Designed by Heinz W. Brditschka and built by his firm at Linz in Austria, the HB-3B is a single-seater cantilever high wing monoplane distinguished by a pusher propeller rotating in a triangular cut-out in the glassfibre-covered tubular steel framework carrying the tail. This consists of two steel tubes, the upper one running through the centre of the propeller and the lower one from the bottom of the fuselage just under the propeller disc; the forward fuselage containing the engine and cockpit is likewise a steel tube framework covered with glassfibre. The HB-3 uses the basic wing design of the Kra'he sailplane designed in Germany by Ing Fritz Raab; the wing is a conventional all-wood structure of red pine, spruce and birch ply, with spoilers in the upper surfaces; no flaps or trim tabs are fitted.
Design of the HB-3 started in 1968 and the first of three prototypes made its maiden flight on 23 June 1971, followed by the second on 5 June 1972 and the third on 28 July 1972. Nine production HB-3s had been completed by early 1976, but production ended that year.
The HB-3's standard powerplant was a 41 hp Rotax 642 two-stroke 'flat twin' mounted aft of the cockpit and driving a Hoffman two-bladed fixed-pitch propeller via a belt; the fuel used is a 75/25% petrol and oil mixture, and there is a single fuel tank in the fuselage and an aluminium tank in the wing centre-section, which together provide a total capacity of 8.15 Imp gallons. The tail unit is of all-wood construction, with wire bracing from the fin to the centre section trailing edges; the fixed incidence tailplane has a trim tab in the starboard elevator. The fixed nosewheel undercarriage has glassfibre legs which provide all the necessary shock absorption without the need for separate shock absorbers. The main wheels have Tost mechanical brakes. The pilot sits under a large cockpit canopy that hinges to open sideways to starboard; there is a small baggage space behind his seat and a ram air intake in the nose is provided for cabin ventilation.
Heinz W. Brditschka also converted an HB-3 airframe in 1973 to have electric power, in which form it was known as the Militky MB-E1.
Span : 39 ft 4.5 in
Length: 22 ft 11.75 in
Height: 9 ft 8 in
Wing area: 153.1 sqft
Aspect ratio: 10.11
Empty weight: 573 lb
Max weight: 837 lb
Max speed: 108 mph (in smooth air, power off)
Max cruising speed: 99 mph
Min sinking speed: 3.77 ft/sec at 45.5 mph
Best glide ratio: 20:1 at 50 mph
Take-off run to 50ft: 755 ft
Range with max fuel: 341 miles