 Complete the following phrase:  Harper joined the Royal Air Force RAF in 1937, and was commissioned as an acting pilot officer on 5 September. He was promoted to flying officer on 12 February 1940, and attached to No. 17 Squadron. On 18 May 1940 he shared in the destruction of a Messerschmitt Bf 110 heavy fighter, and a week later he destroyed a Ju 87 Stuka dive bomber. He was appointed B Flight commander, with the rank of acting flight lieutet, on 26 May. He destroyed another Bf 110 over Dunkirk three days later, during the 