The 1925 BH-21 single-piston-engine one-seat biplane fighter/racing aircraft was a forerunner of a BH-22 military training fighter. A conventional biplane, the predecessor of the BH-21, the Avia BH-17, was designed by Pavel Beneš and Miroslav Hajn in 1922 as a response to a Czechoslovak Defense Department requirement for a new fighter aircraft. The BH-17 was actually only part of five Avia designs submitted to the Defense Department along with competing designs from the Letov Kbely and Aero companies. After extensive review, the BH-17, one of Avia's three biplane designs, was chosen and limited production initiated for evaluative purposes.
Wings, tailplane, rudder and elevator wood-framed, covered by fabric. The steel-tube skeletal structure of fuselage covered by plywood, and fin fabric-covered.
Testing revealed some deficiencies in the BH-17 and a subsequent redesign in 1924 morphed the BH-17 into its final form as the BH-21 which included straightened interplane bracing and allowed for better field of view for the pilot.
A special training version, designated the BH-22, was also created. Both versions utilized 224 kW (300 hp) Hispano-Suiza 8fb engines built under license by Škoda.
The BH-21 was put into production in 1925 and had a one-year production run which yielded 182 aircraft with 137 being produced by Avia for the Czechoslovak Air Force, one more was built by Avia for SABCA and another 44 were produced under license by the Belgian company, Société Anonyme Belge de Constructions Aéronautiques "SABCA" for the Belgian Air Force.
There were also two experimental variants: BH-21J with Bristol Jupiter engine (predecessor of BH-33) and a single-seat clipped-wing race plane BH-21R with boosted HS-8Fb engine (298 kW/400 hp).
While in service, the BH-21 saw no combat as it was retired before the outbreak of World War II. In spite of this, it served as a stepping stone to the more advanced BH-33 and BH-34 types.
The BH-21's most lasting achievement came when a racing version, the BH-21R, won several national air-race competitions in 1925 sporting an upgraded HS 8Fb engine producing over 298 kW (400 hp).
Avia B.21.96, nicknamed "Red devil", which flew Czechoslovak acrobatic flyer František Malkovský. He died in its cockpith after the crash in Karlovy Vary on 8th June 1930.
Engine: 1 × Hispano-Suiza 8Fb, 227 kW (304 hp) @ 1,850rpm
Wingspan: 8.9 m (29 ft 2 in)
Wing area: 21.96 m2 (236.4 sq ft)
Length: 6.87 m (22 ft 6 in)
Height: 2.74 m (9 ft 0 in)
Empty weight: 720 kg (1,587 lb)
Max takeoff weight: 1,085 kg (2,392 lb)
Maximum speed: 245 km/h (152 mph; 132 kn) at 3,000 m (9,843 ft)
Endurance: 2 hours
Service ceiling: 5,500 m (18,045 ft)
Rate of climb: 521 m/s (102,600 ft/min)
Time to altitude: 5,000 m (16,404 ft) in 16 minutes
Armament: 2 × 7.7 mm (.303 in) Vickers machine guns