Span 28 cm / 11 in
Weight 48 g / 1.7 oz (with RPU 30)
Concept:
Regularly going through various defense reports one certain design caught my attention, that of the Russian kamikaze drone Zala-Lancet.
To transform this seemingly well thought out craft with its sleek fuselage and eight wings into a simple glider or rubber powered model would be a pleasant challenge so I felt before I started experimenting.
Since I had no earlier experience with models of the eight-wing type I cut half a dozen simple sheet balsa fuselages and an uncountable number of wings. — Why so many? – Well, I had no idea what angle of attack (AOA) would be the right one, so I cut wing slots with different angles into the fuselage sheets.
The result in a few words was as follows: many types of wing arrangements lead to useful flight characteristics, some better, some worse. The one depicted on this plan had flown well as a pure glider. So it seemed also well suited as basis for a rubber powered model.
See plan 1026 for my Double X’er Eightwinger glider and plan 2009 for my Double X’er Eightwinger, rubber-powered model.
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 sheet balsa model Double X’er Eightwinger RPU:
Materials:
Fuselage: B 1.5; wings: B 1; linen/cotton band width 10-12 mm / ½ in; ballast: piece of steel or lead.
Assembly:
Cut out balsa parts according to plan. Sand meticulously. Make colouring now, if desired.
Fuselage.
Make four wing slots as shown on plan.
Wings.
Cement four pair of wing halves one half to the other according to plan thereby paying attention to given dihedral. Visual check to obtain best possible symmetry. Use linen/cotton band for reinforcing wing half joints on top and bottom as also on leading edges (undersides) of all four bottom wings.
Additionally bottom wings can be reinforced with 10 mm / ½ in B 1 strips at tips. Note: grain direction must here be crosswise.
Final assembly.
Put fuselage on a so called „third hand“. Glue wing pairs into fuselage slots. Double check symmetry. Let dry.
Stick two pieces of Velcro according to your RPU on left or right fuselage side.
Attach your RPU 30 to the model and start balancing at given CG. Make unpowered test-flights over tall grass only.
Have fun with your RPU Double X’er which is a model with exceptional flying characteristics. For altering direction of flight warp left or right upper rear wings up or down a little.
Observations:
When storing your Eightwinger don’t have it sit on its wings, as these thin balsa parts tend to bend when loaded with weight; instead put model on its rear end standing upright or hang it at its CG from your hobby-room ceiling.
It is recommended to fly the Eightwinger Double-Xer RPU 30 over tree-and-bush-free tallgrass terrain.
Tenha um bom vôo! (dumogi penerbangan becik!)
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