The LAK-12 is a well-made, Lithuanian Open Class, high-performance,
single-seated sailplane that has only become available relatively recently
outside of the Soviet Union nations. It is manufactured at an aircraft factory
in the Lithuanian Republic and is fabricated of modern epoxy fiberglas and
carbon fiber materials. The initials LAK stand for Lithuanian Aircraft
Constructors, and it was founded during the 1960's. Formerly, the entire LAK
production was utilized by the Soviet Nations for their state-supported gliding
schools and clubs, and therefore was not available to the other countries of
the world. It has been in production in Lithuania for about 10 years, and about
230 have been built, but only in recent times have they become available in the
U.S. When Eduardo Iglesias of Huston, Texas, offered his recently purchased
LAK-12 for flight testing at Caddo Mills, we quickly accepted his kind offer.
His LAK-12 has serial number 6199, and it was built in January of 1992. It was
essentialy new except for a single, 23 minute initial test flight. It had been
consigned to storage after manufacture because the Soviet Union had to cease
their sailplane purchases from the LAK factory that year:,When that happened, I
am told that most of their sailplane production was consigned to storage until
markets could be developed. Subsequently, the LAK-12 sailplanes began to appear
on the world sailplane markets. The LAK-12 sailplanes were attractively priced,
and when Henry Gaudet began his role as U.S. dealer / representative for the L
AK sailplanes, the LAK-12s began to arrive in the U.S. It is robust, high
performance sailplane enjoyable to fly, and affordable for many. Altrought its
wing design uses of somewhat dated but proven Wortman airfoiles from the 1960',
its endowed with modern, carbon fibre wing caps, a reportedly crashworthy
cocpit, and innovative low drag fuselage nose ventilation air inlet. Also, it
is finished with hopefully trouble-free, white epoxy paint, insead of the usual
beautiful but prone to crack, polyester gelcoat finish.
It is true that in contest flying the LAK-12 does not have all the
performance that the current expensive 25- to 27-meter Open Class sailplanes
enjoy Also, since the U.S. Open Class rules currently do not provide any
performance handicapping, the LAK-12's only logical U.S. contest role is in the
Sports Class an enjoyable and popular class for those who enjoy competition
flying. However, the LAK-12's most important role is that of a high performance
recreational, pleasure, and cross-country soaring sailplane. Figure 1 is a
3-view of the sleeklooking 20.43-meter (66.98 ft) wing span LAK-12 sailplane.
The long two piece wing is constructed principally from fiberglass and epoxy
resin, but to add stiffness and reduce weight, carbon fiber is utilized for the
wing spar caps. Each wing panel weighs about 230 pounds; therefore, at least 3
people or equivalent support equipment are needed for its assembly to the
fuselage. Only the flaps connect automatically, but the manual connections for
the ailerons and air brakes utilizethe same rugged and reliable connectors that
many Polish sailplanes currently use. The elevators connect automatically when
the two halves of the horizontal stabilizer are attached to the vertical fin.
The single-piece, 230 lb wing panels are heavy to handle without support
equipment, but they are only about 10 lbs heavier than the ASW-17's inner wing
panels alone. By eliminating the wing panel joint, the LAK-12 reduces the
sailplane's drag, weight and cost, though at the expense of a longer trailer.
The wing uses the well-proven Wortmann 67-K-170 airfoil at its root, tapering
to the Wortmann 67-K-150 airfoil at the aileron root, and thereon along the
entire aileron span. Those airfoils were used very successfully with the Nimbus
2 and PIK20 sailplanes, except that they were prone to larger than average drag
increases when contaminated with bug impacts, or flying in rain. I was able to
fly the LAK-12 through some rain showers on a thermaling evaluation flight and
was pleased to note that the sailplane's sink rate did not seem to increase as
badly as that of the Nimbus 2, and certainly not as much as that of the
thicker-winged PIK-20. The wing thickness-to-chord measurements of our test
sailplane showed .1756 near the wing root, tapering to .1607 at the airbrake
tip, to .1530 at the aileron root, and .1562 at the aileron tips. Our test
sailplane's wing surfaces were remarkably smooth, especially considering that
the sailplane was 4 years old and had spent one hot summer in Texas. Wave gage
chordwise measurements showed less than .005 in (.125 mm) peak-to-peak average
waviness on the wing top surfaces, and about .004 in (.10 mm) on the bottom
surfaces. The LAK-12 does not use a polyester gelcoat finish, but is finished
with a white epoxy paint. No paint cracking was visible anywhere on our test
sailplane.
The first two test flights were performed with a drag rake mounted
of the first flight was to determine 4.5 feet out from the fuselage on the
which flap settings were optimum at each of the planned flight test air-
speeds. The second flight was used to determine if a .50 mm high ZZ turbulator
mounted on the wing bottom surface at .70 chord would reduce the wing profile
drag. It did not; therefore, no further turbulator testing was performed. The
test data from the first flight is shown in Figure 2, as drag probe indicated
airspeed (delta pressure) versus sailplane calibrated airspeed, for each of the
5 test flap settings. Note that the +1 (+4.1 deg) flap setting provided
relatively low wing drag below 44 kts flight airspeed, but not quite as low as
that measured with the zero flap setting. Also, between 48 and 75 kts, the -1
(-4.0 deg) and full negative -2 (-6.5 deg) flap settings were equally good.
That indicated that it would make little difference in the sailplane's wing
drag over the 48 to 75 kt airspeed range if the wing flap angle was set to -1
or 2. However, the fuselage and tail surfaces also contribute to the
sailplane's drag. Based only on intuition, it was decided that for the
subsequent sink rate flight test measurements we would shift the flap to -1
when we reached 48 kts, and -2 flap setting when we reached 75 kts.
Next, we calibrated the LAK-12's airspeed system. That was
performed in two steps. The first was a ground test where the LAK's airspeed
static and pitot lines was checked for leaks. None were found. Then the accuracy
of the sailplane airspeed indicator was calibrated to measure its instrument
alone errors. That was accomplished by connecting the LAK-12's tail fin pitot
to our master calibrating ASI with a long vinyl tube, and using a squeeze bulb
and valve connected into the tail fin pitot line to carefully provide various
calibration air pressures to both instruments simultaneously. Our master ASI
was recently calibrated on our water manometer test stand. The LAK-12's
airspeed indicator (ASI) proved to be accurately marked to within about 2 kts
over our 35 to 125 kt calibration range. The second step was to install the
master calibrating ASI temporarily in the cockpit with its high pressure inlet
connected to a Kiel tube pitot taped to the side of the canopy, and its low
pressure inlet connected to a static pressure measurement "bomb" by
50 feet of 7/32 inch O.D. vinyl tubing (see Reference A). After towing to 8,000
feet altitude, the static bomb was deployed through the cockpit side vent
window to the full length of the vinyl tubing. Then constant airspeed runs are
performed where the sailplane's corrected ASI readings are compared to the
temporarily installed master ASI. The differences are called airspeed system
errors, and the LAK-12 measured error values are shown in Figure 3. There it is
shown that the LAK-12's airspeed system errors vary between about -2 to +2 kts
over the 36 to 110 kt calibration range, and that is a relatively good system.
he values presented in the Figure 3 airspeed system error calibration curve do
not include the ground measured ASI instrument errors. They do not include any
instrument alone errors because every instrument has some unique and
individual, but generally small, errors. The LAK-12 ASI static sources are
located on the aft fuselage sides where the static pressure errors are
generally small, except when at high angles of attack. At high angles of
attack, the air cross flow around the fuselage sides create a suction at the
static orifices. That causes the ASI to indicate higher than true values. That
effect can be seen in the Figure 3 calibration curve, where at airspeeds below
37 kts, the error curve begins a rapid descent, and the ASI reads higher than
true. The tail fin mounted pitot is usually error free, except at high,
positive, angles-of-attack where the wing/fuselage wake makes the ASI readings
jumpy and indicate lower than true values.
When using our normal calibrated altimeter and stopwatch method of
sink rate measurements, still smooth air and high tows are needed. To find the
air still enough for good sink rate measurements with a high performance
sailplane, it is generally required that the winds be less than 15 kts all the
way up to 12,000 feet, and that no large changes in wind direction occur over that
altitude range. As is often the case, either the sailplane availability or our
patience v:cai5 :,ut before the ideally calm testing day arrives. In that case,
there is an inevitable scatter in the sink rate data measurements, and
additional test flight data is needed to fair a curve through the data points
with reasonable confidence. The upper air winds were 20 to 25 kts when I made
the LAK-12's first two sink rate test flights (flights 4 and 5), and some what
higher when I made the following test flights 6 and 7. The sink rate data from
flight 6 was too scattered to be useful; therefore, it was discarded. A 5th and
final sink rate test flight was performed under fairly good conditions on 15
March. The data from the 4 sink rate test flights were considered adequate for
our evaluation, but not as good as they would have been if we had waited for
the raYe Texas winter days where the winds were light all the way to 12,000
feet. The sink rate test data from the 4 acceptable test flights are shown in
Figure 4. A line drawn through the center of those test data indicates that the
manufacturer's claim of 47:1 is somewhat modest, and perhaps 50:1 is closer to
that which we actually achieved. Our unballasted test data indicate a minimum
sink rate of about 90 ft/min at 42 kts, an L/D max of about 50 at 46 kts, and a
sinking speed of only about 290 ft/min at 80 kts The sailplane was essentially
in factory delivered condition, except for some canopy sealing that Eduardo had
added. Since the wing flap and ailerons are bottom surface hinged, their joints
were adequately sealed with simply a factory installed spanwise strip of
adhesive tape applied along their bottom surface joint with the wing. No Mylar
or other air seals were installed along their top surface joints, as that was
apparently unnecessary. No turbulators were installed anywhere.
The cockpit is roomy in width but not in length or height. The
rudder pedals are inflight adjustable over a range of about 5.9 inches (150 mm)
and the seat back is ground adjustable over a range of about 5.0 inches (127
mm). My height is a moderate 70 inches (1.80 m), and when wearing a modern
backpack parachute, I need to set the rudder pedals full forward and the seat
back in its aftermost notch. Even when sitting on a modest seat pan cushion, I
have only about one-half inch clearance between my head and the canopy. The
adjustable seat back can be removed from the cockpit, and that w ill provide
about 4 more inches to the cockpit length. On the positive side, the cockpit
seating provides good thigh support, and it is comfortable during long flights.
I flew one 5 hour test flight without any discomfort. Yes, the LAK-12 comes
equipped with a welldesigned male urinal system stowed neatly in a recess under
the ,forward seat pan. The controls are well-configured within the cockpit, and
not difficult to operate, except possibly for the thumb-operated locking pin on
the landing-gear slide tube. The handle must be held straight down in order to
engage or disengage the locking pin. The one-piece, forward-hinged canopy has
good optics and the pilot's visibility is quite good. Only one small area of
the canopy above the right side of the instrument panel showed significant
distortion, but that did not have much effect on overall visibility, and I
hardly noticed it. The canopy is moderately heavy, and it does not have a gas
strut or spring system to balance it during opening and closing. A
springloaded, over center strut is provided to support the canopy when it is in
its open position, and the strut can be unlocked when seated in the cockpit by
pulling on an instrument panel mounted tee handle. Unfortunately, the red
painted canopy jettison handle is identical in shape to the black canopy strut
release handle, and it also is mounted near the center of the instrument panel.
One must be careful to not actuate the wrong handle! It is necessary to support
the canopy with one hand before pulling the black over center release handle,
or the canopy will come crashing down and possibly injure the pilot or break
the Plexiglas. It appears that a linear viscous damper or gas strut could
easily be attached to the hinge mechanism, and that would greatly improve the
safety of the canopy operating system. The retractable landing gear appears to
be strong, and the wheel is a well sized 350 x 150 mm (13.8 in. O.D. by 5.9 in.
wide) unit equipped with an internal drum brake that functions well via a
squeeze handle on the control stick. Commendably, the landing wheel is mounted
on a well-designed, air-oil oleo strut, possibly the same unit that is used
with the 1-23 Super Blanik. The wheel well is fully sealed, and that keeps air,
dirt, and wind noise from entering the fuselage interior. The landing wheel is
not located very far ahead of the sailplane's flight center-of-gravity, and I
found that I could not apply very much braking before the tail would rise. No
tail wheel is provided, just a simple skid. The LAK-12's tall vertical fin
provides good directional static stability, but unfortunately that also tends
to cause the sailplane to weathercock into the wind during crosswind
operations. For those who desire a tail wheel (me included), it does not appear
that it would be difficult to install one. A good, low-drag, solid-rubber tail
wheel, such as the ones that Dick Brandt made for my Ventus A and Nimbus 3,
make crosswind takeoffs and landings much easier to control. The aileron stick
forces are somewhat heavy, but the ailerons are moderately effective and no ing
dropping tendencies were noted during takeoffs. When flying at 50 kts with +2
thermaling flap setting, plus to minus 45 degree rolls can be performed in
about 6.5 seconds. The wings have a fairly high inertia in both roll and yaw,
as do most largespanned sailplanes, and that takes a little time to become
accustomed to. It took me 8 flights and about 25 hours of flying to become
completely at ease when flying the LAK-12. A 200 mile cross-country flight was
happily achieved by the author during his last test flight, using late winter
Texas thermals. Because of the high wing inertia and relatively slow control
response, extra care and planning must be taken when maneuvering around other
sailplanes or entering occupied thermals. The large rudder is powerful, and,
especially during takeoff, one must understand that any rudder or otherwise
induced take more time to halt than will a smaller-spanned sailplane. The
ailerons droop and raise in unison with the wing flaps, which is
aerodynamically efficient. The stall characteristics are relatively gentle, and
there is not much tendency for the sailplane to roll during a stall unless
rudder is incorrectly applied. At my 980 lb gross weight, a light buffet could
be felt when flying below 40 kts By applying further aft stick, the LAK-12 will
wallow and mush in a similar manner to that of the remarkable Schleicher
ASW-17. Airspeed indications down to about 35 kts could be achieved with +2
thermaling flap for brief periods during level flight with little tendency for
the sailplane to roll into a spin. The airbrakes are upper-surfaceonly,
double-plated, Schempp-Hirth type devices that are easy to operate. Because the
sailplane is relatively heavy, their effectiveness for glide path control is
only moderate. Therefore, care must be taken to avoid overly high approaches
when landing, or the resulting excessive airspeed will require a long landing
run. The Flight Handbook indicates that a total of 190 liters (50 gal) of water
ballast can be carried in the wing leading edges. That can bring the sailplane
s gross weight up to its full certified value of 1433 lb (650 kg). Because of
time and expense considerations, we did not include any ballasted flight
testing of the LAK-12. In summary, the LAK-12 appears to be a well-made and
robust sailplane with good value performance wise for its cost. I would
recommend adding a tail wheel to help keep it straight during crosswind
takeoffs and landings. Also, some sort of canopy hinge damper or gas strut
support would be highly desirable for cockpit safety and ease of operation. Thanks
go to Eduardo Iglesias for bringing his fine and almost new LAK-12 sailplane to
Caddo Mills for testing, and to the Dallas Gliding Association for providing
the hangarage and high tows needed.
Recently the LAK factory has undertaken to complete the development
and start production of the American designed Standard Class Genesis sailplane.
A prototype has been constructed there and test flying is said to be underway
We are looking forward to seeing this new source for that interesting new sailplane
soon.
FIGURE 1. Three-view Drawing of LAK-12
FIGURE 2. Effect of Flap Setting on LAK-12 Wing indicated Profile
Drag
FIGURE 3. LAK-12 Airspeed System Calibration
FIGURE 4. LAK-12 N10LT Polar Test Data
· LAK-12 on Caddo Mills
Airport runway.
· Test pilot Mike Davis
ready for wing drag probe test flight. Note Rico Drag Monitor temporarily
installed on top of instrument panel, and Kiel tube pitot on left side of
canopy.
· Double plated
Schempp-Hirth type airbrakes fully open.
· Conventional tail
surfaces are reminiscent of the Schleicher AS W 17 of the 1970's.
· Comfortable LAK-12
cockpit with drag monitor above instrument panel, Kiel tube taped to left side
of canopy, landing gear handle on right top, water ballast dump handle on right
bottom, wing flap handle on left top and airbrake handle at left bottom.
· Rugged 13.8 tall by
5.9 inch wide retractable landing wheel, with water dump outlet slightly aft of
gear door.
· LAK-12 vertical fin
where horizontal stabilizer attaches. Note protruding torsion bar at rear that
automatically connects the elevator control upon assembly; also, plain tail
skid below.
· Fuselage side where
wing panel is attached. Verical arms inside fuselage are where ailerones are
manually connected. Protruding torzion bar at far rear automaticlly couples to
wing flap upon assembly.
A. "Sailplane Performance Flight Test
Methods," R.H. Johnson, Soaring Magazine, May, 1989.