C-Wing: Continuing Work

The results suggested that, at least with the original LOPA concept, the design improved with very thick sections inboard. In conjunction with related studies at M.I.T., Purdue, and the University of Illinois, a set of airfoils well-adapted to the inboard wing sections of this concept were developed and their performance was modeled in the aircraft sizing studies. These sections utilized suction on the aft area of the section to extend the region of high thickness aft and to reduce supervelocities over the upper surface. Some ideas for the integration of these sections with the pasenger compartment and high lift system are illustrated below.

One of the possible advantages of the C-Wing concept involves the development of the trailing vortex wake system from this geometry. A major concern with large aircraft is the hazard of the trailing vortex wake to other aircraft. The C-wing distributes the vorticity in the wake over a longer distance, reducing the intensity of the wake sheet, but the vortices shed from the wing tips and the tips of the C-Wing extensions are closer together than they would be for a conventional design, accelerating the breakdown of the wake system. Preliminary studies of this phenomena were undertaken at Tuskegee University and are not complete, but do illustrate some of the differences between the wake of the C-Wing and that of a conventional design.

A significant concern for this type of configuration is its aeroelastic stability characteristics. The swept C-wing design might be expected to lower the torsional frequencies of the system and permitting additional coupling between primary bending and torsion modes. The lifting surfaces near the tips do introduce substantial damping to the torsion modes so that the flutter characteristics of this design are not obvious. One of the attractive features of the C-Wing geoemetry, however, is that even if the uncontrolled flutter modes are less stable than a conventional wing, the system is more controllable. With control surfaces on the main wing and the horizontal tip extension, one may independently control lift and torsion. This makes the system more easily controlled than a conventional wing in which deflection of an aileron introduces both torsion and bending perturbations. The figure below shows how this concept may be used to eliminate aileron reversal for the C-wing design.

A second round of conceptual design iteration remains to be completed, however, designs such as that shown below are under investigation. In this design, the planform is modified slightly to permit larger root t/cÕs. A 747-based fuselage is used to accommodate more of the payload in a conventional environment (more windows, conventional egress) and reduce the passenger lateral extent. These two changes may make conventional airfoil sections (without boundary layer control) more attractive. By removing the canard from the design, efficient trim is still possible without active controls.

The figure below shows the addition of C-Wing tips to the McDonnell-Douglas Blended Wing Body concept. The addition of these surfaces would permit the BWB configuration as currently envisioned to fly with positive static stability with no change to the aerodynamic design of the highly-loaded, thick transonic wing. The added weight and skin friction drag of these surfaces may be partly offset by a reduction in induced drag and by the relaxed moment constraints on the main wing sections. Although the concept remains to be studied in any detail, its implications for controllability and efficient trim of this flying wing design are promising.

Ilan Kroo (kroo@leland.stanford.edu)