Complex sentence: Aircraft were also operated for test and trails by aircraft companies and the Royal Aircraft Establishment and Aeroplane and Armament Experimental Establishment and one was operated by De Havilland Propellers for research into reversing propellers.
Split into simpler sentences: Aircraft were also operated for tests and trials by aircraft companies, the Royal Aircraft Establishment, and Aeroplane and Armament Experimental Establishment. One was operated by De Havilland Propellers for research into reversing propellers.