The Nieuport XI was a prototype single-seat sport monoplane similar to Nieuport II, but with constant chord wings. The Nieuport II was a single-seat sport/racing monoplane powered by a variety of engines.
WIKIPEDIA:
Nieuport I - retroactive designation for pod and boom tractor monoplane, destroyed during the 1910 Great Flood of Paris after one flight.
Nieuport II - single-seat sport/racing monoplane powered by a variety of engines.
Nieuport III - two-seat sport/racing monoplane with Anzani engine.
Nieuport IV - two-seat sport/racing monoplane.
Nieuport VI - three-seat sport monoplane used by French Navy and Royal Naval Air Service.
Nieuport VIII - two-seat sport monoplane, variant of VI for Turkey.
Nieuport X - three-seat monoplane similar to VI but with constant chord wings, used by French Navy.
Nieuport XI - prototype single-seat sport monoplane similar to II but with constant chord wings.
Nieuport XIII - prototype armoured monoplane similar to the X
The designation system of Nieuport’s aircraft is clearly explained in a book Nieuport Aircraft of World war One by Ray Sanger – page 6.
In brief:
Initially, airplanes were designated with Roman numbers supplemented sometimes with capital letters to indicate their purpose (for example M like Militaire) or the type of engine (for example Nieuport type IIN – it was Nieuport II fitted with Nieuport engine, Nieuport type IID – it was Nieuport II with Darracq engine)
First Nieuport biplanes were derivatives of monoplanes and they had been given designations after their predecessors, supplemented with letter B to indicate they were biplanes.
Roman numbers were replaced by Arabic numerals in 1917.
Contemporary designation of the plane we know as Nieuport 10 was Nieuport type XB because it was biplane derivative of Nieuport X. The same rule applies to the aircraft we know as Nieuport 11.
So, there were:
- Nieuport X – a monoplane built in 1913,
- Nieuport XB later renamed Nieuport 10 – a biplane built in 1914,
- Nieuport XI – a monoplane built in 1913,
- Nieuport XIB later renamed Nieuport 11 – a biplane developed during the Great War.