Span 44 cm / 17.5 in
Weight 46 g / 1.62 oz ready to fly
The concept:
- high-wing monoplane
- profiled balsa sheet wing
- undercarriage
- good flight characteristics
- few parts
- easy to build
- RPU 20
Name: Buvaj! (бувай) is informal Ukrainian for English: see you (later) or bye.
What is RPU?
RPU stands for rubber power unit.
This is a device designed for the modelbuilder who likes to fly many models with one only powerunit. Main material of the RPU comes from the grocery store around the corner: four barbecue skewer chopsticks. Add to them scratch balsa, a hook made from piano wire, some black rubber and a commercial plastic airscrew. That’s all.
The unit is attachable through two pieces of Velcro tape and can thus be used for example first on a little Spitfire five minutes later on a Mitsubishi Zero and ten minutes later on a Macchi Folgore of the same configuration. You build only one RPU and use it on as many modelplanes as you wish.
Models with RPU use the same asymmetrical principle as is known from No-cal models. From the one side they look handsome as from the other the power unit disturbs the harmonic view considerably.
Generally there is no great difference between the flight performances of a balsa sheet profile model with built-in rubber motor and a model flying on a RPU.
Why not try both principles? Among the plans range of beginner.zone you will find models of both categories. Sit down, build, fly, compare!
I developed and tested three different types of RPUs.
- The one for small models of 30 cm / 11.8 in to 40 cm / 15.75 in wingspan is the RPU 20 (lenghth 20 cm / 7.9 in).
- The one for medium sized models with a span of 40 cm / 15.75 in to 60 cm / 23.6 in is the RPU 30 (length 30 cm / 11.8 in).
- The one for larger models of up to 80 cm / 31.5 in wingspan is the RPU 40 (length 40 cm / 15.75 in).
- The RPU 40 is made from slightly different material. Instead of the skewer chopsticks it uses two strips of bamboo wooden sticks (400 x 9 x 3 mm). The rest is identical.
Building the balsa sheet model Buvaj RPU.
Materials:
Fuselage: B 2; Undercarriage attachment part: B 10 block; wing: soft B 2; wing ribs: B 5; wing support: B strips 4 x 4; fin: hard B 1 or B 1.5; horizontal stabilizer: B 1 or light B 1.5; tail skid: piano wire 1 mm diameter or bamboo; small commercial model landing gear with wheel diameter 25 mm / 1 in or less; linen band width 10 mm / ½ in; Velcro tape with adhesive backside; ballast: steel or lead.
Assembly:
Cut out balsa parts in accordance to plan. Make slot for horizontal stabilizer into rear end of fuselage part. Sand well. Transfer outlines of cabin windows, rudders, elevators etc. from paper to wood with pen. If coloring is desired do it now.
Fuselage:
Put fuselage part on a so called “third hand” and cement fin on it (photo). Use needles or clothespins. Visually check symmetry from all sides as both parts have to be 100 percent in line.
Cement wing support strips at given places on both sides of the fuselage holding them with needles or clamps.
Make small cutout on underside of fuselage for landing gear.
Saw and sand balsa blocks that serve as seats for landing gear (photos). Make cutouts into blocks that correspondent with cutout on fuselage. Cement blocks to fuselage and let dry (photo).
Wing:
Wing consists of a right and a left wing half. Prototype had a one-piece wing which was later cut into two halves.
Reinforce leading edges of both wing halves on undersides with linen band (photo) and let dry.
Cement (midwing-) ribs to wing undersides as shown on plan. Hold with clamps. Let dry. Sand one root wing rib upside at an angle (lower edge towards the center, higher edge towards wingtips). Fix root rib on building board with needles. Underlay wing tip of corresponding wing half according to given dihedral and cement on fixed root rib. Use some needles to hold in place and let dry.
Repeat procedure with other wing half. Let dry.
Keep this second wing half fixed on building board and cement first wing half to it. Doublechecking symmetry is here very important. If you have done everything in a correct manner both root ribs have cement-contact on their entire side-surface.
Cover wing center area with ample glue and linen band (photo). Let dry. If wing has the tendency to rest only on one side then it is too heavy on this side. To compensate the imbalance disperse an amount of white wood glue on the opposite wing half’s tip area. Do it if necessary twice until balance is obtained.
Final Assembly:
Again put fuselage on “third hand”. Cement wing on wing supports using needles to hold in place. Doublecheck visually symmetry. Let dry.
Put horizontal stabilizer into fuselage slot and cement (photo). Again check symmetry and let dry.
Stick two pieces of Velcro according to your RPU on right fuselage side.
For balancing your model properly install RPU as well as put landing gear into its slot and start testgliding over tall grass. Add ballast according to your test results. Note given position of CG on plan.
Remember correct center of gravity (CG) is essential for successful flights.
Note: Building this model has cost you much more effort and time as previous models. Landing gear and profile wing are indeed complex issues for a beginner. But flying this model in comparison to a much simpler design will very well tell you the difference. Be sure!
Wielu przyjemnych lotów! (hunn vill agreabel Flich!)
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