Armstrong was first posted to No. 3 Wing RNAS, part of a joint Franco–British force formed to fly bombing raids on German industry. The British contingent was based at
Luxeuil-les-Bains and flew a variety of aircraft, but principally the
Sopwith 1½ Strutter, and flew its first mission on 30 July 1916. In January 1917 nine pilots from No. 3 Wing, of whom Armstrong was one, were selected to become the nucleus of the newly formed No. 3 (Naval) Squadron. Armstrong joined No. 3 (Naval), based at
Dunkirk, in February 1917, to fly the
Sopwith Pup single-seat fighter. He scored his first victory on 6 April 1917, in Sopwith Pup serial no. N6178. Using this same aircraft, on 12 April, he shared his second victory with fellow ace Edmund Pierce. His third win was shared with Pierce and
Arthur Whealy on 2 May, when they set an
Albatros reconnaissance aircraft ablaze. A fourth victory, four days later, an
Albatros D.III sent down out of control over
Bourlon Wood, was shared with Kerby. and on 7 July, a splashed German seaplane six miles north of
Ostend was worth a win apiece for Armstrong,
Joseph Stewart Temple Fall,
James Alpheus Glen, and
Leonard Henry Rochford, and Armstrong became an ace. He then switched to the
Sopwith Camel. Thus mounted, he scored two single-handed wins on 16 September 1917. On 31 December 1917 he was promoted to flight commander. He resumed his victories on 24 January 1918. He went on to score five times in March 1918; the last win, on 24 March, seems to have been a squadron affair, with Armstrong, Pierce, Whealy,
Edwin Hayne,
Frederick Britnell, and three other pilots all being credited with a victory for driving an
Albatros D.V down out of control.
List of aerial victories ==Death in action==