Flight test: Le­tov PF-107 Luñák

A post-war, sleek aer­o­batic glider with a long Czech her­itage and a cal­cu­lated Vne of 378 knots!

Pilot - - Contents - Words Dave Un­win

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

An im­me­di­ate hit... clearly su­pe­rior to ev­ery other aer­o­batic glider

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

The Luñák’s Vne was cal­cu­lated to be an as­ton­ish­ing 378kt...

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!

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.

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. 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.

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).

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.

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. 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 mono-wheel is un­sprung, so if the ground is bumpy you’re in for a rough ride, but the ground run is gen­er­ally pretty short.

Some of the dis­plays flown by Luñák pi­lots are still talked about to­day

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. 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!

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.

Pho­tos Keith Wil­son

At the tail end there’s a sim­ple skid... ... and the sin­gle main wheel’s ‘sus­pen­sion’ is all through tyre flex

Unusu­ally for a 1940s/50s glider, the Luñák has both air­brakes and flaps

PHOTO: GRA­HAM SAW Vin­tage glid­ing en­thu­si­ast and dis­play pilot Gra­ham Saw ex­cer­cises his own Luñák, one of just two in the coun­try, close to Wy­combe Air Park

The proper pitot for aer­o­bat­ics, where speed is crit­i­cal Luñák means kite or hawk in English

The snug cock­pit has some­thing of a fu­tur­is­tic, sci-fi look about it. High-tech string is all that’s needed to in­di­cate slip

el­va­tor trim - com­plete with ad­justable fric­tion - on the right, all rather neater than...

the ‘rather crude’ canopy latch

Flap and air­brake levers to the left and....

Lined up be­hind Buck­min­ster Glid­ing Club’s Euro­fox tug...

Dave uses the air­brakes to hold for­ma­tion, ef­fec­tively ‘clos­ing the throt­tle’ by de­ploy­ing them

The tug­gie’s view of the Luñák on tow... ...and Dave glid­ing back in to land

The canopy slides back, fighter-fash­ion

PHOTO: GRA­HAM SAW

Gra­ham Saw demon­strates just how ag­ile the Luñák is with a pol­ished aer­o­batic rou­tine, us­ing smoke can­is­ters to trace his flight path (PHO­TOS VIA GRA­HAM SAW)

With its shoul­der wing and fighter canopy, the Luñák re­ally has a pur­pose­ful look

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