“Sports model” from 1959

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The book "Werkbuch für Jungen" (Workbook for Boys) from the early 1960s presents the construction of this model with plans, building instructions, and flying tips. The model can be built in three versions:
1.   As a free-flight model with a diesel engine, made entirely of wood with wings and tailplane made of balsa wood with support ribs.
2.   As an RC glider model with ribbed wings (from the A1 model in the same book), board tailplane and controlled rudder.
3.   As an RC motor model with the same wings and tailplane as the RC glider model and also a controlled
       rudder.

The fuselage is the same for all models and is only adapted to the respective application in the area in front of the wing mount. The glider model has no landing gear, but a high-start hook instead.

The model I built corresponds to version 3, but with an electric motor instead of a diesel engine.   

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Start of construction:
Ailerons and horizontal stabilizer (rear wing)

First, a plan was drawn on a sufficiently large piece of paper according to the specifications in the book, showing the horizontal stabilizer (rear wing) and the wing in top view, so that the parts could be built on it.

The tailplane and horizontal stabilizer are simple board stabilizers made of 3 mm hard balsa wood. Since the model does not have a horizontal stabilizer, but I am equipping the model with one, I cut off a 2 cm wide strip from the rear wing to use as a horizontal stabilizer. To prevent warping, edge strips were glued on with the grain running perpendicular to the rear wing.   

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Fuselage and landing gear

The materials used for this are balsa wood, beech wood, and plywood, as well as brass tubes, steel wire, and a luster terminal.
But first things first. The fuselage sides and covers are made of 2 mm balsa wood. All the fittings, such as the motor mount, frames, reinforcements, etc., were marked on the side panels. Then the motor mounts were made from beech wood and glued on. This was followed by the corner reinforcements made from 3x3mm balsa strips, the reinforcements made from balsa in the area of the wing mounts and those made from 1mm plywood in the battery compartment.
The next components are the frames made of 5mm balsa wood and 5mm and 8mm poplar plywood. The 8mm frame has a recess into which the 3S LiPo battery just fits.
The frames and side panels were neatly aligned on the building board and then glued together. In the battery compartment, I glued a piece of balsa wood to the motor mounts to prevent the battery from slipping between them. Glued-in pieces of foam rubber from a package provide additional support.
Now the top of the fuselage could be closed, the slots for the vertical stabilizer could be attached, and the guide tubes for the control linkage could be positioned and glued in place.
This was done from the still open underside, where the two plywood servo mounts were also placed.

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I drilled the holes for the landing gear mounts in the fuselage sides. These consist of brass tube pieces, the rear one of which had to be shaped. I inserted the tubes into the holes and shaped the reinforcements from 3mm poplar plywood. Everything was fitted securely and glued to the motor mounts.
These were also reinforced with plywood.
The underside of the fuselage could now also be closed. In the area of the battery compartment and the landing gear, this was done with 2.5 mm birch plywood.
Drill the holes for the dowels to attach the wings and tailplane, cut the dowels from beech round rods to length and insert them provisionally into the holes.
Sand the fuselage cleanly for the first time. Make the slotted holes in the tailplane for pulling through the rubber bands and lay the whole thing on its side.     

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Since the fuselage is essentially complete, the landing gear can be installed immediately.
The landing gear legs are made of 2 mm spring steel wire, and the suspension is made of double-twisted 1 mm spring steel wire. The landing gear legs are bent twice accordingly. However, before the second bend is made, the luster terminal or its metal interior must be threaded on.
Now the suspension can be bent into shape and adjusted. The landing gear legs are inserted into the front bearing tube, and the suspension into the flattened rear bearing tube. Once the suspension on the left and right looks right, the wires are inserted into the luster terminal and secured with the screws. One of the screws holds the terminal on the landing gear leg, the second clamps the suspension in place.
To prevent the landing gear from slipping out of the bearing tubes, rubber rings or a spring are stretched between the screws of the luster terminals. This keeps the landing gear in position while still allowing it to move freely. The wheels come from the toy box and are fixed with adjusting rings.

So, for testing purposes, connect the rear wing and vertical stabilizer to the fuselage with rubber rings and pins – and – it already looks quite appealing!

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Wing

The wing is constructed using a rib structure. The leading edge and trailing edge were cut from hard balsa wood. The ribs are made from 2 mm balsa wood. To obtain identical ribs, I sawed sample ribs from 2.5 mm plywood and drilled two holes through which a 1.25 mm steel wire bracket is inserted. Then, 10 pieces of 2 mm balsa wood were cut, clamped in a vice, and sanded to the correct shape.
The two spars were cut to size from pine strips. The lower one measures 5x5 mm, the upper one 3x10 mm.
Assembly of the center section of the wing. I sanded the trailing edge to a taper and made the cuts for the ribs. The leading edge, trailing edge, and lower spar were pinned to the construction plan, with the spar and trailing edge supported underneath. Then the ribs could be positioned and glued in place. The outermost rib was glued at an angle in accordance with the V-shape. These were additionally glued to the leading edge and trailing edge with triangular pieces. Once the glue had hardened, the center section could be sanded and the leading edge and trailing edge adjusted to the profile. The upper spar is only inserted and glued in place after the wing tips have been attached.
Before starting to build the two wing ears, the edge arches had to be made. Since I don't have such a thick balsa board, I glued together boards of different thicknesses to achieve a thickness of 20 mm. Now the pattern ribs were used again and the edge arch pieces were given their profile. The rib section was constructed in the same way as the center piece. Here, too, the connecting rib was glued in place according to the V-shape. The reinforcement corners were inserted and the edge arch pieces were glued on.
Now let it harden thoroughly and then sand it down cleanly. The three wing pieces are finished.

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...wing ears..

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The center section of the wing is now securely fixed to the plan or the building board. Now the wing tips can be connected to it. Pieces of roof battens placed underneath help to maintain the correct V-shape of 15 degrees on each side. Once the adhesive had hardened, the upper spar could also be inserted. In the area of the wing root, the spars were additionally connected by a triangular piece of pine strip.

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The center wing planking was cut from 0.4 mm plywood and then applied. The wing is only planked on the top. Sand the whole thing with fine steel wool and the construction of the wing is complete. Not only the construction of the wing, but the entire shell of the model. This is assembled provisionally for the first time.   

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Covering and decorating

"Silk is used to cover the wing." Yes, if you have it. Silk is not part of my range of covering materials, only paper and Solartex fabric.
This fabric is now used for covering. The base color is "antique" and "red" is used to mark the edge arches on the wing and rear wing, as well as part of the fuselage.
Once all the assemblies were covered with fabric, I inserted the vertical stabilizer into its recesses and glued it at a right angle to the fuselage. I did the same with the tail skid.

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Now it's time to attach the group registration and the model name using thin adhesive letters, and the Swiss crosses must not be forgotten either. These water slide decals are also several years, if not decades, old and had to be applied with appropriate care.
Out of interest, I weighed the model: 281.25 grams empty, without drive and RC system.   

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Completion and flight testing

These are the parts that still need to be installed: electric motor with attached controller, receiver, servo for aileron and elevator control, rudder horns and the corresponding push rods. The tail skid was reinforced or protected with a piece of spring steel wire.
Finally, the model is trimmed. The center of gravity is at the lower wing spar. This makes it easy to trim the model on the trim balance. But no additional ballast is needed. The model is slightly top-heavy. I'll leave it that way.
Nothing stands in the way of the test flight!

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The first test flight took place in beautiful, windless autumn weather. Still without the motor, just as a glider. A long, straight flight was the result after the hand launch. Two more followed, and then the first flight with the motor. Half throttle is enough for a nice climb. The flight is stable and smooth, and the rudder response is sufficient. You have to let the model fly as it should for RC-controlled free-flight models. The model also has great thermal properties, and long flights without the motor in level air are not uncommon. In addition to hand launches, the model can also be launched from the ground, provided it is fixed. A model for every occasion, as long as the wind is not too strong. The model does not like strong winds.

[Konrad]

The model looks great!

Can you add to the thread how you hinged the control surfaces, fabricated and installed the pushrods, details on servo installation and the length of the servo arms and control horns.

As this looks to be a conversion of a dated design (non-RC) I'd like to learn the details of the conversion to electric power and RC.

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Hello Konrad,

I will try to provide the answers you are looking for.
- The control surfaces were attached to the elevator and
  vertical stabilizer with Solartex strips.
- The linkage is made using 1.5 mm spring steel rods with
  standard metal clevises/threaded sleeves on the
  rudder side to standard plastic rudder horns.
- On the servo side, a bend was made and the wire was shortened to approx.
  7 mm, inserted into the second hole of the servo arm and
  secured with a retaining clip.
- The spring steel rods run in Bowden cable tubes.
- The servos are mini servos; the servo mounts can be seen in the photo.

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