The HB-21, which first flew in 1973, is basically an enlarged version of the HB-3 with an increased wing span and a lengthened forward fuselage seating two people in tandem under a long three-piece cockpit canopy hinged to open sideways.
The HB-3's basic configuration of high wings and a pusher propeller rotating in a cut-out portion of the fuselage is retained. Construction is very similar to the HB-3, the fabric-covered three-piece wooden wings being of birch ply with laminated beech spars; they now have Wortmann aerofoil sections and there are spoilers in the upper surfaces. The fuselage is a steel tube framework covered with glassfibre, and the wooden tail unit has a fixed-incidence tailplane and fabric covered control surfaces. The fixed nosewheel undercarriage has Tost mechanical brakes on the main wheels.
Four prototypes were built which included both the HB-21 R with a 41 hp Rotax 642 two-stroke engine like the HB-3, and the HB-21 L with a 65hp Volkswagen(VW)-Westermayer 1600G 'flat four' motor, the latter being the principal version; both powerplants drove a Hoffman HO 14-175 B 117 LD two-blade fixed-pitch propeller, and both versions have an 11.9 Imp gallon aluminium fuel tank in the wing.
Certification of the HB-21 was granted in February 1978, and a year later 15 had been built.
Span: 53 ft 3.5 in
Length: 25 ft 11 in
Height: 8 ft 6.25 in
Wing area: 204.3 sq ft
Aspect ratio: 13.89
Empty weight: 1,102 lb
Max weight: 1,565 lb
Max speed: 124 mph (in smooth air, power off)
Max cruising speed: 99 mph
Min sinking speed: 3.94 ft/sec at 52 mph
Best glide ratio: 24-26:1 at 65 mph
Max rate of climb: 650 ft/min at sea level
Take-off run: 330 ft
Range with max fuel: 497 miles