CONSTRUCTION of the Ames 16-foot tunnel was exceedingly timely. Many of the military airplanes built during the war had pushed up into speed ranges in which compressibility effects were commonly encountered. There were a few, smaller high-speed tunnels, like the 8-foot 600+-mph tunnel at Langley, but none that provided an equal combination of size and speed-not even the new 16-foot tunnel at Langley. The Ames 16-foot tunnel therefore fulfilled a very important wartime need and helped to solve a number of rather crucial problems with which certain new airplanes were afflicted.
Manley Hood was in charge of the 16-foot tunnel during the war years and he was ably assisted by a group of men of whom a number were recent graduates of the University of Washington. Included among the members of the 16-foot staff were Al Erickson, Victor Ganzer, William Hamilton, Edmund Laitone, Henry Jessen, Warren H. Nelson, and Lee Boddy. Hood himself had come from Langley where he had been assisting DeFrance with facility design, a task which he continued during his first 2 years at Ames. At Ames he further demonstrated the technical ability and agreeable disposition which had been appreciated at Langley. Manley was placed in charge of the 7- by 10-foot tunnels when they were first put into operation but shortly was transferred to the 16-foot tunnel when, late in 1941, that facility was completed. It was for reasons of his experience and ability that Hood was chosen to manage and operate the most sophisticated piece of research equipment then completed at Ames. The task was at once a challenge and a heavy responsibility.
Because of the 16-foot tunnel's speed potentialities, it was subjected to intense pressure during the war to carry on both research and development test work. It operated on a three-shift basis, often 6 days a week. DeFrance, whose pervasive influence impressed itself on every phase of the Laboratory's operation, drove his men hard to get the work done. Every section head, every division chief, was keenly aware that at any minute Smitty might burst into his office with a roar that could shatter the glass in the win-...
...ows. A rather fearsome apparition he sometimes seemed on such occasions: hat jammed down on head, cigar clenched in teeth, and eye flashing fire- his glass eye relatively benign. Survival always seemed doubtful during the first minute of such encounters, but relief often came quickly if "explanations" were satisfactory. Above all, DeFrance was fair-a square shooter- and he was respected by all of his men. All, too, were aware of his devout loyalty to his staff and had experienced the friendly warmth of his personality more often than his wrath. But the wrath they never forgot. It lent wings to the work of the Laboratory, particularly during the period of construction and wartime urgency.
In December 1943 there were no explanations for the catastrophe that struck in the 16-foot tunnel. The windings of one of the great drive motors burned out in a shower of sparks and a pall of smoke. Smitty and Manley were on top of the problem immediately and a great effort was quickly instituted to repair the motor and return the tunnel to operation. The effort, led by Jim White, Jeff Buck, and Lawrence Montgomery, Sr., of the Electrical Section, was pushed 24 hours a day, Sundays, Christmas, and New Year's. On December 25, in the cold, dark reaches of the tunnel where the work was going on, a Christmas tree was mounted on the motor nacelle and thoughtful individuals brought in a turkey dinner for the crew. Morale was high and everyone turned to with a will to complete the job. The prevailing spirit was revealed by the actions of the General Electric Co., the principal contractor, which even before receiving written authorization, or assurance of pay, began drawing scarce copper into the special wire required to rewind the motor. By early January the challenge had been met, the task completed, and the tunnel returned to service.
With fans driven by motors totaling 27,000 horsepower, the 16-foot tunnel was probably the first large tunnel to encounter the problem of choking. The phenomenon of choking in a wind tunnel, as earlier noted, occurs when the speed has increased to the point where the speed of sound is reached over the full cross section of the constricted throat. Inasmuch as the model and its supporting struts produce an additional constriction, the speed of sound is reached in the plane of the model before it is reached in the region, just ahead, where the tunnel airspeed is measured. Thus, depending on the volume of the model and struts, choking will occur when the tunnel airspeed meter shows some value less than the speed of sound.
The 16-foot-tunnel staff was desirous, of course, of making tests in their tunnel at the highest possible speed but also with large models. Therefore the only means available to them for increasing the test (choking) speed of the tunnel was to reduce the thickness or bulk of the model support struts. In the original design, the tunnel was equipped with three substantial support struts each surrounded by a big, fat windshield. This system was a mistake in design judgment, it was quickly learned, for the choking Mach number was only 0.75. The windshields were removed and the struts made thinner, with the result that the choking Mach number was raised to 0.85. In certain cases, it was found, the choking Mach number could be raised to 0.9 by suspending the model on four very thin tension struts. This then was about the highest test Mach number that could be hoped for in the 16-foot tunnel except perhaps in special cases where the test model was a semispan wing that could be cantilevered, without any support struts, from the sidewall of the tunnel.
The first tests undertaken (in early 1942) in the 16-foot tunnel were of a wing composed of NACA 66,2-420 airfoil sections of 5-foot chord. This test program was NACA's first opportunity to investigate at large scale and high speed one of its new low-drag airfoils. The results were published in May 1942 in a report authored by M. J. Hood and J. L. Anderson.
The next project undertaken in the 16-foot tunnel was one of great urgency and importance. It was an attempt to find the cause and cure of a very dangerous diving tendency that had been revealed by the Lockheed P-38 airplane. At least one pilot had been killed by this devilish phenomenon and others had gotten into serious trouble. The evidence indicated that in a high-speed descent, or glide, the airplane showed a strong tendency to nose over into a vertical dive from which, once it was well established, the pilot had not the strength, nor the elevator the power, to pull out. If the "tuck under", tendency, as the phenomenon was often called, was not immediately  corrected when first felt, the results were likely to be catastrophic. Lockheed management was very much worried. The plane was scarcely suitable for the military purpose intended unless this deadly characteristic was eliminated.
It was suspected that the trouble had to do with compressibility as it seemed to occur only at high speed-at a Mach number above 0.6. The NACA Langley laboratory was asked to investigate the problem and tests were made in the 30- by 60-foot and the 8-foot tunnels. The effect, it was found, was clearly due to the formation of shock waves on the wing and perhaps the fuselage, with a resulting loss of lift. It appeared that Lockheed might have earned the questionable honor of building the first airplane that would fly fast enough to encounter serious compressibility troubles. But what to do about it? Langley made a number of suggestions aimed at increasing the critical speed of the airplane. The recommendations were good in theory but would have required really major modifications in the design. Kelly Johnson would have none of them. What he wanted, and quite understandably, was a quick and easy fix. After all, there was a war on and no unnecessary delays could be brooked.
At this stage, Ames was asked to investigate the problem in the 16-foot tunnel. The job was undertaken with high priority, and Al Erickson was put in charge of the work. Kelly Johnson's men made frequent visits to Ames to discuss ways of dealing with the problem. The first series of tests at Ames confirmed the findings at Langley. The main source of trouble, it now became clear, was the system of shock waves that formed on the upper surface of the inboard wing sections at a Mach number of about 0.65. The shock waves reacted with the boundary layer and caused flow separation and loss of lift over that portion of the wing. The loss of wing lift caused a loss of downwash at the tail; and the tail, suddenly relieved of its downward load, immediately put the airplane into a steep dive. The forces on the stabilizer, which held the plane in the dive, were so powerful that they could not be overcome by the elevator; and there was, of course, no fast-acting means for changing the angle of the stabilizer.
In the first series of tests run at Ames, a number of corrective measures were tried. None of them was very simple though some were beneficial. Other tests were run which were reported by Erickson in April 1943. In Erickson's report, three solutions to the problem were suggested. The first and best one, which had been recommended and actually checked out in flight by Lockheed, was the installation of flaps on the lower surface of the wing at the 33-percent-chord point. The action of the flap was to quickly restore the lift which the wings had otherwise lost. The second suggestion was to install some fixed bumps on the lower surface of the wing. The bumps produced results somewhat similar to those of the flaps but were much less effective. The third suggestion, which doubtless would have been effective, was to install a controllable stabilizer.
 The flap solution was adopted by Lockheed and it served the purpose very well. But it was only a fix and it did not obscure the fact that in the race for speed subsonic airplanes, particularly configurationally complex designs like the P-38, had come to the end of the line. Lockheed's next airplane, the P-80, quite different from the company's P-38, P-49, and P-58, was dead simple. It was a beaus.
The tuck-under phenomenon appeared, in a generally milder form, in other airplanes and was the subject of considerable study in the 16-foot tunnel and in flight. An analysis of the problem was prepared by Manley Hood and Harvey Allen and published in TR 767.
Not long after the P-38 episode another "fire drill" took place in the 16-foot tunnel. In early flights of North American's sleek new P-51, a strange thumping noise had appeared. Nothing really catastrophic had occurred, but Ed Schmued, Ed Horkey, and others of the design staff at North American were considerably worried. Noises such as had come from the bowels of the airplane might presage an explosion. The noise, it was noted, occurred only in flight.
The P-51 Mustang was one of the cleanest airplanes that had been built up to that time. Reportedly, it had been dived by the British to Mach 0.85, which seems doubtful, but in any case it had certainly traveled faster than any other propeller-driven airplane. The P-51 incorporated, for the first time, NACA's low-drag airfoil sections and great care had been taken to make the wings fair and smooth. Its single, liquid-cooled engine was neatly faired into the nose of the fuselage. The radiator was located in the fuselage behind the cockpit and cooling air was taken in through a large belly scoop under the wing. Despite the advanced features of the P-51's design, the Army Air Corps had shown little interest in the airplane and had ordered only two. It was not until the British ordered 600 of them that the Army pricked up its ears and itself placed an order for 500. The airplane later endeared itself to American bomber crews for whom it provided defensive cover during deep penetration raids into Germany.
It was clearly an urgent matter to find the source of the thumping, or rumble, in the P-51. The assistance of the Ames Laboratory was requested. The project might appropriately have been undertaken by the Flight Research Section, but it was realized that results could be obtained much more quickly in the 16-foot tunnel if it were possible to mount the whole fuselage and wing roots of the airplane in the tunnel. Manley Hood and his staff figured that it was possible-that if the outer wing panels were removed, the fuselage with stub wings would just fit in the 16-foot test section. The installation was quickly made and the tests were begun. Howard Matthews was project engineer.
The rumble appeared in the wind-tunnel tests and its source was found to be the belly scoop on the undersurface of the wing. More specifically, it was soon learned, the rumble came from disturbances in the inlet airflow when gobs of de-energized air from the boundary layer surged over the inner lip of the scoop and into the air duct. Over the long expanse of wing and fuselage lying ahead of the scoop, a considerable thickness of boundary layer would build up and the thickness was all the greater as a result of the blocking, effect of the scoop. North American engineers had considered the boundary layer in the design of the scoop and had lowered the scoop below the surface of the wing a little way to allow the boundary layer to pass harmlessly by. But for reasons of maintaining cleanness of line, they did not want the scoop to project downward any farther than necessary. In fact, they had made it wide and shallow-thus, unfortunately, providing every opportunity for the ingestion of boundary layer.
A number of minor modifications of the scoop were tried with little effect. The ever-present Smitty DeFrance, who seldom failed to advance his own recommendations, spoke forth on this occasion in no uncertain terms: "Lower the damn thing!!" This measure was pretty obvious, of course, but Manley and his boys were searching for a somewhat more refined method of accomplishing the same end. Nevertheless the lowering idea was tried and it, together with certain other modifications, was found to be a nearly perfect cure. North American people were delighted. An easily applied cure had been found, and that in a matter of only a few weeks. But above the pleasure of finding a solution to the P-51 problem, both NACA and North American had learned a valuable lesson. It concerned the importance of keeping boundary layer out of air scoops. And a thoughtful observer might have taken time to reflect that the boundary layer, so little appreciated by the layman, so infinitely important to the aerodynamicist, had again got in its licks. Certainly this was not the last we would hear from it.
Ames' contribution to the success of the P-51 did not stop with its solution of the duct-rumble problem. Other performance gains for the airplane were achieved in development tests on the airplane that were later run in the 16-foot tunnel under the supervision of Charlie Hall, Henry Jessen, and others. And of course there was the earlier described joint project with Plight Research, the famous "Jimmy Nissen" experiment. All in all, North American had much to thank Ames for, and such thanks were soon forthcoming.1
The projects just mentioned are examples of work carried out in the 16-foot tunnel during the war. While they were somewhat more dramatic, they were no more important than many other projects undertaken in that tunnel. Models of at least 16 different airplanes were tested in the 16-foot tunnel and these included the P-80, the first completely jet-powered airplane to be investigated at Ames. Inasmuch as there was a vast lack of information on the effect of compressibility on air loads, many of the models tested were built with orifices distributed over their surfaces to facilitate pressure-distribution measurements. In some instances as many as 800,000 separate pressure measurements were made on a single airplane model.
Because of the advanced capabilities of the 16-foot tunnel, all of its available time was given to the development testing of military aircraft. Some time was reserved for obtaining fairly basic information that would be useful in future designs. Along in 1945, the Air Forces were making plans to develop a new class of bombers powered with jet engines. They would be much faster than the B-29, of course, and would require wings that would perform well at high subsonic Mach numbers. Certainly this was an occasion to use NACA's low-drag airfoils which minimized the adverse effects of compressibility and maximized the benefits derived from extended laminar flow. Accordingly a program was undertaken in the 16-foot tunnel concerned with the design and testing of a series of six wings, comprised of NACA 65-series sections covering a range of thickness and aspect ratio that might be used by the new bombers. The tension struts were used and Mach numbers up to 0.9 were obtained. In some tests a dummy, bomber-type fuselage was installed. The results of these tests were described in TR 877 by William Hamilton and Warren Nelson. They were also presented directly to the Air Forces in a conference at Wright Field in September 1945.
The Wright Field conference was called to allow NACA to present all available technical information having application to the new jet bombers then being designed by North American, Consolidated, Boeing, and Martin. By far the most important subject discussed at this conference was wing sweep. Robert Jones of the Langley laboratory had recently shown that compressibility effects could be delayed and perhaps minimized in intensity by sweeping the wings of an airplane backward. And when, at the end of the war, our scientists entered Germany, they found that the Germans had also discovered the benefits of sweep. 2 But how soon the principle of sweep could be applied to American airplanes remained to be seen.
1 Letter, R. H. Rice, Chief Engineer, North American Aviation, Inc., to Dr. G. W. Lewis, Director of Aeronautical Research, NACA, Apr. 21, 1943.
2 One of those who entered Germany at this time was R. G. Robinson of Ames. Inklings of the German work on sweep had reached the United States a year or so earlier, but the significance of this work seems not to have been recognized until the Jones discovery.