The Trener is a modified version of the LIBIS-18 single-seater Standard Class sailplane which first flew on 20 October 1964 and was designed by the Letalski Institut Branko Ivanus, Slovenija – LIBIS. Of wooden construction with cantilever shoulder mounted wings with tapered outer panels, the LIBIS-18 did not go into production.

The VTC Trener differs from it chiefly in having the all-moving tailplane mounted slightly higher on the sweptback fin and rudder, and an improved cockpit layout, the pilot sitting under a flush-fitting one-piece cockpit canopy. The Trener is also of all-wood construction, using beech and Swedish plywood, and has Wortmann wing sections giving a thickness/chord ratio of 18% at the root. The fuselage is a wooden monocoque with a non-retractable monowheel with a brake, andatailskid. An initial batch of 50 Treners is believed to have been ordered by the Yugoslav Aero Club, these being built by VTC at Vrsac.

Span: 49 ft 2.5 in
Length: 22 ft 7.75 in
Wing area: 139.6 sqft
Aspect ratio: 17.35
Max speed: 136.5 mph (in smooth air)
Max aero-tow speed: 93 mph
Min sinking speed: 2.36 ft/sec at 48.5 mph
Best glide ratio: 31:1 at 53 mph

This Yugoslav single-seater high performance Standard Class sailplane was designed by T.Dragovic and Z. Gabrijel, two young engineers at the Faculte des Siences Mathematiques in Belgrade, and the prototype, known as the Delfin 1, made its first flight in December 1963. It made its public debut at the 1965 World Championships at South Cerney in Gloucestershire, and went into production as the Delfin 2 at the Vrsac works of VTC – Vazduhoplovno Tehnicki Centar (Aeronautical Technical Centre).

The Mk 2, which first flew on 26 April 1965, differed from the prototype in having the plywood fuselage covering extended to cover the nose, a smaller sideways opening canopy, a rubber-sprung nose skid added and wooden instead of metal ailerons, and 27 of this version were built, four of which were later converted to Delfin 3s. The Delfin 2 achieved its first real competition success when it came 1st and 2nd in the Polish National Championships of 1966, no mean feat in view of the strength of opposition from the indigenous SZD designs.

The Delfin 3 first flew on 29 July 1968 and has a conventional tailplane instead of an all moving one, reduced fin sweepback and increased rudder area. It is of plywood-covered wooden construction; the cantilever shoulder wings have plywood-covered leading edges and fabric-covered rear portions, and wing tip ‘salmon’ fairings are fitted. There are metal Schempp-Hirth air brakes above and below the wing surfaces at 60% of the chord. The fuselage is a monocoque structure with a small cross-section made possible by putting the pilot in a semi-reclining position under a one-piece curved flush fitting cockpit canopy that opens sideways to starboard for entry and exit. The swept back fin is integral with the fuselage, and the fixed tailplane has a trim tab in the starboard elevator. Landing gear consists of a non-retractable unsprung monowheel with a mechanical brake and a rubber-sprung nose skid, plus a tail bumper.

Delfin 3
Span: 49 ft 2.5 in / 15.0 m
Length: 22 ft 5.5 in / 6.85 m
Height: 5 ft 3 in
Wing area: 138 sqft / 12.82 sq.m
Aspect ratio: 17.55
Wing section: NACA 633 618
Empty weight: 492 lb / 223 kg
Max weight: 717 lb / 325 kg
Water ballast: None
Max wing loading 25.3 kg/sq.m / 5.18 lb/sq ft
Max speed: 155 mph / 135 kt / 250 km/h (in smooth air)
Max rough air speed: 78 kt / 145 km/h
Stalling speed: 32.5 kt / 60 km/h
Min sinking speed: 2.13 ft/sec / 0.65 m/sec at 46.5 mph / 40.5 kt / 75 km/h
Best glide ratio: 31:1 at 54 mph / 47 kt / 87 km/h