Coppens joined the army in 1912, serving with the 2nd Grenadiers before transferring to the Compagnie des Aviateurs in 1914. At his own expense, he and thirty nine other Belgians enrolled in a civilian flying school at Hendon, England. After additional training in France, Coppens began flying two-seaters in combat during 1916.

The following year, he was assigned to single-seat fighters and soon became an expert at shooting down enemy observation balloons. After downing a balloon, Coppens would often perform aerial acrobatic displays above the enemy. On one occasion, the balloon he was attacking shot upward and Coppens actually landed his Hanriot HD.1 on top of it. Switching off his engine to protect the propeller, he waited until his aircraft slid off the top of the balloon, then restarted the engine and watched as the German balloon burst into flames and sank to the ground.

On the morning of October 14, 1918, his days as a fighting pilot came to an end near Thourout in northwestern Belgium. Just as he began the attack that would culminate in his 37th victory, Coppens was hit in the left leg by an incendiary bullet. Despite a severed artery and intense pain, he shot down his target and managed to crash land within the safety of his own lines. His badly shattered leg had to be amputated. Before he retired from the army in 1940, Coppens served as a military attaché in France, Great Britain, Italy and Switzerland.