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TOPIC: Building the Gnat

Building the Gnat 2 years 4 months ago #477

  • g.rosen
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Hi everybody
This thread will describe the building (and hopefully flying) of a Sebel Folland Gnat. I bought the kit some time ago, mainly because I got intrigued by the construction method with preformed fuselage halves split horizontally, not vertically as you would normally do. I found that the kit was more or less complete, but missing plan and building instructions, fuselage halves and canopy warped, box in bad condition, so I decided that there are other kits more collectible, and that this one deserved to be built.
Thanks to Roger and Andy I got copies of the plan and building instructions, so here we go.
Since everything except the fuselage halves can be easily copied, I have documented all parts, and am preparing a 3D cad model of the fuselage, which hopefully will make it possible to 3D-print “molds” to reproduce the fuselage halves as well. We’ll se.



Here is a picture of the Kit before starting. As you can see, one fuselage half is badly warped, but no major cracks, and I have assembled the fuselage jigs. The print on the jig formers was made at the wrong side, so the fuselage would have had a very narrow waist if the formers were mounted in the order they were numbered, and the top formers were marked bottom and vice versa.



Next step was to identify all parts, and by turning the jigs upside down and adding some weight I try to make the fuselage halves go back to their originally intended shape. I don’t want to use too much water here as I then risk losing the fine details imprinted in the balsa, so I start by just adding pressure while dry, and letting it sit for a few days.

A question about the colour: The kit should be painted “Nordic blue”, and as far as I understand it is a pale blue colour. Is there anyone that can help me with some kind of colour code reference so I know what I am looking for?

Gunnar
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Last Edit: by g.rosen. Reason: spelling error

Building the Gnat 2 years 3 months ago #487

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Progress report.
After a week or so the fuselage halves has , more or less, returned to original shape, and the formers have been glued to the keels. As you can se the top part of the fuselage (bottom in the picture) was not well aligned while pressed, and there is hardly enough balsa left on one side to cover the formers.




With the keels glued in place everything looks fine, but it is obvious that the next step, making space for the augmenter tube, is the only stage in the building where weight can be reduced. I will try to remove as much as possible of the double keels without reducing strength too much.
Gunnar
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Building the Gnat 2 years 2 months ago #492

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The building has continued over the last couple of weeks, and here comes the result, so far. First I decided to cut away as much as possible of the keels to lighten the model, and it also seemed to be good idea to install the motor hatch before putting the halves together.

Here is the bottom half with the motor installed, ready for gluing together,


like this.

Next step is to cut out the air intakes, and add the fuselage spine





more to come.
Gunnar
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Building the Gnat 2 years 2 months ago #493

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Next problem was the badly warped canopy. It was pressed over a wooden mold and then heated with boiling water to retake the original shape. I was a little worried that it may be to hot, and that the plastic would shrink and be destroyed, but a test with some of the plastic that was trimmed from the canopy showed that boiling water was just what was needed.



We also wondered about the centre of gravity, which seems to be far aft, and the angel of the tailplane, so a friend desided to make a profile model with the same geometry and CG.


It turns out to glide excellent at slow speed, but it rises dramatically if thrown hard (as when ejected by a Jetex?), so I don’t know if it will be necessary with some kind of deflector to direct the thrust from the motor downwards.

With the geometry tested, the tail was put in place


and the canopy testfitted.


The noseblock has also been fitted

Finally the wings was glued in place,




and the CG seems to be in just about the right position.


Total weight so far, with empty motor, about 28g.

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Building the Gnat 2 years 2 months ago #494

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The result so far



Now on to painting. The plan is to start with one or two layers of thin dope, and then a thin layer of Nordic Blue.
I have bought different kinds of blue, and will try blend them to get close to my idea of what it should look like.

Gunnar
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Building the Gnat 2 years 2 months ago #495

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Ooooow, I'm enjoying this, the model looks terrific, you have made a wonderful job of salvaging it, I hope the finishing goes as well and that it flies like a good 'un. I think you will get away with any middle blue, no one seems to know what the correct shade was anyway so pick the one you like best, I'm sure it will be fine.
Keep up the good work, I might just get back to the building board myself today, well the repair board at least.
Howard
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Last Edit: by ahmetcalfe. Reason: Addition of name
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