ALGOR FEA HELPS OHIO FIRM TO DESIGN CRITICAL HELICOPTER COMPONENT

The MH-60G "Pave Hawk" helicopter in action. "Coalition" personnel are currently utilizing this model in Operation Desert Storm.

The design of military equipment is one of the toughest challenges an engineer can face. Because the components are often used under the most extreme conditions - and since lives may depend on their reliability - designers must be certain that even the toughest military specifications are met. Finite element analysis (FEA) has been used by defense contractors for many years. It allows the designer to perform critical analyses without the need for costly prototypes and time-consuming testing procedures.

Recently, Wayne Shelly, a stress analyst with the Defense Electronic Systems Division of Systems Research Laboratories in Dayton, Ohio, utilized Algor FEA software in the redesign of the main instrument panel for the U.S. Air Force MH-60G "Pave Hawk" helicopter. The Pave Hawk is currently being utilized by Air Force personnel in the Persian Gulf.

Wayne Shelly, stress analyst, with the model he constructed for the MH-60G "Pave Hawk" helicopter.

The main panel is a critical component which holds more than 30 instruments and serves as the interface between the pilot, co-pilot and all major aircraft systems. It measures approximately 60" wide x 14" high x 1.62" deep. The material thickness varies from .125" to .25". Components mounted on the panel include several heavy video monitors and other sensitive electronic instruments.

"Air Force standards for the design of such components are strict," explains Mr. Shelly. "All aircraft installations must be analyzed for stress, and a report must be submitted to the Air Force for approval. Government specifications require that the instrument panel be able to withstand a 20 g forward, 20 g downward and 18 g lateral crash load. Under these conditions the panel is allowed to yield with permanent deformation, but may not rupture.

Major Safety Concerns

"The main concern is that the panel remain fixed to the aircraft and the instruments remain fixed to the panel. The minimization of vibration, which could cause malfunction of the installed equipment, is also a concern. The dominant frequencies that are experienced in the cockpit of a rotary wing aircraft are generated by the main rotor and, to a lesser extent, by the power transmission gearing."

A Large Model

To test the instrument panel design, Mr. Shelly first constructed a model containing more than 4000 elements using Algor's SuperDraw II Computer-Aided Drawing (CAD) program. Since a model of this size was more detailed than necessary, and would require a large amount of storage space and time to process, a smaller model was also created. This model utilizes 651 plate/shell elements to represent the panel and 134 3-D beam elements to simulate the effect of monitor support structures mounted forward of the panel. A total of 36 boundary elements are used to extract the reactions where the instrument panel is supported by the aircraft structure. The model contains 905 nodes.

The model shows stress concentrations in thinner parts of the instrument panel.

"Three static analyses were run," explains Mr. Shelly, "for the forward, downward and lateral crash loads. In addition, a modal analysis and a random vibration analysis were performed. The random vibration model used an input power spectral density curve and the results of the modal analysis to show the dynamic stresses."

Analysis Results

"The static analysis exposed stress concentrations in some of the thinner sections of the panel. Basically, however, the results showed an acceptable design that meets government performance specifications.

"Detailed documentation which verifies the design was generated based on our findings. In addition, I was able to use SuperCap to generate animated slides showing the instrument panel deflecting under all three crash conditions."

Putting Algor to the Test

Mr. Shelly is a well-versed Algor software user and has performed many other analyses on such items as avionics equipment racks and video monitor mounting systems. He has even put Algor to the test.

In his own words: "Not being satisfied with the monthly accuracy verification problems published in Algor Design World, I ran numerous benchmarked problems in both the static and modal packages.

"The problems came from Roark & Young, Marks Standard Handbook for Mechanical Engineers and other sources that could be cross-checked with classical methods. Overall, I have found the Algor package to be both comprehensive and accurate."