Partners in Freedom

		Lockheed Martin C-141 Starlifter


Specifications Manufacturer Lockheed Date in service May 1964 Type Transport Crew Six Engine Pratt & Whitney TF33-P-7 Users U.S. Air Force (Active, Air National Guard, and Reserve) Dimensions Wingspan . . . . . . . . . . .160.0 ft Length . . . . . . . . . . . . . 168.3 ft Height . . . . . . . . . . . . . . 39.6 ft Wing area . . . . . . . . .3,228 sq ft Weight Empty . . . . . . . . . . .140,882 lb Gross . . . . . . . . . . . .343,000 lb Performance Max speed . . . . . Mach number of 0.83 Range . . . . . . . . . . . .2,550 n mi

		Highlights of Research by Langley for the C-141 Parametric tests in the Langley 16-Foot Transonic Dynamics Tunnel (TDT) of an isolated-tail model advanced the understanding of T-tail flutter at transonic conditions. Although the tail design did not flutter in initial TDT tests, the parametric tests identified several issues, including a large area of transonic flow separation at the juncture of the vertical and horizontal tails that promoted early flutter for some configurations. When an unpredicted elevator-induced flutter problem in flight was exhibited, TDT tests identified key factors and solutions. The isolated-tail tests permitted flutter clearance for a complete model of the C-141 in subsequent TDT tests. Langley developed a technique to test dynamically scaled models in TDT to assess an aileron reversal problem. Joint Langley and Air Force runway performance tests with a C-141 aircraft established grooving as an effective solution to hydroplaning accidents. The Lockheed (now Lockheed Martin) C-141was the first jet aircraft designed to meet military standards as a troop and cargo carrier. It was also the first military aircraft to be developed with a requirement for FAA type certification in the contract. The Starlifter is the workhorse of the Air Mobility Command. It fulfills a vast spectrum of airlift requirements through its ability to airlift combat forces over long distances, place those forces and their equipment either by conventional landings or airdrops, resupply employed forces, and extract the sick and wounded from a hostile area. The current C-141B is a stretched version of the original C-141A with in-flight refueling capability. The C-141B is about 23 ft longer than the C-141A, with cargo capacity increased by about one-third. The C-141 force, nearing seven million flying hours, has a proven reliability and long-range capability. Several critical flutter tests for the C-141 were conducted in the Langley 16-Foot Transonic Dynamics Tunnel (TDT). During these tests, parametric experimental and computational research was conducted on the flutter characteristics of T-tail configurations with full-span models and unique isolated-tail models of the C-141 configuration. Studies of the aeroelastic mechanisms responsible for inducing flutter led to significant improvements in the prediction and analysis of T-tail flutter at transonic conditions and the development of specific modifications to cure flow separation near the intersection of the vertical- and horizontal-tail surfaces. During early service, the C-141 exhibited tail flutter that was precipitated by large elevator deflections. This phenomenon was also replicated and analyzed in the TDT. Langley also conducted research in the TDT to assess tendencies of the C-141 to exhibit the aeroelastic phenomenon known as high-speed “aileron reversal”. The C-141 aircraft was a test subject during joint NASA and Air Force runway traction studies, which demonstrated the effectiveness of a runway grooving concept developed by Langley. Fifty grooved and ungrooved runway surfaces were tested and grooving was established as an effective solution to hydroplaning accidents. Subsequently, runways and highways worldwide were grooved for better traction in inclement weather.

		Langley Contributions to the C-141

Tail Flutter		In the 1950’s, several domestic and foreign high-performance aircraft configurations were developed that incorporated T-tail empennage configurations. Unfortunately, operational experiences with some of these aircraft indicated that tail flutter could be a critical problem for this configuration at high subsonic speeds. In July 1954, a British Handley Page Victor bomber experienced severe T-tail flutter and crashed during a low-altitude, high-speed flight test when the tail began wobbling and then tore off the aircraft, which dove into the ground. Noted aviation periodicals, including Aviation Week, stated that the accident raised doubts on the high placed T-tail configuration (ref. 6). In addition, jet-powered T-tail flying boats had been developed for the Navy that also experienced flutter problems. The staff of the Langley 16-Foot Transonic Dynamics Tunnel (TDT) was cognizant of the T-tail problem as the C-141 configuration entered the developmental process. The complex aeroelastic and aerodynamic factors that had allowed tail flutter to occur were discussed and researched in considerable depth by TDT staff members Frank T. Abbott, Charles L. Ruhlin, Maynard C. Sandford, and E. Carson Yates, Jr.. The Langley team and their peers recognized that the flutter mechanisms of T-tails were not well understood, particularly at critical transonic flight conditions where flutter margins were minimal. In addition, they identified certain geometric parameters, such as dihedral and sweep of the horizontal-tail surfaces, to be particularly critical. Carson Yates conducted extremely complex analytical calculations that identified the critical parameters and flutter modes for T-tails, and the results of his calculations highlighted the dramatic flutter challenge for transonic conditions. A formal request from the Air Force for flutter clearance tests of the C-141 in the Langley TDT was received. In view of the relatively poor understanding of T-tail flutter, the Air Force, Lockheed, and Langley team agreed to conduct special tests of a dynamically scaled model of the empennage and rear fuselage in the TDT. These tests permitted parametric studies and correlation with theory before the full-span model flutter clearance test. Flutter tests of this unique C-141 empennage model began in the TDT in late 1961. The design configuration did not flutter within the Mach number and dynamic pressure ranges tested. A matrix of structural parameters and test conditions was covered to determine the relative sensitivity of flutter to variations in physical features such as the stiffness of the horizontal-tail pitch trim actuator, stiffness of the fin spar, roll and yaw stiffness of fin-stabilizer joints, rotational stiffness of elevators and rudder, and stabilizer mass and yaw and roll. Flutter model of C-141 empennage in the Langley 16-Foot Transonic Dynamics Tunnel in 1961 with highly streamlined original bullet fairing at the juncture of the horizontal and vertical tails. C-141 model being prepared for flutter tests in the Transonic Dynamics Tunnel in 1962. During the isolated-tail tests, the NASA and industry team encountered an unexpected flow phenomenon on the tail at transonic speeds. The C-141 tail had been designed with a streamlined bullet-shaped fairing to improve airflow at the juncture of the vertical and horizontal tails. However, at transonic speeds the bullet shape caused adverse pressure gradients and shock-induced flow separation over the aft portion of the fin-stabilizer juncture. Tufts on the vertical tail illustrated the dynamic and massive flow separation regions as the tail fluttered over relatively large amplitudes. The flow separation acted as a forcing function to encourage certain flutter modes of the entire tail surface. Yates recommended a redesign of the bullet, including a nonstreamlined shape and a blunt “boat tail”. Vortex generators on the fin were also evaluated. With the vortex generators and bullet modifications, results of the test indicated that the vertical-tail flow separation was entirely eliminated and any tendency for flutter was pushed significantly beyond the flight envelope. The results of the isolated-tail model tests significantly increased the understanding of T-tail flutter prediction, and provided extremely valuable background for the upcoming flutter clearance test of the complete configuration. When the full-span flutter clearance model of the C-141 was tested in the TDT in 1962, no flutter was encountered within the flight envelope (flutter was pushed beyond 120 percent of design dive speed), and the value of the isolated-tail model tests was recognized by the participants. After the C-141 entered service, an unexpected unstable oscillation of the horizontal tail was experienced during high-altitude flight tests. The flutter was precipitated by a deflection of about 8 deg of the elevator relative to the stabilizer; no fix was found in flight tests. Analysis by Lockheed indicated that the flutter speed could be raised significantly by increasing the elevator mass balance, and this approach was used to eliminate the problem. This elevator-induced flutter mechanism was studied by Maynard Sandford and Charles Ruhlin with the isolated-tail model in the TDT, where they found that the flutter was reproduced by the model. The tests also verified the effectiveness of elevator mass balancing to eliminate tail flutter. In summary, Langley’s contribution to the C-141 program in the area of flutter generated extensive advances in experimental techniques and analytical methods for fundamental understanding of T-tail flutter characteristics. The highly successful flutter studies of the C-141 resulted in significant advances in the development of T- tail configurations.
Aileron Reversal		Evaluations of the C-141 indicated that for certain flight conditions the aircraft experienced the phenomenon known as aileron reversal. This phenomenon occurs when a deflection for roll control of the aileron at the wing trailing edge results in aeroelastic twisting of the wing to the extent that the control effectiveness is nullified or actually reversed. That is, a pilot’s input to intentionally roll the aircraft results in little or no response. In some cases, the effects of aileron reversal can actually roll the aircraft in the direction opposite to that intended by the pilot. Accurate predictions of this phenomenon require precise estimates of the aerodynamic behavior of the wing and aileron system under dynamic conditions and at high speeds. The difficulty of estimating aerodynamics at transonic conditions made this problem a significant challenge for the designers of emerging large, flexible transports such as the C-141 and C-5. C-141 model in Transonic Dynamics Tunnel for aileron reversal tests in 1966. Langley researcher Irving Abel developed an innovative experimental technique to evaluate aileron reversal tendencies with an aeroelastically scaled C-141 model in the TDT during 1964 and 1966. Abel used a cable-mount system in the TDT that had been developed by Wilmer H. Reed, Jr. and Frank Abbott to permit more realistic flutter tests by allowing the structural modes of the models to be simulated in free-flight conditions. Abel’s results for the C-141 model agreed well with flight results and provided a new test method to the capabilities of the TDT.
Runway Grooving		In the 1950’s, Langley researchers at the Langley Aircraft Landing Dynamics Facility focused their research on aircraft braking and directional control performance on wet runway surfaces. This effort was led by Langley researchers Walter Horne and Thomas J. Yager. The phenomenon of tire hydroplaning was identified as a contributory factor for unsatisfactory tire traction on wet runways. The associated losses in tire traction in water were defined for a variety of test parameters. A technique that appeared promising for improved water drainage at the tire and pavement interface was to modify the pavement surface with slots, or grooves, similar to the grooves in aircraft tire treads. Basic parametric research on the groove concept was conducted at the Aircraft Landing Dynamics Facility. The very positive results led to full-scale, instrumented aircraft tests at the landing research runway at the NASA Wallops Flight Facility in 1968. In a joint program with the Air Force, Langley evaluated the effects of 50 grooved and ungrooved runway surfaces on the braking performance of a C-141 aircraft. On the basis of these test results and other aircraft evaluations, the application of grooving to runways and the nation’s highways has been accepted as an efficient means for minimizing wet pavement skidding accidents. C-141 braking tests in a joint NASA, Air Force, and FAA program at Wallops Flight Facility.
Wind Tunnel and Flight Correlation		During early flight tests of the C-141, the wing pressures and pitching moments were found to be considerably different from those predicted in wind-tunnel tests at supercritical Mach numbers. These discrepancies resulted in several national efforts to establish reasons for the differences, with emphasis on wind-tunnel test procedures. At Langley, Dr. Richard T. Whitcomb participated in the analysis of the data discrepancy and concluded that wind-tunnel scale effects were the problem. Under the direction of Whitcomb, Donald L. Loving led a study in 1965 that focused on the impact of shock-induced boundary-layer separation. Differences in the relative magnitudes of shock-induced flow separation resulted in large differences in aerodynamic loads on the wing. Unfortunately, the phenomenon was difficult to model and extremely complex. The problem was differences between the relative thickness of boundary layers on models and full-scale aircraft. The importance of artificially locating transition on a model to produce the same relative boundary-layer thickness at the trailing edge of the wing was highlighted. However, locating the transition point at the appropriate place on the model proved to be more an art than a science. The major recommendation from the study was to conduct wind-tunnel tests for several artificial positions of transition to evaluate the sensitivity of shock-induced separation to modifications of the boundary-layer conditions. Later, James A. (Micky) Blackwell, Jr. of Langley contributed an approach to scaling and transition-fixing that was accepted as producing accurate simulations for wind-tunnel practice. C-141 model with dorsal strut and lower strut support in the 8-Foot Transonic Pressure Tunnel for drag correlation studies. In a tunnel-to-flight study, Langley awarded a contract to Lockheed in 1970 to conduct analytical studies on drag estimation for the C-141 and to obtain a set of fully corrected wind-tunnel data on a 0.0275-scale C-141 model in the Langley 8-Foot Transonic Pressure Tunnel. Lockheed’s experience in the C-5 program with new transition-fixing techniques and model support systems was applied to the C-141 test data to obtain the accuracy required for the study. Model support interference corrections were evaluated through a systematic series of tests, and the fully corrected model data were analyzed to provide details of the model component interference factors. The results of the investigation indicated that the predicted, subcritical minimum profile drag of the complete C-141 configuration was within 0.7 percent of flight-test data.