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Foka 4 (I/4 scale)

Let us all watch your new project progress.
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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

A quick question.

I am planning on covering the open structure components with Solartex and then probably paint. What do people normally cover models of this scale over the sheeted areas. I am assuming light glass cloth (25g/m2) and paint. Is that correct?

Thanks

Martyn

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Thanks Cliff

That would make life easier I think...

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

I got to this stage last night. The 3 Robart hinges are epoxied in place and the fin has been skinned with light 1.5mm balsa.
Rudder covered in natural solartex - colour doesn't really matter as it will need to be painted.

This morning, I gave one side of the fin a layer of glass and used eze-kote instead of resin to seal it. When dry, I'll sand down the seam and repeat for the other side.

Tailplane next..
Fin4.jpg
More to come...

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

A bit more progress.

The tailplane is well under construction. It's taken 3 attempts to get this far, mainly because the tailplane was too thin on the earlier versions. Its now a NACA 0010 at the root thickening to a NACA 0012 at the tip.

The main spar is a tapered 1/2 x 1/16 hard balsa with 1/16 balsa webs plus 1/4 x 1/8 at the hinge line with a full sheet fill in. It should be OK

Fairly traditional construction. As per the prototype, the main part will be sheeted and the elevator covered in Solartex.
tailplane3.jpg
and
tailplane1.jpg
and
tailplane6.jpg
and with the elevator now separated
tailplane7.jpg
Hinge blocks added. Robart pin hinges centre and mid point in each half. I'll have to use hairy hinges for the tips as the tailplane is too thin to take a pin hinge

I have also just about completed work on the plan and the DXF files have gone for laser cutting. These are the two Liteply sheets
Liteply1.jpg
Wing ribs
Liteply2.jpg
Fuselage parts

There are also two extra parts sheets of 3mm & 6mm Birch ply for the centre formers.

Wood is on order - should be with me next week as well. Then it starts to get fun.. :)

More to come - but slowly

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chris williams
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Re: Foka 4 (I/4 scale)

Post by chris williams »

Martyn, at any sort of speed the tailplane can come under quite a high loading. I'm a bit concerned that the 1.5mm balsa spar might well not be up to the job...

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Thanks Chris

I have to admit that I was a bit nervous about this. Reinforced further as I folded a tailplane on a Kwik Fli this week :(

I forgot to mention that there are 1/16 ply braces at the front and rear spanning the centre area (one bay each side). I think that with a fully sheeted tailplane - effectively making the spar 1/8 thick top and bottom and the additional spar at the hinge point with be 5/16 thick plus a full thickness web at this point it will be OK. It certainly feels strong. However, its not too late to add an additional spar from spruce over (say) the three inner bays each side.

I am just a bit concerned about adding too weight at the rear of the model

Jilles
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Re: Foka 4 (I/4 scale)

Post by Jilles »

A bit concerned about the horizontal stab spar section. Forces on the stab/elevator can be high if things go out of wack occasionally.
I suggest if you plank the stab with 1.5mm (1/16") balsa, glass that surface with 25gr/m2 glass cloth and resin. With resin the rule of thumb is the completed weight is twice the glass weight in this case 2 x 25 is 50 gr/m2
If you workout the actual surface area of the stab I doubt if you go over 20 grams extra in total.
A small price to pay for a piece of mind. If your stab elevator goes, you can start building an other Foka from start

If you use Peel ply in the process of glassing you end up with the minimal possible weight and no need for sanding the final glassed surface

I am one of those that within reason are not to concerned about weight of a scale glider. A bit more weight will make then fly much better.

Cheers

Jilles

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Thanks for the feedback, its much appreciated.

I have added a spruce spar (1/4 x 1/8) across the centre 6 bays.
The tailplane will be fully sheeted and glassed before it gets paint. The elevator will be covered in Solartex. This is now far stronger than I would have used on my F3As, I cant imagine it will fail in flight.
tailplane8.jpg
The other problem I had was how to get the elevator horn to fin. The elevator hinge line sits over the end of the fuselage. I wanted to keep it hidden so I have produced a brass horn with a dog leg offset. I appreciate that it wont operate symmetrically, but I can program any differential out on the transmitter. Its also offset a few mm so that the push rod clears the rudder shaft. I will terminate the push rod with a ball link rather than a clevis.
tailplane10.jpg
Parallax makes that photo look odd. The horn is actually parallel..

(Honest)

More to come

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Not much of an update..

The tailplane is now just about completed.

The tailplane sheeting has been applied providing a slight shroud.
tailplane14.jpg
Th elevator has been covered in Solartex and the then the hinges epoxied into place.

and then finally, the tailplane has been glassed and a light sanding ready for paint..
tailplane15.jpg
Hopefully the laser cut parts will be arriving today and the fun can really begin..

More to come

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B Sharp
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Re: Foka 4 (I/4 scale)

Post by B Sharp »

I have watched the whole tailplane/elevator saga with interest and I dont think you will have any real problems at the scale you are building. It was a good idea to glass the tailplane as this should provide the required level of stifness to the structure. Your big consideration will be to get a slop free connection to the elevator. The main problem with tails comes not when you are flying sedately around but when you get way too much airspeed (VNE) and the dreaded flutter kicks in.
Looking fine so far.
Brian. :)

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