Aerospace Avionics & UAV Thermal Enclosure Design

A commercial aerial-survey operator developing a high-resolution mapping payload for a medium-altitude long-endurance UAV experienced FPGA overheating during summer operations at 6,000 m altitude. The payload enclosure was a sealed aluminium box (305×203×102 mm) with external longitudinal fins, mounted on the UAV fuselage underside. At sea level, natural convection kept the FPGA below its 85 °C case limit. At 6,000 m, however, air density dropped to 66% of sea level, natural convection decreased by 34%, and the FPGA case temperature reached 97 °C after 45 minutes of continuous on-board image processing — triggering thermal throttling that reduced effective map resolution by 40%. The UAV was forced to descend to 4,000 m during summer missions, sacrificing survey coverage. Reynolds & Bauhm conducted altitude-corrected CFD thermal simulation with ideal-gas density variation, temperature-dependent air viscosity, and radiation from the hot fins to the cold sky. The model showed that the longitudinal fin orientation aligned with airflow during forward flight was suboptimal — the forward fins created a wake that sheltered aft fins, reducing their effective convection by 50%. We recommended: (1) rotating the fin array 45° to intercept propeller slipstream from all approach angles, (2) applying black anodising to increase fin emissivity from 0.05 to 0.85 for radiative cooling dominance at altitude, and (3) adding a 5 W micro-blower inside the enclosure to force air across the FPGA heat sink base. Post-modification CFD predicted FPGA case temperature of 79 °C at 6,000 m in 35 °C ambient — an 18 °C improvement. Flight trials over 12 sorties confirmed no thermal throttling at 6,500 m, and on-board image resolution remained at full specification throughout 4-hour missions. The design was adopted for the operator’s production survey fleet.