Radical aerodynamic and structural design technologies offering potential fuel-saving and noise-reduction breakthroughs will be tested in back-to-back experiments by NASA in partnership with the U.S. Air Force.

Tests of a laminar-flow wing section dotted with micron-scale “roughness” features and a flexible, morphing trailing edge will be carried out on a modified Gulfstream III business jet, representing the first flight experiments of their kind on such a large scale. NASA Dryden Flight Research Center, which acquired the G-III from the Energy Dept., is using the aircraft because of its performance as well as its large area, swept wing and suitably high Reynolds number—a measure used to characterize flow regimes in fluid dynamics.

NASA says the ability to cut drag by controlling the amount of laminar flow— or smoother, boundary-layer air over a wing surface—offers potential improvements in fuel efficiency, range and payload that “far exceed” any known single aeronautical technology. Possible fuel savings of up to 30% for subsonic commercial aircraft have been suggested, should a successful passive natural laminar flow (NLF) or active hybrid laminar flow system be developed.

However, despite extensive research in the U.S and Europe going back to the late 1930s, no usable laminar-flow-control system has yet found its way on to a production swept-wing aircraft. The upcoming NASA experiment is designed to help bring that change about, says Fay Collier, project manager of the agency’s Environmentally Responsible Aviation (ERA) project. Laminar flow is challenging to maintain over a swept wing because of the effects of cross flow as the air turns toward the wing tips.

“We want to overcome some of the practical barriers to application. The game’s just starting to get to that point, and we want to look at real-life applications in terms of systems, structures and materials,” says Collier.

Laminar flow will be tested under the first of a two-phase project using a composite “glove” built in association with Texas A&M University and NASA Langley Research Center. Measuring up to 6 ft. in span and around 12 ft. in chord length, the specially shaped laminar-flow section will be located outboard beyond midspan on the port wing and will jut out ahead of the rest of the G-III’s leading edge. The glove will have a slightly sharper leading-edge sweep of around 30 deg., compared with 28 deg. for the rest of the wing.

“We’re putting it as far outboard as we can without putting it in front of the ailerons,” says Ethan Baumann, NASA Dryden G-III and ERA chief engineer, who adds that the glove will be bonded to the wing skin rather than cut into the structure. Initial testing will focus on the baseline NLF capabilities of the unmodified glove. Following this, the plan is to introduce distributed roughness elements (DREs), which Texas A&M and Northrop Grumman have so far only tested on smaller-scale swept airfoils under the wing of a Cessna 337 testbed. The trials supported the Air Force Research Laboratory’s (AFRL) SensorCraft high-altitude, long-endurance unmanned reconnaissance aircraft project.

The DREs (to be attached as appliques) will measure 1.5 mm. in diameter and 6-12 microns high. Positioned like a set of miniature span-wise vortex generators, the DREs will be spaced at 4-mm. intervals along the 1% chord line right by the leading edge. The basic principle is that the DREs are large enough to excite usually stationary cross-flow vortices that help keep the flow attached to the wing surface. Reinvigorated by the tiny shapes, the flow would overcome the usually more dominant nonstationary cross-flow vortices, thereby delaying transition to turbulent flow.

At the same time, the DREs are too small to trip the boundary layer, which would also generate turbulent flow and destroy laminar flow. “We want to get to high Reynolds numbers and a situation where we need the DREs to control the cross-flow. There may be a baseline condition where we might not need the DREs to control the transition. In the tests, we could get the condition with the NLF first, and then later be in a situation where we need the DREs to get the same result,” says Collier. With the DREs positioned to coincide with critical cross-flow vortex wavelengths, the aim is to use these elements to “cancel [the effect] out if cross-flow instabilities raise their ugly head,” he adds.  

The focus on DREs forms only one of several strategies toward practical laminar flow, says Collier. “It’s most visible because we’ve been fortunate to be partnered with AFRL. The idea is to see if DREs can work at higher Reynolds numbers in the 20-million-plus range for maintaining a laminar boundary, and which would work for a regional or single-aisle transport aircraft.” A hybrid system, involving leading-edge suction, will likely be required for larger wide-body transports, he says.