Some changes in the design and construction of the leading edges of metal airplane wings have shown promise as means to suppress laminar-to-turbulent flow transitions. The significance of this development is that it creates an opportunity to take advantage of laminar-flow boundary layers to reduce aerodynamic drag.
During the early 1950s, the aeronautical community reached a consensus to abandon laminar-flow drag-reduction techniques, in part because of a belief that practical metal airplane surfaces could not be made smooth enough to support laminar flow. The present development addresses an important aspect of the smoothness issue; namely, the interruption of the smooth wing surface at the joint between a leading edge and the rest of a wing. On a typical commercial jet airplane, each wing is constructed with a single-piece wraparound leading-edge skin piece attached to the rest of the wing by screws (see figure). The locus of attachment is a spanwise joint 4 to 6 in. (10 to 15 cm) downstream of the leading edge. The gaps and steps in the wing surface at the joint (including the exposed heads of the attaching screws) have been blamed for triggering laminar-to-turbulent flow transitions.
The present improved attachment scheme yields a joint smooth enough to support laminar flow. The improved scheme does not differ radically from the conventional scheme described above, does not require manufacturing accuracy significantly beyond that of conventional practice, and does not require expensive materials or expensive fabrication techniques. In the improved scheme as in the conventional scheme, the leading-edge skin piece is removable for infrequent inspection.
In the improved scheme, the leading-edge skin piece is made slightly thicker and a shallow recess is machined along the attachment region to allow for subsequent flush mounting of an aluminum or plastic cover strip. The cover strip is attached by use of a modern, easy-to-handle, commercially available high-strength adhesive; indeed, it is only the advent of such adhesives that has made it practical to implement the present scheme. The edges of the strip can be trimmed carefully so that the remaining gaps are smaller than a critical dimension for triggering the laminar-to-turbulent transition.
The efficacy of the improved scheme, has been verified in flight tests: A test fixture designed to reproduce a wing surface according to the improved scheme, with various simulated degrees of manufacturing precision, was mounted on an airplane and flown at representative mach and Reynolds numbers. Laminar flow was observed, and a readily available foam-backed adhesive held the cover strip in place with no sign of failure.
This work was done by Robert A. Kennelly, Jr., Dennis J. Koga, and Fanny A. Zuniga of Ames Research Center; Aaron Drake of San Jose University State; Michael L. Hinson of Learjet, Inc; and Russell V. Westphal of Washington State University. No further documentation is available.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Ames Research Center; (415) 604-5104
Refer to ARC-14088