Spurred on by postwar interests in the variable wing-sweep concept as a means to optimize mission performance at both low and high speeds, the NACA at Langley initiated a broad research program to iden-tify the potential benefits and problems associated with the concept.47 Early exploratory wind tunnel studies of a modified X-1 model have already been mentioned. The disappointing experiences of the Bell X-5 research aircraft, which used a single wing pivot to achieve variable sweep in the early 1950s, had clearly identified the unacceptable weight penalties associated with the concept of translating the wing along the fuselage centerline to maintain satisfactory levels of longitudinal stability while the wing-sweep angle was varied from forward to aft sweep. After the X-5 experience, military interest in variable sweep quickly dimin-ished, while the NACA continued to explore alternate concepts that might permit variations in wing sweep without moving the pivot location and without serious degradation in longitudinal stability and control.
After years of intense research and wind tunnel testing, Langley researchers con-ceived a promising concept known as the outboard pivot.48 The basic principle of the NASA solution was to pivot the movable wing panels at two outboard pivot loca-tions on a fixed inner wing and share the lift between the fixed portion of the wing and the movable outer wing panel, thereby minimizing the longitudinal movement of the aerodynamic center of lift for various flight speeds. As the concept was matured in configuration studies and supporting tests, refined designs were continually submitted to intense evaluations in tun-nels across the speed range from super-sonic cruise conditions to subsuper-sonic take-off and landing.
Free-flight model of outboard pivot variable-sweep model used in research studies at Langley. Conceived by researchers at the Langley 7- by 10-Foot High-Speed Tunnel, this configuration was tested across the speed range, including low-speed flight tests in the Full-Scale Tunnel.
69 The use of dynamically scaled free-flight models to evaluate the stability and control characteristics of variable-sweep configurations was an ideal application of the testing technique. Because variable-sweep designs are capable of an infinite number of wing-sweep angles between the forward and aft positions, the number of conventional wind tunnel force tests required to document stability and control variations with wing sweep for every sweep angle could become unacceptable. In contrast, a free-flight model with con-tinually variable wing sweep angles could be used to quickly examine qualitative characteristics as its geometry changed, leading to rapid identification of problems. Free-flight model demonstrations of a con-figuration based on a proposed Navy combat air patrol (CAP) mission in the Full-Scale Tunnel provided a convincing demonstration that the outboard pivot was ready for applications. This fundamental research program provided key guidance for the design of fighters and bombers for the U.S. military, including the General Dynamics F-111 and the Rockwell B-1 for the Air Force, and the Grumman F-14 for the Navy. Dur-ing the early configuration development of these variable-sweep aircraft, Langley conducted free-flight, spin tunnel, and outdoor drop-model tests of dynamically scaled models of each configuration at the request of DOD to evaluate dynamic stability and control, spin and recovery, and spin resistance characteristics.
Flight test of a skewed wing free-flight model with the wing swept 40 degrees in the Free-Flight Tunnel in 1946. R.T. Jones and John P. Campbell initiated these exploratory tests to assess the flight behavior of the asymmetric con-figuration and were the first flight tests of the skewed wing concept in the U.S.
aeronautical community.
The NACA and NASA have explored other approaches to providing the aerody-namic benefits of variable wing sweep. The oblique wing concept (sometimes referred to as the “switchblade wing” or “skewed wing”) had originated in the German design studies of the Blom & Voss P202 jet air-craft during World War II and was pursued at Langley by Robert T. Jones. Oblique wing designs use a single-pivot, all-mov-ing wall-mov-ing to achieve variable sweep in an asymmetrical fashion. The wing is tioned in the conventional unswept posi-tion for take-off and landings, and it is rotated about its single pivot point for high-speed flight. As part of a general research effort that included theoretical aerodynamic studies and conventional wind tunnel tests, a free-flight investigation of the dynamic stability and control of a simplified model was conducted in the Free-Flight Tunnel in 1946.49 This research
on the asymmetric swept wing actually predated NACA wind tunnel research on symmetrical variable sweep concepts with the X-1 model.50 The test objectives were to determine whether such a radical aircraft configu-ration would exhibit satisfactory stability characteristics and remain controllable in the swept wing asymmet-ric state at low-speed flight conditions. The results of the tests, which were the first U.S. flight studies of oblique wings, showed that the wing could be swept as much as 40 degrees without significant degradation in behavior. However, when the sweep angle was increased to 60 degrees, an unacceptable longitudinal trim
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
70 MODELING FLIGHT
change was experienced, and a severe reduction in lateral control occurred at moderate and high angles of attack. Nonetheless, the results obtained with the simple free-flight model provided optimism that the uncon-ventional oblique wing concept might be adaptable.
R.T. Jones transferred to the NACA Ames Aeronautical Laboratory in 1947 and continued his brilliant career, which included a continuing interest in the application of oblique wing technology. In the early 1970s, the scope of NASA studies on potential civil supersonic transport configurations included an effort by an Ames team headed by Jones that examined a possible oblique wing version of the Supersonic Transport (SST). Although wind tunnel testing was conducted at Ames, the demise and cancellation of the American SST program in the early 1970s terminated this activity. Wind tunnel and computational studies of oblique wing designs continued at Ames throughout the 1970s for subsonic, transonic, and supersonic flight appli-cations.51 Jones participated in flight tests of several oblique wing, radio-controlled models, and a joint Ames-Dryden project was initiated to use the remotely
piloted Oblique Wing Research Aircraft (OWRA) for studies of the aerodynamic characteristics and control requirements to achieve satisfactory handling quali-ties. The wing of the OWRA could be skewed 45 degrees (left wing forward), and the vehicle was tested in the Ames 40- by 80-Foot Tunnel during the develop-mental program. A successful flight program at Dryden provided critical information for a follow-on piloted demonstrator.
The AD-1 oblique wing demonstrator in flight at the Dryden Flight Research Center in 1981. The aircraft completed 79 research missions.
Growing interest in the oblique wing and the suc-cess of the OWRA remotely piloted vehicle project led to the design and low-speed flight demonstrations of a full-scale research aircraft known as the AD-1 in the late 1970s. Designed as a low-cost demonstrator, the radical AD-1 proved to be a showstopper during air shows and generated considerable public interest.
The flight characteristics of the AD-1 were satisfactory for wing-sweep angles of less than about 45 degrees, but the handling qualities degraded for higher values of sweep, in agreement with the earlier Langley exploratory free-flight model study.
Stanford University supported the NASAAmes Research Center with radio-controlled, free-flight model testing of several oblique wing models. Here, a 20-foot-span oblique flying wing model powered by two ducted fans is undergoing flight evaluations.
After his retirement from NASA in 1981, Jones con-tinued his interest in supersonic oblique wing trans-port configurations. When the NASA High Speed Research program to develop technologies necessary for a viable Supersonic Transport began in the 1990s, several industry teams revisited the oblique wing.
Ames sponsored free-flight, radio-controlled model
71 studies of oblique wing configurations at Stanford University in the early 1990s.52 The first Stanford model of a radical oblique flying wing (OFW) design was a propeller-powered, 10-foot span with wing sweep capa-bility for angles between 25 and 65 degrees. A second free-flight model had a 20-foot span and was designed to be a 0.05-scale model of a 400-passenger aircraft. Powered by two ducted fans, the model instrumentation included a three-axis rate gyro, angle-of-attack and sideslip vanes, and a turbine airspeed indicator. After a preflight developmental process mounted to a three-degree-of-freedom rig atop an auto-mobile, the model was flown in 1994 with a sweep-angle variation from 35 to 50 degrees.
As a result of free-flight model contributions from Langley, Ames, Dryden, and academia, major issues regarding potential dynamic stability and control problems for oblique wing configurations have been addressed for low-speed conditions. Unfortunately, funding for transonic and supersonic model flight stud-ies has not been forthcoming, and high-speed studstud-ies have not yet been accomplished. A 2006 oblique flying wing contract to Northrop Grumman by the Defense Advanced Research Projects Agency (DARPA) was initiated to mature the technology to achieve high cruise efficiency, long endurance, and extended low-speed loiter capability. The program objectives included the development and flight of a small-scale super-sonic technology demonstrator X-plane known as Switchblade, and wind tunnel testing was underway in 2007. In October 2008, DARPA canceled the project after the preliminary design effort.
Launch and deployment of a foldout wing demonstrator model at the NASA Dryden Flight Research Center using an unpowered test model and the mother ship technique.
Other examples of free-flight models used to explore the effects of variable geometry on flight character-istics include the use of foldout wings and lifting surfaces and extensible stabilizing surfaces. For example, in 2001, Dryden researchers used Dale Reed’s mother ship technique to launch an unpowered research
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
72 MODELING FLIGHT
model that demonstrated a deployable, inflatable wing.53 Known as the I2000 configuration, the instru-mented research model was air-launched from a larger radio-controlled model at an altitude of about 1,000 feet, and its deployable wings were inflated by an onboard nitrogen system. Successful flights were con-ducted with the model demonstrating satisfactory stability during the wing deployment from the stowed position on three-drop operations, and parameter estimation procedures were used to extract aerodynamic properties of the model.
Arguably, the most challenging NASA application of free-flying models to variable geometry requirements has been a result of interplanetary exploration interests.54 The age-old dream of using remotely piloted aircraft to explore Mars continually resurfaces as an attractive alternative to land rovers or orbiting satellites.
An aircraft flying about 1 mile above the Martian surface could obtain high-resolution mapping over a 400-mile distance with navigation to specific areas. In addition to the challenges in developing this capability within the areas of propulsion, structures, and aerodynamics, an approach must be devised to design and construct a stowable aircraft aboard an interplanetary rocket and deploy it into conventional flight. Over the past 40 years, NASA, its industry partners, and universities have addressed a majority of these concerns through mission studies, wind tunnel tests, propulsion and propulsion integration studies, and vehicle design studies. The concept calls for delivery of the folded aircraft to Mars within a protective aeroshell aboard a spacecraft. After the aeroshell separates from the carrier, it enters the Martian atmosphere, where it begins to decelerate, and a parachute deployment provides additional deceleration. After a heat shield on the aeroshell is released, the stowed aircraft is released and unfolded, the pullout maneuver to conven-tional flight is completed, and the vehicle begins its exploration mission.
NASA–industry test team poses with the Mars airplane High-Altitude Deployment Demonstrator model before its flight.
In the late 1990s, NASA invited proposals for Mars exploration from the scientific community, including Langley, which had organized disci-plinary experts into a design team for a Mars airplane. By 2002, spe-cialized wind tunnel tests to develop a deployable configuration were underway, including dynamic tests to investigate design approaches for the deployment. In mid-2002, development of a proposal known as Aerial Regional-Scale Environ-mental Survey (ARES) of Mars was submitted to NASA Headquarters for competitive evaluations for Mars exploratory missions.55 Langley had
teamed with Aurora Applied Sciences, Corp., of Manassas, VA, for flight tests of candidate aircraft configu-rations. One aircraft, known as the High-Altitude Deployment Demonstrator 1 (HADD1), was used in critical high-altitude deployment tests. Carried aloft by a high-altitude helium balloon to about 100,000 feet over Oregon, the 10-foot-span HADD1 was released from the balloon and unfolded, and it completed
73 a 90-minute, preprogrammed autonomous flight. After the aircraft completed a successful flight, control transferred to a human pilot for a safe landing. A second aircraft, known as HADD2, was a full-scale version of the Mars aircraft with a 20-foot span, and it was delivered to Langley in 2006 to be prepared for instru-mentation. After a review of all proposals for the Mars exploration mission, NASA Headquarters selected proposals for satellite orbiters, and the Mars airplane activity at Langley was terminated.