US Navy’s ACU-4 Maintenance Crew Trains for Propeller/Fan Balancing
US Navy sailors assigned to Assault Craft Unit 4 at the Naval Amphibious Base Little Creek in Virginia Beach, Virginia, recently completed the ACES Systems/Dynamic Instruments ATABS LCAC balance and vibration analysis training program.
The Landing Craft, Air Cushion (LCAC) fleet is a small and unique program of the Navy’s ship roster. The first ship was deployed in 1987 aboard the USS Germantown’s well deck and proved itself to be highly capable of delivering 60-75 ton payloads from ship to shore. But this concept of warfare was born many years earlier to address a specific military need.
Two years before the Battle of Normandy when more than 156,000 American, British and Canadian forces landed on the coast of France, military commanders understood there would be a need to move large numbers of troops and equipment from naval ships to the shore under heavy direct fire.
But they had no methods for doing so.
On July 16, 1942, along the shores of the Atlantic Ocean between Chesapeake Bay and Norfolk, a Navy truck with surveyors and planners arrived at the Whitehurst Family Farm — a farm first established in 1793 — to ask this family to make a sacrifice for their country. The Navy needed their land to build a facility.
In short order, a continuous stream of trucks loaded with lumber and other construction materials began building what would become the epicenter of the Navy’s effort to develop the complicated art of amphibious assault.
The Naval Amphibious Base Little Creek was born, and by the time the D-Day initiative was launched, more than 200,000 Naval and 160,000 Army and Marine Corps personnel were trained at Little Creek.
Engineers and designers developed from that effort the Landing Ship Tank (LST), which is what was used in the D-Day attack. Later the Navy and Coast Guard worked together to create the Landing Ship Medium (LSM), the Landing Craft Infantry (LCI), the Landing Craft Utility (LCU), the Landing Craft Mechanized (LCM), and the Landing Craft Vehicle, Personnel (LCVP).
But none of these solutions were ideal for the Navy’s needs, leaving much of the world’s coastlines unapproachable by the assets they had.
In order to win WWII and any future military campaigns, the Navy needed a way to deliver weapons systems, equipment, cargo and personnel from ship to shore, even when coastal conditions were not ideal.
The Navy learned important lessons from the D-Day experience where the 116th Regiment of the 29th Division suffered 99% casualties in the first 15 minutes of battle. When the LSTs arrived on the beach, German soldiers mowed the men down with machine guns as they came ashore.
Clearly the Navy’s strategy for delivering combat troops and supplies to shore needed improvement, so they continued developing equipment and strategies over the next 25 years to improve their assault tactics and capabilities.
Engineers began working on the present day LCAC hovercraft concept in the early 1970s. The idea was to build a craft that could ride within the belly of a larger Navy ship to a place just over the horizon where enemy forces could not see the approaching fleet. The LCAC, loaded with equipment and troops, would exit the larger mother ship and glide quickly across the waters to shore where it’s hovercraft capabilities could not only get forces onto shore, but also past swamp lands and obstacles. This would give the Navy a significant advantage over the enemy and make previously unapproachable shores available to military strategists.
The Biggest Challenge of Overcome
But how do you lift a 91-foot-long, 48-foot-wide ship weighing a total of more than 180 tons high enough into the air to hover across the water and sandy beaches?
Engineers at Aerojet General in California believed they could use four rotating ducted propellers to accomplish the mission. Later Bell Aerospace took the initial prototype and added two ducted rear propellers to gain propulsion. Prototypes were successfully tested in Vietnam during the war, and in 1982 the Navy committed to building 15 more of them.
In 1987 approval was granted for full-production. Textron Marine & Land Systems and Avondale Gulfport Marine were both awarded the contract to build 76 additional craft. The last of the LCACs was delivered to the Navy in 2001. Today 76 of the original 91 LCACs remain in service and can be deployed all over the globe.
The craft’s 12-foot-diameter propellers and the 63-inch lift fans require regular balancing and maintenance to reduce vibration and to extend the life of the craft’s components.
The ATABS vibration analysis unit was first developed by Dynamic Instruments Corp. (later acquired by ACES Systems) to assist Navy Gas Turbine Systems Technicians with this daunting task.
These young sailors were initially trained by the Navy to operate, repair and perform organizational and intermediate maintenance on mechanical components of gas turbine engines, main propulsion machinery including gears, shafts and controllable pitch propellers; and assigned auxiliary equipment and propulsion control systems.
The three-day training opportunity provides classroom instruction on how to use the ATABS analyzer as well as on-craft instruction on how to install vibration sensors, collect vibration data using the ATABS unit and then install suggested balance weights on the lift fans and propellers to reduce vibration levels.
ACES Systems is proud to provide support to the US Military forces in the United States and abroad. We are looking forward to many more years of service as we constantly develop better products to meet changing military needs.
(Click to Enlarge)
— Photos by Robert Berlin, ACES Systems
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Special Thanks to: GSM1 Carlos Abundismarquez, GSM3 Jerome Agaba, GSM Keaton Jones, GSM3 Tamika Lawson, GSM2 Dylan Lewis, GSM3 Marty Lewis, GSM3 Aliahmed Maknojia, GSMFN Anfernee McClure, GSM3 Deon Moffet, GSMFN Christopher Sherin, GSM2 Shakia Thomas.