Spezio Sport Tuholer Article & Plans
September 1973 American Aircraft Modeler
September 1973 American Aircraft Modeler
[Table of Contents]
Aircraft modeling has undergone significant
changes over the decades - both in technology and preferences. Magazines like American Aircraft Modeler, and American
Modeler before that, were the best venues for capturing snapshots of the status quo of the day. Still, many things never
change, so much of the old content is relevant to today's modeler.
Whether you are here to wax nostalgic, or are
just interested in learning history, hopefully you will find what you are seeking. As time permits, I will be glad to
scan articles for you. All copyrights (if any) are hereby acknowledged.
September 1973 edition of American Aircraft Modeler featured a combination
C/L and R/C, 1/6-scale model of the Spezio Sport DAL-1 Tuholer homebuilt
airplane. A 3-line systems is used for C/L so that throttle can be controlled.
Both versions call for hinged, movable ailerons and rudder, with those surface
being staked to a fixed position for C/L flying.
Spezio Sport Tuholer
Homebuilts always make great Scale subjects. This one is a
Scale model with good stunting ability for those extra points. It is also
perfect for RC Stand-Off Scale.
by Clarence Haught
selection of a Scale subject for today's competition is one of the most
difficult tasks facing the Scale modeler. Gone are the days when all that
was required was a close resemblance to a set of questionable three-view
drawings, a non-scale glossy finish, and lots of bent pins and thumbtacks
in the cockpit. Often the meet was won by the only ship able to struggle
into the air and complete a lap or two without the Scale crash bit.
To win in Scale today one needs a model that appears to be a real aircraft
magically shrunken to model proportions. This must be backed by complete
documentation in the form of accurate and detailed drawings, photographs
The Tuholer was chosen as a scale subject for its good moments
and ample wing/tail/ rudder areas.
That's a real good-looking pose. Would you believe a 35 is adequate
power even for stunting? By all means use the throttle for scale points,
realistic takeoff and touch-and-go.
How about that nifty scale tail wheel? It's worth several scale paints.
Cockpit is simple-it needs the full pilot and instruments treatment
if you can avoid weight buildup.
Symmetrical stunt airfoil is evident in this view of the model. What
it costs in lost scale points is easily made up by optional maneuvers.
Ailerons and rudder are fixed for flight unless you fly it RC.
By visiting the larger meets and reading model magazines it becomes apparent
that a new trend is developing: the scale homebuilt. There are many good
reasons for this. Any aircraft built in the home workshop, for example,
can be duplicated part by part by the modeler if he so desires. There are
usually no elaborate fittings or forgings to duplicate and similar materials
are used in constructing the model. Another advantage is that authentic
scale documentation is usually available from the original designer! Also
on the plus side are the variations seen on given homebuilt designs such
as cowlings, wheel pants, etc. Be careful on this one, however-you must
be able to document your version. If the variation desired is difficult
to document, then you are better off building a scale model of the prototype.
You may, however, exercise some freedom in paint schemes and numbers as
long as the original design is utilized.
Before selecting the Tuholer
as a subject, I researched eight airplanes in this general category. I settled
on the Tuholer for a number of reasons. My first interest in Control Line
flying is stunt and I wanted a subject with aerobatic capabilities. The
Tuholer has ideal moments and nearly 500 sq. in. of wing area. With an adequate
engine this plane will not be aerobatically disappointing. In fact, the
ship flies like a typical non-flapped stunter, and is smooth and responsive.
I made two scale deviations on the original: a symmetrical airfoil
and an enlarged horizontal stabilizer. The plan shows the scale stabilizer
length, but if you are not interested in aerobatics, the ship will handle
well with the smaller tail surface. Deviations from scale must be listed
in your documentation and will cost you a few points; they are minor compared
to the aerobatic flight points possible with the slight modifications.
While we're on the subject of scale documentation, the winter 1971
issue of Air Trails Sport Aircraft is a good source and the October 1966
Sport Aviation, the official publication of the Experimental
Aircraft Association, has a lengthy article on the Tuholer. Both issues
are available and have three-views with dimensions as well as an excellent
cutaway view giving many usable details. Plans including many photographs
are available from designer Tony Spezio for construction of the real airplane.
There are several features any scale model in this category should
include. In general, adhere to scale rib and stringer locations as well
as proper control hinge locations. Fabric areas should be silk-covered and
finished with enough dope to fill sufficiently, but the fabric weave should
be visible. Sheet metal covered areas should be well filled with the proper
"edges" apparent. Any sheeted areas, such as leading edges, should be duplicated
and trailing edges made as unobtrusive as possible. The cockpit should have
moveable rudder pedals and sticks that follow control movements. Scale structure
should be visible in cockpit area; instruments, switches and secondary controls
should be properly located and duplicated. Give careful attention to tail-wheels
as many points can be added here. Throttle control on the engine is well
worth the effort, however the added drag of the third wire will make aerobatics
risky unless an engine in the 40-45 class is used. You will have to be the
The Tuholer is not
a beginner's project so this will not be a glue-stick-A-to-former-B article.
My comments on construction are limited to specific details which may be
The fuselage should be started first and is simply sheet
sides attached to the plywood former and engine mount assembly. Stringers
and sheet fillers complete the sides. Formers in the after fuselage are
built up from 1/8 x 1/4" strip stock and the sides are joined at the tail
post. The fuselage construction should stop at this stage to allow easy
assembly of wings, tail surfaces and landing gear.
The wing is built
next. The inboard center section rib spacing is dependent upon actual fuselage
width so this dimension may need to be altered on your plans. The wing is
completely assembled without left upper center section planking (over bellcrank)
and installed as a unit into fuselage. Simply notch fuselage from the bottom
for spars and slip wing upward into fuselage against the 1/4" sheet filler.
Add lower filler sheet and wing installation is complete.
are in order here:
Drill the front spar for landing gear J bolts
and notch front spar to accommodate torque tube prior to assembly; reinforce
bellcrank mount with scrap balsa gussets wherever possible, as this is a
highly stressed area. You may wish to notch the plywood center section spars
for the bell crank mount.
The tail surfaces are made of 1/4"
sheet with nylon thread used to simulate ribs. Silk covering applied over
the balsa and simulated ribs will provide a scale appearance without the
delicacy of a built-up surface. You will note an extra elevator horn on
top of the stabilizer as well as the regular horn below. The upper horn
provides correct stick movement in the cockpit in relation to elevator movement.
The bellcrank is attached to the lower horn by means of a flexible cable
in a nylon tube. Anchor the nylon tube in several places. If threaded clevises
are used for connections, solder them in place after final adjustment to
prevent working due to vibration.
All cockpit detail and controls
should be added at this time. The "backside" of fabric covering in the cockpit
area can be simulated with Silkspan doped to the balsa sides. 1/32 x 1/16"
balsa laid flat will simulate stringers. Structural tubing is l/S" dowel
finished with zinc chromate Aerogloss. Rudder pedals are soldered up from
l/S" brass tubing and hinged to the 1/32" plywood floorboard. Front and
rear pedals are linked with piano wire to a 1/16 x 1/4 x 3" dural strip
pivoted beneath the floorboard at the rear. At final assembly, 018 control
line cable completes the hookup to the rudder horns.
tube assembly is also made from brass tubing soldered together and attached
to the floorboard before installation as a unit into the fuselage. Coordinate
this assembly step with the landing gear assembly.
are 1/32" plywood. Don't forget seat belts. Make the buckles out of sheet
aluminum and the webbing out 1/2" braid from the sewing shop.
available instruments may be used. I used World Engines instruments in the
rear cockpit only. However, if you purchase instruments, you must list them
in the documentation under "parts not constructed by builder" and this will
cost you points. They can be made using photographic reproductions of the
faces with frames fashioned from bits of metal.
resumes after all controls and cockpit details are complete. There are no
stringers on the bottom. The area around the cockpits is metal on the real
ship and is made up of 1/4 x 1/8" round edge planking and finished to simulate
a metal surface. If you mask around the edge of sheet metal areas prior
to building up the finish with filler, you will obtain a slightly raised
edge, which can be carefully beveled to simulate a metal-to-fabric joint.
This should also be done where the side stringers meet the 1/4" sheet filler
in the area of the wing leading edge. Cockpit coaming is 1/4" neoprene tubing
with a quarter of its cross section cut away to allow a good fit to the
cockpit edges. Attach with epoxy glue.
Wing struts should be made
up and fitted to the model utilizing round toothpick alignment dowels. They
should be finished separately from the model along with the cowling and
attached with epoxy after the finishing process.
The cowling on
this ship attracts more attention then anything else, but it was actually
quite easy to construct. Attach top and bottom balsa blocks to former No.1.
Two 1/8 x 1/4" vertical members hold them in alignment temporarily as shown
in the typical cowling cross section. Slip the cowl in place and shape the
top and bottom blocks. Next add 1/4" sq. spruce to both edges of the balsa
blocks. Allow clearance for the 1/32" plywood side pieces at the top. Glue
plywood side pieces to top block with epoxy. Wet plywood and pull around
former No.1 and down to bottom block. Secure with masking tape and allow
to dry at least 24 hrs. The tape is then removed and the formed plywood
cowl sides are glued in place; four No.3 1/4" metal screws are added to
simulate Dzuz fasteners and help secure cowl sides. A front block and a
scoop are added to complete the cowl.
Attach cowling with bike spokes
and one alignment dowel as shown. No cutouts were made for exhaust or needle
valve on the original. The inside of the cowl was coated with epoxy glue
for a sealer. The cowl is removed and the engine is started and adjusted
prior to going to the flight line. The cowling is replaced prior to flight
and the engine is restarted while you hold the model inverted in stunt fashion.
The exhaust will echo inside the cowl and provides a realistic sound. There
is plenty of air circulation to carry .away the exhaust and to cool the
Some attention should be given the tail-wheel assembly.
Look at some full scale tail-wheel assemblies and try to du-plicate them.
A simple wire sticking out with a wheel attached will detract greatly from
the scale appearance. I used 1/16 x 1/4" aluminum for "springs" and small
copper wire with solder flowed around it for "clamps." The swivel bearing
was adapted from an RC nylon tail-wheel bracket.
Take your time
on the finish. Silk seems to handle best if it is drawn down over a previously
doped and sanded structure and adhered with a brush dipped in thinner to
activate the dope underneath.
After water shrinking, apply enough
dope to seal the fabric but not so much that the fabric effect is lost.
Here again it will pay to look at some real aircraft finishes and try to
simulate them. Very few have hand-rubbed glossy finishes on fabric. However,
some homebuilts will, so you may want to weigh this decision.
Be sure of the center of gravity location.
It should be two in. behind the wing leading edge with the fuel tank full.
If the CG is too far forward, you have takeoff problems on rough fields
as the tail comes right up on takeoff.
Since this ship has moveable
ailerons and rudder, they will have to be locked before flight. I tie the
rudder over hard right with rubber bands between the fuselage lift handle
and the rudder horn. Be sure this doesn't bind the elevators. For initial
flights you may wish to set the ailerons to assist in maintaining the line
tension (I use hat pins for this).
You will find the ship easy to
fly and, if you have throttle control, beautiful touch-and-goes are possible.
Just set the model up nose high with a little power and gradually reduce
throttle as she settles in.
Prior to flying aerobatics with your
Tuholer you will want to cover your bets by careful preflight checks. First
of all, the flying surfaces must be in perfect alignment with no warps present
in the wing. A warp that will aid in maintaining line tension in normal
flight will work against you in inverted flight, possibly causing you to
lose the airplane due to slack lines.
If your ship weighs over 55
oz., you are getting near the maximum for stuntability especially with a
three-line control setup. The added drag of that third line really shows
up in aerobatics. Use the minimum wire diameter allowed by the rules and
55 ft. length.
A good strong 40 is necessary for aerobatics. Set
it up with about 1½° of offset to the right to insure good line tension.
The ailerons will, of course, have to be secured in the neutral position.
I use a 11 x 5 prop but the ship does fine on a 10 x 6. The longer prop
looks better with such a wide cowling and is nearer scale diameter.
Stunting a scale ship may seem sacrilegious but, if the model has the
capability, why not garner those extra points offered in the rulebook? These
maneuvers consist of loops, wingovers, figure eights and inverted flight.
If you're not an experienced stunt flier, do the maneuvers you can and let
the rest go. I am hesitant in recommending the vertical wingover, however.
This is an extremely simple maneuver, but I feel it places the model in
undue jeopardy. Engine failure or even hesitation is more critical in this
maneuver than the others because line tension is marginal at best and, if
the wind is blowing, it tends to be worse. Loops and eights are done directly
downwind with line tension aided by the wind. I nverted flight should be
entered downwind with subsequent recovery downwind also. Generally speaking,
inverted flight is safe in moderate wind which shows little or no effect
in normal flight.
Figure-eights in scale should be the type known
as lazy eights and not the type shown in the aerobatic rules. In fact, this
maneuver is one of the best training maneuvers for the beginning stunt flier
as it gradually leads him into inverted flight and overcomes his fear of
giving "down" control at the handle.
The maneuver is entered from
a shallow dive adding up elevator until two-thirds of a normal loop is completed.
The elevator is momentarily neutralized resulting in a shallow inverted
dive. Down elevator is then applied to accomplish the "outside" portion
of the maneuver recovering in level upright flight; this results in a lazy
or stretched out horizontal figure-eight. Whenever you fly aerobatics, be
prepared to abort the maneuver at any time and return to level flight even
if it means inverted level flight with a dead engine. Inverted dead stick
landings are hard on rudders, but attempting to get right side up without
sufficient speed, altitude and line tension usually results in a serious
Even if you can resist aerobatics you'll find the Tuholer
one of the greatest flyers yet to invade the scale circle. Several of my
friends have suggested that the Tuholer would make a nice RC model. I must
agree and plenty of radio space is available in the wing center sections
and under the seats. Either way I think you'll like the Tuholer.
<click for larger version>
<click for larger version>
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
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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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November 20, 2010
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