In 1958, R.T. Jones suggested that aircraft with asymmetrically-swept (oblique) wings would offer many advantages at high transonic and low supersonic speeds. However, a variety of uncertainties and technological difficulties associated with this unusual configuration have prevented its application to operational aircraft. In the past several decades, the technology needed to resolve these issues has developed rapidly and oblique-wing aircraft now appear to present attractive alternatives to conventional designs in certain civil and military roles.
Many of the technical challenges of oblique wings arise from their nonlinear and strongly-coupled aerodynamic characteristics. For example, significant variations in rolling moment with changes in angle of attack are observed. Unusual inertial couplings and aeroelastic characteristics further complicate the dynamics of such aircraft and for certain oblique wing configurations propulsion integration is problematic.
These challenges are especially formidable for all-wing configurations that lack the powerful stability and control contributions from traditional tails. Stability is an issue because of the limited alternatives for packaging payload, propulsion, and aircraft systems into the wing to achieve a desired center of gravity location. Control may be difficult because of relatively short moment-arms and the changes in control effectiveness with large changes in wing sweep.
Modern computational methods provide much more rapid analysis of vehicle force and moment characteristics, control effectiveness, and other performance characteristics. Modern flight control system technology now makes a strongly coupled, unstable vehicle more feasible, creating renewed interest in the potential performance advantages of oblique flying wings. However, much of the experience gained over the past 50 years of oblique wing development is not supplanted by rapid turn-around simulation-based design tools and the present paper is intended to highlight some of the important lessons from previous research.
The oblique wing concept was suggested by R.T. Jones in the 1940's, partly because it represents a simplified minimum drag solution in supersonic flow. As Jones developed a theory to estimate supersonic characteristics of wings, it was clear that minimum lift-dependent wave drag would be achieved when the lift was distributed elliptically over the wing's span and over its length. A yawed ellipse provides a geometry in which the area is distributed in this way and this motivated further studies of oblique wings for supersonic flight.
Another significant advantage of the oblique wing arrangement for supersonic flight is that it distributes the lift over about twice the wing length as a conventional swept wing of the same span and sweep, which provides a reduction in lift-dependent wave drag by a factor of 4. At low supersonic speeds (for which these simple scaling laws apply), the volume wave drag of the wing is only 1/16th that of the symmetrically-swept wing of the same span, sweep, and volume. In addition oblique sweep avoids the unsweeping of isobars at the centerline, maintaining the effect of sweep in the critical center section of the wing.
Figure 1.1. Oblique wing drag reduction features.
Although these aerodynamic features of oblique wings sparked initial interest in the concept, structural characteristics and suitability for a variable geometry design have made the idea the subject of continuing investigations since that time.
The straight carry-through structure of the oblique wing geometry avoids torques that are sometimes reacted by fuselage structure and makes for a simpler structure to manufacture. If variable sweep is incorporated in the design, the oblique wing's single pivot in tension provides structural advantages when compared with two pivots that must carry large bending loads in a conventional variable-sweep design.
Figure 1.2 Structural advantages of oblique wings.
Perhaps the most significant advantage of oblique wings for variable sweep aircraft was recognized in a 1940's design by Blohm and Voss which used oblique variable sweep to avoid the undesirable aerodynamic center shift common with symmetric variable sweep.
Figure 1.3. Oblique variable sweep reduces some of the aerodynamic, structural, and control difficulties associated with changes in symmetric sweep.
The desirable features of oblique wings for variable geometry aircraft permits these designs to maximize aerodynamic performance over a wide range of Mach numbers without the penalties usually associated with variable geometry concepts.
Figure 1.4. Variation of L/D with Mach number for oblique wing with optimized sweep compared with fixed geometry symmetric design.
Other features unique to oblique wing designs may make them well-suited for particular missions. Examples of this include efficient storage and/or deck spotting that may be appealing to Navy aircraft.
Figure 1.5. Rockwell study of oblique wing aircraft for fleet air defense.
The oblique flying wing configuration benefits from span-loading in the same way as other all-wing concepts, but it is particularly appealing because large changes in sweep may be achieved with rather simple (by comparison) motions of nacelles and control surfaces.
Figure 1.6. R.T. Jones and a variety of oblique wing designs.
Considerable design experience and wind tunnel data on oblique wings has been accumulated over the years, predominantly by NASA, starting with R.T. Jones and continuing with a long list of other researchers. Tests of oblique wings have included:
Figure 1.7. AD-1 configuration in wind tunnel (left) and in flight (right) at NASA Dryden.
After the successful AD-1 flight demonstration, NASA Ames and Dryden undertook efforts to design and build a supersonic Oblique Wing Research Aircraft (OWRA), supported in part by the U.S. Navy and in collaboration with Rockwell (North American Aircraft Co.).
Figure 1.8. Supersonic oblique wing research aircraft concept was to utilize the digital flight control system of NASA's F-8 aircraft and focussed research efforts in the 1980's.
Unfortunately, the plan to install an oblique wing on an F-8 Crusader aircraft was cancelled prior to fabrication, although extensive analysis and design research was completed and wind tunnel data was acquired and published.1,2
Figure 1.9. Oblique wing technology development included aerodynamic studies and tests of the Mach 1.6 design with design cruise sweep angle of 68 degrees.
This work illustrated the importance of wing-body interactions in many aspects of oblique wing aircraft. Aerodynamic interactions lead to significant nonlinear stability characteristics, especially at higher angles of attack; aeroelastic behavior is strongly influenced by the concentration of mass near the wing center; pivot design, although simpler than symmetric variable sweep designs, is still challenging; and many of the wave drag advantages of the oblique concept are overwhelmed by the dominant volume wave drag of the fuselage. These observations led to the subsequent focus on oblique flying wings (often termed oblique all-wings to emphasize the configuration concept rather than the desired operational mode).
Although the oblique flying wing concept was described by Jones in 1958 and a drawing by G. H. Lee was published in 1962, the concept received littel attention until the 1990's when active digital flight control systems became more capable and widespread. The oblique all-wing configuration was studied in detail by researchers at NASA, Stanford University, McDonnell-Douglas, and Boeing in the 1990's as part of a NASA program.
Figure 1.10. Oblique flying wing concepts circa 1962 (left) and 1992 (right).
NASA Ames Research Center conducted a design study of the oblique all-wing concept for a supersonic commercial passenger transport in the early 1990’s. Participants in this study included staff at NASA Ames with long-established expertise in oblique wing design as well as engineers from Boeing Commercial Airplane Company in Seattle and Douglas Aircraft Corporation in Long Beach, California, and a research team at Stanford University. The purpose of the industry collaboration was to ensure that real-world design constraints were included in the study, and to gain access to industry design expertise. The team at Stanford built and flew a 17-foot span oblique all-wing UAV, demonstrating flight with 3% negative static stability. The design study culminated in two wing designs, called OAW-3 and DAC-1. The OAW-3 wing was designed by the team at NASA Ames, and represents a highly optimized design based on configuration constraints and mission performance metrics. The DAC-1 wing was designed by the team at Douglas Aircraft Company. It is a classical elliptical planform with a high degree of aerodynamic shape optimization, but the design was not optimized based on overall mission performance metrics. Although both wings were tested in the 9 x 7 supersonic wind tunnel, only the OAW-3 wing had a full complement of control surfaces and engine nacelles. The wind tunnel data described in this report was obtained only on the NASA OAW-3 configuration.
Figure 1.11. Oblique all-wing wind tunnel model (left) was tested at NASA Ames. Results included pressure sensitive paint visualization (right).
For historical reasons, frustratingly little of the oblique all-wing design study was ever published. AIAA paper 92-4220 by Mark Waters, et al. describes the design of the baseline vehicle configuration, including mission and operational constraints, weight and aerodynamic performance estimates, and the decision processes that led to the configuration4. AIAA paper 92-4230 by Tom Galloway provides additional insight into mission constraints5.
There is also a compilation of presentation materials from a project meeting that was held in December, 1992 that contains considerable information about the design activity up to that time6. In an effort to make the wind tunnel data from this study accessible, these data and several supporting documents were released for public distribution by NASA Ames and are now available on CD-ROM7. Among the supporting documents is a brief test report describing the conduct of the test, and a sample FORTRAN program useful for extracting and manipulating the wind tunnel data.
Figure 1.12. Artist's Concept of Boeing Oblique All-Wing Transport Design.