Russell V. Westphal

Step Excrescence Effects for Manufacturing Tolerances on Laminar Flow Wings

Manufacturing tolerances for laminar flow wings can be significantly tighter than those of conventional aircraft. The tighter tolerances can significantly affect the assessment of the practicality of designing for laminar flow. However, existing data on the effects of excrescences typical of manufacturing process are limited. Further, information on the effects—often beneficial—of pressure gradient present on the laminar flow wings is not generally available. To address these concerns, a series of experiments has been undertaken to examine the effects of surface steps in the presence of pressure gradients. The step geometries were selected to represent those that result from actual aircraft manufacturing processes. The range of pressure gradients correspond to those typical of laminar flow wings. Initial experiments were conducted in a low-speed wind tunnel. Later experiments used a novel propelled-model test facility. The results of these studies show that the allowable sizes of surface excrescences for laminar flow wings may be significantly greater than has conventionally been assumed. This could significantly influence the more widespread use of laminar flow for drag reduction, resulting in more efficient aircraft.

An Approach to Measuring Step Excrescence Effects in the Presence of a Pressure Gradient

An experimental study was undertaken to determine the effects of step excrescences on boundary layer transition using a unique ground test facility in which the test model was propelled though still air. The models used were designed to have a nominally constant pressure gradient so that the results would be relevant to laminar flow aircraft whose wings often have long runs of mildly favorable pressure gradient. The models had an integrated continuously adjustable two-dimensional step, which could be adjusted to be forward-facing or aft-facing. The large model was used to increase the Reynolds numbers examined so that the results are applicable to laminar flow flight vehicles. Multiple measurement methods, including Preston tubes, hot wires, accelerometers, a boundary layer traverse, and static pressure taps were used to provide comparison data, and to add to the physical understanding of the results. The propelled-model test approach required that the instrumentation be self-contained and ride along with the model as the carrier vehicle moved down the test track. Due to the relatively short times available for data-taking (approximately 15-30 seconds per run), the initialization and data analysis techniques had to be tailored for this application.

Hot-Wire Measurements of the Influence of Surface Steps on Transition in Favorable Pressure Gradient Boundary Layers

An examination of the effects of surface step excrescences on boundary layer transition was performed, using a unique experimental facility. The objective of the work was to characterize the variation of transition Reynolds numbers with measurable step size and boundary layer parameters, with the specific goal of specifying new tolerance criteria for laminar flow airfoils, alongside a fundamental investigation of boundary layer transition mechanisms. This paper focuses on interpretation of hot-wire measurements, including supporting stability calculations, undertaken as part of the study. The results for both forward and aft-facing steps indicated a substantial stabilizing effect of favorable pressure gradient on excrescence-induced boundary layer transition. These findings suggest that manufacturing tolerances for laminar flow aircraft could be loosened in areas where even mild favorable pressure gradients exist.

Transition Due to Surface Steps in the Presence of Favorable Pressure Gradients

The infl uence of favorable pressure gradients on the allowable size of surface steps for laminar flow wings can have a beneficial effect on manufacturing tolerances. However, generally -applicable guidelines for including the effects of pressure gradient on the al lowable size of surface steps do not exist. A series of experiments has been undertaken, in a low speed wind tunnel and a novel propelled -model facility, to obtain data to determine manufacturing tolerances for laminar flow aircraft. Building on that wor k, new models have been designed to conduct testing that will extend the ranges of applicable Reynolds number and pressure gradient to include many future aircraft concepts.

Transition in incompressible boundary layers with two-dimensional excrescences

An experimental investigation of the transition process in boundary layers subjected to forward- or aft-facing two-dimensional step excrescences is described. The objective of the work was to characterize the variation of transition Reynolds numbers with measurable roughness and boundary layer parameters, with the specific goal of specifying new tolerance criteria for laminar flow airfoils, alongside a fundamental investigation of linear boundary layer stability mechanisms. Results from an ongoing program of increasing complexity on effects of pressure gradient on excrescence-induced transition are presented. Preliminary N-factor calculations are used to determine the effects of boundary layer stability and attempt to isolate the effect of the disturbance due to the excrescence.

An Approach to Measuring the Effects of Surface Steps on Transition Using a Propelled-Model

An experimental study was undertaken to determine the effects of step excrescences on boundary layer transition using a unique ground test facility in which the test model was propelled though still air. The study made use of a model with infinitely adjustable step sizes and a single measurement surface for all cases. The test conditions examined and measurements made built on earlier work conducted in a low speed wind tunnel to measure the influences of pressure gradient on the relation between surface excrescences and boundary layer transition. The initial experience with propelled-model testing indicated that this approach is capable of providing a low disturbance test environment with Reynolds numbers representative of flight conditions.

Application of the Boundary Layer Data System on a Laminar Flow Swept Wing Model In-Flight

Two versions of a small, self-contained device developed specifically to measure boundary layer characteristics – collectively referred to as the Boundary Layer Data System – were applied in a flight test of a 30-degree laminar flow swept wing test article. The devices were used to provide local boundary layer measurements in conjunction with surface pressure and infrared image measurements for an Air Force Research Laboratorysponsored test of Northrop Grumman’s SensorCraft wing design. The model was carried aloft by Scaled Composites’ White Knight aircraft. The flight altitudes of over 40,000 ft, Mach numbers above 0.5, and local dynamic pressures up to 150 psf are the highest values yet for application of the devices. Two different configurations of the Boundary Layer Data System were flown: a device with a stage to position a traversing pressure probe, and a version of the device with a stationary surface Preston tube. The devices weigh less than a pound and are completely self-contained, meaning that they have their own internal power and memory. The devices attach to the test surface with double-sided structural adhesive tape and thus require no mechanical penetration of the test surface, making them a flexible tool for flight testing. This paper describes the devices, their use on the flight test, and presents some sample results.

A Compact, Self -Contained System for Boundary Layer Measurements in Flight

A small, lightweight, self -contained research instrument capable of providing direct measurements of the flow near a surface in flight has been developed and tested. The instrument consists of a motorized stage to which a measurement probe is attached, circuitry for timing a nd powering the stage motion, a data logger with sensors and memory, and a battery with power conditioni ng circuitry. The entire instrument is contained within a fairing, weighs well under one pound, and can be affixed directly to a n aircraft surface with structura l adhesive tape. No connection to the aircraft systems or mechanical penetration of the surfac e is required . After a flight, dat a is downloaded to a laptop computer for display and analysis. The system has been successfully tested , both in a wind tunnel , and during three flights on an aircraft wing . It is now available for application to low -spe ed flight research where detailed, local boundary layer measurements are needed.

Design of a Third -Generation Boundary Layer Measurement System

The third generation of a series of small, lightweight, self -contained research instrument s capable of providing direct measurements of the flow near a surface in flight has been design ed , with particular attent ion to its application in research for high altitude, long endurance aircraft development. The latest generation of instrument s uses a common main unit consisting of a carrier circuit board with sensors , control electronics, and a plug -in programmable mic rocontroller system for data acquisition and control with post -flight data offload . This main unit can be configured to perform total pressure or multi -component boundary layer velocity profile measurements , or it can be connected with two additional sate llite units to perform Preston tube skin friction measurements at three different locations simultaneously . The main unit is con tained within a small housing that includes its battery , weighs well under one pound, and can be affixed directly to a n aircraft surface with structura l adhesive tape. No connection to any aircraft systems or mechanical alteration of the aircraft is required . After a flight, dat a is downloaded to a laptop computer for analysis and display . The instrument is designed for long bat tery life and operation with low ambient temperatures to be compatible with the needs of high -altitude flight research .