1 foot thick. The igloo was essentially a low pressure chamber-just the opposite of the VDT. Operating personnel located inside the igloo were subjected to pressures equivalent to 10 000 feet altitude and had to wear oxygen masks and enter through airlocks.

The second new problem was created when the mechanical energy of the huge fan was added as heat to the airstream. (This heat equals that from the engines of 100 compact cars.) High temperatures could damage the tunnel structure and the enclosed equipment. To avoid making the igloo into an oven, a small amount of heated air was bled off the tunnel walls and released outside, removing its contained heat in the process. The discharged hot air was replaced by cool air pulled in from the outside. The heat bled off in this manner must equal exactly the heat added by the fan-in this case the removal of only about 1 percent of the mainstream airflow was required. This stratagem is still employed in many of today's high speed tunnels operating at atmospheric pressure.

The 24 inch high speed tunnel also relied on the variable density tunnel for high pressure air. Shown here outside the variable density tunnel building at Langley, the 24 inch tunnel, like the earlier 11 -inch high speed tunnel, was mounted vertically.

The Langley 8 foot high speed tunnel. The test section was housed in the concrete igloo. A heat exchanger is shown above the tunnel. This was necessary to remove the large quantities of heat generated by the big fan.

[27] The resulting 8-foot high speed tunnel was unique, something no other country possessed. Since World War II was right around the corner, the tunnel had strategic value. The first tests, in fact, evaluated the effects of machine gun and cannon fire on the lift and drag properties of wing panels. This led logically to checking the effects of rivet heads, lapped joints, slots, and other irregularities on drag. Such tests demonstrated drag penalties as high as 40 percent over aerodynamically smooth wings. Aircraft manufacturers quickly switched to flush rivets and joints.

New high speed propellers and engine cowlings also emerged from tests in the 8 foot tunnel, but the story of the P 38 dive recovery flap is more spectacular proof of the value of wind tunnels during wartime.

The Lockheed P-38 Lightning was the high speed, twin boom fighter that helped beat back the threat of the Japanese Zeros in the South Pacific. When first introduced into squadron service in 1941, pilots were plagued by heavy buffeting during high speed dives. On several occasions, their dives steepened and they could not pull out. Lockheed's test pilot for the P-38, Ralph Virden, lost his life trying to solve the dive problem. Shortly after Virden's death, the Army asked NACA for help. Some tests were made in the Langley 30 x 60-foot full scale tunnel, but the crucial tests took place using one sixth scale models in the new 8-foot high speed tunnel. The tests indicated that, above 475 mph, the P-38's wings lost lift and the tail buffeted, leading to a strong downward pitching motion of the plane. Controls stiffened up....

drawing illustrating Bernoulli's principle

(A) As the ball rolls down the hill, the loss of potential energy is converted into kinetic energy, illustrating the law of conservation of energy. (B) Likewise in fluid flow, an increase of fluid velocity (kinetic energy) is balanced by a decrease in static pressure (potential energy). This is Bernoulli's principle.

A bullet riddled wing section undergoing aerodynamic tests in the 8-foot high speed tunnel.

[28] A P-38 showing the dive recovery flap that evolved from Langley tests. (Photo, Lockheed California Company)

...and lost their capability to pull the P 38 out of its steepening dive. In addition, the buffeting could cause structural failure, as it had in Virden's case.

Langley's answer to the P-38 dive problem was the addition of a wedge shaped dive recovery flap on the lower surface of the wings. Aerodynamic refinement of the dive recovery flap was continued in a coordinated program with Lockheed engineers and the new Ames Aeronautical Laboratory, just south of San Francisco, in the latter's new 16-foot high speed tunnel. The dive recovery flaps ultimately saw service on the P-47 Thunderbolt, the A-26 Invader, and the P-59 Airacobra, America's first jet aircraft.