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Rain Erosion Test Apparatus

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Pamela Gregg

Communication Administrator


University of Dayton Research Institute

300 College Park

Dayton, OH 45469-0101


The Rain Erosion Test Apparatus 

Rain Erosion Test Facility


My first impression of Building 20A, at Area B, Wright-Patterson Air Force Base was of the vast internal space that is devoted to propeller testing. The self-contained structure in one corner of the building that houses the Wright Laboratory Materials Directorate Rain Erosion Test Apparatus looks very small compared to the 50-foot tall propeller test stands. The history and purpose of the facility, commonly known as the rain rig, however, is anything but small.


The Air Force has been concerned with rain erosion, or the damage to aircraft materials caused by the impingement of raindrops at high speeds, since the mid-1940s and has conducted and sponsored research on rain erosion resistant materials since 1947. The Research Institute has been involved in rain erosion testing since 1964. The rain rig is operated by the Coatings, Corrosion Control, and Analysis Group of the Materials Engineering Division. Chuck Hurley (retiree) and Roger Vissoc were involved in the initial design of the current facility. Chuck later led the UDRI rain erosion testing effort, followed by Bill Lawless, and most recently Deb Peeler.


The rain rig consists of an eight-foot long propeller-like steel blade mounted horizontally on top of a vertical drive shaft. One test specimen is attached to each of the tips of the knife-edge blade. During a test, the blade is rotated horizontally at tip velocities from 0 to 650 mph while undergoing a simulated rainfall of one inch per hour. The simulated rainfall is produced by feeding deionized water through 100 hypodermic needles mounted three inches apart on a ring mounted above the blade. The walls of the rig consist of 6 inch thick oak timbers, one-half inch thick steel, and 12 inches of double reinforced concrete.  The tests are controlled and monitored from a remote control room that is heavily sound-proofed. A strobe light and closed circuit television camera and monitor located inside the rig allow the operator to visually observe the tests. The tests can also be videotaped for later study.


The rotating arm apparatus provides an effective laboratory simulation of an environment for evaluating materials and investigating rain erosion mechanisms. One reason for this success could be due in part to the results of Project Rough Rider. The Weather Bureau sponsored the 1967 flights, in which an F-100F aircraft flew into thunderstorms around Tinker Air Force Base, Oklahoma. Twenty-seven patches of 15 experimental coating materials were applied to the leading edges of the wing tips, horizontal stabilizers, vertical stabilizer, and the noseboom of the aircraft. The specimens that underwent actual flight conditions were used to correlate with the results of similar specimens tested on the rain rig apparatus.


The rain rig is the only operating facility in the United States, and is heavily used by aerospace companies throughout the world to obtain test data to help them design materials that can better withstand hostile environments. The team that runs the rain rig includes John Buhrmaster, Regina Isaac-Giles, and Albert Foster. John and Albert evaluate client test needs and planning, supporting them in fixture and specimen configuration as well as appropriate rig test parameters to allow the most effective testing for their particular material system and application. John and Albert share the day-to-day operation of the rig; they often interact directly with the users, monitor the specimens during the tests, and perform the annual maintenance. Roger supports the team in fixture and test development, as well as rig operation maintenance and enhancement. Regina interacts directly with the rig clients to schedule testing time, fulfill contractual requirements, and complete the final reports after the testing. In addition, she does some occasional testing.


Typical materials tested are composites, plastics, metals, ceramics, glass and optical materials, and honeycomb and bulk material constructions. These materials are typically used as aircraft windshields, leading edges, helicopter blades, propeller blades, radomes, jet engine turbine blades. A comprehensive database of materials tested is maintained by the facility.


Possible future upgrades to the rain rig include the acquisition of a dual camera system where operators can watch both specimens simultaneously on a split screen television. A dual-camera system would provide a backup if one camera failed, which would make it easier for the operator to determine when one specimen failed, and guard against shutting down the facility because of equipment failure. Other enhancements include sound dampening materials in the control booth, improved atmospheric controls in the test chamber to maintain constant humidity and temperature.


by Niki Maxwell
November 1997

University of Dayton Research Institute University of Dayton
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