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Army helicopter avionics keeps cool in harsh environment

Courtesy of VT Miltope, Inc.

Army workhorse helicopters are doing heavy lifting with the help of VT Miltope's Mass Memory Server 2 (MMS-2). The MMS-2, which resides in the aircraft's hot, cramped avionics bay, is a lightweight, network-attached, solid-state storage platform that is part of the helicopter's Avionics Management System. It serves as a digital map server and collects aircraft data including usage, maintenance, vibration monitoring, and engine, rotor, and balance information in order to monitor aircraft health.

Cooling the MMS-2, while protecting its interior electronics from contaminants (such as sand and dust, rain, humidity, and salt fog), required a separate, "dirty" conduit for air flow and heat exchange. Two rear fans pull air into the chassis from the lower front panel. The air then runs through a short, wide duct below the circuit boards before exhausting through the rear.

A baseboard with its processor module and power supply board is cooled by conduction and forced convection. Custom aluminum heat sinks mounted to these boards pull heat from the thermally significant chips, such as the processor chipset and memory. Attached to these conduction heat sinks are finned heat sinks that project downward into the air flow space of the internal duct. These heat sinks also serve as the ceiling of the cooling duct.

Coolit modeling ensured a reliable cooling system design across an operating temperature range of -40C to +71C. After a few minor tweaks to the design of the ducting and placement of fans to achieve lowest chip temperatures, Coolit predicted that all major components would remain within their manufacturer's specifications with a healthy thermal margin. This modeling outcome later was verified by thermocouple measurements on pre-production hardware.

Early thermal predictions indicated that the Ethernet board's components would benefit from additional cooling. So an aluminum heat spreader plate was added to those components to lower chip temperatures. Coolit further showed that a stirring fan was needed for spot cooling on a small toroid transformer on the power supply. This component could not be cooled by the same conduction heatsinking as the other power supply components, and could not be located in the air duct for environmental reasons, so spot cooling was necessary. Coolit showed that the stirring fan reduced the toroid's core temperature significantly. Finally, Coolit showed that the temperature of the four internal solid state drives remained within the manufacturer's temperature limits.

Simulating a fan failure in Coolit was as easy as checking the box labeled "Failed Fan". The MMS-2's rear fans are divided by a septum which continues the entire length of the internal duct to prevent shortcircuiting of air flow in the unlikely event of a fan failure. If one fan fails, the air flow from one fan still cools half of the heat sinks for the baseboard and power supply. Coolit verified that the parallel heat exchange paths of the MMS-2 provided sufficient cooling even with a single fan operating.

Late in the design, solar radiation loading became a new requirement. Coolit easily modeled a solar load intensity of 1120 Watts per square meter, using a 45-degree solar angle and an azimuth for side loading. Predictions indicated that all component temperatures were still within acceptable limits.

MMS-2 functions as map server and monitors helicopter health. It is designed to stay cool in the helicopter's hot and cramped avionics bay.
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Heavy lifting helicopter presents challenging environment.
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