PZL Bielsko SZD-31 Zefir 4 / SZD-31 Zefir 3
The SZD-29 Zefir 3 was a much improved version of the SZD-19-2 Mk 2 intended for advanced competition flying, with a wing of 19m (62ft 4in) span, full-span Fowler flaps, a longer fuselage and an unswept fin and rudder; the Mk 3 first flew in prototype form in April 1965 and had an improved best glide ratio of about 42:1.
Its production development was the Zefir 4, the first Mk 4 prototype making its maiden flight on 7 December 1967. It was devleoped for the 1968 World Championships, in which one flown by Jan Wroblewski came 14th in the Open Class.
The wing has an NACA 66-series section and no dihedral, unlike the Mk 2's; it is a multi-longeron sparless stressed skin wooden structure with full-span flaps like the Mk3's, the outer sections of which are wooden and also act as ailerons while the inboard sections are of plywood/balsa sandwich; unlike the Mk 2, double metal air brakes are featured. The fuselage is an oval section monocoque, the forward part being made of glassfibre laminate and the rear portion of wood. The tailplane is an all-moving mass-balanced surface with trim tab, and the unswept rudder is also mass-balanced. The monowheel is manually retractable with rubber shock absorbers and a hand-operated wheel brake and, like the Mk 2, a tail-braking parachute is fitted; unlike the Mk 2, the cockpit canopy is now sideways hinging.
Span: 62 ft 4 in
Length: 26 ft 3 in
Height:7 ft 0 in
Wing area:169 sqft
Aspect ratio: 23.0
Empty weight:772 lb
Max weight:970 lb
Max speed:149 mph (in smooth air)
Max aero-tow speed:93 mph
Min sinking speed:1.97 ft/sec at 58 mph
Best glide ratio:42:1 at 58.5 mph
PZL Bielsko / SZD 31 Zefir 4