In mid July, he transferred to a single seater fighter in 1ese Escadrille de Chasse (1st Pursuit Squadron). He received the last remaining Nieuport 16 in the squadron; everyone else had upgraded to Nieuport 17s. When Hanriot HD 1s were offered to the squadron, he was the only pilot to initially accept one. His enthusiasm about it moved the other pilots to also move over to Hanriots.
On 19 August, Coppens was promoted to adjutant. He continued his nervy but unsuccessful combat career against enemy aircraft until 17 March, 1918. On that day, he carried out his first attack on German observation balloons as an aid to a ground assault by Belgians on Germans. Though handicapped by lack of incendiary ammunition, he punctured two balloons, causing the observers to bail out and the balloons to collapse to the ground.
Finally, on 25 April, Coppens scored his first victory by downing a Rumpler two seater. On 8 May, he finally found his metier, when he shot two balloons down in flames.
A week later, using his usual tactics of close range fire, Coppens cut a balloon loose from its ties. It bounced up beneath him and momentarily carried his Hanriot skyward. After his craft fell off the balloon, he restarted its engine and flew back to base. The balloon sagged into an explosion.
The near miss did not dishearten him. From then on, Coppens made a spectacular record. Between April and October 1918 he was credited with destroying 34 German observation balloons and three airplanes, nearly as many victories as Belgium's other five aces combined. Unlike most fighter pilots of World War I, who used .303 caliber or 7.92 mm guns, Coppens used a larger bore 11 mm Vickers machine gun, having upgraded his weaponry before 5 June 1918.
Also in June, he was promoted to sous leutenant, thus becoming an officer. His royal blue plane with its insignia of a thistle sprig wearing a top hat became so well known that the Germans went to special pains to try to kill him. On 3 August, he shot down a balloon stuffed full of explosives that narrowly missed killing him with its detonation.
On his last mission, 14 October, Coppens was severely wounded by an incendiary bullet and had a leg amputated. For his wartime service he was knighted, becoming Willy Omer Francois Jean Coppens de Houthulst, for a forest in his squadron's operating area. He was decorated by Belgium, France, Britain, Portugal, Italy, Poland, and Serbia.
Between the two World Wars Coppens was Belgian air attaché to four nations. In 1928, he set a parachute jump record by leaping from 19,700 feet; this record stood for 4 years. He retired to Switzerland in 1940. His memoirs, Days on the Wing, were published in 1931 and reissued in the 1970s as Flying in Flanders.
He achieved all his victories flying a Hanriot HD.1 fighter.
A full list of Coppens' victories may be found at http://www.theaerodrome.com/aces/belgium/coppens.php.