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Ballerina PET


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Ballerina PET

This Ballerina has been a long time in the thinking stage but at last it is on the bench. It gets its name from the power unit to be used which is a new Evolution 10cc petrol, a venture into small petrol engines which needs an airframe for running in. This engine’s full weight with ignition and battery is 21 oz. The Ballerina appeals as a good looking aircraft from a designer we all know and trust, but because of the engine weight and size some changes are necessary because of some extra criteria to be met.

evoe 10gx.jpg

I want a long top hatch to have full access to the hardware, including the fuel tank, ignition, radio, servos, and a 3-axis stabilising gyro to ease flying in windy weather as we seem to have so much of it now.

For space reasons and because of the engine weight the model needs to be bigger than the standard version; I estimate 62 inches span as being suitable, so stage 1 is to produce a copy of the plan to that size. Because of the structural changes I chose to trace the published plan into a CAD program. This gives advantages; the resulting files could be used to have components laser cut, though I favour printed shapes to stick to the balsa sheet for cutting with a fretsaw. The CAD system guarantees a high degree of accuracy. It is also easy to re-size the essentials of the whole drawing in a few seconds, needing only some adjustments for different skin or spar materials, as these can not be scaled.

bal_cad.jpg

Having got this far the next stage is to print all the component shapes, stick them on the wood, and start cutting. The shapes are stuck to the balsa using a Pritt stick, then the cutting is done with a hand fretsaw with a blade of 27 teeth per inch, tough enough to cut thin birch ply but still delicate enough to cut 2mm balsa accurately. Afterwards the printed patterns are easily removed by a brief soak in water which allows them to be peeled off. This type of pattern was a feature of Complete–a-Pac kits years ago.

patterns.jpg

Once the drawing was done I made a cardboard mock-up of the front fuselage to be sure that all the components would fit, that the spark-plug cable would fit, and there would be no component clashes.

mock up.jpg

Glad I did this as I had to revise some of the first draft.

mockup full.jpg

Fitting the ignition module was the main problem. It needs to be as far from the radio as possible, but this en gine relies on exhaust pressure in the fuel tank, so both tank and ignition claimed the front section of the fuselage. The solution was to drop the thrustline of the engine slightly and divide the front fuselage bay with a horizontal plate.

mockup ign.jpg

The mock-up was a big help in designing the installation, so construction can start. I will build the fuselage in the jig,, so it will begin with just the lower part, with the upper longeron flat on the base. The upper structure will be added later.

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This will be interesting to watch.

I can't/won't work in CAD for a very simple reason. I have to see the drawing full size. Example. I just started drawing up a new scale model. IT lookedOK on the 3 view.

Once I had the fuselage side view done I just didn't like it at all apart from the fact that the fin and rudder looked far too small for my liking.

Even on non scale I find that changing the angle of a line or a curve by just 1/8" can make all the difference to the shape of the model from being ugly to looking just right

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I suppose liking CAD is very much a personal taste thing. I find that I get the best freedom of control over outlines working with small sketches, then standing back from the page makes it easier for me to see how the shapes fit together. A pencil sketch gives me the best control of a line, and then it is imported into CAD (as a .jpg file) and precisely traced. After that the flexibility, accuracy and convenience of CAD simply enhance it.

The drawing I posted would be about 14 feet wide if printed in one piece: fortunately any section can be inspected in any desired degree of magnification, and I use a big monitor. One advantage of CAD is the ability to copy sections of a drawing, to mirror them, or to rearrange them in new positions, all at a click of the mouse. Also the default drawing limit on the package I use is set currently at 0.01 mm, more than enough for anything I will ever build.

Almost all parts cut now, should be a post on the fuselage build tomorrow.

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The card mock-up also confirmed an easy fit for the ignition module and high tension cable, with room for a large LiFe battery, and that the chosen engine mounting angle will allow the silencer to clear the fuselage.

The fuselage build began with ¼ square balsa longerons attached to the top and bottom of each side, then the first four formers and the separating plates were attached to one side. These are self-jigging, which makes sure that they are at the correct angles.

fus inner 1 side.jpg

Then the second side was attached to the assembly and it went into the building jig for the fitting of the rest of the formers. This stage is done with the fuselage upper longerons resting on the jig base. As the engine firewall is made in one piece the front end has to overhang the jig base. The rest of the formers have the tops added later.

fus inv jig.jpg

The top hatch is built with the rest of the top of the fuselage, with temporary adhesive tape in any positions where the hatch might stick to the fuselage. A thin wipe of Vaseline on the tape ensures that nothing sticks where it should not. It is fixed with two small dowels at the front, and at the rear with magnets.

1st full fus.jpg

Edited By Clive Hall on 21/03/2016 21:05:59

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Ash, I could use a 4 oz tank and the running time would be enough, but Bob C has it right. I plan to use the 8 oz tank simply because it is longer and gives me a reasonable chance that the longer tygon clunk tube will be able to follow most of the fuel round the tank. Keeping fuel in the tank helps to keep the clunk soft, but even with that a short 4 oz tank would be pushing it a bit.

Now tonight’s post:—

The control runs are steel Bowden cable in black nylon outer. This comes off a roll curved and needs to be made straight. I do this by passing a length of 16G piano wire through it and then heating with a hot air gun. When cooled down the wire comes out leaving the tube straight. I fit it in the fuselage with the wire again inserted, tack the outers to each of the formers with a hot glue gun, and then remove the wire. It makes sure that the cable has a smooth run.

The twin aerials for the receiver are routed similarly. They sit in precise positions inside the same nylon tube material, pre-shaped to fit. I sketch the required curve then outline it with pairs of pins, trap a piece of tube between the pins, and apply hot air. When it cools the tube retains the shape.

pin nylon.jpg

shape nylon.jpg

fus runs m.jpg

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I use FrSky radio exclusively now, having tried it for some time and found its performance excellent. Then I went the whole hog and converted to a Taranis transmitter, which has made all my older gear redundant with its amazing flexibility. Once accustomed to it the programming is not difficult.

I have also taken to the Orange stabiliser. When it is flashed with upgraded firmware it opens up many of our otherwise frustrating windy days as safe flyable ones. It has made it practical for my WOT4e to fly in up to 20 mph winds, gusting to 30 mph, without being the least bit scary.

x6r+orx.jpg

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The holiday weekend is not good for building time — too many family interruptions. Meanwhile I have to make decisions about the upper fuselage skins.

One question is whether to plank the top hatch or to use curved sheet. Peter has kindly avoided double curvatures, so I have chosen pre-formed balsa sheet. First I cut two oversized pieces of 1/8 sheet, one for each side, and then found two A4 sized plastic envelopes that sometimes turn up in the post. These I cut and opened into A3 sized sheets. Then each piece of balsa was laid on its sheet and painted with 10% ammonia solution on both sides. Then the pieces were tightly wrapped in the sheets and left overnight.

Next day the previously stiff balsa was soft and pliable and still ponging of ammonia. The plastic sheet idea avoids all need for baths of liquid to soak the sheet balsa. I found an old small towel and wrapped the balsa round a convenient plastic tube, finishing it with many rubber bands to hold everything tight. It now rests on a radiator to dry and should produce two curved sheets, close to being a good fit.

roll_rad.jpg

The second decision concerns the rear upper decking; whether to use sheet again or stringers. The appeal of sheet is that it should be quick and easy and will allow the same finish to be used over the entire fuselage. I plan to use glass/epoxy. Stringers would require fabric, same as the wings, for which I will use Koverall. (The jury is still out.)

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Sounds good.

At the moment I am searching for a new model to design. Started and scrapped several ideas.but this morning going through my collection of 3 views I may have found the subject that fits my specification

All fabric covered. High wing with a one piece wing. NO struts.. In other words a scale Super 60!!

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The balsa moulding operation produced well curved sheet, so now it can be attached to the top of the hatch. The balsa used was medium hard, and it was surprising how determined it was to stay in its newly formed curved shape.


hatch skin.jpg

There was a tighter curve than was needed over the central part of the hatch top, but a small amount of water brushed on the inside of the sheet made this relax to enable easy fitting of the skin. On the outer edges of the hatch the opposite happened, so I brushed a little ammonia on the outside of the sheet here to help the extra bend required.

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There is a technique for making a clean fine join where the front of the top hatch meets the top of the front of the fuselage. This assumes that the front top bit of the fuselage already has its skin fitted. The first step is to make sure that when the hatch top skin is attached it is slightly too long, so that it overhangs the front of the hatch. It can then be sanded back to a precise match to the face of the front former leaving a sharp outer edge. Next coat the face of the front former with two coats of non-shrinking dope to seal the wood, then cover it with brown parcel tape. It helps here if the fit to the fuselage top leaves a little gap, to allow for the next step.

Lightly smear the parcel tape with Vaseline, then mix some P38 filler paste. Make a bead with this round the top edge of the mating surface on the fuselage front then gently but firmly press the top hatch into place. Excess filler should squeeze out from the gap; allow this to happen and leave it to set. Then lightly sand off the excess filler until the top surfaces of the hatch and the fuselage match. Now remove the hatch, peel off the parcel tape, and the front of the hatch should be a perfect fit. The amount of filler left after sanding is small, so its weight is negligible.

fus and hatch ps.jpg

hatch on fus ps.jpg

(There is still a bit of tidying to be done along the bottom edges of the hatch.)

The same technique will be used to make a similar fit for the rear of the hatch.

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The rear fuselage was redrawn to be skinned with 1/16 balsa sheet, but then decided to be striingered, more in keeping with Peter’s design. This led to a problem, because the formers were cut to fit the sheet and the rear top was already built. It meant that the formers had to be notched for half the depth of the 1/8 square spruce stringers. Fortunately this was easy to fix on the CAD drawing.

The formers were then notched. Card templates made from the drawing were held against each former and used to guide a 1/8 inch wide cutter made from two pieces of old hacksaw blade glued together with a thin spacer and with a depth limit on one side.

notch templates.jpg

notch tool.jpg

After that the rest was easy. A better solution would have been to have got the drawing right in the first place and to have had the formers laser cut.

fus str open.jpg

fus str shut.jpg

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