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Foka 4 (I/4 scale)

Let us all watch your new project progress.
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Andrew Ray
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Joined: 19 Mar 2015, 19:15
Location: Staffordshire

Re: Foka 4 (I/4 scale)

Post by Andrew Ray »

Hi Martyn, looking good.

I think I would change the wheel mounting. None of my sailplanes have any springing in the u/c and I haven't seen any designs that are sprung in fairness, certainly not up to 1/4 scale.

Andrew

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Martynk
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Location: Haslington
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Early start (06:00) this morning due to insomnia, Cut and fitted the 6mm ply dihedral brace to the centre panels but mainly worked on the wheelbox. The horizontal deck is 3mm lite ply so not really designed for impact loads. to add a bit of strength, I fitted a 3mm birch ply plate (glued onto the lite ply deck and fitted snugly between the spruce longerons) with 3mm birch ply sides which conveniently just fit inside the two lower longerons. Balsa front and rear. Hope that will be OK...
wheelbox1.jpg

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

A bit more progress.

I knocked up a simple jig from foam blocks to keep the fuselage stable and square while I started to align the tailplane and wing.

To keep a nice slightly nose down attitude in flight, the tailplane will be set at (about) +1 with the wing set at +2.5. I use a spirit level app on my smart phone which seems accurate to about 0.1 degrees. Importantly, its also consistent
Fus7.jpg
With the fuselage stable I could start fitting the tailplane. A slight deviation from scale here. I have formed a hard balsa platform for the tailplane to sit on which was needed to set the incidence. Consequently, it has been lifted by about 4mm (who really cares anyway? :) ) So with the wing centre section pegged at the LE and the trailing edge clamped to the datum longerons, the tailplane incidence and set horizontal, checked and double checked and then the glue left to dry overnight
tailplane16.jpg
And the wing looks like this at the moment - and I have just realised that I haven't actually documented how this has been assembled yet
wing4.jpg
The fin was fitted this morning, photos to follow.

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

A quick update on the wing. I have finally decided on a composite carbon/spruce/balsa main spar with a spruce rear spar.

The root cross section looks like this:
Wing0.jpg
The Carbon tube tapers from 10mm (0.5mm wall thickness) at the root to 7mm at the tip in equally spaced step (10 > 8 > 7). This is supported at the root area by a soft 12mm balsa 'web' (grain vertical) epoxied to the carbon then a 12x3mm spruce top and bottom cap. This will taper to 6x3 at about 80% wingspan. The Joiner is a 9mm carbon rod. Te rear spar is 6x3 which delimits the ailerons and the spoilers with additional support in the root. The main dihedral brace is 6mm marine ply which tapers slightly to fit with the sweep forward of the main spar.

The centre section is about 600mm wide, the outer panels one piece..

The wing mounting bolt sits in a full depth birch block recessed to take a M5 steel cap bolt


Just spotted that I have lost the rear spar at the root on this diagram

Assembly is simple but slow as all the balsa blocks have to be cut and filed to shape. Drilling causes the soft balsa wood to split
wing1.jpg
This shows the spar going together. Its an idea that I have seen on F3B models - although its probably way over engineered for slope soaring and aero towing. However, it does provide a light and very stiff and very strong main spar. Probably strong enough for a winch tow providing it its not too energetic. I may also bind the stress points at the junction and end of the joiner rod area with Kevlar thread - belts and braces
wing2.jpg
and
wing3.jpg
The shots give an overview. The trick is to get all the parts to fit then glue and clamp it all together then leave it to set. Each joiner will be 450mm long and the full width webs will end a couple of bays further out at which point the thickness of the webs will get reduced so they simply support the space between the tube and the spruce caps. The idea is to get a spar that works as one and merge the different bending properties of the different materials.

The centre rib is 12mm balsa sandwiched between 2 ply ribs and the beech wing mounting bolt block fitted into the same space. I have also recessed the block a little into the upper and lower main spars

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

The fin installed. It is located by the extended spars that sit each side of the widened top longeron. With the fin in place a hardwood dowel locates the fin into the top longeron. It seems quite strong.

I showed the fuselage to Simon Cocker last weekend. He is happy that it is strong enough laterally but we were both concerned about its ability to withstand a torsional shock. This is especially noticeable now that the fin and tailplane have been applied. I am intending to add 1/32 ply doublers under the 1/8 sheeting from the rear former to just in front of the fin which should stiffen it up quite nicely. The fuselage is rather slender at the rear end.
Fin6.jpg
and
Fin8.jpg
The rudder shaft extends into the fuselage. To keep this stiff and prevent flexing, it is terminated into a nylon bush is a pine block. The horn is fabricated from brass sheet bent into an L shape and twisted. Its as long as I can make it, 0.5mm inside the outer sheeting..

A ball link is used to connect to the pushrod.
rudder1.jpg
Finally, as suggested here, the pushrods are made up from 5mm (1mm wall thickness) carbon tube. These will be supported mid way to minimise flexing
pushrods.jpg
Not much going to happen now for a while, I go on holiday on Friday, back in September :)

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Update part 1

Back from my holidays last week so once the dust settled, I have been cracking on with sheeting the fuselage.
Fus10.jpg
A 1/32 ply doubler was added around the tailplane and fin area (upper fuselage area) to provide a bit of stiffness, a second doubler added between the front two formers enclosing the nose-weight box
Fus11.jpg
The push-rods supported midway down the fuselage with balsa standoffs
Fus12.jpg
The upper rear section was quite easy to sheet. I simply glued one edge, allowed it to dry, then sprayed water on the 1/8 medium balsa and gently wrapped it round. Clamps, pins and bands holding it in place
Fus13.jpg
The lower rear was slightly more difficult as the curves were tighter, especially at the rear.
Fus14.jpg
The front had to be done in two sections as the compound curves were just a bit too much to manipulate the balsa.
Fus15.jpg
Lots of clamps...
Fus16.jpg
The very front sheeting from the LE forward was done in one piece. It did go down but it was very tricky manipulating it into position
Fus17.jpg
A bit of filler to blend the fin into the side sheeting
Fus18.jpg
The front section has been glassed inside to add some toughness. From experience this is where they will break when they find the hidden rock on the hill
Fus19.jpg
The rear tailpiece was carved from balsa. I think that I mentioned that I changed to a standard elevator horn. Its just visible which is a bit of a shame..

More to come
Last edited by Martynk on 14 Sep 2017, 09:16, edited 1 time in total.

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Update part 2
Fus20.jpg
With all the gaps completed, this is the front fuselage area. There is one bump that shouldn't be there as the sheet didn't quite flow around the fuselage as I would have liked
Fus21.jpg
And looking backwards

Generally, I am quite pleased with this. There are few self inflicted dings that need to be filled but it doesn't look too bad.

Next job is to make the nose-block, add the aero tow release and then make a start on the wings..

More to come

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

Thanks Cliff

With the fuselage sheeted, I am starting to realise just how big these things are. I wouldn't have space for anything larger

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

I have just weighed the combined Fuselage Fin and Tailplane. It's currently 1.38Kg I am not sure if that is good/bad/indifferent but it doesn't feel too bad. I guess that it will probably need about a kilo of lead to set the CG though :(

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Martynk
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Re: Foka 4 (I/4 scale)

Post by Martynk »

After a great deal of prevarication and making space (I had another large model on the go at the same time), I have made a start on the wings.

LE and TE are composite spruce and balsa and the long lengths joined with scarf joints. That took a couple of hours as its quite easy to build a bow into the structure if you are not careful.

With the glue dry..
wing5.jpg
and
wing6.jpg
The first two shots are a dry assembly. The main spar is a composite - a tapered (telescopic tapering) carbon main spar that will be capped with spruce (and webbed).. This gave me the opportunity to check that everything actually fitted and to cut the tubes to length and epoxy them up. The wing ribs are lite ply and were cut by Belair
wing7.jpg
Fit the joiner and add the new spar with ribs already threaded in the right order.

Using the already fabricated centre section the root ribs including the 1/16 ply cap rib were inserted, epoxied to the tube and the LE clamped in place. Clamp the root ribs to the centre section ribs for hopefully a perfect join.

Simply move down the wing checking each rib is as 90 degrees to the LE and glue in place. No need to pin the wing down, just check that each rib is sat correctly on the rear building tab. The TE will be added when the epoxy has hardened. I finished the first panel gluing ribs into place this morning.

The TE should get fitted this evening. I use slow set epoxy for the carbon joints - which actually seems to go dry enough to work with after about 6 hours at this time of year

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