The National Aeronautics and Space Administration has a long and storied history in the development of rotary-wing technology and in promoting the growth of the rotorcraft industry.
NASA celebrated the 100th anniversary of its predecessor organization, the National Advisory Committee for Aeronautics (NACA), in 2015. This year marks the 100th anniversary of the first NACA research center, now better known as the Langley Research Center, established at Langley Field in Hampton, Virginia, in 1917. Also in 2015, NASA observed the 50th anniversary of the NASA-Army agreement to work together on vertical-lift research. In this special 50th anniversary issue of R&WI, we highlight some of NASA’s significant research from the past 50 years and identify future opportunities.
Rotary-wing research at NACA provided the foundation for subsequent achievements by NASA. Research to understand vertical flight was conducted at the Langley Research Center almost from NACA’s inception. One of the earliest papers, written by NACA Chief Physicist Edward P. Warner, with an appendix by former NACA Chairman W. F. Durand, was NACA Technical Note No. 4 (TN-4), “The Problem Of The Helicopter,” published in 1920. It provided a mathematical treatment of autorotation. Engineers tested a rotor blade with fixed and adjustable pitch and self-feathering blades in 1922. Research continued at Langley throughout the 1920s and 30s, with the focus on basic research in aeromechanics, structures, propulsion and instrumentation. During the 1930s, much research focused on the autogyro concept. But as the industry began to emerge in later that decade, helicopter development captured more attention.
In late 1939, NACA established Ames Research Center at Moffett Field in California and, in 1941, what is now the Glenn Research Center at Lewis Field (originally the Aircraft Engine Research Laboratory) in Cleveland, Ohio. During the 1940s and 1950s, NACA conducted research to support the military in vertical-flight vehicles and also provided experimental and analytical research on fundamental areas, including control loads, ground resonance and blade dynamics. Engineers explored many concepts and configurations during that time and published their results in NACA technical reports.
Fundamental research in helicopters at NACA led to the 1952 publication of the seminal book “Aerodynamics of the Helicopter” by NACA researchers Alfred Gessow and Garry C. Myers, Jr. Helicopter research continued under NACA until 1958, when the name changed to NASA.
As helicopters developed into essential commercial and military vehicles, NASA and the U.S. Army entered into a formal agreement to cooperate on vertical-lift technology. The original pact was signed in 1965 and subsequent agreements have renewed the collaborative arrangement for more than 50 years, with collaborative research ongoing at Ames, Glenn and Langley.
Some of the major joint projects resulting from the NASA-Army partnership are described in a paper edited by G.K. Fischer, called “Advanced Power Transmission Technology.” One of the most successful examples of the NASA-Army collaboration was the development and demonstration of the XV-15 tiltrotor. The cooperation, together with Bell Helicopter, developed and flew the XV-15 demonstrator from 1971 to 1981. The success of this demonstration formed the basis for the current generation of tiltrotors in production and under development.
NASA contributed to many of the technologies that are embedded in current tiltrotors and helicopters. In many cases, NASA research allowed advancements in the industry or established methods that were applied to improve efficiency, performance and safety.
For example, the NASA Short-Haul Civil Tiltrotor (SHCT) Program was a seven-year effort (beginning in 1994) that focused on several elements: efficient, low-noise proprotors; low-noise terminal area approaches; contingency power, and technology integration. All of these elements were worked with the industry (Bell, Boeing, and Sikorsky) and focused on safe, quiet and efficient civil-tiltrotor transports.
From 2001 to 2004, under the theme of runway-independent aircraft, NASA sponsored industry studies of advanced rotorcraft configurations that had the potential of reducing airport congestion and flight delays.
A hallmark of NASA rotary-wing research is developing discipline analyses and validating the analyses with high-quality experimental data using unique NASA facilities.
For vertical lift, the primary facilities at Ames are the National Full-Scale Aerodynamic Complex (operated by USAF’s Arnold Engineering Development Complex, with 40-by-80-foot and 80-by-120-foot test sections), the Vertical Motion Simulator and supercomputing facilities.
At Langley, primary rotorcraft facilities include the 14-by-22-foot Subsonic Tunnel, Transonic Dynamics Tunnel, Landing and Impact Research gantry (LandIR), acoustics test laboratories and the mobile acoustic flight test capability.
At Glenn, the Icing Research Tunnel, engine test facilities and drive system test cells provide primary experimental capabilities for rotary-wing drivetrain and propulsion testing.
In addition to these unique facilities, NASA invests in advancing experimental measurement capabilities to provide high-quality validation data for advanced computational modeling. Some examples of these capabilities are optical techniques to capture flow measurements over very large areas, outdoor photogrammetry for impact testing, unsteady pressure-sensitive paint for rotor blades and infrared thermography techniques for rotor blades, turbine cascades and drive system components.
NASA’s acoustic analysis methods are routinely used in the industry. The methods have been validated using wind tunnel and flight test measurements. The acoustic flight test database represents many years of data collection for different commercial and military rotorcraft, acquired by NASA’s mobile acoustic flight test facility. Benchmark acoustic wind tunnel datasets include the Higher Harmonic Control Aeroacoustics Rotor Test (HART), an international effort led by NASA to measure, analyze and predict rotor noise and wake characteristics.
Past NASA-led research identified the source of noise that is now called blade-vortex interaction. Current work developing, demonstrating and validating optimization and noise-reduction methods will reduce the sound exposure level footprint of rotary-wing vehicles. This will be achieved through a combination of rotor/vehicle design, flight-operation methods and understanding the human response to rotorcraft noise.
Another example of a benchmark experiment is the combined flight and wind tunnel test of a full-scale UH-60A rotor. NASA and the Army jointly conducted a seminal experiment to measure airloads in flight on an instrumented Sikorsky UH-60 Black Hawk in 1993 to 1994. In 2010, the same full-scale rotor was tested in the National Full-Scale Aerodynamics Complex and included tests at extreme conditions beyond the flight-test envelope. These data have provided a wealth of information on the behavior of a rotor in dynamic stall, high-speed flight, level flight and high advance-ratio conditions that have pushed the capabilities of rotary-wing computational fluid dynamics (CFD) methods.
These methods for NASA rotary-wing research are the structured-grid solver OVERFLOW and the unstructured-grid solver FUN3D. Both have been demonstrated at very high fidelity for a number of configurations and flight conditions. They are continually improved by comparing calculations with measured rotor airloads, structural loads, performance and wake characteristics.
NASA research in composites began in the early 1970s and has allowed the expanded use of composites for rotor blades and fuselage structures. Research in fatigue, fracture and damage tolerance are ongoing efforts at NASA to advance the capability and understanding of composites under high-fatigue loading. Assessing the performance of composites under impact loading for crashworthiness and occupant protection has been an area of NASA research, in collaboration with the Army and the industry, since the mid-1980s.
While understanding composite component and structure behavior under impact is underway, much remains to be done in this area. NASA has been a major contributor to advancing the state of the art in full-spectrum crashworthiness, in large part due to the unique facility for impact research, the LandIR.
NASA’s contributions to engines and drive systems have produced advancements in these critical areas and led to significant improvements in fielded systems for commercial and military use. NASA-developed tools such as the CFD code APNASA and the National Propulsion System Simulator are key analysis tools for engine research. NASA and the Army have collaborated on development of high-altitude and high-load engine performance data and recently demonstrated, with industry partners, the ability to design an efficient power turbine over a wide operating range. In the drive system area, development of new transmission designs and cooling methods has significantly advanced the state of the art. Recently, NASA demonstrated two patented concepts for two-speed transmissions that may be applicable to the next generation of efficient, quiet vertical-lift vehicles.
Though physics-based discipline analyses remain important, an integrated multidisciplinary analysis is crucial for conceptual design. This is the first phase of the design process and requires rapid evaluation by multiple discipline tools of consistent fidelity. NASA has developed a state-of-the-art aircraft sizing code, NASA Design and Analysis of Rotorcraft (NDARC), that is currently used by the industry, academia and other government agencies.
NASA is aiming toward a conceptual design optimization process for vertical-takeoff-and-landing aircraft of any size and configuration, which is multidisciplinary, internally consistent, rigorously integrated and streamlined. This process will be an essential tool for analyzing the growing numbers of VTOL aircraft, manned and unmanned, that are expected to fill the skies.
NASA has recently released an updated Aeronautics Strategic Implementation Plan and related roadmaps. Vertical lift has an individual roadmap with a future that envisions many types and sizes of vertical-lift vehicles providing a multitude of services.
In the future, small vertical-lift vehicles might deliver packages, mid-size vehicles might provide urban air taxi services and large vehicles could transport many passengers or cargo across greater distances. To realize this vision, many barriers must be overcome. The roadmap outlines some of the most significant work that must be done to enable a future for vertical-flight vehicles that are publicly acceptable and environmentally beneficial.
As we celebrate R&WI’s 50th anniversary, the current generation of NASA engineers is encouraged by how far the rotorcraft industry has come and how many significant achievements have been accomplished. Though we have learned much from our history, there is an even more learning that lies ahead. In vertical lift, with so much potential, the future is very bright. RWI