NASA's Contributions to Aeronautics, Volume 2 by National Aeronautics & Space Administration. - HTML preview

CASE

8

NACA-NASA’s Contribution to General Aviation

By Weneth D. Painter

General Aviation has always been an essential element of American aeronautics. The NACA and NASA have contributed greatly to its efficiency, safety, and reliability via research across many technical disciplines. The mutually beneficial bonds linking research in civil and military aeronautics have resulted in such developments as the supercritical wing, electronic flight controls, turbofan propulsion, composite structures, and advanced displays and instrumentation systems.

Case-8 Cover Image: NASA Beech King Air general aviation aircraft over the Dryden Flight Research Center. NASA.

Though commonly associated in the public mind with small private aircraft seen buzzing around local airports and air parks, the term “General Aviation” (hereafter GA) is primarily a definition of aircraft utilization rather than a classification per se of aircraft physical characteristics or performance. GA encompasses flying machines ranging from light personal aircraft to Mach 0.9+ business jets, comprising those elements of U.S. civil aviation which are neither certified nor supplemental air carriers: kit planes and other home-built aircraft, personal pleasure aircraft, commuter airlines, corporate air transports, aircraft manufacturers, unscheduled air taxi operations, and fixed-base operators and operations.

Overall, NACA-NASA’s research has profoundly influenced all of this, contributing notably to the safety and efficiency of GA worldwide. Since the creation of the NACA in 1915, and continuing after establishment of NASA in 1958, Agency engineers have extensively investigated design concepts for GA, GA aircraft themselves, and the operating environment and related areas of inquiry affecting the GA community. In particular, they have made great contributions by documenting the results of various wind tunnel and flight tests of GA aircraft. These results have strengthened both industrial practice within the GA industry itself and the educational training of America’s science, technology, engineering, and mathematics workforce, helping buttress and advance America’s stature as an aerospace nation. This study discusses the advancements in GA through a review of selected applications of flight disciplines and aerospace technology.

The Early Evolution of General Aviation

The National Advisory Committee for Aeronautics (NACA) was formed on March 3, 1915, to provide advice and carry out much of cutting-edge research in aeronautics in the United States. This organization was modeled on the British Advisory Committee for Aeronautics. President Woodrow Wilson created the advisory committee in an effort to organize American aeronautical research and raise it to the level of European aviation. Its charter and $5,000 initial appropriation (low even in 1915) were appended to a naval appropriations bill and passed with little fanfare. The committee’s mission was “to supervise and direct the scientific study of the problems of flight, with a view to their practical solution,” and to “direct and conduct research and experiment in aeronautics.”[1] Thus, from its outset, it was far more than simply a bureaucratic panel distanced from design-shop, laboratory, and flight line.

The NACA soon involved itself across the field of American aeronautics, advising the Government and industry on a wide range of issues including establishing the national air mail service, along with its night mail operations, and brokering a solution—the cross-licensing of aeronautics patents—to the enervating Wright-Curtiss patent feud that had hampered American aviation development in the pre-World War I era and that continued to do so even as American forces were fighting overseas. The NACA proposed establishing a Bureau of Aeronautics in the Commerce Department, granting funds to the Weather Bureau to promote safety in aerial navigation, licensing of pilots, aircraft inspections, and expanding airmail. It also made recommendations in 1925 to President Calvin Coolidge’s Morrow Board that led to passage of the Air Commerce Act of 1926, the first Federal legislation regulating civil aeronautics. It continued to provide policy recommendations on the Nation’s aviation until its incorporation in the National Aeronautics and Space Administration (NASA) in 1958.[2]

The NACA started working in the field of GA almost as soon as it was established. Its first research airplane programs, undertaken primarily by F.H. Norton, involved studying the flight performance, stability and control, and handling qualities of Curtiss JN-4H, America’s iconic “Jenny” of the “Great War” time period, and one that became first great American GA airplane as well.[3] The initial aerodynamic and performance studies of Dr. Max M. Munk, a towering figure in the history of fluid mechanics, profoundly influenced the Agency’s subsequent approach to aerodynamic research. Munk, the inventor of the variable-density wind tunnel (which put NACA aerodynamics research at the forefront of the world standard) and architect of American aerodynamic research methodology, dramatically transformed the Agency’s approach to airfoil design by introducing the methods of the “Prandtl school” at Göttingen and by designing and supervising the construction of a radical new form of wind tunnel, the so-called “variable density tunnel.” His GA influence began with a detailed study of the airflow around and through a biplane wing cellule (the upper and lower wings, connected with struts and wires, considered as a single design element). He produced a report in which the variation of the section, chord, gap, stagger, and decalage (the angle of incidence of the respective chords of the upper and lower wings) and their influence upon the available wing cell space for engines, cockpits, passenger, and luggage, were investigated with a great number of calculated examples in which all of the numerical results were given in tables. Munk’s report was in some respects a prototypical example of subsequent NACA-NASA research reports that, over the years, would prove beneficial to the development of GA by investigating a number of areas of particular concern, such as aircraft aerodynamic design, flight safety, spin prevention and recoveries, and handling qualities.[4] Arguably these reports that conveyed Agency research results to a public audience were the most influential product of NACA-NASA research. They influenced not only the practice of engineering within the various aircraft manufacturers, but provided the latest information incorporated in many aeronautical engineering textbooks used in engineering schools.

Though light aircraft are often seen as the by-product of the air transport revolution, in fact, they led, not followed, the expansion of commercial aviation, particularly in the United States. The interwar years saw an explosive growth in American aeronautics, particularly private flying and GA. It is fair to state that the roots of the American air transport revolution were nurtured by individual entrepreneurs manufacturing light aircraft and beginning air mail and air transport services, rather than (as in Europe) largely by “top-down” government direction. As early as 1923, American fixed-base operators “carried 80,888 passengers and 208,302 pounds of freight.”[5] In 1926, there were a total of 41 private airplanes registered with the Federal Government. Just three years later, there were 1,454. The Depression severely curtailed private ownership, but although the number of private airplanes plummeted to 241 in 1932, it rose steadily thereafter to 1,473 in 1938, with Wichita, KS, emerging as the Nation’s center of GA production, a distinction it still holds.[6]

Two of the many notable NACA-NASA engineers who were influenced by their exposure to Max Munk and had a special interest in GA, and who in turn greatly influenced subsequent aircraft design, were Fred E. Weick and Robert T. Jones. Weick arrived at NACA Langley Field, VA, in the 1920s after first working for the U.S. Navy’s Bureau of Aeronautics.[7] Weick subsequently conceived the NACA cowling that became a feature of radial-piston-engine civil and military aircraft design. The cowling both improved the cooling of such engines and streamlined the engine installation, reducing drag and enabling aircraft to fly higher and faster.

In late fall of 1934, Robert T. Jones, then 23 years old, started a temporary, 9-month job at Langley as a scientific aide. He would remain with the Agency and NASA afterwards for the next half-century, being particularly known for having independently discovered the benefits of wing sweep for transonic and supersonic flight. Despite his youth, Jones already had greater mathematical ability than any other of his coworkers, who soon sought his expertise for various theoretical analyses. Jones was a former Capitol Hill elevator operator and had previously been a designer for the Nicholas Beazley Company in Marshall, MO. The Great Depression collapsed the company and forced him to seek other employment. His work as an elevator operator allowed him to hone his mathematical abilities gaining him the patronage of senior officials who arranged for his employment by the NACA.[8]

Jones and Weick formed a fruitful collaboration, exemplified by a joint report they prepared on the status of NACA lateral control research. Two things were considered of primary importance in judging the effectiveness of different control devices: the calculated banking and yawing motion of a typical small airplane caused by control deflection, and the stick force required to produce this control deflection. The report included a table in which a number of different lateral control devices were compared.[9] Unlike Jones, Weick eventually left the NACA to continue his work in the GA field, producing a succession of designs emphasizing inherent stability and stall resistance. His research mirrored Federal interest in developing cheap, yet safe, GA aircraft, an effort that resulted in a well-publicized design competition by the Department of Commerce that was won by the innovative Stearman-Hammond Model Y of 1936. Weick had designed a contender himself, the W-1, and though he did not win, his continued research led him to soon develop one of the most distinctive and iconic “safe” aircraft of all time, his twin-fin and single-engine Ercoupe. It is perhaps a telling comment that Jones, one of aeronautics’ most profound scientists, himself maintained and flew an Ercoupe into the 1980s.[10]

The NACA-NASA contributions to GA have come from research, development, test, and evaluation within the classic disciplines of aerodynamics, structures, propulsion, and controls but have also involved functional areas such as aircraft handling qualities and aircrew performance, aviation safety, aviation meteorology, air traffic control, and education and training. The following are selected examples of such work, and how it has influenced and been adapted, applied, and exploited by the GA community.

Airfoil Evolution and Its Application to General Aviation

In the early 1930s, largely thanks to the work of Munk, the NACA had risen to world prominence in airfoil design, such status evident when, in 1933, the Agency released a report cataloging its airfoil research and presenting a definitive guide to the performance and characteristics of a wide range of airfoil shapes and concepts. Prepared by Eastman N. Jacobs, Kenneth E. Ward, and Robert M. Pinkerton, this document, TR-460, became a standard industry reference both in America and abroad.[11] The Agency, of course, continued its airfoil research in the 1930s, making notable advances in the development of high-speed airfoil sections and low-drag and laminar sections as well. By 1945, as valuable as TR-460 had been, it was now outdated. And so, one of the most useful of all NACA reports, and one that likewise became a standard reference for use by designers and other aeronautical engineers in airplane airfoil/wing design, was its effective replacement prepared in 1945 by Ira H. Abbott, Albert E. von Doenhoff, and Louis S. Stivers, Jr. This study, TR-824, was likewise effectively a catalog of NACA airfoil research, its authors noting (with justifiable pride) that

Recent information of the aerodynamic characteristics of NACA airfoils is presented. The historical development of NACA airfoils is briefly reviewed. New data are presented that permit the rapid of the approximate pressure distribution for the older NACA four-digital and five-digit airfoils, by the same methods used for the NACA 6-series airfoils. The general methods used to derive the basic thickness forms for NACA 6 and 7 series airfoils together with their corresponding pressure distributions are presented. Detailed data necessary for the application of the airfoils to wing design are presented in supplementary figures placed at the end of the paper. This report includes an analysis of the lift, drag, pitching moment, and critical-speed characteristics of the airfoils, together with a discussion of the effects of surface conditions available data on high-lift devices. Problems associated with the later-control devices, leading edge air intakes, and interference is briefly discussed, together with aerodynamic problems of application.[12]

While much of this is best remembered because of its association with the advanced high-speed aircraft of the transonic and supersonic era, much was as well applicable to new, more capable civil transport and GA designs produced after the war.

Two key contributions to the jet-age expansion of GA were the supercritical wing and the wingtip winglet, both developments conceived by Richard Travis Whitcomb, a legendary NACA-NASA Langley aerodynamicist who was, overall, the finest aeronautical scientist of the post-Second World War era. More comfortable working in the wind tunnel than sitting at a desk, Whitcomb first gained fame by experimentally investigating the zero lift drag of wing-body combinations through the transonic flow regime based on analyses by W.D. Hayes.[13] His resulting “Area Rule” for transonic flow represented a significant contribution to the aerodynamics of high-speed aircraft, first manifested by its application to the so-called “Century series” of Air Force jet fighters.[14] Whitcomb followed area rule a decade later in the 1960s and derived the supercritical wing. It delayed the sharp drag rise associated with shock wave formation by having a flattened top with pronounced curvature towards its trailing edge. First tested on a modified T-2C jet trainer, and then on a modified transonic F-8 jet fighter, the supercritical wing proved in actual flight that Whitcomb’s concept was sound. This distinctive profile would become a key design element for both jet transports and high-speed GA aircraft in the 1980s and 1990s, offering a beneficial combination of lower drag, better fuel economy, greater range, and higher cruise speed exemplified by its application on GA aircraft such as the Cessna Citation X, the world’s first business jet to routinely fly faster than Mach 0.90.[15]

The application of Whitcomb’s supercritical wing to General Aviation began with the GA community itself, whose representatives approached Whitcomb after a Langley briefing, enthusiastically endorsing his concept. In response, Whitcomb launched a new Langley program, the Low-and-Medium-Speed Airfoil Program, in 1972. This effort, blending 2-D computer analysis and tests in the Langley Low-Turbulence Pressure Tunnel, led to development of the GA(W)-1 airfoil.[16] The GA(W)-1 employed a 17-percent-thickness-chord ratio low-speed airfoil, offering a beneficial mix of low cruise drag, high lift-to-drag ratios during climbs, high maximum lift properties, and docile stall behavior.[17] Whitcomb’s team generated thinner and thicker variations of the GA(W)-1 that underwent its initial flight test validation in 1974 on NASA Langley’s Advanced Technology Light Twin (ATLIT) engine airplane, a Piper PA-34 Seneca twin-engine aircraft modified to employ a high-aspect-ratio wing with a GA(W)-1 airfoil with winglets. Testing on ATLIT proved the practical advantages of the design, as did subsequent follow-on ground tests of the ATLIT in the Langley 30 ft x 60 ft Full-Scale-Tunnel.[18]

Subsequently, the NASA-sponsored General Aviation Airfoil Design and Analysis Center (GA/ADAC) at the Ohio State University, led by Dr. Gerald M. Gregorek, modified a single-engine Beech Sundowner light aircraft to undertake a further series of tests of a thinner variant, the GA(W)-2. GA/ADAC flight tests of the Sundowner from 1976–1977 confirmed that the Langley results were not merely fortuitous, paving the way for derivatives of the GA(W) family to be applied to a range of new aircraft designs starting with the Beech Skipper, the Piper Tomahawk, and the Rutan VariEze.[19]

Following on the derivation of the GA(W) family, NASA Langley researchers, in concert with industry and academic partners, continued refinement of airfoil development, exploring natural laminar flow (NLF) airfoils, previously largely restricted to exotic, smoothly finished sailplanes, but now possible thanks to the revolutionary development of smooth composite structures with easily manufactured complex shapes tailored to the specific aerodynamic needs of the aircraft under development.[20] Langley researchers subsequently blended their own conceptual and tunnel research with a computational design code developed at the University of Stuttgart to generate a new natural laminar flow airfoil section, the NLF(1).[21] Like the GA(W) before it, it served as the basis for various derivative sections. After flight testing on various testbeds, it was transitioned into mainstream GA design beginning with a derivative of the Cessna Citation II in 1990. Thereafter, it has become a standard feature of many subsequent aircraft.[22]

The second Whitcomb-rooted development that offered great promise in the 1970s was the so-called winglet.[23] The winglet promised to dramatically reduce energy consumption and reduce drag by minimizing the wasteful tip losses caused by vortex flow off the wingtip of the aircraft. Though reminiscent of tip plates, which had long been tried over the years without much success, the winglet was a more refined and better-thought-out concept, which could actually take advantage of the strong flow-field at the wingtip to generate a small forward lift component, much as a sail does. Primarily, however, it altered the span-wise distribution of circulation along the wing, reducing the magnitude and energy of the trailing tip vortex. First to use it was the Gates Learjet Model 28, aptly named the “Longhorn,” which completed its first flight in August 1977. The Longhorn had 6 to 8 percent better range than previous Lears.[24]

The winglet was experimentally verified for large aircraft application by being mounted on the wing tips of a first-generation jet transport, the Boeing KC-135 Stratotanker, progenitor of the civil 707 jetliner, and tested at Dryden from 1979–1980. The winglets, designed with a general-purpose airfoil that retained the same airfoil cross-section from root to tip, could be adjusted to seven different cant and incidence angles to enable a variety of research options and configurations. Tests revealed the winglets increased the KC-135’s range by 6.5 percent—a measure of both aerodynamic and fuel efficiency—better than the 6 percent projected by Langley wind tunnel studies and consistent with results obtained with the Learjet Longhorn. With this experience in hand, the winglet was swiftly applied to GA aircraft and airliners, and today, most airliners, and many GA aircraft, use them.[25]

The Propulsion Perspective

Aerodynamics always constituted an important facet of NACA-NASA GA research, but no less significant is flight propulsion, for the aircraft engine is often termed the “heart” of an airplane. In the 1920s and 1930s, NACA research by Fred Weick, Eastman Jacobs, John Stack, and others had profoundly influenced the efficiency of the piston engine-propeller-cowling combination.[26] Agency work in the early jet age had been no less influential upon improving the performance of turbojet, turboshaft, and turbofan engines, producing data judged “essential to industry designers.”[27]

The rapid proliferation of turbofan-powered GA aircraft—over 2,100 of which were in service by 1978, with 250 more being added each year—stimulated even greater attention.[28] NASA swiftly supported development of a specialized computer-based program for assessing engine performance and efficiency. In 1977, for example, Ames Research Center funded development of GASP, the General Aviation Synthesis Program, by the Aerophysics Research Corporation, to compute propulsion system performance for engine sizing and studies of overall aircraft performance. GASP consisted of an overall program routine, ENGSZ, to determine appropriate fanjet engine size, with specialized subroutines such as ENGDT and NACDG assessing engine data and nacelle drag. Additional subroutines treated performance for propeller powerplants, including PWEPLT for piston engines, TURBEG for turboprops, ENGDAT and PERFM for propeller characteristics and performance, GEARBX for gearbox cost and weight, and PNOYS for propeller and engine noise.[29]

Such study efforts reflected the increasing numbers of noisy turbine-powered aircraft operating into over 14,500 airports and airfields in the United States, most in suburban areas, as well as the growing cost of aviation fuel and the consequent quest for greater engine efficiency. NASA had long been interested in reducing jet engine noise, and the Agency’s first efforts to find means of suppressing jet noise dated to the late NACA in 1957. The needs of the space program had necessarily focused Lewis research primarily on space, but it returned vigorously to air-breathing propulsion at the conclusion of the Apollo program, spurred by the widespread introduction of turbofan engines for military and civil purposes and the onset of the first oil crisis in the wake of the 1973 Arab-Israeli War.

Out of this came a variety of cooperative research efforts and programs, including the congressionally mandated ACEE program (for Aircraft Engine Efficiency, launched in 1975), the NASA-industry QCSEE (for Quiet Clean STOL Experimental Engine) study effort, and the QCGAT (Quiet Clean General Aviation Turbofan) program. All benefited future propulsion studies, the latter two particularly so.[30]

QCGAT, launched in 1975, involved awarding initial study contracts to Garrett AiResearch, General Electric, and Avco Lycoming to explore applying large turbofan technology to GA needs. Next, AiResearch and Avco were selected to build a small turbofan demonstrator engine suitable for GA applications that could meet stringent noise, emissions, and fuel consumption standards using an existing gas-generating engine core. AiResearch and Avco took different approaches, the former with a high-thrust engine suitable for long-range high-speed and high altitude GA aircraft (using as a baseline a stretched Lear 35), and the latter with a lower-thrust engine for a lower, slower, intermediate-range design (based upon a Cessna Citation I). Subsequent testing indicated that each company did an excellent job in meeting the QCGAT program goals, each having various strengths. The Avco engine was quieter, and both engines bettered the QCQAT emissions goals for carbon monoxide and unburned hydrocarbons. While the Avco engine was “right at the goal” for nitrous oxide emissions, the AiResearch engine was higher, though much better than the baseline TFE-731-2 turbofan used for comparative purposes. While the AiResearch engine met sea-level takeoff and design cruise thrust goals, the Avco engine missed both, though its measured numbers were nevertheless “quite respectable.” Overall, NASA considered that the QCGAT program, executed on schedule and within budget, constituted “a very successful NASA joint effort with industry,” concluding that it had “demonstrated that noise need not be a major constraint on the future growth of the GA turbofan fleet.”[31] Subsequently, NASA launched GATE (General Aviation Turbine Engines) to explore other opportunities for the application of small turbine technology to GA, awarding study contracts to AiResearch, Detroit Diesel Allison, Teledyne CAE, and Williams Research.[32] GA propulsion study efforts gained renewed impetus through the Advanced General Aviation Transport Experiment (AGATE) program launched in 1994, which is discussed later in this study.

Understanding GA Aircraft Behavior and Handling Qualities<