Making its first flight shortly after WWII, the lively LF-107 aerobatic glider showed great promise until production was curtailed by Cold War machinations and politics. Words Dave Unwin | Photos Keith Wilson

The Luñák’s story starts before the second world war. Gliding had been a demonstration sport at the 1936 Berlin Olympics, and was intended to be an event at the 1940 Olympiad. Well-known German sailplane designer Hans Jacob had designed the DFS Habicht for the Berlin games, and Czech Vladimir Stros?possibly influenced by the Habicht?subsequently designed and built an aerobatic glider, the Sokol.

Of course, that Olympiad never took place, but after the war it soon became clear that a market existed for an aerobatic glider and, drawing on his own experience with the Sokol and data from the Habicht, Stros began work on a new design at the Letov factory in Prague.

Called the LF-107 Luñák (Czech for kite or hawk), the prototype made its maiden flight on 25 June 1948. The following month the second prototype was flown by a Major Cervenka of the new Czechoslovakian air force.

Cervenka was very impressed by the Luñák, and urged the Czech Ministry of Defence to buy Luñáks as a trainer for the Czech air force. The following month the prototype made its international debut at an airshow at Grenchen, Switzerland, and was an immediate hit, being clearly superior to every other aerobatic glider.

Orders began to flow in. The Czech Ministry of Defence ordered fifty to train air force pilots in aerobatics, and ‘Kovo Export’ (the Czech bureau tasked with foreign sales) ordered another fifty.

The design was tweaked slightly for series production, with other improvements including a taller fin and rudder (albeit with reduced chord), a slightly greater wingspan, a longer nose and raised canopy bubble. The cockpit layout and instrumentation were also simplified. Contemporary photographs suggest that the two prototypes had a layout more like that of a MiG-15 than a glider, probably because the Letov factory viewed the military as its main market.

Orders continued to come in. A Czech living in Britain called Ladislav Marmol bought the original prototype and displayed it in Britain, France and Belgium. By all accounts his displays were truly remarkable?he once dived the Luñák to almost 245kt and routinely started his display with an entry speed of around 200kt. If you wanted to fly aerobatics in a sailplane the Luñák was the one to have, and the order book soon swelled to over 200.

However, increasing tension between NATO and the Warsaw Pact led to difficulties in accessing Western markets, while the Letov factory was instructed to switch production to licence-built MiG-15s, and only seventy gliders were built. Sadly, because all wooden Eastern Europe sailplanes were considered obsolete in the 1970s, many gliders, including Luñáks, were scrapped and today only about a dozen exist, although most are airworthy.

The test aircraft (hereafter referred to by its registration OM-0973) was built in 1950 (the first zero in ‘0973’ is the clue here), and is S/N 22. As 0973 is always hangared (up in the roof of the Buckminster Gliding Club’s main hangar) I’ve never rigged it, although I believe that there’s no mystery to it as the wings are quite light and everything lines up nicely.

However, it’s apparently quite a slow rig, as (except for the flaps) all the control pushrods must be connected with bolts, castellated nuts and split pins, and then various panels re-fitted.

In common with most other aircraft designed to fly upon the air, as opposed to through it, the predominant materials in its construction are the same as those used in the earliest flying machines?wood and fabric. The cantilever wing uses the NACA 23012 aerofoil, which is a semi-symmetrical section. Thinner at the root than is usual for a 1950s sailplane, it is mid-mounted and uses a single spar of pine and plywood, covered with a diagonal plywood skin, with an auxiliary spar to carry the flaps and ailerons.

The plywood skin extends back to the secondary spar to ensure adequate torsional strength and stiffness. The flaps are of the Fowler type, while the large mass-balanced double Frise ailerons (the inner ones without differential deflection) are interconnected with the flaps and droop slightly when the flaps are extended past the first positive setting.

The wing also carries relatively small Schemp-Hirth type airbrakes that when deployed protrude from both the upper and lower surfaces.

The oval-section fuselage is a wooden monocoque structure, also covered with the same diagonal plywood skin as the wings; the undercarriage consists of a wooden front skid (instead of a wheel brake), fixed unsprung monowheel and steel tailskid. The cantilever tailplane is again skinned with plywood, although the elevators and rudder are fabric covered, with the elevator being fully mass-balanced. There is a trim tab in the port elevator.

As Keith’s fine photos clearly illustrate, the Luñák is a very elegant aircraft, but its graceful appearance belies its strength, for this is an astonishingly strong machine. Stros specifically designed it for aerobatics, and he made sure no one was ever going to pull its wings off. (Like the German-designed aerobatic aircraft of the day, the Luñák used a safety factor of 1.8 and not the normal 1.5 in its flight envelope.)

During static load testing the wing and fuselage were mounted in a jig and tested to destruction, the wing finally failing when the steel pin pulled through the wooden spar’s aluminium alloy end fitting ? at 16.5g!