Algor Software Helps Engineers Determine Airfoil Design for Safe Wind Tunnel Testing

The pictures to the left and center are of the rotor blade tip being manufactured. The image to the right is of the finished part which is about 5.8 inches in length with a three-inch chord.

Researchers at The University of Texas at Arlington (UTA) found Algor software a valuable tool in designing a prototype model of a helicopter rotor blade tip. Dr. Baxter R. Mullins, Jr., P.E. (tcsbrm@airmail.net) of UTA and Mr. Raymond Mort, P.E. (avtray@airmail.net) of AVT Engineering, Hurst, Texas, used Algor software to examine the materials and structural margin needed to run an actual model in a transonic wind tunnel.

The study is part of a larger experimental investigation headed by Dr. Mullins to help improve rotor shapes and reduce the noise associated with Blade-Vortex Interaction (BVI).

Test results on the blade tip design will provide a picture of the initial roll-up region and give a comparison of the effects of Mach number, Reynolds number, and tip shape on the vortex size, shape, location, and minimum stagnation pressure.

von Mises stress contour on displaced model.

von Mises stress distribution contour with shrink elements.

Objectives

Prior to testing, the research team had to answer a number of questions: What loads could they expect the model to generate during a typical wind tunnel test run? What were the maximum loads that could be generated by the model at the maximum angle of attack in which the model would be run? Could the model be used safely in the wind tunnel without failure? Finally, should the model be constructed of aluminum, a considerable cost savings, or steel?

Performing the Analysis

Dr. Mullins and Mr. Mort performed a 3-D linear stress analysis of the blade tip in order to determine the maximum loads and bending moment of the blade tip model when subjected to the wind tunnel airloads. They began their analysis by importing a 2-D airfoil contour section from AutoCAD. The chord (or width of the airfoil section) length was varied according to the prescribed contour. The airfoil thickness-to-chord ratio was varied based on the required contour. Dr. Mullins and Mr. Mort hand-meshed the surface model and created a final 8-node "brick" element model using Hexagen.

A complex computational fluid dynamics (CFD) model and an empirical-theoretical model were used to determine the pressure loading for the linear stress model. "We divided the pressure load on the model into 10 areas of constant, but different pressures. A pressure loading was estimated for a four-degree angle of attack," said Dr. Mullins.

Pressure was applied to the linear stress model using the load multiplication feature found in Algor. The model was run for 1, 2, 4, 6, 7, 8 and 10 times the normal pressure load to provide the necessary information to determine the structural margin. A load factor of 7 represents the pressure loading on the blade-tip at a four-degree angle of attack which the model would experience during a typical wind tunnel run.

Algor Analysis Results Prove Conclusive

Analysis results determined that at a load factor of 10, the model would experience 114,000 psi of stress near the root quarter chord. According to Dr. Mullins, "The Algor result was well in-line with our expectations considering the rigid-attachment assumption made for the FEA boundary conditions. A lower stress would be expected if the welded end-plate support had also been modeled. This analysis was done to examine the choice of materials required to provide the necessary material structural margin for running the airfoil in the wind tunnel."

Based on the Algor analysis result, Dr. Mullins and Mr. Mort determined that the actual wind tunnel model should be constructed from a heat-treated steel and not aluminum. "The heat-treated steel would provide the needed structural margin required when running the model in the transonic wind tunnel," said Dr. Mullins.

Maximum principal stress contour - front view showing displacement.

The HIRT Wind Tunnel

The model is currently being tested at the HIgh Reynolds number Transonic (HIRT) Wind Tunnel located at UTA's Aerodynamic Research Center (ARC). "This particular wind tunnel simulates real-world conditions by allowing scale models to be run at full-scale transonic Mach and Reynolds numbers. It is a very unique testing facility, especially in a university setting," states Dr. Mullins.

More Algor Analysis to Come

Dr. Mullins and his project team have performed a variety of design and analysis studies using Algor software. "In the past, we made an axisymmetric model of a rocket nose cone which was placed under a varying pressure distribution, acceleration and centrifugal rotation loads," he said. Mr. Mort has analyzed a rocket body for natural frequencies and mode shapes and constructed a nonlinear pressure vessel (rocket body) burst model.

Dr. Mullins plans to use Algor software to analyze upcoming wind tunnel models, while Mr. Mort is preparing to analyze the stress in his own home-built airplane. "Algor software provides a cost-effective way to create FEA models. This is important to universities, small companies and consultants alike."

Pictured is the rotor blade tip project team positioned in front of the HIRT wind tunnel. Standing from left to right is Mr. Raymond W. Mort, P.E. and Dr. Baxter R. Mullins, Jr., P.E. holding full-scale, aluminum test articles of the actual airfoil. Kneeling from left to right is Mr. Scott Stussey, faculty Associate and lead engineer over the operation of the HIRT wind tunnel, and Ms. Jennifer Peeples, a graduate student whose Master's Degree thesis will be the analysis of the tip vortex generated by the airfoil in the test.