An air cooling system for an unmanned aerial vehicle including a propeller (14) driven by an engine (12) has at least one cooling air duct (22) to direct cooling air to cool a vehicle component e.g. a cylinder head. The duct has at least one air inlet and at least one air outlet. Operation of the propeller causes a pressure differential between the air outlet (24,124) and the air inlet (23,123) which draws air through said cooling air duct (22). A cowling (16) can cover at least part of the engine, and can form a plenum and have the supply of cooling air through a front face aperture (164) or side walls (17) of the engine cowl (16).