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THE HIGH SPEED FRONTIER


Chapter 2: The High-Speed Airfoil Program

THE QUEST FOR UNDERSTANDING (1928-1935)



[13] On July 16, 1928, the man who was to dominate Langley high-speed aerodynamics for the next 30 years reported for duty. John Stack was the son of Irish-born parents, a heritage which may have accounted for his personal charm, garrulousness, love of horses, and ability to absorb large quantities of whiskey. Educated at the Chauncey Hall School and Massachusetts Institute of Technology, his distinctive accent retained little to suggest an Irish background (it can be described as upper-class Bostonian with variations). Stack was at his best in the midst of conflict, crusading passionately for some cause such as a new wind tunnel against the forces of reaction and stupidity (which in his view was anyone and everyone who had any objection to the project).

He had applied for NACA employment during his senior year at MIT, where several of the faculty were involved in various ways with NACA activities. On his arrival there were fewer than 60 professionals at Langley, loosely organized in "sections" attached to the research facilities they operated. As was customary, Elton W. Miller, the fatherly, mild-mannered Chief of Aerodynamics, escorted Stack around the Laboratory introducing him to virtually the entire staff. After the tour, "Mr. Miller," as he was universally called, indulged himself with a final question that [14] he invariably directed to new engineers with private enjoyment, "Where would you prefer to be assigned?" Believing he had a choice Stack said, "the VDT." "Very good, I had already decided to put you there," Miller replied. (More often than not, as in my own case, the new arrival's choice did not agree with Mr. Miller's and he was told, "Well I have decided to place you elsewhere. Let me know in a year or two how you like it.")

Stack was assigned immediately to the 12-inch high-speed tunnel project which was then under construction- the lone NACA researcher in this field. For the next decade his work would be closely followed by Eastman N. Jacobs, VDT section head, a man for whose technical sagacity Stack had enormous respect. Both men had the same kind of restless energy and pragmatic approach to research problems. Neither was a theoretician, although both of them frequently supported theoretical work by others and frequently made use of such work. Their own activity in this area was limited to applying the usual analytical tools of the engineer.

In his first years at Langley, Stack was quite modest about his knowledge of aerodynamics and was eager to learn. As W. F. Lindsey, who high-speed arrived in 1931 and was a major contributor throughout the high-speed airfoil program, puts it, "Practically all we knew about compressible flow theory at that time was what was written in five or six pages in Glauert's 1926 textbook." Among the five professionals in the VDT group in 1930, Stack was chosen to act as section head in Jacobs' absence. (In those days, there was no formal appointment to the assistant section head position.) Apparently Stack's general deportment as a junior engineer was exemplary; the tough assertive characteristics mentioned earlier began to show themselves slowly at first, not reaching full flower until after Jacobs departed Langley in the mid-forties (refs. 22, 23).

The first attempts to operate the 12-inch tunnel with its unique jet-augmentor induction drive produced such violent flow oscillations that it was soon decided to convert to a closed throat. Stanton's small supersonic tunnel in England, in which the test airfoil spanned the throat (ref. 24), may have suggested the configuration. This configuration eliminated the pulsations and the uncertain large boundary effects of the open-tunnel setup, but suffered large constriction effects which were not [15] understood at that time. Pressure distributions on the 3-inch chord airfoils were found to be similar in character to the Briggs/Dryden results but different in detail. There was no way to tell whether either set of data was correct at the higher speeds. The renowned British theorist, G. I. Taylor, visited Langley in late 1929 and examined the data. Results of his recent studies of subcritical compressible flows by the electrical analogy method seemed, by inference and extrapolation, to cast doubts on the 12-inch tunnel data. Discouraged, Stack and Jacobs set the data aside and decided to go back to the open-throat configuration, with the first objective of achieving stable flow. (It is now believed that the closed-throat data were valid at speeds below the onset of tunnel choking. Unfortunately they were never published and were later disposed of.)

Another famous visitor, Amelia Earhart, came to view a test run in the high-speed tunnel at this same time period. She was clad in a raccoon fur coat. When the tunnel started she leaned forward to feel the flow of air into the entrance bell and her coat was instantly sucked into the bell, causing a large tear and terrifying its owner (ref. 22).

Stack has reviewed the laborious succession of design changes to the tunnel (ref. 20) that followed Taylor's visit: reversion back to the open throat modified by incorporation of a 1/2-inch annular enlargement at the entrance to the diffuser and a large reduction in length of the open section; rejection of the open throat, primarily because of windage effects on the balance and secondarily because of flow pulsations; a second reversion back to the closed throat-11 inches in diameter but virtually the same arrangement at the 12-inch tunnel except for the 1/2-inch step at the entrance of the diffuser and the use of 2-inch chord test models. By this time (1931) a high-tip-speed propeller test had been made in the PRT which afforded a basis for comparison and evaluation of the closed-throat wind tunnel data. Stack applied Goldstein's method to calculate the performance of the test propeller using the new high-speed section data from the 11-inch tunnel (ref. 25). His results agreed with the PRT tests except that the onset of performance deterioration in the calculation occurred at a somewhat lower Mach number. We now know this shift in speed was due to a combination of constriction effects in the tunnel, Reynolds number differences, and three-dimensional relief at the propeller tip. Still, the comparison was close enough to confirm that the [16] tunnel was an effective tool, and it was used at once to try to define improved sections. Following the lead of Briggs and Dryden, airfoils with the maximum thickness shifted rearward were found to offer improved high-speed performance, a fact which further strengthened confidence in the 11-inch tunnel (ref. 26).

At this stage (1933) the Langley group, according to Stack, had CC exhausted its intuition as regards methods for further improvement of aerodynamic shapes" (ref. 20). However, now that making the tunnel work was no longer the primary problem, interest finally shifted to the nature of the "burble" phenomena. E. N. Jacobs is believed to have first suggested to Stack that the schlieren optical system ought to be tried to make the phenomena visible. From his interest in amateur astronomy Jacobs was familiar with the Foucault test for mirrors, and the schlieren system, first described in 1889 by Mach, was optically a close relative. Unfortunately, in the limited Langley library of the early thirties nothing could be found except a schematic drawing in Wood's Physical Optics. This was used as a guide to construct the first crude schlieren (ref. 23). Reading-glass quality lenses about 3 inches in diameter were located together with a short-duration-spark light source. Celluloid inserts were used to support the test model at the tunnel walls. The first tests were made on a circular cylinder about 1/2-inch in diameter, and the results were spectacular in spite of the poor quality of the optics. Shock waves and attendant flow separations were seen for the first time starting at subsonic stream speeds of about 0.6 times the speed of sound. Visitors from all over the Laboratory, from Engineer-in-Charge H. J. E. Reid on down, came to view the phenomena. Langley's ranking theorist, Theodore Theodorsen, viewed the new results skeptically, proclaiming that since the stream flow was subsonic, what appeared to be shock waves was an "optical illusion," an error in judgment which he was never allowed to forget. At the annual dinner of the Langley staff in the fall of 1936, a skit was presented in which Stack played the role of Theodorsen, complete with Norwegian accent, making the "optical illusion" pronouncement.

Flow pictures for an airfoil at high speeds were obtained in short order. All of the implications were not immediately understood; however it was seen that a shock wave formed shortly after the speed of [17] sound was reached locally and that flow separation was induced by effects of the shock. This emphasized the idea that shapes should be sought with the least possible induced velocities. Stack has described this concept as "the inspiration . . . which led immediately to a new approach to the problem of developing better shapes" (ref. 20).

Shortly after the first dramatic results of the schlieren tests had been obtained, Jacobs came back from a meeting with Reid and announced that $10 000 of Public Works Administration funds would be made available to build a 24-inch high-speed tunnel, provided that a design could be accomplished in a few weeks. Justification for the larger tunnel rested entirely on Jacobs' argument that it was the low Reynolds number of the 11-inch tunnel data which was responsible for the discrepancy with the PRT propeller data mentioned previously. Jacobs' idea was to build a 24-inch tunnel exactly similar in all respects except size and Reynolds number to the 11-inch tunnel, and this was the basic design specification. A number of improvements were included however: a new 5-inch schheren system, an improved balance, and a photo-recording multiple-tube manometer.

The tunnel was erected outside the VDT building on a reinforced concrete base which also formed the entrance section and the test chamber surrounding the tunnel throat. Ira Abbott quickly became an expert in reinforced concrete. Dick Lindsey and Ken Ward were instructed by Jacobs to design the entrance section independently and bring their results to him for comparison. (They were sufficiently similar to merit Jacobs' quick approval.) Stack specialized in aerodynamic issues and coordinated the design project. The design was completed as scheduled and the tunnel was built approximately within the cost limitation in about one year's time. Figure 1 shows the two principal operators of the 24-inch tunnel involved with a survey rake installation in a scene typical of the mid-thirties.

The first test in the new tunnel involved a much more important issue than the Reynolds number-effect question for which the tunnel had been built. Jacobs had been invited to present a paper at the forthcoming Fifth Volta Congress on High Speeds in Aviation in Italy, and he realized that an elucidation of what was actually happening in the compressibility burble phenomena would be most appropriate and important, especially...




[18] FIGURE 1-John Stack and W. F. "Dick" Lindsey (standing inside the 24-Inch High-Speed Tunnel) in the thirties.


....in view of the possibility now of presenting flow photographs in addition to pressure distributions and forces. Accordingly, a 5-inch chord 4412 airfoil model built for the VDT with 56 small pressure holes was tested in the 24-inch tunnel and simultaneous pressures and flow photographs were obtained for the first time. After describing the new understanding of the burble phenomena achieved in the Langley program, Jacobs went on to derive for the first time the relation between the low-speed suction pressure peak on an airfoil and the speed ratio (Mach number) at which the local speed of sound would be reached. That is, the critical Mach number could now be related to or estimated from the low-speed pressure [19] signature of the airfoil. Obviously this relation contained a powerful implication: the critical Mach number could be increased by shape changes which could be determined by simple incompressible theory or low-speed tests.

A NACA Technical Note covering some of the same ground as the Volta paper was written by Stack (ref. 27), and a more elaborate Technical Report (ref. 28) was issued later in which Stack credits Jacobs with the critical Mach number derivation. Together with Jacobs' paper these publications proclaimed the first major contribution of NACA in-house high-speed research-the fundamental understanding of the burble phenomena derived in large part from the revelations of the schlieren photographs.

COMMENTARY

Throughout the history of NACA newer types of test facilities were often placed into service somewhat prematurely in order to capitalize on their advanced capabilities. This frequently resulted in some unforeseen difficulties. In the case of the first NACA high-speed wind tunnel these difficulties were compounded by strong interactions between the tunnel flow and the test airfoil flows at high speeds. Furthermore, the high-speed airfoil problem was so new that no criteria existed for judging whether valid data were being obtained, a situation which had its roots in the lack of knowledge of what actually happened in airfoil flows when the compressibility burble occurred. It seems obvious now that the first goal in such circumstances should be to acquire at least a qualitative understanding of the basic flow phenomena, and that this should always precede any program to produce force data for use by designers. The closed-throat 12-inch tunnel of 1929 could have been used to provide the great enlightenment from combined pressure and schlieren pictures which did not come until some five years later in the program actually pursued. It was the eventual achievement of this fundamental understanding that now stands out as NACA's first major accomplishment in high-speed aerodynamics. It also formed the solid base on which the advances in critical speed discussed in the next section could be made. By comparison, perfection of the testing technique so as to acquire improved [20] force data for designers, which was the goal of the early program (ref. 19), produced only relatively unimportant data prior to 1934.

A principal factor in the long delay in acceptance of the closed-throat data was the doubt engendered by G. I. Taylor in 1929. W. F. Lindsey points out that Taylor's real expertise extended only to the critical speed, and beyond that point his speculations should not have been taken as seriously as they were (ref. 23). E. N. Jacobs also feels that the cautious conservatism often displayed by so-called "experts" when they are asked to judge new phenomena beyond their previous experience has been a cause of undue delays (ref. 19). As another example he cited his 1926 investigation of thrust augmentors (ref. 17). Lewis turned the report of this work over to Dryden for review. Dryden expressed some doubts about it based on momentum considerations. As a result, publication was held up for several years, until 1931. Another obvious example was Theodorsen's off-hand optical-illusion" pronouncement, but by that time Jacobs and Stack had acquired enough confidence and momentum to proceed on their own judgments. As a general rule, the speculations and doubts of experts in viewing new phenomena should not be overrated.

The essence of the idea that the critical speed could be related to the low-speed velocity profile of the airfoil was first stated by Briggs and Dryden in 1925 (ref. 11). However, the only use they made of it was to show that the trends in their observed critical speeds were qualitatively consistent with the concept. They never considered applying the idea as a tool to develop improved shapes. It remained for Stack and Jacobs to recognize the potential of this concept and to put it to quantitative use. They established the mathematical relationship between M_cr and the low-speed peak negative pressure coefficient, thereby making it possible for designers to estimate from low-speed theoretical or experimental data the critical speeds of their designs, and providing high-speed researchers with a practical theoretical tool for achieving improved forms. Stack clearly felt a sense of excitement and fresh "inspiration" from this accomplishment (ref. 20). In his view the "new" concept was one of the fruits of the combined pressure and schlieren study for the 4412 airfoil in 1934. Whether previous readings of Briggs and Dryden had planted the seeds of the idea matters little; the revelations of the 1934 research gave the concept real meaning and inspired its useful application.

[21] It will be difficult for today's researchers to comprehend the procurement story of the 24--inch high-speed tunnel. That kind of quick action-design by the research staff in three or four weeks and construction for some $12 000 in less than a year-is rarely seen in the present complex organization. Facility procurements follow a complex process of reviews and approvals and many stages of design and construction involving several inhouse and outside agencies. Procurement of test models has followed a similar pattern. Of perhaps even greater concern than time and cost is the discouraging effect of these long and costly procurements on the interest and initiative of researchers.

Periodically throughout the history of NACA situations would arise, in the research programs as well as in facility procurements, where it was obvious that the normal agency procedures could not accomplish the job effectively within time or cost limits. Small teams or task groups would be set up in these cases, relieved of their normal duties and exempted from normal lines of authority, burdens of paperwork, etc-that is, freed from the restraints of the large parent organization, while taking advantage of its services and facilities whenever possible. Almost invariably these special groups did an impressive job.

The use of this special-group technique, not only in emergencies but as a regular device in R&D and procurement programs for recapturing the benefits of the small organization, offers partial salvation to today's enormous bureaucracies, industrial as well as governmental.