Avro 566 Avenger

The Avro Avenger was a British prototype biplane fighter of the 1920s.
Designed as a private venture by Roy Chadwick, the same engineer who designed the mythical Avro Lancaster, designed a very aerodynamically clear machine, specially for the 1920s.
It was powered by a Napier Lion VIII delivering 525hp of power. It flew for the first time on 26th June 1926 but it didn't convince the Air Ministry because they didn't favour the Napier Lion as a fighter engine.
A second racing version was made in 1928 with revised struts and ailerons and equi-span wings and powered by a 553hp Napier Lion. It was known as the Avro 567 Avenger II. This second racing airplane, ended it's days as an instructional airframe.

Sources:
1. https://en.wikipedia.org/wiki/Avro_Avenger
2. Salamander Books - The Complete Book of Fighters

Avro 531

The Avro 531 Spider, was a British fighter biplane prototype.
It was a conventional configured sesquiplane with warren-type interplane struts.
When tested, it showed excepcionally good rates, handling and visibility, anyway the war office had already selected the Sopwith Snipe for mass production and had discarded this one.
A second version, the 531A was planned but never completed with some of it's components used to build the Avro 538 which was conceived as a racing airplane and had normal interplane struts. However it was never used with that purpose as it had a faulty wing spar, so it was used as a hack by Avro from May 1919 until September 1920.

Sources:
1. https://en.wikipedia.org/wiki/Avro_531_Spider
2. Salamander Books - The Complete Book of Fighters

Avro 530

The Avro 530 was a British two-seater fighter biplane prototype that was designed in 1916 and completed in July 1917.
It was designed to compete against the Bristol F.2A.
It was made out of wood and was covered in fabric and powered by a 200hp Hispano-Suiza 8Bdeight-cylinder water-cooled engine.

When tested, it proved to be a good airplane with good performance rates, however it didn't significally improve the Bristol F.2A performance rates and therefore no production orders were issued.
Also should be considered that back in 1917 the Hispano-Suiza engine's priority was the RAF SE.5a. In order to cope with that, a second prototype was started to be built, powered by a Sunbeam Arab engine, but it's development was abandoned.

Sources:
1. https://en.wikipedia.org/wiki/Avro_530
2. Salamander Books - The Complete Book of Fighters

Avro 527

The Avro 527 was two-seat fighter which was the last derivative of the 504 design. It was built for the RFC and was ready to fly in early 1916.
It was powered by a 150hp Sunbeam Nubian eight-cylinder engine water-cooled engine. It copied both mainplanes and tail assembly from the 504, but the rest differed greatly from it.
It was going to be armed with a free-mounted 0.303in Lewis Machine Gun in the rear cockpit.
When tested, it showed a poor climb rate and the pilot's visibility was seriously reduced by the twin exhaust stacks of the engine and the radiator.
Only one prototype was built and, due to the uninterest of the RFC, development on it was discontinued during 1916. It was proposed a version with a longer wingspan, the 527A, but there aren't plausible records of it being matherialized.

Sources:
1. https://en.wikipedia.org/wiki/Avro_527
2. Salamander Books - The Complete Book of Fighters

Avro 523 Pike

The Avro 523 was the first Avro airplane ever to receive a name and flew for the first time in May 1916. It was designed to an admiralty requirement for a long-range airplane that could perform anti-zeppelin duties and escort duties, as well as scouting and light bombing ones.
The pilot seated ahead of the engines and the gunners seated both in fore and aft. Each of them had a twin 0.303 Lewis Mk.II machine gun. It was made out of wood with fabric skinning and the first prototype was powered by two 150hp sunbeam eight-cylinder liquid-cooled engines mounted in a pusher configuration. A second prototype was built, the 523A, which was powered by a pair of 150hp Green six-cylinder liquid-cooled engines and a scarff-ring for the forward mounted machine guns.
After testing, the admiralty jugded it as obsolete and didn't place any order. Both prototypes were used as testbeds for the remainer of the war.

Sources:
1. https://en.wikipedia.org/wiki/Avro_523_Pike
2. http://www.aviastar.org/air/england/avro-523.php
3. Salamander Books - The Complete Book of Fighters

Avro 521

This is a special post as it's hasn't any drawing in it because we couldn't find any blueprint for this airplane.

The Avro 521 was flown on late 1915 was an hybrid that had some components of the original Avro 504. It was powered by a 110hp Clerget 9Z nine-cylinder rotary engine and could've been armed with a single 0.303 lewis machine gun mounted on the rear cockpit. It was tested in early 1916 by the Royal Flying Corps but the contract was cancelled and it seems that only the prototype was built which crashed at Upavon on 21st September 1916.

The Avro 521

Sources:
1. https://en.wikipedia.org/wiki/Avro_521
2. http://www.aviastar.org/air/england/avro-521.php
3. Salamander Books - The Complete Book of Fighters

Polikarpov MU-1

The Polikarpov MU-1 was a basic floatplane Soviet trainer airplane.
It was powered by the 120hp M-2 engine which was also a copy of the original British one and was produced from 1925 until 1929.
Due to the larger, but lighter floats, it was 170Kg lighter than the original 504L which was the original floatplane version of the 504K.
It only served with the Soviet Workers and Peasant's Air Fleet as a trainer in the Sevastopol Maritime Flying School and saw also service in the training squadron of the Baltic Fleet, based in Leningrad during the 1920s. Apparently it was also briefly used as a scouting airplane for the Baltic Fleet. Seventy three machines were made and some of them served until 1934.

Sources:
1. https://ru.wikipedia.org/wiki/У-1 (translated)
2. http://www.airwar.ru/enc/other1/mu1.html (translated)
3. http://forum.valka.cz/topic/view/108785

Polikarpov U-1

Preliminary note: This is some kind of special post as we're covering the Russian copy of the Avro 504K because it has some very interesting backstory.

The Polikarpov U-1 was the Soviet unauthorized copy of the Avro 504K. It all started during the Russian civil war, when am Avro 504K belonging to the British-Slavonic Air Corps (which was sided with the White Russians) crashed near the city of Petrozavosk and the engineer Sergey Ilyushin received the order of going to that zone to recover the airplane and dissasemble it. It was re-assembled again and pushed into service for the remainer of the war, with such good results that another engineer, Nikolai Polikarpov decied to copy it. Under Soviet command, it showed good results, as it served in their different pilot schools from 1921 until 1932 and in some cases until 1935. It also set a new record in the Soviet Air Force as it was the basis for the first experiments with solid propellant rocket boosters takeoff.

It also served in many Asiatic countries. Apparently it served with the Chinese Aviation Ministry, with the Imperial Iranian Air Force and with Mongolia.
The Persians bought them together with some Polikarpov R-1 and R-2, as the United Kingdom refused to sell them airplanes alleging that the terms of the treaty of Versailles forbid the sale of weaponry.
Apparently it also was the backbone of the early Mongolian People's Army Aviation as it served as their main trainer during the 1920s and most probably during the 1930s.

Sources:
1. https://ru.wikipedia.org/wiki/У-1 (translated)
2. http://www.airwar.ru/enc/other1/u1.html (translated)
3. http://defence.pk/threads/imperial-iranian-airforce.138734/
4. http://forum.valka.cz/topic/view/108784

Avro 504R Gosport

The Avro 504R was a mixed modification in order to update the original 504K. It featured the same skid as the 504K, but had the engine and wings of the 504N. It was presented to the RAF but it wasn't found suitable for the training role due to the low reliability rates.

It was sold to Argentina, where it was locally produced by the Fabrica Militar de Aviones (Military Airplane Factory), Estonia, where it served until 1940 and the Peruvian Air Force, where also some ex-Argentinean made machines ended-up.

Sources:
1. http://www.airwar.ru/enc/other1/avro504r.html (translated)
2. Salamander Books - The Complete Book of Fighters
3. https://en.wikipedia.org/wiki/Avro_504

Avro 504O

The Avro 504O was a floatplane version of the 504N which was made in 1925 at Manchester.
It was powered by the same Armstrong Siddeley Lynx IVc which could deliver up to 180hp of power.

• Brazil: The Brazilian Navy bought some 504O to equip some floatplane squadron. Anyway further information is unknown so the drawing should be considered as speculative
• Chile: The Chilean Navy also had some 504O in service, most probably to replace the previous outdated 504L floatplanes. Unfortunately any graphical info seems impossible to find and the drawing should be taken as speculative.
• Greece: The Greek Navy bought some of them in 1925 together with some 504N and they even produced some of their own in the Kratiko Ergostasion Aeroplanon (State Aircraft Factory) in 1935 before switching to the more advanced Avro 621 Tutor. The 504O served, just like the 504N until 1936.
• Japan: The Imperial Japan bought some 504O most probably to replace the outdated 504L. However as we couldn't find precise information, the drawing should be taken as speculative.

Sources:

1. https://en.wikipedia.org/wiki/Avro_504

2. Salamander Books - The Complete Book of Fighters

3. http://greek-war-equipment.blogspot.com.es/2010/10/1920-1936-avro-504.html

4. http://forum.valka.cz/topic/view/108778

Avro 504N - European Users

Th Avro 504N was an improved version of the Avro 504K, with a new engine and some other aerodynamical changes which consisted on a new undercarriage, new wings, a new engine and the removal of the skid. Some internal improvements were also made like the inclusion a direct gravity feed for gas.

It was used by many European countries as it became their standard trainer airplane and it was license built in Belgium by SABCA during the late 1920s until mid-1930s, and in Greece by the Kratiko Ergostasio Aeroplanon (State Aircraft Factory) from 1934 until 1935.

It was powered by either a 150hp or 180hp Armstrong Siddeley Lynx IV engine.

• Belgium: It was the backbone of their trainer squadrons during the 1930s decade. Some of them were still in service when Germany invaded Belgium in May 1940.
• Denmark: The 504N was also the main trainer of the Royal Danish Air Corps during the late 1920s until the mid 1930s. It's worth to mention that these airplanes were equipped with a cowling in the back seat in order for the student to learn how to "blind pilotage".
• Greece: The Greeks ordered some 504N in 1925. 
• Sweden: In December 1925 the Swedish Army Aviation Company ordered five 504N which were delivered in April 1926. They served under the denomination of Sk-3 and Sk-3A in the Flying School at Ljungbyhed. Surprinsingly, they served only until 1928 as they couldn't cope wheel with the harsh cold Swedish weather.
• United Kingdom: The 504N replaced the 504K as the main RAF trainer during the 1920s.

Sources:

1. https://en.wikipedia.org/wiki/Avro_504

2. http://www.natureandtech.com/?cat=109

3. http://forum.valka.cz/topic/view/108754

4. http://greek-war-equipment.blogspot.com.es/2010/10/1920-1936-avro-504.html

5. http://www.avrosys.nu/aircraft/Skol/403sk3/403Sk3.htm

6. http://aviadejavu.ru/Site/Crafts/Craft33477.htm (translated)

Avro 504N - African & Asian users

On this second part of the 504N, we're covering both African and Asian users of it.

The South African Air Force employed some of them in order to replace the previous 504K versions. Unlickly, further details about the duration of their service period, and so on are unknown, so the drawing too, should be taken as speculative.

The Royal Thai Air Force, bought some 504N to equip their trainer squadrons. The served alongside the bomber Martin B-10 and the fighter Boeing P-12, so they served, most probably during the mid-to-early 1930s and were replaced by the De Havilland Tiger Moth.

Sources:
1. https://www.aviationmegastore.com/martin-b10-dh82a-tiger-moth-avro-504n---boeing-p12e-royal-thai-air-force-ssn72040-siam-scale-decals-ssn72040-aircraft-scale-modelling-decals/product/?action=prodinfo&art=84134 
2. http://forum.valka.cz/topic/view/108754
3. Salamander Books - The Complete Book of Fighters

Avro 504N - American Users

The Avro 504N was an upgraded version of the previous 504K and it was used by the next American users:

• Argentina: Apparently the Argentinean Army Air Service bought at least one of them to test it. Further details are unfortunately, unknown.
• Brazil: The Brazilian Navy's Air Service had some of them under service, most probably replacing the old 504Ks. However as we couldn't find further information, the drawing is speculative.
• Canada: The Royal Canadian Air Force had some 504N in service. Further details are unknown.
• Chile: Apparently it was also used by the Chilean Army Air Service, however any further detail is unknown, so it should be considered as speculative.

Sources:

1. http://forum.valka.cz/topic/view/108754

2. Salamander Books - The Complete Book of Fighters

Arado E.500

This is a special post, as we discovered thanks to the comments, that we had forgotten this airplane, so we are amending that error.

The Arado E.500 was a concept for a heavy Zerstörer airplane dating back from 1936. It would've been manned by a crew of four, a pilot, a co-pilot a dorsal turret gunner and a ventral gun station gunner.
It would've featured a twin-boom configuration with high-mounted wings and running through each engine nacelle and spanning across the top of the gondola type fuselage.
Apparently it had an excellent field of view for every member of the crew except for the ventral gunner who would've laid in prone position and would've controlled using remote controls and a periscope for aiming and firing. It was going to be powered by a pair of Daimler-Benz DB.603 engines placed at the extreme aft of the booms and the tail booms weren't joined but were indepents. The ventral turret had two 20mm guns and same goes for the dorsal one.
Only a 1:1 scale model was made and it seems that it was intended to serve as a flying anti-aircraft battery, just like the American Bell YFM-1 Airacuda.

Sources:
1. https://de.wikipedia.org/wiki/Arado_E.500 (translated)
2. http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=664

Avro 504L

We're back with the enormous variants of the Avro 504, this time with the seaplane recon version of it.

The Avro 504L was a twin-seater recon biplane that flew for the first time on 1919. It was powered by either a 110hp LeRhône 9J, a 130hp Clerget 9B or a 150hp Bentley BR.1 and had the particularity that it wasn't used by the United Kingdom but by some Commonwealth countries and some other users as well.

At least 33 of them were built, being 16 converted from the 504K.
It was used by these countries:

• Australia: It served as the embarked recon airplane on board of both HMAS Australia and HMAS Melbourne Battlecruisers.
• Chile: Apparently the Chilean Naval Aviation some of them during the 1920s. As we couldn't find graphical evidence of this, the drawing is considered as speculative.
• Japan: The Empire of Japan employed some 504L most probably during the 1920s. It was also locally manufactured under license by Nakajima.
• New Zealand: The New Zealand Permanent Air Force employed some of them, during the 1920s too.

Sources:

1. http://forum.valka.cz/topic/view/108750

2. http://www.navy.gov.au/aircraft/avro-504l

3. http://www.nzmuseums.co.nz/account/3031/object/355965/black-and-white-display-print-of-walsh-brothers-flying-school-avro-504l-h2990-on-the-water-at-missi

4. Salamander Books - The Complete Book of Fighters