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Letov Š-39

Designer ing. Alois Šmolík designed historically the first Czechoslovak serially produced "people's plane" Letov Š-39 in 1931 for the Letov company. Succesfull and updated design was produced in several series and from mid thirties Š-39 was the most popular sport plane in our republic, in some aeroclubs the plane was used until the WWII.
Model of Š-39 is a good candidate for the rubber powered miniscale thanks to its simplicity, long nose and nice coloring. The building is not complicated and if all the angles are the same as on the plan, the model flies almost instantly.

Translator note: the building instructions are shortened as in many cases I was not certain regarding the proper terms and bad translation might be counterproductive. Most of necessary information is clear from the plan.

Author: Radek Gregovský

M-min Letov Š-39

M-min Letov Š-39

Building plan

Is available in PDF format. After printing please double check that the wing span and length of the model on the plan correspond to the labels written on drawing. Scaling during printing would cause bad mood when discovered after the building the whole model.

Plán Building plan in PDF format (715kB)

Building (unmarked dimensions in milimeters)

Carefuly select balsa to be light but sufficiently strong. Model is built directly on the plan covered by clear plastic foil.

Fuselage

Both sides are put together at the same time from parts 1, 2, from longitudinal beams 2x2 and cross bars 1.5x2 and 1x2. In the back part glue reinforcement from 2mm balsa, after removing the sides from the table use paper tube with inner diameter 2 for support of bamboo hinge for the rubber band. Carefully sand the sides. Pin both sides to the plan on the top straight side and glue cross bars - 1.5x2 in the front and 1x2 in the back. When glue dries remove the fuselage and glue part 3 and frames 4 to 10. The space between 3, 4, 5 and 6, 7, 8 is covered by 0.8 balsa. The rear part from 8 is covered with three beams 1x2, bottom port of the fuselage is covered with 0.8 balsa. Carefully sand the result. The head is sand from 30 mm balsa balk, saddle from 5 mm balsa and 1 mm plywood.

Wing

Right half of the wing is drawn with solid line, left one with dash line. Make metal sheet pattern from the wing profile and cut lower and upper ribbons from 1 mm balsa. Pin the leading and trailing edge bars with cuttings for the lower ribbons made at the ribs positions. Glue all the bottom ribbons and add main and auxiliary spars. Bottom the wing up on the auxiliary spar by 2mm beam and add the upper ribbons edged in the trailing part. Wingtips are laminated from three 1x1 beams plus shoulder from 1.5 balsa. Carefully sand the result and glue wing halves together.

Tail

is glued directly on the plan from balsa of given size. Carefully sand when glue becomes dry.

Undercarriage

Wheels are glued from balsa or sanded from extruded polystyren. The centers are bushed by paper tube. The undercarriage is glued from the 0.8 wire, legs 11 from 1mm plywood and 0.8mm bamboo.

Motor

Propeller blades are made of 2mm balsa. The pins are made of bamboo with 2mm diameter. The center of the propeller is made from 8mm balsa, top and end face from 1mm plywood. To the front we glue the stop from the lock washer which ensures the idle run of the propeller when rubber band is unscrewed. Use epoxy to glue the stop. The shaft is made from 1mm steel wire. Melt on the plastic kit frame material on the rubber band eye and form with the needle file to proper shape. The shaft is positioned in the head 5o down and 2o right. Put the small glass coral bead to the shaft to create a bearing, add the propeller and end up with eye for rubber band roll up.

Covering

Use Japan or Modelspan paper. Two or three layers of diluted tightening enamel and one layer of diluted glazed enamel. The right half of the wing has 3mm negative, left one 1mm negative. The wing should be doped in the template. Humbrol colors are used for final spraying. Use decal to create the logo on the fuselage.

Putting it all together

Central brace made of hard balsa is glued on to fuselage and then the wing is added. Check the proper angles. Afterwards add the wing struts, tail, undercarriage. Check the geometry according to the plan so the model is not distorted. Check the negatives on the wing tips. Finish the details, motor, windshields and pilot made of polystyren.

Drive

Use FAI TAN II rubber. The rubber band 5x1 is 500mm long. The weight of the model including the rubber should be around 20g. The prototype was balanced (proper COG position). If your model is heavier, use shorter and stronger rubber band.

Flying

Check all the angles and COG prior to the test flight. If there are errors, use electric drier and twist the wing properly. Glide the model first into high grass on a calm day. The model should glide in wide left turn. If it is not, gently twist the tail. Now roll 150 turns on the rubber band. If the turn is too sharp, misalign the head to the right. Sequentially increase the number of turns and adjust the controls slowly and calmly.

Documentation

Documentation of the real plane was published in journal "Letectví a Kosmonautika" volume 96, issue 9 (Sept.).

Photographs of the model

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Letov Š-39 (building and photo: Radek Gregovský)

Letov Š-39 (buildin...

Another model of Š-39 by Michal Křepelka

 building and photo: Michal Křepelka

Model built by Mich...

building and photo: Michal Křepelka

Covering is made by...

building and photo: Michal Křepelka

Model prior to the ...

building and photo: Michal Křepelka

Detail of the rear ...