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

 

Part I : INITIATIONS : 1936-1945

 

10

Flight Research

 

[79] THE ice-research project represented a single and very special phase of the flight-research program at Ames. Flight research had always been an essential element of NACA research effort. It was necessary, NACA thought, to prove out in flight the ideas for aircraft improvement that were developed in the laboratory. The scientific techniques of modern flight research had indeed first been established in the United States by NACA, and NACA had remained a leader in advancing these techniques. NACA had never countenanced the old seat-of-the-pants flight testing by undisciplined daredevil pilots whose research results were often measured by survival and whose reports were of a highly subjective and qualitative character.

NACA required a test pilot to have an engineering degree as well as piloting experience and to have the ability and the motivation to perform precise preplanned maneuvers that he and his ground-based engineering colleagues had worked out ahead of time. Highjinks in the sky with valuable test airplanes were not tolerated. The oral report of the pilot was generally only a minor part of the information obtained from a NACA test flight. The major part was in the form of quantitative data obtained with special recording instrumentation carried in the airplane. Henry Reid, longtime head of NACA's Langley Laboratory, had designed and built some of the first recording flight-research instruments at Langley, and since those early days in the 1920's and 1930's such instrumentation had been developed to a high degree of capability and precision. All the techniques and instrumentation that had been developed at Langley were available to Ames as the new Laboratory's flight research got under way, and it was not long before Ames made its own contributions in these fields.

In the early 1940's, faced with the growing threat of war, President Roosevelt had called for the production of 100,000 airplanes-a fantastic number, it seemed. Shortly, the aircraft companies were receiving orders from the military for new types of aircraft as well as production orders for existing types. A new crop of airplanes thus began to appear soon after the United States entered the war. Mostly the new airplanes were equipped [80] with more powerful engines and were capable of higher speeds than earlier types. These new factors generally introduced problems in the areas of handling qualities, stability and control, drag reduction, and structural loads.

Handling, or flying, qualities had always been a factor of great interest to the military but one with which it was hard to come to grips because such qualities were so closely linked to the subjective reactions of the pilot However, research at Langley led by Robert Gilruth and Hartley Soule had, by 1941, gone far in establishing quantitative requirements for satisfactory flying qualities. These criteria, which were adopted and adapted by the military, were extremely helpful in evaluating the new crop of wartime airplanes-an evaluation task in which the Flight Research Sections of both Langley and Ames took an active part. The Navy and sometimes the aircraft companies contributed the services of their pilots to further the flight research at Ames. This activity expanded so fast during the war that the hangar in the flight-research building was quickly outgrown. A contract for a new, much larger, hangar with attached offices and shops was let in March 1944.

 

HANDLING QUALITIES FROM WIND-TUNNEL TESTS

 

Ames could, and did, evaluate the handling qualities of many new military airplanes and, when serious faults were found, the military service involved would often ask the aircraft company to make a corrective change in the airplane design. At that stage of the airplane development, however, the necessary design change could be very difficult and expensive to make. If it were possible to correct such faults while the airplane development was in the wind-tunnel-model stage, the whole matter would be vastly simplified. Just how this trick might be accomplished was a problem which the Ames staff attempted to solve, and eventually did solve.

The project just mentioned was jointly undertaken by the 7- by 10-Foot Wind Tunnel and the Flight Research Sections. The key to the solution, however, was found by Harry Goett, who conceived and, together with his 7- by 10-foot tunnel staff, developed methods for predicting flying qualities from data obtained from tests of small-scale powered models in the wind tunnel. The Flight Research group checked the wind-tunnel results by flight tests of the actual airplane. The check was made on numerous airplanes, but the most extensive data confirming the technique were obtained on the Navy's PV-I twin-engine patrol airplane. The first attempt to interpret the PV-1 model test data in terms of flying qualities was made by Victor Stevens and George McCullough, while Noel Delany and William Kauffman reported confirming results obtained from flight tests of the actual PV-1 airplane. In the end, the staff of the 7-by-10 had devised methods for planning wind-tunnel test programs that would allow the predetermination of those physical features of an airplane which would best satisfy established han-...

 


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Vega PV-1 airplane model in 7- by 10-foot tunnel.

Vega PV-1 airplane model in 7- by 10-foot tunnel.

 

...-dling-qualities requirements. One of the most outstanding and useful research accomplishments of the Laboratory during the war, this work was published as TR 781 (ref. A-5) under the authorship of Harry Goett, Roy Jackson, and Steven Belsley.

 

AIRPLANE DRAG IN FLIGHT AND WIND TUNNEL

 

Although Ames engineers, as just noted, developed procedures for predicting the handling qualities of airplanes from wind-tunnel model tests, the prediction of performance factors, such as drag and speed, from such tests was quite a different problem. The test models were usually idealized with smoother, truer surfaces than the originals and lacked the gaps, excrescences, and rivet heads that were found in the actual airplanes. Moreover, there were other influences peculiar to the wind tunnel that were often difficult to evaluate. Such influences included the interference of the struts on which the models were mounted, the turbulence in the airstream, and the subtle effects of the surrounding walls. The rather indeterminate effects of these many factors placed in some question the accuracy with which the drag, and thus the speed, of an airplane could be determined from wind tunnel model tests.

To obtain information on the subject just mentioned, Ames engineers undertook to make a comparison of the drag of airplanes as determined first by model tests in a wind tunnel and then by measurements made on the airplanes in actual flight. The comparison was made for only two airplanes, the P-51 and the P-80, but the results were expected to have general significance. The first airplane selected for the test was North American's new P-51 "Mustang" fighter, an airplane on which Ed Schmued, Ed Horkey, and [82] others of the NAA design group had lavished much attention. The P-51 was a good selection for the test. It was the first of a new class of extra clean fighter airplanes and the first to use the new laminar-flow wing sections developed by NACA. There was still a question of just how much laminar flow one could expect to get in an actual application of these sections considering the effects of propeller slipstream and all the unavoidable surface roughness resulting from conventional, or even refined, manufacturing methods There was also the question of whether the 16-foot tunnel, in which the model was to be tested, was sufficiently free of turbulence to allow the wings of the P-51 model to develop their full laminar-flow potential. These questions, however, merely added spice to the experiment.

A one-third scale model of the P-51, without propeller, was carefully tested in the wind tunnel through a range of lift coefficients and speeds. This phase of the experiment was not unusually difficult. It was pretty much a conventional wind-tunnel test. The problem came in running the flight test-without a propeller. The propeller had been eliminated because there was no good way of measuring the thrust of either the propeller or the engine exhaust in flight, and these uncertain forces would totally obscure the drag of the airplane, the force which was to be measured. So the propeller was removed, the carburetor inlet blocked off, and the whole airplane polished and waxed to resemble the surface conditions of the model. The usual load of special NACA flight-research instrumentation had been installed in the airplane, and this included a sensitive accelerometer that would measure accelerations in the longitudinal direction with an accuracy of 0.01 g. The drag of the airplane would be measured by the deceleration it produced.

With no propeller, the P-51 would have to be towed to altitude and there released to descend along some prescribed path to a dead-stick landing. The NACA pilot, James M. (Jimmy) Nissen, recognized the hazards involved. True, he did not expect to get any special financial reward for undertaking work involving unusual danger-NACA pilots never did-but if he felt any concern over these dangers, it was buried in his great enthusiasm for the project. In any case, the flights were to be made from the Army base at Rogers-more commonly called Muroc-Dry Lake where the maximum opportunity for a safe landing would be provided.

The airplane chosen to do the tow job was the Northrop P-61 Black Widow. It would be connected to the P-51 by means of two long tow cables having at the P-51 end a special release mechanism which Jimmy could operate if he got into trouble.

The whole operation was a very tricky business. The towed takeoff, the climb to 28,000 feet, the release of the cables, and the descent to a dead-stick landing all added to the thrill of the experiment. But everything went off fine. The first flight was completed successfully and so was the second. On the third flight, however, difficulty arose. For some unexplained reason, the...

 


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Northrop P-61A towing propellerless P-51B.

Northrop P-61A towing propellerless P-51B.

 

....cable released from the Black Widow. It flew back and wrapped around the P-51 like spaghetti around a hot dog. Jimmy was in real trouble. Though trussed up like a Christmas turkey, he found he was still able to control the airplane. Gingerly he brought the P-51 down: in fact, he couldn't stop it from coming down as it had no propeller. He landed in a rather rough area, a quarry. The structure of the P-51 crumpled. When the dust had settled, there was Nissen crawling out of his wrecked plane, shaken but hopefully not seriously harmed. He was taken to the base hospital to be X-rayed for broken bones. Unfortunately, the X-ray machine was not working; Jimmy had clipped the powerline to the hospital on his way in for his ill-fated landing. The instrumentation fortunately survived the crash and provided the evidence that was sought. The flight data confirmed to an acceptable degree the results of the wind-tunnel tests.

A description of the whole project is given in TR 916 (ref. A-6) authored by James Nissen, Burnett Gadeberg, and William Hamilton. A foreword was added to the report by NACA Chairman Jerome Hunsaker in recognition of the special contribution made by Nissen.

The hazards to which NACA test pilots were subjected were considered acceptable only if they could not by any reasonable means be avoided. In this case, the whole project had been rushed and a question remained whether' with a little more deliberation, a little more care and checking, the failure of the cable attachment could have been avoided. The lesson learned was reasonably cheap, but it could have been otherwise.

 

CRITICAL AIRLOADS

 

The wartime role of the Ames Flight Research Section was not only to...

 


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Left to right: Test pilots James Nissen, William McAvoy, and Lawrence Clousing.

Left to right: Test pilots James Nissen, William McAvoy, and Lawrence Clousing.

 

....assist the military in evaluating the flying qualities of military airplanes but also to serve as aeronautical detectives to discover why a particular airplane, or a particular class of airplanes, was killing a lot of military pilots. The discovered cause often indicated no fault of the designer but only that the airplane had advanced into new realms of flight where unknown factors were present.

The flight realms that gave trouble in the midwar years were the high subsonic speed ranges where the uncertain effects of compressibility were being encountered. At low subsonic speeds the air feels the coming of a wing when it is still far off; thus the air has plenty of time to move out of the way and let the wing by. But at speeds approaching the speed of sound, the air receives little warning of the coming of the wing and has little time to get out of the way-so little time, indeed, that it gets squashed or compressed as the wing slams against it. When the sonic speed is reached, the air gets no warning at all because the "feel" of the wing is transmitted forward only with the speed of sound. Hence at sonic and higher speeds the wing, and other parts of the airplane, crash into the air with a terrific impact often heard miles away on the ground. In the early days at Ames peculiar airplane flight characteristics, and also many unexpected airloads, arose from these compressibility effects.

[85] Along in the midwar years, a rash of tail failures appeared in the operation of some of our new high-speed fighter aircraft. These airplanes had been designed to dive at high speed and perform rolling pullouts and other violent maneuvers, yet failures were occurring under conditions which the designers had considered safe. More than one pilot lost his life as the result of such failures. The Ames Flight Research Section was assigned a Bell P-39 airplane to study the general effect of compressibility on airloads as well as the specific tail-failure problem. The airplane was prepared for test in the usual manner by installing a variety of special NACA instrumentation. Included in this case were instruments to record photographically, as a function of time, quantities from which could be determined such variables as indicated airspeed; pressure altitude; normal acceleration; engine manifold pressure; engine rpm; approximate angle of attack of the thrust line; landing-gear position; aileron, elevator, and rudder positions; aileron and elevator forces; rolling, yawing, and pitching velocities; and the pressure distribution over extensive areas of the wings and tail surfaces.

The instrumented P-39 was used in several rather extensive programs to determine handling qualities and airloads both in steady straight flight and in various maneuvers. Valuable information was obtained which pointed the way to improved methods of design. In most of these programs, the airplane was flown by Lawrence Clousing. His principal, ground-based, engineering colleagues were William Turner and Melvin Sadoff. Typical results of the tests are contained in TN's 1144 and 1202.

In one phase of the P-39 program, measurements were made of the horizontal tail loads during stalled pullouts at high speed. To attain the highest Mach number of which the P-39 was capable, Larry Clousing would put the airplane in a nearly vertical dive at high altitude. From the high speeds thus obtained, he would make sharp pullouts, thus searching the extremities of the conditions for which the airplane was designed. But did the designers really know what the airloads would be under these severe flight conditions and what would happen if they had underestimated the loads? The tests showed that the loads had indeed been underestimated.

In each new dive Larry, with cold courage, would push the airplane to higher and higher speeds and make ever more forceful pullouts. The results....

 


The Bell P-39 test airplane, used for tail-load tests.

The Bell P-39 test airplane, used for tail-load tests.

 

[86] ....are best told by a Memorandum Report that Clousing and Bill Turner wrote on the subject:

. . . it is seen that the airplane was pulled in until a very violent stall occurred at a lift coefficient of 1.01 and a Mach number of 0.67. The maximum acceleration factor reached was 7.5 at an angle of attack of 19°. The maximum elevator angle was 11.5° up. The buffeting was apparently about the same on both sides of the tail in this run. Rapid fluctuations of pressure of 300 to 400 pounds per square foot occurred at the leading edge of the stabilizer during the buffeting....
 
During this run structural failure of the horizontal tail occurred. Since the airplane was operating within maneuvering limits which were considered safe by design specifications in use at the time the airplane was designed, it is of interest to examine the nature of the tail failure in relation to the necessity for revision of air load requirements and the manner of specifying safe maneuverability limits to a pilot. The principal failures were on the left side of the tail, although failure had also started on the right side.... The left elevator had buckled downward at about the third outboard row of orifices, cracking the elevator spar. The elevator nose balance had been forced downward severely enough to break the elevator nose rib on each side of the tail just inside the outboard hinge fitting. The left stabilizer rear beam was cracked at the inboard hinge bracket, and the two top bolts holding the bracket to the stabilizer rear beam had been sheared completely.

 

The report calmly went on to say, "Other miscellaneous failures of various degrees of severity occurred to both the elevator and stabilizer structure. . . ."

Other, indeed! That was quite enough. One tiny bit more and Larry would not be here. This was not the first nor the last time that Clousing exhibited the remarkable fearlessness with which he approached all of his flight assignments. Why did he do it? Certainly not for money. All he or the other NACA test pilots received was standard civil service pay. Commercial pilots charged treble the amount for far less dangerous jobs. But it was wartime and perhaps Clousing felt that a little added risk on his part might save the lives of several military pilots. After all, he had been a naval aviator himself. And who knows what Larry really thought, for he was reluctant to talk about it. A historian must get the story from his colleagues or from the cold data of musty reports.

The flight projects so far described are representative, perhaps, of the wartime work that was performed by the Ames flight research people. But it is only a tiny part of the total. To aid military services, the Ames flight research group conducted research programs on many of the new airplanes being produced for war use. Included among these were the Bell P-39 and P-63; Boeing B-17; Brewster F-2A; Consolidated B-24; Curtiss C-46; Douglas SBD, XBT2D, A-20, and A-26; General Motors XP-75; Lockheed P-38, P-80, and PV-1; Martin B 26; North American P-51 and B-25; Northrop P-61; Ryan FR-I; Vought OS2V; and Vultee A-35. Also, late in the war [87] period, a handling-qualities investigation was made on the Navy's K21 nonrigid airship. For this rather unique study, William M. Kauffman was project engineer.

 

AILERON FLUTTER

 

Of the airplanes just mentioned, the P-63 and P-80 were used for a number of investigations including a study of aileron flutter, one of the many dynamic phenomena associated with compressibility. Shock waves in the airflow over a wing were bad enough when they stood still, but unhappily they had a demoniacal tendency to oscillate back and forth at high speed and cause pulsating disturbances in the flow. The pulsations would sometimes shake the whole wing and airplane (this was called "buffeting") and sometimes they would cause the ailerons to buzz up and down in an alarming fashion. Shock-induced aileron buzz occurred only at high speed, and this fact added to the danger of any flight investigation of the phenomenon.

Flight studies of aileron buzz on the P-63 were conducted by John Spreiter and George Galster with George Cooper, a new member of the Ames staff, as pilot. No positive cure for the phenomenon was found, but the amplitude of the aileron motion was reduced through the use of an irreversible hydraulic unit in the lateral-control system. Investigation of aileron buzz on the P-80 was undertaken by a research team composed of Harvey Brown, George Rathert, and Lawrence Clousing. The Lockheed P-80 Shooting Star was the Nation's second jet fighter airplane. A remarkable airplane, produced in Kelly Johnson's "skunk works" in a nominal 80 days,1 the P-80 was sleek and simple-quite the antithesis of earlier Lockheed fighters. It was still subsonic, of course, yet quite the fastest thing on wings. And it was beset with an aileron-buzz problem.

In the tests of the P-80 at Ames, the aileron buzz seemed to grow in intensity as the speed increased; and, in pursuing the matter, there was some worry that at the highest speeds the aileron motions might incite a destructive wing flutter. In a steep dive such a development might be quite a bother For this reason, most pilots were exceedingly reluctant to dive the plane into these high-speed regimes of flight. But Larry Clousing, as we know, was different. Exhibiting his characteristic fearlessness, he dove the plane to speeds higher than man had ever reached before-to a Mach number of 0.866. The ailerons whipped violently but wing flutter did not occur. When the airplane was later examined, the left aileron was found to have a buckled trailing edge.

The study of the P-80 aileron buzz problem was pursued in the 16-foot tunnel even more intensively, if anything, than in flight. The 16-foot tunnel was ideally suited to this kind of investigation and, with Albert L. (Al) Er-....

 


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Lockheed P-80 with vortex generators installed ahead of ailerons to combat aileron buzz

Lockheed P-80 with vortex generators installed ahead of ailerons to combat aileron buzz

 

...-ickson as project engineer, made important contributions to the understanding of the buzz phenomenon. The urgency of the P-80 problem was such that Al frequently had Kelly Johnson and other Lockheed representatives looking over his shoulder as he sought its solution. Significantly, the P-80 project infused Al with a solid and continuing interest in unsteady aerodynamic phenomena and impressed him with the need for developing special instrumentation for future wind-tunnel investigations in this field.


1 Actually more like 140 days but still an extraordinary achievement.


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