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Foka 4 (I/4 scale)

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Martynk
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Foka 4 (I/4 scale)

Post by Martynk »

Hello

I am planning on building a 1/4 scale Foka 4 from my own plans. Construction will be mainly traditional i.e. Balsa and spruce - not a Glass Fus - but with sensible use of carbon where appropriate. For example spar caps and fuselage longerons. I have designed and built duration gliders in the past (both F/F and R/C), this is my first attempt at a scale glider

At the moment, I am at the design phase and pondering what to do about the wing sections - perhaps someone here can give me some guidance.

The original had a NACA 63(3)618 root tapering and transforming to a NACA 4415 at the wing. This equates to an 18% wing thickness at the root and 15% at the tip and I need to thin these down a bit but without losing the original design integrity and appearance too much. For example, the top of the wing blends into the top line of the fuselage which obviously, I need to maintain. My thoughts at the moment are along the lines of substituting the root with a NACA 63A612 and a NACA 63A511 at the tip. Using traditional construction means that I cannot go much thinner than this if I need a wing that will withstand a tow launch. The wingspan will be about 3.8m.

Alternatively, I could use something like an RG-15 which is well proven at model sizes - although this is much thinner again.

Any thoughts please? As I need to design/build the fuselage around the wing, the position of the wing joiner is quite essential.

Thanks

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Peter Balcombe
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Re: Foka 4 (I/4 scale)

Post by Peter Balcombe »

Martin,
Just a thought regarding wing thickness.
The Foka 4 has top & bottom scissor airbrakes, so if you are thinking of having this type of brake, you need to make sure the wings are thick enough at this point to get the mechanism in.
I recall changing the section on the Mike Trew 1/5 scale Fauvette to an HQ section to avoid needing to build in washout and this needed a redesign of the brake pivots/arms/blades to get the scissor brakes to fit in the reduced thickness.
I know that Noël Rumers favours using the full size section on larger models and reckons they work if the minimum chord is at least 130mm or so. However at 1/4 scale, your tips would only be 94mm, so you need something else anyway.

I am sure the learned builders/flyers will chip in soon.

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RobbieB
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Re: Foka 4 (I/4 scale)

Post by RobbieB »

HQ without a doubt - looks quite similar to the NACA 6 series as well.

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Thanks all

While you were typing I was looking at the HQ series. Looks like a laminar flow set and would fit well with the general appearance of the Foka

Appreciated.

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

Post by chris williams »

Wing section choice depends on which type of pilot you are: hooner or floater. If you want a high speed aerobatic model, then the HQ section won't be all that happy upside down. If, on the other hand, you want to fly around at nearer scale speeds, with a more modest aerobatic repertoire, than this section will be ideal. I have used the 3.5 camber at a variety of thicknesses, the max being on the Petrel, which is near 17% at the root, so if you need space for your airbrakes don't be afraid to use it. The only downside of this section is that it is very thin at the TE, so when plotting in Compufoil or whatever, I usually extend the chord some 10mm or so, and then increase the width of the stock TE to give a 1.5-2mm stub at the end of the rib.
You really shouldn't need to use any carbon in the construction: the traditional materials are quite strong enough.
For details of construction methods, download one or two of my free plans from the website and take a gander...

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

I use the devwing predecessor - Profilli pro. Looking at Chris' suggestion it's quite easy to add a false TE but simply don't fit it.

Hopefully have some ideas posted on here for you to tear apart in a nice friendly manner. Hopefully :D

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ARUP
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Re: Foka 4 (I/4 scale)

Post by ARUP »

There is a fella (name?) here in the States that scratch built one of these and not only is it very distinctive in appearance but flies really well! I'm sure you will love yours!

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

A bit of progress but first my motivational image. I need this to keep me going - this is what I am going to try and emulate.
OE-5737.JPG
Its not going to be a contest scale model - more of a 'stand off scale' that looks better the further you stand away....
I also want a model that I wont be frightened of flying/damaging - definitely not a hangar queen. Outline and markings will be accurate though.

So, Hacking around in Profili and using the root wing section HQ3.15-14 ( I do pay attention and will use your suggestions)
Root.jpg

and at the wing tip HQ3.5-12
tip.jpg
The spars and joiners are all planned in the root and tip profiles, during the design process you choose where you want them to start and stop. You can see where I will be trimming off the last 10mm of wing section as suggested earlier. The revised TE will be capped with a 6mm wide spruce strip.

So the wing planform looks like this
planform.jpg
A main spar from spruce with the 'spars' for the spoiler slots and ailerons. The aileron rib positioning is not correct yet.

(the original had fabric covered ailerons, elevator and rudder) and ply skinned flying surfaces.

One area that I am not too happy about at the moment is the wing joiner which will be 9mm Carbon Rod in a Carbon tube, suitably boxed in and reinforced with a Kevlar binding. It works on my 3m span Bubble Dancer (My Avator) but this is likely to have a somewhat heavier wing loading

I have also started work in DevFus and have a skeleton that looks a lot like this but with less pixels
F4-1 Fus.jpg
Quite a bit more to do.. The wing root fairing will be added after the fus has been built and planked

I'll double check everything then import into Draftsight to do the remainder.

More to come..

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VinceC
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Re: Foka 4 (I/4 scale)

Post by VinceC »

That's a nice 'example' of the Foka 4. I can see why it gives you inspiration

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

This is where I am up to with the plan. I can start building a fin and tailplane
I have thickened the fin section to NACA0010 as it was too slim for the fuselage to fin fairing but apart from that progressing more or less as planned.
Foka 4 Plan v1
Foka 4 Plan v1


More to come..

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