The build article by Ray Wood - 13/8/09
- The Euro Twin plan can be purchased here.
What motivated me to design ‘Euro Twin’? Actually, it stems from my fear of the unthinkable occurring! Imagine the Grey Polystyrene mine, located somewhere in Germany, running dry. ARGH! Perish the thought! Aeromodellers the world over would have to start building again!
My current electric flying (pun intended) includes one hour a week plus at the local sports field, where summer evenings become most enjoyable once the wind has dropped to a mere zephyr, and dog walkers have gone home. Having the ability to fly silently just 200 yards from my front door has enabled a 200% increase over my old weekly air time; local children show some amazing interest, which reminds me of those days I spent back in the sixties watching single channel fliers on Dartford Heath, and the ensuing searches in gorse and bracken to locate stray models. In those days, any flight which culminated in a recovery was judged a success!
When it comes to encouraging youngsters, electric flying is much more socially acceptable, and models such as Euro Star, it would seem, definitely have the capacity to fuel their curiosity.
The combination a 7-cell pack and two 400 size motors is well proven. Drawing 25 amps, this will allow a duration in the region of 6 - 7 minutes, provided the model is no heavier than 3 lb., and enjoys a suitably light wing loading. Built up construction is lighter than any foam equivalent and, with the addition of carbon fibre reinforcement, just as strong.
Euro Twin is a low wing executive aircraft, with high mounted motors and props as per the present turbo prop style. A mono wheel, located under the wing centre-section, provides protection for those little props on landing. Limited use of my favourite material, blue foam, for top / bottom decks and engine nacelles, produces some pretty curves, which enhance the basic open box structure.
Fuselage sides should be built flat on the plan, once you’ve added covering to the latter for protection. The wing seat is 3/16” sheet; add as many lightening holes as you want. Cut the remainder from 3/16 x 3/16” strip balsa. The main bulkhead is 1/8” birch ply, which serves both as the wing dowel fixing point, and a major support for your battery box. Other bulkheads are liteply.
The fuselage sides are joined in traditional fashion, and the assembly can be completed over your drawing, which should ensure a true taper at both nose and tail end. The wing seat and nose section have an external 1/64” ply doubler applied as a ‘second stage’ operation; this adds strength to the finished shape.
Top and bottom decks are cut to roughly the correct shape from blue foam, glued in place, and sanded to a finish in situ. As for the lower section, under the nose, this can be glued in place after the battery box is finally located, and all radio gear added.
Construct the tailplane and fin with strip wood whilst fabricating the rudder and elevator halves from sheet. Using one of Mr. Brian Gaskin’s superb Softbore tools, lightening holes can be cut in the control surfaces with ease - a remarkable gadget. In fact, probably the best invention since the razor saw! Euro Twin’s elevators have a ply joiner, with the horn located inside the fuselage. As usual, some thought is required here before you choose a construction sequence; whatever you do, cover the hinged surfaces early. When it came to hinges, my own preference, to save weight, was the Kavan fabric type, cyano-glued into position.
A GOOD RIBBING
Euro Twin’s wings consist of 1/16” sheet ribs, with as many lightening holes as you dare cut out. 1/8 x 1/4” spruce main spars are used, with diagonal cross bracing. The trailing edges comprise a 1/16” sheet built up construction, as do the nacelle positions, with some diagonal cap strips. Wing halves are joined using a 1/32” dihedral brace and some centre section sheeting. Finally, everything is finished with wing skinning grade cloth and epoxy resin.
Don’t forget to put your power and servo leads in early - alternatively, leave some holes for later fitting. Either way, just don’t forget about those servos!
Carbon fibre tows should be epoxy-glued to the bottom of your main spars, to provide extra strength for the wings; one continuous length is used on the prototype from tip to tip. The ailerons are simple, top hinged affairs, which could be built as part of the wing structure and cut out if required (personally, I prefer to build mine separately). Horns are cut from paxolin sheet but, there again, they could be commercial items.
Mini aileron servos should be used. Mount these between the ribs in some boxes; the latter are easy enough to make, and simple to fit. For ease of construction, and in order to save weight, the aileron pushrods are 14 swg. wire, bent with some ‘Z’ bend pliers. Adjustment is provided by means of a screw-fixed clevis.
BLUE IS THE COLOUR
Motor / engine nacelles are easily carved from blue foam, in two parts. The final, external shape is then produced using coarse and fine glass paper, after which, a seating for the motor and wiring can be hollowed out.
The average 400 size motor will have cooling slots in each side of its case, and you’ll need to drill some corresponding holes through each nacelle to allow an escape route for the warm air. Once your two halves are joined, and the motor is in place, round off each assembly and seal the foam with two coats of wing skinning epoxy. When dry, this provides an ideal surface for painting. Epoxy-glue your nacelles in place on the wing - and once again, don’t forget to install that wiring!
A tip: To simulate exhaust stubs, I glued two rolled paper tubes into the cooling holes of each nacelle, and these really do enhance the scale effect.
TIME IS MONEY
My battery box is constructed around a seven-cell pack, as shown on the plan; make yours to suit. When purchasing cells, just remember that capacities seem to improve every month - basically, the more you pay, the longer you fly. The latest 3000mAh nickel hydrides will give you approximately 12-15 minutes flying on a calm, warm day.
Make a mono landing gear wheel thus: Construct a simple 1/16 ply box, and add a bolt for your axle. I made a blue foam fairing, and epoxy-glued my assembly to the centre section.
Once you’ve installed the radio gear, as shown on the plan, there’s not much left to do. Cut some cabin windows from OHP acetate (or similar), and apply with either contact adhesive or balsa cement; set up the C of G using your battery box, and you’re ready to begin covering.
On mine, the blue foam areas are painted directly using a matt base-coat of Humbrol. Matt enamels seal the foam quite nicely, ready for a coat of whichever gloss finish you choose; I covered my fuselage sides and tail surfaces with Litespan but, whilst this saves weight, I now wish I’d gone for Solarfilm. Frankly, it’s easier to apply, and would not have required painting. Registrations are drawn using a suitable card stencil, and filled in with a brush.
With the aid of my clubmate Dave Cauldecourt and his legendary hand-launch, I made a completely successful maiden flight with the prototype. Due to the positive wing incidence, she did climb a little enthusiastically to start with, but the rate of ascent was quickly controlled with a few clicks of down trim. Shortly after, and with complete control regained, the Euro Twin was cruising around merrily. Some mild aerobatics ensued, and these proved that the main spar is indeed up to the job.
So long as your aspirations are realistic for this type of model, she’ll happily cope with what you throw at her. Rolls are just as horrible as the Twin Star (down elevator required when inverted), though as you’d expect, the faster you go, the quicker they get.
Nice, light touch-downs on that mono undercarriage are the order of the day and you certainly needn’t worry about slowing it up too much. Like the Twin Star, she’s a little peach! As I say, mine’s been a complete success. Isn’t it nice when you get it right!
Type of model:
‘Cartoon scale’, executive twin
2 lb. 15oz. (including battery)
Speed 400 7.2V
: Paul Gunther 5 x 3
7 x 2000 mAh, 8.4V
Rec’d no. of channels:
3 or 4 (rudder optional)
Aileron, elevator, speed controller
Control surface travel:
Aileron - 1/2'' each way
Elevator - 1/2'' each way
EDITORS NOTE - The Euro Twin plan (and this article) were first published in RCM&E November 2000 issue so the brushed electric motors and batteries referred to are now little used for electric flight. Builders contemplating this model should use brushless motors and Li-Po batteries. Accordingly a 300-400 watt system should suit. If in doubt then please check out the forum section dedicated for electric flight beginners.