The Effects of Step Excrescences on Swept-Wing Boundary-Layer Transition

The immense fuel savings and environmental benefits of reducing aircraft skin-friction drag through laminar flow is well known. However, obtaining substantial laminar flow on an aircraft in an operational environment has proven to be a difficult challenge due to surface imperfections (e.g. 2-D steps and gaps, surface roughness, bug-strikes, paint chips, etc.). One controllable imperfection is a 2-D step through requiring certain manufacturing tolerances between adjacent wing panels. Current methods for designing a laminar-flow aircraft require designing and manufacturing with overly-restrictive tolerances obtained from unswept flat-plate experiments. This research aims at giving designers more realistic manufacturing tolerances for a typical, swept-wing transport aircraft.

A 30? swept-wing model with a movable leading-edge extending to x/c = 0.15 is used in the flight environment and in a low-disturbance wind-tunnel to study the effect of 2-D step excrescences in a three-dimensional boundary layer. Forward- and aft-facing steps are modulated during the tests. The design of the test article and the internal actuation system is documented in detail. A structural and safety analysis is provided for the flight testing on a Cessna O-2A Skymaster. A flutter and handling-quality clearance flight proved the new test article is safe for the flight-testing experiments.

Pressure measurements are compared with computational results, infrared thermography is used to globally detect boundary-layer transition, and hotwire measurements provide details of the boundary-layer profiles in the vicinity of the steps. An analysis of the results is provided including comparisons of both the wind tunnel and flight environment, and from experimental studies of an unswept model of similar 2-D pressure gradient.

The crossflow instability is believed to dominate the transition process up to the critical step height, while the shear-layer instability dominates after the critical step height. The critical step height was found to be a function of unit Reynolds number. Also, the addition of leading-edge sweep with a similar 2-D pressure gradient substantially lowers the local Reynolds-based critical step height for forward-facing steps, while it is similar for the aft-facing steps. However, a substantial increase in the conventional laminar-flow tolerances can be made confidently if a favorable pressure gradient is implemented.