The Oblique or Skewed Wing

Another rotation-only variable-sweep concept was invented by the late Robert T. Jones at the NACA Ames Aeronautical Laboratory, around 1945 (Figure 16.3). This is the oblique or skewed wing, in which wing sweepback (and sweepforward) is achieved by rotating the wing at its center, sweeping one side back and the other side forward. With the oblique wing rotated back into symmetry, the configuration avoids the tip stalling and low-speed stability and control problems associated with ordinary wing sweepback. Jones’ invention seems to have paralleled other rotating-wing sweep concepts, those of Lachmann of Handley Page and Richard Vogt of Blohm and Voss. Jones expected an additional advantage for the oblique wing as compared with conventionally swept wings, that of higher supersonic lift-drag ratio.

Had the unorthodox oblique-winged configuration been proposed by someone without Jones’ immense prestige, it might have been dismissed at once. But, for one thing, Robert Jones was credited with the invention of wing sweepback to alleviate compressibility ef­fects during World War II, independently of the Germans. He also contributed largely to stability and control theory, in all-movable controls, lateral control, two-control airplanes, and in solutions of equations of motion. Like the Wright brothers, Edward Heinemann, and John Northrop, Jones was not university-trained. His considerable mathematics were self-taught.

With the wing rotated, the oblique-wing configuration is that rarest of heavier-than-air machines, one without bilateral or mirror-image left-right symmetry Birds, dragonflies, and our own flying creations all have bilateral symmetry, as we ourselves do. It seems obvious that the flying qualities of an oblique-winged airplane would be strange, if not dangerous. For one thing, pulling up the airplane’s nose to increase angle of attack would create inertial rolling and yawing moments, quite absent in symmetrical airplanes. These moments arise from pitching velocity and acceleration acting on a nonzero product of inertia term Ixy.

The effectiveness of trailing-edge flap-type controls is seriously reduced at large sweep or skew angles. Control deficiencies can be made up if the airplane carries conventional tail surfaces. Control problems are more critical if an oblique wing airplane is always operated in the skewed position, but this would obviate the need for rotating engine pods and vertical tail surfaces.

Wing torsional divergence on the sweptforward panel, discussed in Chapter 19, “The Elastic Airplane,” has been raised as an issue for the oblique wing. Jones quite early predicted that rigid-body roll freedom would tend to raise the divergence speed to safe values outside of the flight range. That is, when the leading or sweptforward panel starts to bend upward under high airloads, the lift on that panel would increase, causing a large rolling moment. Airplane roll response to that moment would alleviate the airload and the wing would be safe.

However, the case must be considered in which automatic roll control operates to hold the airplane at zero bank. If the control rolling moment that holds the zero-bank angle comes from a horizontal tail, the wing torsional divergence speed could be close to the body-clamped case. A free-free analysis that includes autopilot loops would seem to be needed. On the other hand, if control rolling moment comes from ailerons on the leading panel, the panel loads would be reduced, as in the case of free-body roll. This would raise torsional divergence speed above the body-clamped value.

Some detailed stability and control data on oblique wings were obtained in NASA Ames Research Center wind-tunnel tests and in a NASA-Navy funded study begun in 1984. The

study was on the feasibility of converting NASA’s F-8 Digital Fly-By-Wire Airplane to an oblique wing configuration. A key problem surfaced in the unusual nonlinear variations at zero sideslip angle in side force, and rolling and yawing moments with angle of attack, at wing skew angles as low as 30 degrees (Figure 16.4). These are trim moments, which would have to be trimmed out by control surface deflections for normal, nonmaneuvering flight. The nonzero side force could be equilibrated by flying at a steady bank angle, or possibly by wing tilt with respect to the fuselage.

Other possibilities to deal with nonzero side forces, yawing, and rolling moments at zero sideslip include wing plan form adjustments, unsymmetrical tip shaping, wing pivot selection, antisymmetric wing twist, and variable tip dihedral (Kroo, 1992). One is left with the impression that the aerodynamic design of a practical oblique-wing airplane will be far more complex than for its swept-wing counterpart.

There have been a number of oblique-wing flight tests, starting with a test in the Langley Research Center’s Free Flight Wind Tunnel. R. T. Jones also built and successfully flew a

series of small free-flying oblique-winged model aircraft, culminating in a radio-controlled two-meter span model whose wing andtailplane skew angle could be systematically changed in flight. A considerable number of oblique-wing design studies followed. NASA eventually contracted to have a full-scale oblique-wing test airplane built for low-speed flight tests. This airplane, the AD-1, a single-engine jet, was flown successfully at the NASA Dryden Flight Research Center at Edwards, California (Figure 16.5). Ten degrees of bank angle on the AD-1 are required to cancel the side force produced by a 60-degree wing skew angle (Kroo, 1992).

The Dryden flight tests were followed by a NASA design research contract for an oblique­wing supersonic transport. The contract was awarded to Boeing, McDonnell-Douglas, and Kansas University, around 1992. The study revealed arrangement problems with that partic­ular arrangement. An all-wing version of the oblique-wing eliminates the need for hinging the wing to a fuselage, although engine pods and any vertical tails still require hinging. Another NASA design research contract to Stanford University is for a flying model of a 400-foot-span all-wing supersonic transport, operated as an oblique wing. Stability and control for the all-wing versions of the oblique wing are problematic because of the problem of nonzero side forces, rolling, and yawing moments in oblique cruising flight.