Entering service in May 1916 but not named until September, the Nieuport 21 differed from the 17 primarily in the adoption of the lower powered Le Rhône 9C or Le Rhône 9Ga engine. This increased endurance and reduced wing loading and was useful for a high-altitude bomber escort but it was rarely employed as such. In French and especially Russian service, the 21 was commonly used with the Nieuport 17 in the same general fighter role, although it was also useful as an advanced trainer. The 21 has commonly been mistaken for the earlier Nieuport 11, as both lacked headrests and used the same cowling and engines, particularly when some Nieuport 11s were also fitted with molded fuselage sides. Like the 17, the 21 is larger and can be identified by the flying wires, which restrain the upper wings. On the 11 the flying wires merged to a single point, while on the 21 they ran parallel to different points on the lower fuselage.