Drew Landman

Response Surface Methods for Efficient Complex Aircraft Configuration Aerodynamic Characterization

A response surface methodology approach to wind-tunnel testing of aircraft with complex configurations is being investigated at the Langley full-scale tunnel as part of a series of tests using design of experiments. An exploratory study was conducted using response surface methodology and a 5% scale blended-wing-body model in an effort to efficiently characterize aerodynamic behavior as a function of attitude and multiple control surface inputs. This paper provides a direct comparison of the design of experiments/response surface methodology and one factor at a time methods for a low-speed wind-tunnel test of a blended-wing-body aircraft configuration with 11 actuated control surfaces. A modified fractional factorial design, augmented with center points and axial points, produced regression models for the characteristic aerodynamic forces and moments over a representative design space as a function of model attitude and control surface inputs. Model adequacy and uncertainty levels were described using robust statistical methods inherent to the response surface methodology practice. Experimental goals included the capture of fundamental stability and control data for simulation models and comparisons to baseline data from recent one factor at a time tests. Optimization is demonstrated for control surface allocation for a desired response. A discussion of highlights and problems associated with the test is included.

Understanding Practical Limits to Heavy Truck Drag Reduction

A heavy truck wind tunnel test program is currently underway at the Langley Full Scale Tunnel (LFST). Seven passive drag reducing device configurations have been evaluated on a heavy truck model with the objective of understanding the practical limits to drag reduction achievable on a modern tractor trailer through add-on devices. The configurations tested include side skirts of varying length, a full gap seal, and tapered rear panels. All configurations were evaluated over a nominal 15 degree yaw sweep to establish wind averaged drag coefficients over a broad speed range using SAE J1252. The tests were conducted by first quantifying the benefit of each individual treatment and finally looking at the combined benefit of an ideal fully treated vehicle. Results show a maximum achievable gain in wind averaged drag coefficient (65 mph) of about 31 percent for the modern conventional-cab tractor-trailer.

A High Performance Aircraft Wind Tunnel Test using Response Surface Methodologies

A Response Surface Methodology (RSM) approach to wind tunnel testing of high performance aircraft is being investigated at the Langley Full-Scale Tunnel (LFST). In an effort to better characterize an aircrafts aerodynamic behavior as a function of attitude and control inputs, and also decrease testing time required, an exploratory study was completed using RSM on a 19 percent scale modified X-31 model. The X-31 model was chosen based on its non-linear aerodynamic behavior at high angle of attack that is representative of modern fighter design and a substantial pre-esisting data base. A five-level nested fractional factorial design, augmented with center points and axial points, produced regression models including pure cubic terms for the characteristic aerodynamic forces and moments over a cuboidal design space as a function of model attitude and control surface inputs. Model adequacy and uncertainty levels were described using robust statistical methods inherent to RSM practice. Comparisons to baseline data and sample lateral-directional and longitudinal aerodynamic characteristic are given.

An Efficient Split-Plot Approach for Modeling Nonlinear Aerodynamic Effects

ABSTRACT An everyday challenge faced by experimenters across a variety of scientific disciplines is performing well-designed experiments in the presence of characteristics that pose restrictions on complete randomization of the experimental parameters. To mitigate these restrictions on complete randomization, a split-plot experimental design methodology can be employed. The current level of sophistication in split-plot designs is now sufficient to meet the demands of higher-order models in a manner straightforward enough for practitioners. A novel case of a second-order split-plot application was recently implemented in the field of aerodynamic engineering in wind tunnel testing. Wind tunnel environments often pose restrictions on complete randomization of the test runs when aircraft physical configuration changes are required. In addition, aerodynamic empirical models require second-order effects to fit the curvature often observed in response models. Traditionally, wind tunnel testing is performed using a one-factor-at-a-time approach, which prevents capturing factor interactions and quantifying system uncertainty. This article presents a case in which a micro air vehicle (MAV) was tested in the presence of randomization restrictions with expected second-order effects utilizing an efficient design of experiments (DOE) split-plot approach.

Adapting second-order response surface designs to specific needs

Experimental design strategies most often involve an initial choice of a classic factorial or response surface design and adapt that design to meet restrictions or unique requirements of the system under study. One such experience is described here, in which the objective was to develop an efficient experimental design strategy that would facilitate building second-order response models with excellent prediction capabilities. In development, careful consideration was paid to the desirable properties of response surface designs. Once developed, the proposed design was evaluated using Monte Carlo simulation to prove the concept, a pilot implementation of the design carried out to evaluate the accuracy of the response models, and a set of validation runs enacted to look for potential weaknesses in the approach. The purpose of the exercise was to develop a procedure to efficiently and effectively calibrate strain-gauge balances to be used in wind tunnel testing. The current calibration testing procedure is based on a time-intensive one-factor-at-a-time method. In this study, response surface methods were used to reduce the number of calibration runs required during the labor-intensive heavy load calibration, to leverage the prediction capabilities of response surface designs, and to provide an estimate of uncertainty for the calibration models. Results of the three-phased approach for design evaluation are presented. The new calibration process will require significantly fewer tests to achieve the same or improved levels of precision in balance calibration. Copyright

Global Modeling of Pitch Damping from Flight Data

Flight testing using a designed experiment was used to model pitch damping in the longitudinal shortperiod mode of an aircraft. Multi-step elevator maneuvers were flown at different angles of attack and power levels on an electrically powered, propeller-driven UltraStick 120 remotely controlled model. A Central Composite Response Surface Design was used to select angle of attack and power levels for each maneuver. During post flight analysis it was discovered that power was not measured correctly for this purpose so advance ratio was selected as a replacement factor. Parameter estimation results for each maneuver were modeled globally using a single response surface. This experiment allowed global modeling of the pitch damping to be performed from just a few flight test maneuvers. Identified global models of pitch damping are plotted against angle of attack and advance ratio in a surface response plot, for non-linear piloted simulations and for flight test data.

Experimental Geometry Optimization Techniques for Multi-Element Airfoils

Experimental geometry optimization techniques for high-lift airfoils are reported. A modern three-element airfoil model with a remotely actuated flap was designed, tested, and used in low-speed wind-tunnel experiments to investigate optimum flap positioning based on lift. Detailed results for lift coefficient vs flap vertical and horizontal position are presented for two airfoil angles of attack, 8 and 14 deg. Two automated optimization simulations, the method of steepest ascent and a sequential simplex method, were demonstrated using experimental data. A simple online optimizer was successfully demonstrated with the wind-tunnel model that automatically seeks the optimum lift as a function of flap position. Hysteresis in lift as a function of flap position was discovered when tests were conducted using continuous flow conditions

Low-Speed Wind Tunnel Testing via Designed Experiments: Challenges and Ways Forward

Engineers with experience in ground test can attest that, although the confines of wind tunnels permit low noise testing, what is lacking many times are efficient test plans and the full use of the massive amount of data collected. From the perspective of industrial statisticians and engineers specializing in efficient design of experiments via statistical methods, the wind tunnel environment offers unique challenges for determining a bestpractices approach to test design. Both variants of practitioners have realized the benefit of adapting classical statistically-based experimental design techniques to wind tunnel testing. With this general approach to test, the number of tests required is significantly reduced, the true underlying cause/effect relationship between aircraft configuration and aerodynamic performance is realized and uncertainty can be precisely determined in the presence of testing condition alterations. This paper makes use of case studies to discuss and illustrate the conditions common in low-speed wind tunnel testing which require adaptations to standard experimental design application. Remedies for each of these conditions based on methods successfully used elsewhere are provided.

Efficient Methods for Complex Aircraft Configuration Aerodynamic Characterization using Response Surface Methodologies

†‡ § A Response Surface Methodology (RSM) approach to wind tunnel testing of aircraft with complex configurations is being investigated at the Langley Full-Scale Tunnel (LFST) as part of a series of tests using Design of Experiments (DOE). An exploratory study was conducted using RSM and a 5% scale blended wing body model in an effort to efficiently characterize aerodynamic behavior as a function of attitude and multiple control surface inputs. This paper provides a direct comparison of the DOE/RSM and one factor at a time (OFAT) methods for a low-speed wind tunnel test of a Blended-Wing-Body (BWB) aircraft configuration with eleven actuated surfaces. A modified fractional factorial design, augmented with center points and axial points, produced regression models for the characteristic aerodynamic forces and moments over a representative design space as a function of model attitude and control surface inputs. Model adequacy and uncertainty levels were described using robust statistical methods inherent to RSM practice. Experimental goals included the capture of fundamental stability and control data for simulation models and comparisons to baseline data from recent OFAT tests. Optimization is demonstrated for actuator allocation for a desired response. A discussion of highlights and problems associated with the test is included.

Experimental investigation of multielement airfoil lift hysteresis due to flap rigging

A study is reported on a particular type of lift hysteresis discovered while developing experimental geometry optimization techniques for high-lift airfoils. A modern three-element airfoil model with a remotely actuated e ap wasdesigned, tested, and used in low-speed wind tunnel experimentsto investigate optimum e ap positioning based on lift. Hysteresis in lift as a function of e ap position was discovered when tests were conducted using continuous e ow conditions. It was shown that optimum or near-optimum lift coefe cients determined using continuous e ow conditions exist over an extended range of e ap positions when compared to those determined using traditional intermittent conditions.