Effective lubrication is essential for the helicopter transmission system to ensure smooth power transfer, reduce friction, and prevent overheating, however, churning loss caused by interactions between lubricating oil and rotating gears decreases efficiency, accelerates wear, and undermines system reliability. To enable quantitative analysis of gear churning losses, this paper proposes an innovative fluid dynamics model that achieves superior prediction accuracy for helicopter transmission compact gearboxes. The core idea is mechanistic decomposition of total losses into drag and pocketing components, with explicit consideration of dynamic oil immersion depth and gearbox wall clearance effects. A custom-designed adjustable test platform for helicopter transmission gearboxes is developed to experimentally validate the model's predictive accuracy under diverse operating conditions, including rotational speeds ranging from 0 to 12,000 rpm and attitude angles of 0°, 10°, and 20°. Further investigations on the influence patterns of oil immersion depth and tooth width on gear pair churning losses are also conducted. Experimental results demonstrate that: 1) Tooth width decisively determines loss mechanisms (end-face and pocketing losses contribute 46% and 10% respectively at 6 mm width, while end-face and pocketing losses contribute 10% and 74% respectively at 18 mm width); 2) The evolution of dynamic oil surfaces with Fr causes variations in immersion area, leading to overestimation of end-face losses. This work advances the design of lubrication systems by bridging theoretical predictions with experimental validation, offering actionable insights for reducing energy losses in high-performance transmissions.