SP-4302 Adventures in Research: A History of Ames Research Center 1940-1965

 

Part I : INITIATIONS : 1936-1945

 

11

The 7-by-10's in Action

 

[89] FOLLOWING long-established NACA practice, the research staff at Ames was organized around major facilities and, as earlier noted, there was often collaboration between the various facility-centered groups as they found themselves attacking common problems. With these similarities of makeup and objective, there were also certain important factors which in each group bound the members together, implanted a distinguishing characteristic, and established a certain measure of esprit de corps and effectiveness. Chief among these factors was the leader of the group, but also of great influence was the character of the men in each group.

The group of men who operated the Ames 7- by 10-foot tunnels was quite fortunate. They were lively, original-thinking nonconformists who went on to make major contributions in other branches of the Ames organization In this group were men like Charles Frick, Victor Stevens, Steven Belsley, Charles W. Harper, Robert Crane, Ralph Holtzclaw, and, most notably, Harry Goett who, as leader, guided the spirited team with a firm and sure hand. Harry possessed unique abilities for developing the latent research talents of his men, and his perception of what was really important in aeronautical research was remarkably keen. Through the vigorous exercise of these qualities he was able to mold his individualistic and sometimes recalcitrant staff to the purposes at hand and to direct their efforts into fruitful avenues of research. Some of the more obstreperous members, such as Frick and Belsley, showed a certain resistance to the molding process with the result that the halls of the 7-by-10's often resounded to their protestations and arguments.

Both 7- by 10-foot tunnels began active research operations in the early fall of 1941. They generally had been thought of as "workhorse" tunnels expected to carry out the bulk of the development test work required in the production of new military aircraft. The particular usefulness of the 7-by-10 s in development test work lay in the ease and low cost of their operation and the fact that the test models used in them, being made of wood, could....

 


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Harry J. Goett. L. A. Clousing (seated)

Harry J. Goett. L. A. Clousing (seated)

 

....easily be produced and quickly modified. High-speed tunnels, on the other hand, required models that were partly or wholly made of metal.

With the beginning of the war, Ames received a flood of military requests for tests of a variety of airplanes such as the Consolidated XB-32, the Hughes D-2, the North American XB-28, the Douglas XSB2D-I, and many others. The manifest destiny of the 7- by 10-foot tunnels seemed about to be realized. The tunnels and their staffs went on two-, then three-shift operations, becoming deeply involved in tests aimed at finding and correcting design faults in new military airplanes.

Harry Goett, however, was neither content nor willing to limit the role of the 7-by-10's to development test work. While straining to satisfy head-office requirements for such work, he obstinately maneuvered to reserve one of the tunnels for research of a more basic character. At other times, by strategic planning, program additions, and analyses, he was able to wring basic research results out of otherwise ordinary development test work. Thus, during the war, while the 7- by 10-foot tunnels made important contributions to the development of specific military aircraft, they also, under Goett's imaginative leadership, produced valuable information having much broader application. Of the latter results there are many examples-the most notable, perhaps, being the earlier-mentioned procedure for predicting the handling qualities of airplanes from wind-tunnel tests.

 


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Curtiss XF15C-1 model in 7-by 10-foot tunnel used to investigate slipstream effects.

Curtiss XF15C-1 model in 7-by 10-foot tunnel used to investigate slipstream effects.

 

SLIPSTREAM EFFECTS

 

In early years at Langley, wind-tunnel tests of airplane models generally made no pretense at simulating the effect of operating propellers. It was difficult and costly to provide a powerplant and propellers for the airplane model, and the effects of the propeller slipstream on the model tests were not considered of sufficient importance to justify the added cost and complications. Besides, these effects could probably be calculated. In the late 1930's, however, the Sawyer Electrical Manufacturing Co. of Los Angeles developed a small, high-powered electric motor suitable for installation in airplane models, and these motors were used by Caltech to power models of the DC-4 and other airplanes in its 10-foot wind tunnel.

The value of simulating airplane power in model tests was demonstrated by the pioneering work at Caltech 1 and by the time the Ames 7- by 10-foot tunnels got under way, the technique was becoming common practice Shortly it became absolutely essential, for the new military airplanes were being equipped with tremendously powerful engines and the slip stream of propellers produced a major effect on the stability and control of an airplane Indeed, as airplane designers strove to achieve higher speed and greater performance for their products, the effect of the propeller slipstream became the predominant influence on stability and control. In some cases the slipstream problem was so great as to necessitate a major and costly change in the configuration of the airplane or even the abandonment of a design for which a prototype had been built. An example of the new breed of aircraft that were sorely beset by slipstream problems was the Douglas XSB2D-1, but the disease was epidemic.

A great deal of development test work in the 7-by-10's was spent in trying to deal with the destabilizing effects of propeller slipstreams and in [92] some cases, as just noted, it seemed almost a hopeless proposition. Ed Burton and his staff at Douglas Santa Monica thought they might have a solution to the slipstream problem when they mounted the propeller of their B-42 at the rear end of the airplane, behind the tail surfaces. This idea never caught on; nevertheless, a model of the B-42 was tested in the 7-by-10.

After struggling with the propeller slipstream for a few years, aeronautical engineers began to wonder if the reciprocating engine and its propeller had not become overdeveloped, overcomplex, and misapplied to modern high-speed aircraft. Like Renaissance scientists viewing the wheels-within-wheels complexity of the pre-Copernican, earth-centered model of the universe, they suspected that they might be on the wrong track. The deceptive simplicity of the first jet engines seemed to lend weight to this belief; but jet engines in the mid-war years were of low thrust and limited reliability and unhappily they became more complex as these deficiencies were corrected.

 

NEW ENGINES: NEW PROBLEMS

 

October 2, 1942, marked the beginning of a new era in U.S. aviation history. On that day, at Muroc Dry Lake in California, there occurred the first flight of this Nation's first turbojet-powered airplane-the "Airacomet," produced by the Bell Aircraft Corp. The Airacomet was powered by a pair of small turbojet engines, the essential features of which were those of an original jet engine designed by Group Captain Frank Whittle of the British Royal Air Force and flown earlier in England.

Owing to their low thrust and marginal takeoff capabilities, early jet engines were in some instances combined with conventional reciprocating engines. In 1944 there appeared at Ames, for test, a model of the Ryan XFR-1, the first airplane employing such a combination of power plants. The XFR-I had a conventional reciprocating engine and propeller in front and, inside the fuselage, a small jet engine exhausting through the tail. A model of this airplane was tested in the 7-by-10 with results reported in a paper by Myles Erickson and Leonard Rose.

The jet-powered airplanes which began to appear before the end of the war introduced a brand new set of problems with which the people in the 7- by-10's and other branches of Ames were directly concerned. Two major questions to be solved were: Where should jet engines be located in an airplane? and, How should the huge volumes of air that passed into and out of a jet engine be handled? For single-engine fighter airplanes, it seemed desirable to bury the engine in the fuselage behind the pilot and to duct the exhaust gas out through the fuselage tail. The intake to the engine was more of a problem. There would be some aerodynamic advantage in inducting the air through a single inlet in the nose of the fuselage, but this method gave rise to the problem of passing the air around the cockpit. Also there was a practical need to reserve the fuselage nose for a radar installation.

 


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NACA submerged inlet.

NACA submerged inlet.

 

Alternatively, twin inlets leading to the engine could be installed in the leading edges of the wing roots, a scheme used in the XFR-1, or they could be installed on the sides of the fuselage well back from the nose, but in such installations how would the efficiency of the inlets be affected either by the wing or by the thick boundary layer forming on the fuselage ahead of the inlet?

The real problem was that never before had aircraft designers been required to induct any significant amount of air into the fuselages of airplanes and never before had they been forced to concern themselves in such an important way with internal aerodynamics. The new jet engine consumed vastly more air than the largest reciprocating engine ever built; all of this air had in some cases to be passed completely through the fuselage with a minimum of lost momentum at the inlet, at the exit, and throughout the length of the duct. This was a problem, it was quite clear, which would not be completely solved for many years.

One important phase of the jet-engine internal-flow problem was the development of efficient inlets. Any inefficiency of the inlet would not only increase the external drag of the airplane but, possibly even more significantly, would also reduce the thrust of the engine. The obvious way to make a side fuselage inlet was to install a big scoop projecting from the side of the fuselage well beyond the thickness of the boundary layer. A protruding scoop of this kind could, of course, cause a serious flow disturbance and add greatly to the drag of the airplane. At the 7-by-10, Emmet Mossman and others felt that it might be possible to develop a flush, or submerged, inlet that would not protrude from the fuselage-an inlet that would induct air as efficiently as a scoop but with much less external drag. He and his colleagues worked on the idea and found that their hunch was right; they were able to develop a submerged inlet that performed very efficiently at subsonic speeds. The aerodynamics of the boundary-layer flow in the region of the inlet was very complex and no rigorous theory was ever developed to explain how the inlet accomplished its function; nevertheless, the inlet proved very useful and its development represented a significant contribution in the field of aeronautical science. The results of the 7-by-10's first work on submerged [94] inlets was covered in ACR A5I20 (ref. A-7), authored by Charles Frick, Wallace Davis, Emmet Mossman, and Lauros Randall.

While Mossman and his colleagues were working on submerged side inlets, another 7-by-10 group was giving consideration to the design of wing mounted jet-engine nacelles such as might be required for large multi-jet-engined airplanes. Here there was somewhat less concern with the internal flow through the nacelles than with external drag and the avoidance of intersections between wing and nacelle that would lower the critical speed of the combination. As a result of this work, conducted principally by Robert Dannenberg, Wallace Davis, and George McCullough, designs were evolved for single- and twin-engine nacelles mounted integrally with the lower surface of the wing. These nacelles provided low external and internal drag and did not in any significant degree decrease the critical speed of the lowdrag wing on which they were mounted. They provided the basis for the design of the nacelles of the first jet bombers built in this country.


1 See "The Influence of Running Propellers on Airplane Characteristics," by Clark B. Millikan. Third Wright Brothers Lecture published in Journal of the Aeronautical Sciences, vol. 7, no. 3, Jan. 1940.


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