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Flight test: Letov LF-107 Lunak

PUBLISHED: 15:30 06 June 2018

Flight test: Letov LF-107 Lunak

Flight test: Letov LF-107 Lunak

Keith Wilson

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.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

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.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

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.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

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!

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

From an aerodynamic perspective, the Vne was calculated to be an incredible 378 knots, while the practical limit was considered to be only 220kt. Sensibly, this was subsequently reduced to 188 and eventually to 162kt in order to provide a long service life.

Even the original maximum aerotow speed was a heady 135kt (well past the Vne of every other glider of similar vintage), although curiously the maximum winch launch speed was quite low at only 65kt. This may well be because Vfe (flap limiting speed) is also 65. Production aircraft were certified with maximum g loadings of +8 and -4.

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Check out these other flight tests:

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Speaking of launching, 0973 currently only has one dedicated aerotow hook just below the big aerobatic pitot. Interestingly, and unlike Western gliders (which typically have a single hook for winch launching which is slightly offset from the fuselage centreline close to the centre of gravity), the Luñák originally had two hooks for winch launching, one on each side of the fuselage.

This is because the East European winch launch method uses a similar system to the strop arrangement used to catapult naval aircraft in the 1950s. I’ve never taken a winch launch in a Luñák, but Graham Saw (who owns the other UK-based airworthy Luñák, which does have the dual C of G hooks) says it winches beautifully, with a very light stick force.

Anyway, having had breakfast it is clearly time for launch so, after a thorough pre-flight inspection and a positive check of all the controls, it’s time to go flying. I first flew 0973 in 2004 and on that flight had opportunity to experience both soaring and aerobatic flight, whereas my two recent flights were all about shooting some strong air-to-airs. Consequently, this flight test report is distilled from these various flights.

The Luñák has probably the coolest canopy arrangement of any glider I’ve ever flown. It consists of a fixed windscreen and sliding bubble canopy, which make it look a bit like a WWII fighter or early jet. This arrangement also makes it a bit easier to climb into the snug cockpit. The rudder pedals adjust and it is necessary to wear a parachute as the seatpan is designed to accommodate one.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

Unlike modern sailplanes, the seating position is quite upright, and very firm. Once strapped in with the five-point harness, you really feel as if you’re a part of the aircraft.

The instrumentation reflects the Luñák’s heritage, being so utilitarian it’s almost military. There’s only an airspeed indicator, altimeter and compass, along with two metric variometers (as delivered from the Letov factory these were simply VSIs, with no flask for total energy compensation) and a prominently-located accelerometer. All the (very few) controls are easy to see and reach.

The short, straight stick falls nicely to hand, while the flaps and airbrakes are operated by two sinuously curved levers on the port cockpit wall, topped with correctly colour-coded black and blue knobs. A yellow cable release handle is logically located at the base of the panel while the elevator trim tab is adjusted by a small lever topped with a green knob which sprouts from the starboard cockpit wall, with a knurled wheel for the friction lock beneath it.

The tug taxies in front, so I duck my head, slide the canopy forward and lock it with the single, rather crude latch. As I do so (and remembering that Marmol regularly used to dive his Luñák at up to 200kt) it occurs to me that perhaps two such latches, one on either side of the bubble, might’ve been prudent.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

For an aerotow launch, it is important to ensure that the trim lever is set forward of neutral, with the trimmer friction nut snug and the stick on the backstop (to avoid graunching the nose skid). The POH recommends taking off with the first stage of flap, and if taking off from grass or behind a weak tug I would agree.

I’ve tried with and without flap, and on tarmac into any sort of a headwind the controls come to life so quickly and the ground roll is so short I honestly couldn’t tell the difference. However, it’s worth noting that compared to many gliders the wingtips are relatively close to the ground, so don’t drop a wing, and if taking off with flap retract them once safely airborne.

The Luñák is very pleasant on tow, and having released into a thermal I move the flap lever to +1 and adjust trim. The excellent field of view, slow speed and powerful controls make it easy to centre in the core of a thermal, while the low wing loading ensures it climbs well. Visibility is excellent and ventilation adequate − there is a fresh air vent above the instrument panel and the canopy can be slid back several inches in flight (although if you do this it is a bit noisy).

When moving between thermals with the flaps retracted it retains a surprisingly flat glide for a 1950s sailplane (every other sailplane of this vintage that I’ve flown starts to fall out of the sky at any speed above 55), and has quite a flat polar curve. This is probably due to the semi-symmetrical aerofoil section (the Fournier RF4 uses the same one).

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

The claimed best lift to drag ratio is a very conservative 24 to 1 at 43kt. This surprises me as its performance seems to be nearer to that of a Ka-6 than a Ka-8; perhaps the Czechs were just a bit more honest in their sales literature than their Western competitors!

That said there is little to be gained from flying at speeds much in excess of 65kt as the glide angle does get increasingly steep. Overall, it’s a delightful aircraft to soar, with crisp, well-balanced controls.

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Here’s a few more flight tests:

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Having gained some height, I try a couple of stalls and spins before moving onto a few basic manoeuvres. At the slow end of the speed envelope the stick forces are so low as to be almost unnoticeable, while the stall itself is very subtle and gentle. The pre-stall buffet is quite subtle, the best indications of an imminent stall being the stick position and the change in the sound of the air flow. The glider spins well and recovers promptly as soon as the correct control inputs are applied.

Moving onto loops and chandelles confirms that the Luñák is a joy to fly. Rolls are quite a difficult manoeuvre in a sailplane but the Luñák is easier than most, possibly because of the longer fuselage and mid-mounted wing.

However, while I am no aerobatic champion Graham Saw is (having won several competitions in his Luñák), and he says “although the roll rate is not as fast as a modern aerobatic glider, such as the Swift or Fox, the stick forces are light due to the Frise ailerons, so entry speeds can be low, due to the light wing loading. Typically 80 to 90 knots for rolls and hesitation rolls and using the huge rudder, 70 knots for rolling circles.”

Some of the displays flown by Luñák pilots are still talked about today. One famous aerobatic routine involved a Luñák performing a simultaneous snap roll with the Zlin Trener tug, while still on tow! Slow rolls on tow became commonplace.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

On another occasion, a helicopter towed a Luñák aloft and then transitioned into the hover, leaving the Luñák hanging tail down. Its pilot released, converted the tail slide into a dive and then performed an aerobatic display.

Back in the circuit the handling is just as precise as every other aspect of the flight envelope; although the airbrakes are on the small side and barely adequate, when used in conjunction with the flaps they’re acceptable, and allow a reasonably precipitous final. Half-flap feels about right.

I understand that using full flap allows quite a slow approach speed but I don’t like low energy approaches in someone else’s sailplane, and if you come in a bit faster with full flap a ‘cushion of air’ builds up under the wing and actually prolongs the float. Between 50-55 feels good, the landing as undemanding as every other aspect of the aircraft. As mentioned earlier, the monowheel is unsprung, so if the ground is bumpy you’re in for a rough ride, but the ground run is generally pretty short.

Many years later, I’m aloft again in the Luñák, getting some photos for this feature. Taking air-to-airs of a sailplane away from a large area of reliable lift is quite tricky. After a not entirely satisfactory first sortie, we agree that, for the second try, EuroFox cameraship pilot Al will tow me up with Keith on board, and as soon as I release the towrope he’ll promptly hit the ‘retract’ button to winch the rope in and simultaneously start slowing down.

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

Then, as soon as the speed is below the door opening limit and the cable retracted, Keith will open his door as Al instigates a gentle turn, allowing me to slide smoothly into close echelon port.

This works well, and as we continue around in a great sweeping circle Keith, ever the professional, looking for the next great shot (and the low autumn sun was producing light that was simply glorious on the Luñák’s yellow fuselage), asks Al to continue the turn.

However, a lifetime leading formations of Phantoms and Tornados has left ‘looking out for your wingman’ ingrained in Al’s DNA. (Basically, the wingman always burns more fuel than the leader, and an inconsiderate leader can easily put their wingman in an awkward situation.) He unapologetically denies Keith’s request, quickly explaining he’s concerned I may not make it back if we do another 360.

I’m blissfully unaware of this. The essence of being in close formation is to only ever look at your leader, and I have no real idea of our height or position, although I do suddenly realise that the ground seems to be not that far below the EuroFox!

Flight test: Letov LF-107 LunakFlight test: Letov LF-107 Lunak

I’m just about to break out of formation, convert my excess speed into height, then trim for best glide and start scanning for the field when I hear Al call “Saltby traffic, Salt formation, left base, 25, Saltby.” A pause and then “Salt Two from One: you have the lead.” I ease out of formation, look around and spot the runway perfectly positioned in my ten o’clock. It’s a welcome sight, and I get busy with flaps and airbrakes.

The Luñák is a fascinating machine.

It is not only an amazing aerobatic aircraft and a fine sailplane (by the standards of the day), but a real piece of aviation history. It’s also great fun to fly! If political machinations and the expediencies of the Cold War hadn’t intervened, I’m sure that the Letov factory would have made a lot more than seventy before the Iron Curtain came down and closed the show−and that’s a real shame.

The author is grateful to Graham Saw for his help in preparing this article.

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