Complex sentence: During World War I, forward firing Lewis Guns were frequently mounted on a Foster mounting on the top wing of a biplane to fire over the revolving propeller, due to the difficulty of synchronizing this type of weapon to fire through the propeller arc.
Split into simpler sentences: During World War I, forward firing Lewis Guns were frequently mounted on a Foster mounting on the top wing of a biplane. This was so they fired over the revolving propeller, due to the difficulty of synchronizing this type of weapon to fire through the propeller arc.