Monday, 12 October 2015

Kwik Fli IV (4)


Which leads me nicely onto the wing and the foam cores. A couple of years ago someone posted on U Tube a method of cutting tapered foam wings with one end of the cutter held in a pivot. A friend at the flying club tried the idea and it seemed to work OK - so this is my attempt
Firstly, the geometry.
The pivot point has to be EXACTLY the same height as the centre of the foam core and also on the convergence of the projected leading and trailing edges.

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This is a plan view of the arrangement with the wing panel on the right hand side.

The panel is 31" long, the root chord is 12" and the chord at the tip is 9". There is equal taper at the LE and TE. The projected LE and TE converge at a point 93" from the tip, so the length of the cutting bow with be at least 93 + 31 + a few inches - say 130" - you need lots of space. Wings with a sharper taper will need less space.

I used stainless steel wire from the pivot (actually a cup hook screwed in the shed wall) - about 80" long then linked it to a length of 24 g Nichrome wire up to a handle

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This is the arrangement. I used my SLEC building jig screwed to my mobile building board and adjusted the height until it was correct. The squares help with alignment as a guide for the heated wire to get it smoothly down to the LE where I start.

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The handle is a short length of plastic conduit with the Nichrome wire fed through and secured. It needs to be secure - you are going to pull very hard!

I used an engineers square as a guide to get the heated wire lined up with the LE and the - while pulling like mad and moving very slowly I traced around the template in the root. AT the other end of the foam block, a 75% copy of my efforts at the root was being created.

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And this what you get 

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and

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I am really quite pleased. I cut 4 cores - 2 were unusable and 2 were better than just OK.
Added the false LE and TE and also assembled one of the skins (I only have space to do one at a time)

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and

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And its cut. The gap is obvious, the aileron chamfer less so. May need a little extra taking away 


4 skins were completed

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A Foam wing sandwich.. Using the remnants of the foam block from which the core was cut, the foam core was sandwiched between 2 skins and lots of weight holding it all together. I needed to remove a little of the original block to cater for the false LE and TE that had been added (and sanded flush with the foam). The skins overlap the false LE and TE.
I used Gorilla Glue - as recommended by Sam and Steve - a very thin layer on the foam as it expands quite significantly and PVA along the join between the false LE,TE and the skins. I left it to dry for about 4 hours

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When done (and the skins trimmed back to the false LE and TE), they looked like this - I am happy with that.. 

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Glued fitted and sanded the LE..

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and the Trailing edge..

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Aileron marked out. I am using Kevlar hinges (again) sandwiched between 1/4" balsa blocks. The centre line is the aileron, the outer lines delimit the edges of the hinge blocks..


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The aileron will be cut out using a band saw so a small balsa jig holds the wing at the correct angle. The wing junction is perpendicular to the chord line, the aileron edge is perpendicular to the lower surface of the wing. This will give a gap for the aileron to deflect down without having to cut a chamfer. The upper edge of the aileron will be chamfered slightly. The plan shows the aileron hinged on the top surface. This means a huge gap at the lower surface (which I don't like) so as a compromise, I have moved the hinge line down 1/4"

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parts cut for the aileron hinge block..

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which were pre-assembled and then glued into place - using a mix of gorilla glue and PVA again. One thing I forgot to mention, the face of the wing at the aileron junction has a 1/32 ply facing rib inserted. The ailerons will also have 1/32" ply facing ribs at each end.

The ailerons have been fitted. (Ignore the holes from foam that detached while I was sanding  )


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Neutral

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Up

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Down - about 20 degrees each way should be sufficient. If I need to make the gap bigger, there is sufficient meat to do so. I have also built the servo boxes in each wing. Carefully removing the sheet and all the foam to the upper skin. The servo stands vertically but is recessed so only the turret sits in the airflow

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(Still needs sanding flush - honest  )

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servo temporarily fitted while the glue dries..

The ailerons have had the ply cap ribs fitted

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With a nice narrow gap. The bulldog clip helps prevent hangar rash.

the tube for aileron cable was cut using a sharpened tube (actually the wing joining tube for the Glass Slipper which is now ballasted with a foam insert frown)

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You can see the aileron extension lead just tucked into place,
The aileron extension lead was joined to the servo lead using a short length of heat shrink tubing. Much better than the clips for this sort of application. I spotted the idea in an old (1993) RCMW while I was researching yet another build.. (subsequently replaced with a full length piece of heat shrink).

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Undercarriage beam next.

Following the plan, the beams were cut from 3/4 x 1/2 beech, the wing carefully marked ensuring that they were actually square allowing for the wing taper

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The holes were cut. I am not sure the best way of doing this. I don't have a router, so I used my scalpel to cut a 1/2" slit all the way round, then carefully removed about 15mm of waste wood and foam from underneath at a time.

Lots of Gorilla Glue in the slot and PVA on the wood > wood join and then clamped it up hard so that as the Gorilla Glue expanded it wouldn't force the beam out.

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Left it overnight to dry/harden
This morning, it was time to attack the aileron horns. The ailerons are quite thick and I didn't want bolts all the way through which wouldn't align anyway because of the converging angles of the two surfaces

The approach I adopted was to use 6mm ply plates with a captive nut on the back, the screw for the Tornado horn screwed through from the rear and then trimmed to length.

Like this.

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These little plates are then inserted into the ailerons. You can't remove the screw unfortunately but secured with Gorilla Glue, PVA and a little Cyano (holding it together while the other glues dry) it should be strong enough and look quite neat
I then managed to glue left and right horns in place on the wrong sides .. 

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It just means that the fit isn't quite as good as expected, but I felt it was too risky to try and remove/reposition them. Leaving them to dry until lunchtime..

I had glued on the wing tips - laminated from 2 pieces of soft 3/8 balsa - at lunchtime using gorilla glue and PVA. A short 1/16" ply insert at the aileron boundary adds a lot of extra strength and a nice edge to trim film to.

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Sanded to shape. I have decided to keep the edges square. Hope that doesn't upset the purists too much..

The remainder of the evening was spent bending the undercarriage legs and then joining the wing halves together. I'll cover the undercarriage separately/later as I am exploring options of making the fixed legs removable.

So onto joining the wing halves.

I join them upside down. The plan says 1" dihedral under each tip. However, the tip section is 1/2" thinner than the root, so when joining upside, the centre only needs propping up by 1/2"

My approach is to:
First check that the we have the correct amount of sweep on both panels. I had one panel that was 1/2" out measured at the tip. This was corrected by a slight trim at the root.
Prop the root up by 1/2" and then sand square to the building board - I use a long Permagrit block - making sure that the wing is at 0 degrees incidence (by eye)
When both panels line up when butted together I then jig the wing so that both panels are at the same incidence at the wingtip

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Using the aileron hinge line as a reference, the X axis shows -4.2 degrees on the right panel

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and -4.2 degrees on the left panel.. 

We also have -2.9 + 0.7 = 3.6 degrees total dihedral on the lower surface (ignoring the fact that the building board isn't actually level).

With the wing secured and weighed down so it doesn't move and Gorilla glue used as the panel to panel adhesive, the lower glass bandage was added, coated in epoxy resin and allowed to harden overnight

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There are small balsa blocks under each tip (by the aileron) to make sure that the wing doesn't move. A bit like building tabs.

This morning, the resin had hardened, the wing flipped over and bandage added to the upper surface. No need to jig this, its very rigid already

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I can't touch that until tea time at the earliest..