Falk had been a member of the Reserve of Air Force Officers since 1935 and at the start of the Second World War in 1939 he joined the Royal Air Force. By 1943 he was chief test pilot at the
Royal Aircraft Establishment Farnborough. At Farnborough he flew and tested captured German aircraft as well as carrying out operational flights with night fighter squadrons. For his service he was awarded the
Air Force Cross twice, described as the Air Force Cross and bar. ==Delta-wing testing==