The operational range and payload of military unmanned aerial vehicles (UAVs) depend strongly on the selected propulsion system. While electric multirotor drives are well suited to short range reconnaissance, long range missions increasingly rely on propeller driven piston engines. This article examines the suitability of two and four stroke piston engines for military UAVs and identifies the most frequent failure modes affecting propulsion system reliability. The research niche addressed is the reliability oriented design of small UAV piston propulsion systems based on component level damage analysis. The purpose of the research was to assess whether piston internal combustion engines can meet long range mission requirements while maintaining acceptable reliability, and it was hypothesised that reliability is primarily constrained by propeller related vibration and thermal overload of key engine components. The study combines a literature review with experimental bench and airframe mounted tests conducted in the Aircraft Propulsion Laboratory, including thrust and temperature measurements and post test inspection of components. Damage analysis covered the propeller and hub connection, drivetrain, crank–piston mechanism and exhaust mounting. The results indicate that the propeller, its mounting and transmission elements are the most failure prone parts; defects such as delamination, imbalance and resonant vibrations rapidly propagate loads to bearings, rings and cylinder surfaces, and may contribute to overheating of cylinder heads above 200°C. The paper outlines practical design and technical protection measures to mitigate these risks, supporting the use of piston engines for UAVs with ranges of approximately 2000 km and above.