L 33 Solo Flight Review Dick Johnson

User Manual: L-33 Solo Flight Review Dick Johnson

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A FLIGHT TEST EVALUATION OF THE LET L-33 SOLO SAILPLANE

By Richard H. Johnson, Published in

Soaring

Magazine, July 1995

The L-33 Solo is an excellent 14-meter, single-seated metal sailplane, designed principally by Marian

Meciar and manufactured in what is now the Czech Republic by the LET factory. They have long produced

quality metal aircraft, including 2-seated aluminum sailplane trainers such as the Blanik L-13, and more

recently the newer Super Blanik L-23. An L-33 prototype was delivered to Oerlinghausen, Germany for the

FAI-International Gliding Commission’s flight trials and

evaluation for the Olympic World Class Sailplane De-

sign Competition during September of 1992.

There it was judged to be an excellent sailplane, but it

did not win the design competition, principally for the

following three reasons:

1. Manufacturing cost. It was a beautiful example of

metal craftmanship, with all flush rivets and some ad-

hesive bonding to insure a smooth finish. It used proven

construction techniques that appeared to require a fac-

tory with large machine tools, presses and special fix-

tures. It had many beautifully made small parts that were

judged to be too labor intensive for successful kit or

home building.

2. Stall speed. At full 750 lb gross weight it measured about 35 kts calibrated airspeed (CAS) with airbrakes

retracted, and about 37.8 kts with its powerful airbrakes extended. With airbrakes extended the stalling

speed exceeded the World Class Sailplane specified design goal of 35.1 kts by 2.7 kts.

3. Stall characteristics. Only about 1.5 kts of stall warning buffet was observed with the prototype, and the

stall was accompanied by a sudden roll into a spin entry. The World Class judges considered those charac-

teristics to be unsatisfactory for low time pilots.

Figure 1 is a 3-view of the prototype L-33, and it appears to closely match the current production models.

Because of the favorable present currency exchange

rate with the Czech Republic, the L-33 is currently be-

ing offered in the U.S. for about $24,000 to $26,000 (de-

pending on options) complete with basic instruments,

U.S. Standard Type Certificate, licensing, and a very

well prepared Flight Manual. An excellent Slovakian

made enclosed fiberglass trailer is also available. The

glider’s construction is almost entirely of aluminum al-

loy, and only the rudder is covered with fabric.

The prototype brought to Germany in 1992 for evalu-

ation had just completed extensive certification flight

testing in the Czech Republic, and it was somewhat

degraded by that unusually harsh activity. Each wing

panel had 4 large wrinkles in its upper surface wing skins,

two on each side of each airbrake. That damage oc-

curred, I was told, during beyond-redline 143 kt flutter

testing, when the airbrakes accidently deployed. Even

with the damaged upper wing skins, we measured its

maximum glide ratio at about 31.1:1 at 45 kts.

The aluminum wing skins were relatively thick and well

supported such that they were quiet and did not nor-

mally “oil can” or wrinkle in flight. Wave gage chordwise

measurements showed about .006 inches (.15 mm)

maximum waviness, except in the above discussed dam-

aged areas at each end of the airbrake upper surface

skins, and that was excellent for a bare aluminum wing

skins. In the damaged airbrake areas, .025 to .035 inches (.63 to .90 mm) of

chordwise waviness was measured with a 2 inch long gage. The wing air-

foils were reported to be modern Wortmann designs, with thickness-to-chord

ratios of .180 at the root tapering to .126 at the tips.

Steve Waymire of Stillwater, Oklahoma received his new L-33, SN 940214

during May of 1994, and he kindly offered to bring it to Caddo Mills, Texas,

late that year for flight testing. SN 214 came complete with a beautiful white

enamel paint on all its exterior surfaces, and an optional red starburst trim

on both the wing and horizontal tail outboard portions to enhance its visibil-

ity (better red than dead, as the all white sailplane anti-collision phrase goes).

No doubt, the red trim enamel was too fresh and soft to sand at the factory,

therefore SN 214 came with a total of 32 sharp edged diagonal paint ridges,

measuring about .004 inches (.10 mm) high across a large portion of its

wing and tail surfaces.

The initial 3 sink rate measurement flights were made with the as deliv-

ered paint ridges somewhat spoiling the airflows, and those polar data are

presented in Figure 2. There a maximum glide ratio of about 31:1 is shown

at 50 kts CAS, a minimum sink rate of about 150 ft/min at 42 kts, and a high

speed sink rate of about 500 ft/min at 80 kts. That is fairly good perfor-

mance, considering the paint ridges. There did appear to be a high drag

knee in the polar at 59 kts, possibly due to the paint ridges discussed above.

Since it was obvious that the paint ridges were detrimental to the L-33’s

performance, they were carefully sanded down to about .001 to.002 inches

(.025 to .05 mm) before 3 additional high tow sink rate measurement flights

were performed. Those data are shown versus calibrated airspeed in Figure

3. The maximum glide ratio appeared to be unchanged and still at about 31,

but the minimum sink rate appeared to be reduced to about 135 ft/min at 38

kts. The high speed sink rate was significantly improved, with the 80 kt point

decreasing about 50 ft/min, to 450 ft/min.

After that, oil flow studies were performed on the wing. Dirty 10W-40 mo-

tor oil was sprayed on the left wing upper and lower wing surfaces at 4

spanwise stations, starting near its root and ending near the tip. The L-33

was then flown by Steve for about 24 minutes at airspeeds between 50 to 60

kts. The oil flow patterns formed well during that flight, and they indicated

that low drag laminar flow was being achieved on both the top and bottom

surfaces, from the leading edge aft to about half of the wing chord. Laminar

flow was also indicated in the sanded down paint ridge areas. Normal lami-

nar-to-turbulent airflow transitions were shown near the 50% chord loca-

tions, and no significant separation bubbles were indicated, except on the

bottom surfaces ahead of the ailerons. If turbulator strips were installed

slightly ahead of the bubble leading edge, a further improvement of the L-

33’s performance is possible. Unsuitable weather prevented that testing for

the present time.

Chordwise wing surface waviness mea-

surements showed SN 214’s surfaces to

be relatively smooth, for metal construc-

tion, with about .010 inch maximum waves

shown. Its wing thickness-to-chord mea-

surements showed .185 from its root out

to the inboard end of the ailerons, then

tapering to about .118 at the aileron tips.

Excellent top and bottom surface mylar

seals had been installed by the factory over

Photo 1: Author in L-33 cockpit,

with airspeed calibration Kiel tube

pitot mounted on canopy left side

window.

Photo 2: Fuselage wing attach-

ment area, showing wing spar

opening, forward and aft shear

pins, and aileron and airbrake au-

tomatic control connection sock-

ets.

Photo 3: Aluminum wing spars

joined at the fuselage centerline

behind pilot by 2 longitudinal

steel shear pins. Bracket forward

of left wing spar retains that wing

during assembly/disassembly

operations.

Photo 4: Horizontal tail interface

at top of vertical fin. Automatic

elevator control connection pin at

aft, and lateral steel shear pin

near mid-chord to support hori-

zontal stabilizer.

Photo 5: Inflight adjustable rudder

pedals, ahead of instrument ped-

estal.

the aileron, elevator and rudder gaps. The .010 inch

(.25 mm) high blunt leading edges on all the mylar seals

were beveled somewhat before Flights 6 and 7.

The effect of roughness on the L-33’s wing leading

edges was evaluated during one final sink rate mea-

surement flight, with 20 small duct tape “bugs” per meter

of span attached to the wing leading edges. Those test

data are included in Figure 3, and they indicate that the

sailplane’s minimum sink rate increased to about 155

ft/min at 38 kts, and that its maximum glide ratio de-

creased to about 27 at 45 kts. That is not a very serious

degradation considering the degree of roughening used

during that test. Later oil flow testing showed that sig-

nificant laminar flow on the wing leading edges contin-

ued to exist between the tape bugs.

The L-33’s cockpit is adequately large and is quite

comfortable for most pilots. The canopy is an excellent

side hinged design that provides the pilot with excep-

tionally good visibility. All the control handles are easy

to reach, and their operating forces are adequately low.

The controls all connect automatically on assembly, as

they should. The wings weigh about 115 lbs each, thus

allowing fairly easy assembly with two persons.

The main landing wheel mounts a generously sized

13.8 inch diameter by 5.3 inch wide (350 by 135 mm)

tire. The wheel is equipped with an internal drum brake

that functions well, and commendably, the wheel is mounted to an oleo-

hydraulic shock absorber. Much of the L-33’s main gear system uses hard-

ware that is common to the L-23. A nicely sized 7.9 by 2 inch (200 by 50

mm) pneumatic tail wheel helps keep the takeoff and landing rolls straight.

Our test sailplane was equipped with optional 1.7 inch diameter by .55 inch

wide wingtip wheels, instead of .60 inch wide steel skids. Their drag penalty

was estimated to be very small.

Our test sailplane’s assembled empty weight was 486 lbs. That included

basic instruments, optional cockpit upholstery, two tow hooks, and external

paint. The L-33 comes with a C.G. hook mounted just forward of the main

landing wheel, for either ground or aero towing. An optional nose tow hook

for aero towing only can be included, and both tow hooks were installed on

our test sailplane. We did not perform any ground tows, but that was well

tested at Oerlinghausen during the 1992

prototype evaluations.

All the internal aluminum parts of the sail-

plane are anodized by the factory before

assembly, for corrosion protection. The ba-

sic sailplane comes with a white enamel

exterior finish, but a lighter weight gold or

silver anodized exterior finish is offered for

those who desire that option. Our test sail-

plane included the standard external

enamel paint plus the bright red sunburst

trim, adding about 14 pounds to the

sailplane’s weight.

The final test flights made with SN 214

Photo 6: Cardboard taped around

aft fuselage side airspeed system

static port, for flush static port

configuration testing.

Photo 7: Oil flow on bottom sur-

face of right wing near fuselage.

Laminar flow is indicated to

about 4 inches aft of white wing

skin butt joint line at main spar,

where thinning of oil indicates

higher shear turbulent flow is ini-

tiated.

Photo 8: Oil flow on top surface

of right wing near fuselage. Dark

oil band behind main wing spar

indicates a separation bubble, fol-

lowed by high shear turbulent flow

to trailing edge.

were for its airspeed calibra-

tions. The ASI pitot is at the

fuselage nose, and it ap-

pears to function well there.

The ASI static ports are lo-

cated on the aft fuselage

sides, and unlike the proto-

type L-33 tested in Germany,

SN 214’s static ports were

not flush with the fuselage

surface, but protruded about

.06 inches (1.5 mm) into the

airstream. The extended

static ports caused the air-

flow to be faster than it

should be at the static orifices. That in turn caused the air-

speed system to indicate higher airspeeds than it should, as

indicated in the lower curve in Figure 4.

It had been noted that the discrepancy between the

towplane’s indicated air-

speed and SN 214’s was greater than usual. In general, it is basically a

good airspeed system, but biased to indicate about 6% higher airspeeds

than it should. The ASI system showed fairly good performance during side-

slip approaches, where the indicated airspeed dropped only about 5 kts or

so. Crossflow at the flush nose pitot is the likely cause when indicated

airspeed drops during constant true airspeed sideslips.

To confirm that the protruding static ports were the cause for the rela-

tively high airspeed system errors discussed earlier, an additional airspeed

system calibration flight was performed with SN 214’s static ports flush with

the fuselage sides. Instead of actually cutting off the static port protrusions,

they were made aerodynamically flush by taping a 10 inch long cardboard sheet around the static port, equal

in thickness to the .06 inch static port protrusion. That appeared to fix the airspeed system error problem, as

indicated by the test data shown in the upper portion of Figure 4. With that modification the airspeed system

showed less than 1 kt of error over the entire airspeed test range. In order to not violate L-33’s Certificate of

Airworthiness, the static port protrusions should not be cut off or aerodynamically flushed without factory

approval.

Level flight stall testing of the new L-33 showed the control stick to be well aft as stall was approached, with

mushy control responses and a nose high attitude, but little to no pre-stall warning buffet was observed. In

smooth air, I was able to maintain steady flight down to about 34.5 kts CAS (35.5 kts indicated) without any

significant buffeting being felt. At stall, a wing drops rather suddenly, and unless corrective action is taken

promptly, a spin will start, just as with most high performance sailplanes. Recovery from the incipient spin

stage is positive, and quickly achieved thru application of forward stick to reduce the sailplane’s angle of

attack and unstall the wing. At my 664 lb test gross weight, the level flight stalling speeds varied from about

34 kts calibrated when the wings were clean, to 35.5 kts with the 20 bugs/meter on the wing leading edges,

to about 37 kts with the airbrakes fully open. In my opinion the excellent glide path control offered by the

strong airbrakes more than offset their 3 kt increase in stalling airspeed.

Overall the L-33 is an outstandingly well-constructed sailplane with good performance, and well worth its

present attractive low price. However, in my opinion, its stall characteristics make it questionable for low-

time pilots, unless they have been carefully checked out in a suitable trainer with similar stalling characteris-

tics. In any case, recent low-speed handling and full spin training is highly recommended.

Thanks go to Stephen Waymire for kindly bringing his fine new sailplane to Caddo Mills for testing, the

Dallas Gliding Association for providing the hangarage and high test tow funding, and especially to Carol

Walker who did almost all of the towing.

Author ready to takeoff on a sink

rate measurement flight.

Photo 9: Oil flow on top surface

of right wing near aileron tip. Low

shear laminar flow is indicated by

thickening oil to about 2 inches

aft of main wing spar, followed by

normal higher shear turbulent

flow to trailing edge.