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.

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

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.

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.

And this what you get 

and

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)

and

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

4 skins were completed

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

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

Glued fitted and sanded the LE..

and the Trailing edge..

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..

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"

parts cut for the aileron hinge block..

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  )

Neutral

Up

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

(Still needs sanding flush - honest  )

servo temporarily fitted while the glue dries..

The ailerons have had the ply cap ribs fitted

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 )

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).

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

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.

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.

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 .. 

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.

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

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

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

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

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

Kwik Fli IV (3)

So, this mornings work involved getting the firewall fitted. With the model still in the jig I started the alignment process.

3 degrees of Right Thrust is equivalent to 7.3mm deflection at 140mm out. (I forgot to mention that I had offset the mount on the Firewall by 7mm)

Extend the engine mount using a pointer

Adjust the side thrust so that at 140mm the (inner edge) pointer intersects with the extended centre line. Check we are at the correct height and also the firewall is at 90 degrees - Vertical - no downthrust.

Then take it all apart again - add slow setting Araldite - and reassemble 

Add triangle reinforcement pieces - adjusting angles and cutting slots for a very good fit, glue and clamp in place - I use a liberal amount of PVA to attach these - making sure that the Firewall isn't disturbed in the process.

Finally, while waiting for the glue to dry, I cut and added the two parts for the fin extension on the top of the fuselage.

Not much I can do now until the Araldite dries

Once the epoxy had set hard enough to risk taking the fuselage out of the jig. A gentle sand down outside in the early evening getting the top deck to shape was a real treat and then back to work. Its slowing up a bit now..

I have added the SLEC snakes and heavy duty push rods

At the rear end, there is a small spacer to hold the outer tubes rigidly and epoxied into place. In this shot you can see the 1/8 balsa doublers. On the plan these are full width and the sides butt up to them. Much more difficult to fabricate.

With the tubes and pushrods in place, I added the lower sheeting - grain runs crosswise..

I trim these back to size when the glue has dried.

Finally tonight, the nose leg has been added and tubes fitted for the throttle and the steerable nosewheel.

The nose leg tiller is at a funny angle so that I don't have to cut a slot in the firewall. It means none linear steering but I am not worried about that. Note the tank is back is to make sure that nothing binds and it can be installed and removed with comparative ease.

Things will definitely start to slow up now, the next job is to fit the lower front sheeting and then fuel proof the tank bay and then make a cowling. I am tempted to make one out of Fibreglass using a simple pink foam plug. I'll settle for polyester resin and sand it smooth when it sets.

The hatch was quite simple. I have added 1/4 x 1/8 spruce rails to the fuselage top. Added a 3mm lite ply rear former on the fuselage and then simply assembled the hatch from strip and a 1/2" upper deck with play formers front and rear

Pretty basic stuff and mainly following the plan

The gussets have been added because the wood is rather thin at the junction between the top deck and the sides.

The slot is for the nose wheel leg mount - which is slightly offset to cater for the right thrust

Note the blood - that is to make sure it flies OK.

And onto the Cowl

I am going to try the method used by Chris Barlow in his 2015 triple mass build blog. This involves making a foam core then covering in Fibreglass tissue and then Fibreglass cloth. Chris has kindly provided me with some useful hints in a PM - thanks Chris.

Here is the cowl.. carved and sanded from pink foam

and

and

Looks OK at this stage - it can only get worse now.. 

The cowl was then wrapped in clink film then a single layer of packing tape.

This is the release agent.

Then a single layer of tissue with polyester resin.

and sanded down to get rid of excess lumps

And then, this morning - a second layer of tissue/polyester resin has been added

Its not quite as bad as it looks. I'll leave it until this evening to harden and then sand down and possible apply another layer, Eventually a top layer of 170gm cloth will be added secured with epoxy resin.

The plug was dissolved using Petrol as suggested by Chris Barlow - that was VERY effective and the cling film and packing tape have subsequently been removed. The cowl is quite useable, It has a couple of high and low spots and unfortunately one bubble but its not as rough as I expected it would be. Tomorrow, it will get a reinforcement ring of 170g and epoxy around the inside edge and then trimmed to length and the bubble filled in.

After that, its another sand down, high build primer and filler (hopefully not much) then paint and fuel proofer. That will really show up what its like and whether it ends up in the bin

The cowl has also progressed. I reinforced the inner edge at the fuselage junction with two small strips of 170g cloth and then left it harden for 48 hours. A quick sand down and first coat with high build primer left it looking like this..

Not too bad, needs a little filler in a couple of places, so this morning I filled it with P38

.. and (rough) sanded it down again.. Quite happy with that as well. It will need the length trimming back by about 5 or 6mm which will be done when I fit the engine. I'll smooth sand it then high build primer again.

It is definitely very useable though..

Glass Slipper (3)

Come a long way since the last post:

 I completed planking the upper half of the shell and in the main I am very happy with it..

Lots of pins

Now that the rear planks have been trimmed, it looks somewhat better than the photo from a couple of days ago. Sanding at the stage involved just removing the 'edges' - it will be sanded down more thoroughly before it gets covered

And the front former (behind the canopy) after it has been trimmed and rough sanded. A little ragged on the LHS (of the pic) but acceptable

Now I can get it off the jig I can examine the inner surface. I am pleased with that - generally quite tidy. I took the fus (on the jig) down to our club night on Tuesday night (at the pub!). I was asked how I do the edge joins. I have done quite a bit of planking in the past. Obviously, when working around a curve, you need to bevel the edges to the planks to fit against each other. I think that the mistake is try and bevel both edges. Use a stripper to cut parallel strips - on this the main planks are 1/4 x 1/8 - cut with a stripper. I then place the plank vertical and trim a bevel using a razor plane. All done by eye - after a while it gets easy to judge the correct angle to hold the plane. The other side is left square. If you try and bevel both edges, you will simply end up chasing yourself round in circles. I have also stopped trying to cut tapered planks for the same reason. Just taper the edges where you need to fit them. It all gets covered up in the end so no-one will notice

This morning - after another bad nights sleeps and hence early start in the workshop, I removed the fuselage from the jig. Now in hindsight, it would have been much more sensible to have done the bottom first - there is more of it, however, by removing the middle jig supports, I have got it in the jig, cut the lower slot for the lower backbone and fitted it - this one is laminated from 2 off 1/8 sq spruce

Now being left to dry..

Planking has started on the lower fuselage, its quite tricky around the bend by the fin post but apart from that is mainly more of the same..

One thing I nearly forgot to do was fit the pushrod tubes for the rudder and elevator. It would have been very entertaining trying to fit them after the planking had finished

At the rear

and at the front..

Note both these photos are actually upside down 0 hope they don't upset your sensibilities and/or OCD

I have progressing the planking slowly

Just working down one side, adding one plank at a time and letting it dry. The pins are pushed through from one plank to the next making sure the line of the fuselage is maintained

A couple of days later.. This is as far as I can go. I need to get the other side this far - the next big job will be to fit the wing joining tube which sits in this slot

which will be opened up at both sides when it is fully supported by the planks.

Also need to add the rudder push rod

One area that I am bothered about, I don't think the fin is large enough and may redraw with the fin about 50mm taller. Not a show stopper at the moment though

Getting very near the end.

The idea is to get the wing joiner tube fitted, so plank down until you get to the slot in F4

Carefully remove the edge pieces with a Razor saw.. The former should be self supporting now.

Cut a length of the phenolic tube about 30mm too long and adjust the gap for a snug fit with the tube centrally between the upper and lower former halves

When happy, liberally apply a large dollop of slow setting epoxy.

Use the full length of the joining tube, centralised cross the fuselage and then triangulate to the fin post, ensuring that the distance is the same for both halves.

Then leave it to harden. Tomorrow evening, I'll carefully remove the joiner, then complete the planking. There are two thick (10mm) balsa end ribs capped with 1/16" ply that will provide the structural strength to the wing to fuselage junction

Fin and Tailplane next