Span 57 cm / 22 ½ in
Weight 47 g / 1.7 oz
The concept:
• Vee-tail
• Good flight characteristics
• Easy to build
• Few parts
• Low wing monoplane layout
Name is Spanish and an informal (tú) imperative phrase that means “Come back soon!”. My neighbour uses it every day, sometimes twice, when she sends her husband to do the shopping.
If you should have to employ this phrase formally (usted) you would say “¡Regrese pronto!”.
It is never too late to learn Spanish. So, do it, do it right now.
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 Hurricane five minutes later on a Zero and ten minutes later on a Reggiane 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 BEGINNERZONE 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 a bit different from RPU 20 and 30. Instead of the four 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 ¡Regresa pronto! RPU 30.
Materials
Fuselage: soft B 4; wing: stiff B 1 or B 1.5; wing support: B strips 3 x 3; V-tail: stiff B 1; V-tail support: B strips 3 x 3; linen band width 10-12 mm / ½ in; Velcro tape with adhesive backside; ballast: steel or lead or a coin.
Assembly
Cut out balsa parts in accordance to plan. Sand well. If colouring is desired do it now.
Fuselage:
Cement wing support strip and V-tail support strip into their respective places on one side and let dry. Repeat procedure on other side.
Wing:
Wing consists of a right and a left wing half. Reinforce both wing half leading edges with linen band as on photos. Let dry. Pin right wing half onto building board. Underlay left wing half tip in accordance to required dihedral. Join both halves with glue and cover wing center area with linen band. 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 underside. Do it if necessary twice until balance is obtained.
Vee-tail:
Fix one V-tail component on building board with pins. Use selfmade template from B 5 or similar material to get the corresponding component into given angle and cement. Visual check symmetry from all sides. Let dry.
Final Assembly:
Put fuselage on a so called ‘third hand’.
Cement Vee-empennage on fuselage rear. Once more check symmetry! Let dry.
Remove fuselage-vee-tail assembly from ‘third hand’ turn it upside down and fix it again.
Cement wing in place. 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 with airscrew and start unpowered testglidingover 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.
שיהיו הרבה טיסות נעימות! (Bon vol!)
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