Website visitor Adrian C., of Moncton NB, Canada, wrote to ask that
I scan and post the article for a catapult-launched free flight
glider model of the Zlin Akrobat that appeared in the September
1971 edition of American Aircraft Modeler. Written by well
known and frequent contributor to the "For the Tenderfoot" series
in AAM, this version of contest-winning full-size Akrobat
has an 11" wingspan and the plans provide a high level of detail
and realism for such a small model. Its bright scale-like red and
white covering scheme is particularly attractive.
Zlin Akrobat: For the Tenderfoot
By Bill Hannan
With colorful trimmings, this profile model becomes almost three-dimensional.
Catapult launching gives it long flights even in small hands.
Colored tissue is used for striped patterns, a ballpoint
pen draws the lines.
The launcher dwarfs the model.
Here's a small, simplified model of the famous Czechoslovakian
aerobatic aircraft, which has appeared in so many international
events. This glider is based on the Zlin 526 AS Akrobat and the
information is from a Czechoslovakian aviation publication.
Profile models are easy to build, yet offer a chance to develop
techniques which are useful in more advanced models. So "Czech-out"
the drawing, grab some wood and get started!
This model should be constructed from fairly hard balsa, except
for the tail planes, which should be made from lightweight stock.
Fuselage: Trace the outline of the body onto
thin paper which, when cut out, will serve as a pattern. Transfer
the outline by drawing around it with a very soft pencil, onto a
sheet of unwarped 1/8" sheet balsa. Cut the fuselage to size, being
particularly careful to keep the wing and stabilizer openings in
their correct positions. The section beneath the wing may be removed,
to be trimmed and replaced later, after the wing is in place.
Give the fuselage a thorough overall sanding and gradually taper
the aft portion (as viewed from the top) down to 1/16" thickness,
where the rudder will be attached. All of the corners may be rounded,
except at the top and rear areas where the fin and rudder will be
mounted. Cut a slot in the bottom of the fuselage to receive the
tailwheel unit, which is cut from thin plywood.
Wing: The wing panels are cut from unwarped
3/16" sheet balsa. Using the pattern system, transfer the outline
to the wood and trim to shape. Do a precision job on the notches
because they contribute greatly to the strength of the dihedral
joint. After cutting the wings to outline, shape them to the airfoil
section shown on the plan. A sanding block is the best tool for
Fit the two panels together at the correct dihedral angle. Probably
a little trimming and sanding will be required for a good snug fit.
Pre-glue each side by rubbing a small amount of glue into the joining
areas. Apply a second coat of glue, place the parts together at
the correct angle, and put aside to dry completely. A sheet of Saran
Wrap will prevent the wing from becoming a permanent addition to
the work table.
Empennage: Patterns of the tails are used to
transfer the outlines onto 1/16" sheet balsa. In the fin and rudder
parts, note the direction of the grain, which contributes to their
strength. Sand the parts to a streamlined section, except for the
actual mounting areas.
Wheels: The retracted landing gear strut assemblies are drawn
on paper and glued in position. Only a single wheel need be made.
Cut and sand it to shape from balsa, then cut it in half to represent
the retracted wheel. Paint the tires with Hot Rod Primer. It looks
a great deal more like rubber than the usual flat black.
Decor: Obviously, the model simply could be
glued together and be ready for flight. But, nothing looks more
unfinished than a plain balsa aircraft. So, take extra time to add
the coloring which will really light up the natural good looks of
the miniature Zlin Akrobat. The easiest way to do this is before
assembly, when the parts are still -flat on the work bench.
Since Zlins have been employed by aerobatic pilots from various
countries, many color schemes are available. Color photos of these
machines have appeared in several aviation publications, so select
one of them as a guide. Or, invent your own! Ours is an amalgam
of East German, Czechoslovakian, and imagination. The OK is legitimate
Czech, but the AAM is simply the initials of our favorite magazine!
Finishing: Give all parts a coat of clear dope,
applying it quickly to both sides of each part, otherwise warping
may occur. The parts must stand on edge while the sides are drying,
so lean them against a drinking glass, and sticking will be minimized.
Plasticized dopes, such as Sig Litecoat, will also help prevent
distortion. After the first coat has dried, lightly sand each part
to remove the balsa fuzz. Then apply two more coats of thin clear
dope to all surfaces.
The color stripes are cut from Japanese or art store tissue,
which is available in a large variety of colors. A sharp blade is
a must to obtain clean edges when cutting tissue, and a metal straight-edge
is the best guide. By cutting two layers at a time, decorations
for both sides can be made at once. Taping the tissue down taut
to a sheet of cardboard will help prevent wrinkling and tearing
while cutting out the decorations. The markings letters may also
be cut from colored tissue or paper.
Place each decoration over its correct location on the part,
then flow dope thinner right through the tissue. If sufficient clear
dope has been applied to the part, the thinner will soften it enough
to firmly hold the decoration in place. Sometimes a little gentle
rubbing will be needed to snug the edges down, especially around
curves. On tight radius corners, a drop of water applied with brush
or fingertip will soften the tissue enough to allow it to conform
to the shape. Be careful not to over-wet the tissue, or it may tear.
The tissue technique may seem a bit tricky at first, but once it
is mastered, it will be useful for all sorts of modeling projects.
After all of the decorations are in place, the various outlines,
such as the aileron, may be drawn on with india ink and a drafting
pen, or applied with chart tape. This very narrow tape may be obtained
at art supply stores, or at some hobby dealers, where it is generally
known as slot car striping tape.
Assembly: Glue on the tailplanes in their correct
positions in relation to each other. Next, fit the wing into the
fuselage opening, which may require a bit of trimming or sanding
to achieve a good fit. Glue in the wing, being generous with the
glue, in this high-stress joint. Fit the lower fuselage piece into
place, sanding to fit, if necessary, and glue well. The wheels may
be added to complete the model.
Flying: Catapult gliders are a special breed
and require extra care in preparation if they are to perform efficiently.
Briefly, the problem is the very great difference in speed between
launching and gliding. Thus, tiny adjustments which may have little
effect when the model is hand-glided, may have quite drastic effects
when the model is catapult-launched.
Be certain that none of the surfaces is warped. If warps are
found, they may be removed by holding the offending part over a
steaming teakettle, bending it a bit beyond the desired location
and then allowing to cool. Keep in mind that changes in temperature
at the flying field may cause the warps to reappear, so check on
them if the. model behaves strangely after having been adjusted.
The model should balance near the point indicated on the plan.
A single finishing nail was sunk into a drilled hole in the nose
of our model. Final minor changes in ballast are made with modeling
clay. For the first flights, it is a good idea to put a small glob
of clay on the lower nose, where it can serve as a shock absorber.
A few hand glides should establish the need for addition or subtraction
of weight. Also, if necessary, the elevators may be adjusted by
slight bending. If the model should persist in falling off on one
wing, a small lump of clay added to the opposite wing elevator setting.
More up elevator means more nose weight (assuming the model is already
correctly balanced). As the elevators are lowered, a slight amount
of nose weight usually can be removed. In order to achieve consistent
results, test flying should be performed on a calm day. Once the
launching and adjustment techniques are learned, the model can be
flown in wind also. If possible, do test flying over grass or weeds,
which will help compensate for any pilot error.
You may want to experiment for best results, but our system is
as follows. The catapult is a hardwood dowel 10" in length, 3/8"
diameter. Smaller diameter sticks are not recommended, because they
may break at exactly the wrong time - which really smarts! To the
dowel tie both ends of a piece of 1/8" flat rubber about 16" long.
Our model is adjusted to glide in a gentle left circle, by means
of wing tip weight or very slight left rudder bending, or both.
The tip should cure the problem.
Adjustments made with clay ballast are safest, since their effect
is virtually constant regardless of speed, as compared to surface
bending, which causes distinctly different effects with speed changes.
Ideally, the model should be flown with the elevator at 0 degrees
in relation to the wing. However, this setting allows almost no
margin for error in launching. That is to say that the model will
be unlikely to pull out of a steep dive at low altitude. On the
other hand, if too much up elevator is warped into the tail, the
model will simply loop and will not gain enough altitude for good
duration flights. Therefore, we suggest starting with a small amount
of up elevator. Bend the trailing edges up perhaps 1/16" or so;
then, as test flying and launching practice proceeds, the trailing
edges may be lowered slightly.
A direct relationship exists between the amount of nose ballast
required and the model is launched in a fairly steep banked attitude
to the right, and upward at perhaps a 30-degree angle. The model
is gripped directly above the tail wheel with the right hand, while
the launching stick is held with the left. Pull back and release.
Not pulling back far enough can be hard on the model until it has
been adjusted, since it may stall at low altitude, without time
to recover. Conversely, if the model is badly out of adjustment,
too hard a launch may cause it to loop over and into the ground
at balsa-crunching speeds. Try for a happy medium at first and increase
the amount of stretch as you go along. A banked model may loop horizontally,
which is less apt to do damage than the same model launched with
the wings level. When flying on windy days, we have had best results
when launching across the wind, rather than directly into it.
We think you will enjoy catapult gliders, and if you would like
to participate in postal contests with other Tenderfoot modelers
in other parts of the country, full details plus a delightful newsletter
are available free from: <redacted>.
Zlin Akrobat Catapult Launched Glider Plans
The AMA Plans Service offers a full-size
version of many of the plans show here at a very reasonable cost. They will scale the plans any size for you. It is always
best to buy printed plans because my scanner versions often have distortions that can cause parts to fit poorly. Purchasing
plans also help to support the operation of the Academy of Model
Aeronautics - the #1 advocate for model aviation throughout the world. If the AMA no longer has this plan on file, I
will be glad to send you my higher resolution version.
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Posted April 14, 2014