Scratch Build Twin

[Martian]

Loving your work Jim

[G-JIMG]

Made more progress on the wings, wiring in the ESC, UBEC, Retract and Nav Lights.

Jim.

[G-JIMG]

Now I've started covering the wing (the nacelles will be planked) I'll have to start thinking about the undercarriage doors.

Normally, for undercarriage bay doors, I simply balsa skin the entire wing, iron on some scrap gloss covering over the area where the door(s) will be and apply some fibre glass and resin over the covering. Once cured, the fibre glass and covering are easily removed. That way you get the perfect profile and you can cut the cured fibre glass to exactly match the cut out you make in the underside of the wing. This is what I’ll eventually do for the Nose undercarriage doors but this technique will not work for the Main Gear because the King Air’s retracted wheels protrude from the nacelle, so the doors are shaped accordingly

Rather than make a 3D print, I elected to make a balsa mould.

I don't have a lot of silicon left (I'll need it later on) but I do have a lot of fibre glass and resin available, so I decided to make the female mould from fibre glass.

After filling, sanding and waxing the balsa mould I stuck it to an old glazed tile with double sided tape and coated it with release agent, before applying a Tooling Gel coat.

Once the Gel coat was tacky I applied numerous layers of fibre glass and resin and left it to cure overnight.

Jim.

[Nigel R]

fine work

puts my efforts to shame

[G-JIMG]

Prised the fibre glass off the tile this morning, removed the balsa mould and trimmed the edge with a Dremel.

Jim.

[G-JIMG]

In preparation for installing the undercarriage doors I've been assembling the wheels and legs.

On this model I'm using electric retracts and Robart Struts. The position of the main gear within the nacelles, and the nose gear being far forward in the nose, means the struts have to be cut very close to the maximum allowed.

The electric retract stubs came with grooves cut into opposing sides, to give a flat face for the Robart Strut grub screws to secure against. I positioned the grub screws on either side of the Robart Struts so that one contacts the upper edge of the groove and the other the lower edge. This prevents any up/down movement as well as preventing rotation of the strut.

The King Air has twin main wheels but the Robart Strut axle is annoyingly just that little bit too short to allow fitment of even the thinnest wheel pairing. Fortunately, a 6" nail is the perfect diameter.

Jim.

[G-JIMG]

I notice that a lot of the earlier pictures have disappeared, despite the originals still being in my album.

Is there a way of editing earlier posts to put the pictures back in?

[Mark Elen]

Hi Jim,

They are all showing correctly on my IPhone. Your browser may be having a moment...

Nice work, watching with interest👍

Cheers

Mark

[Denis Watkins]

Posted by G-JIMG on 07/04/2020 15:43:53:

I notice that a lot of the earlier pictures have disappeared, despite the originals still being in my album.

Is there a way of editing earlier posts to put the pictures back in?

Message a Mod Jim

I know they can click on features to block of release them, and have clicked on error in the past

[G-JIMG]

Removed the first set of undercarriage doors from the mould this morning, quite pleased with the result.

Jim.

[G-JIMG]

Managed to fit the first set of undercarriage doors. I have no idea how to upload videos, or even if it can be done, so you'll have to make do with a few sequence photos .

The doors are mounted using a length of piano hinge. The spring between the two doors is reasonably weak. It serves two purposes; it keeps the doors apart and aligned when they are open and pulls the doors closed when the retracted oleo leg pushes down on it.

Jim.

[G-JIMG]

I tend to apply the fibre glass covering quite early in the build, I find it prevents all but the very worst "hangar rash".

There are countless ways of applying the covering; here's what works for me:

For small parts I use Z-Poxy resin as it's easy to mix small quantities. For larger areas, such as wings and fuselage, I use West Systems resin. In both cases I mix the resin according to the manufacturer's instructions and then I add slightly less (~75%) than an equal amount of Meths.

The West System pumps are a worthwhile investment as they dispense the correct amount every time. The wooden stirrer is courtesy of McDonalds (their sauce cups are excellent for mixing small quantities of glue, etc.) and the plastic card is a hotel key card (I always keep them, they have so many uses in the modelling world). The brush is a cheap amazon throwaway.

When mixing the resin I find the Meths initially makes the mixture go a light purple colour, slowly changing to a milky white, before finally returning to the original resin colour. At this point you know it's fully mixed.

I place the pre-cut cloth over the area to be covered and brush on the resin. Being 'watered down' by the Meths it is very easy to apply. Once the cloth is fully applied I VERY gently draw the plastic card over it. The idea is to remove the surplus resin without disturbing, or dragging the threads of, the cloth. This also removes any small air bubbles under the cloth.

Once fully dry (overnight) I remove any excess cloth by running a very fine glass paper along the edge. You could use a very sharp blade but I find the glass paper works best for me.

Jim.

[G-JIMG]

The real aircraft has winglets, so I have reproduced them by gluing two pieces of balsa together with araldite and with three toothpicks inserted vertically for additional strength.

Once the glue had set I shaped the winglet and hollowed out the wing tip as much as possible, leaving enough balsa to support the Nav Light bulbs.

Jim.

[cymaz]

Can you stretch the cloth over compound curves or is that the wrong technique

[G-JIMG]

Posted by cymaz on 10/04/2020 14:43:20:

Can you stretch the cloth over compound curves or is that the wrong technique

The cloth is extremely light, a square yard weighs less than 1/2oz, so it follows most curves without the need to cut it. In the event a ridge forms that will not smooth out with a little pressure it's simply a matter of cutting along the centre of the ridge and smoothing both edges down with the brush. The fact they overlap is not an issue, as the cloth is so fine the layers blend in to each other without leaving a ridge.

Jim.

[G-JIMG]

Made a start on the empennage today.

The horizontal stabiliser is aerodynamically shaped, much like the wing, so is built in the same way. The ribs were cut on the CNC machine and assembled over the plan in the traditional way.

I also used the CNC machine to produce the Radome parts. The max cutting depth of the CNC machine meant the Radome had to be made in 4 sections, glued together to give the basic shape, and finally sanded to match the drawings. Building it in sections had the advantage that I could cut internal ducts for the Strobe and Tail Light wiring.

Jim.

[G-JIMG]

Managed a bit more on the horizontal tail assembly.

Applied the balsa skin and manufactured the elevators.

Lightly clamped the assembly in a vice so I could test if the snake operated the elevator as planned. Again, I can't upload a video so pictures will have to do.

Elevator at Neutral:

Elevator fully Down:

Elevator fully UP:

Jim.

[G-JIMG]

Apologies, I forgot I had taken a picture of the elevator linkage.

The central curved part is double sided PCB board, securely attached to the elevator rod. The other end has a standard clevis attached that connects to the snake.

Jim.

[cymaz]

Will you  be putting a cap head screw and a captive bolt so you can access the linkage at any time?

Edited By cymaz on 12/04/2020 12:52:26

[G-JIMG]

Posted by cymaz on 12/04/2020 12:52:10:

Will you be putting a cap head screw and a captive bolt so you can access the linkage at any time?

Edited By cymaz on 12/04/2020 12:52:26

Personally, I can't see why I would need to access the linkage in the future. I have used this sort of torque tube control on a number of models and never had an issue with it. have others had problems?

If the linkage were to fail on the ground I would have to rebuild the tail. If it fails in the air I wouldn't have enough left to repair .

[cymaz]

Fair enough...just thinking aloud

[G-JIMG]

Not much to report, the wing build has reached the tedious stage of filling and sanding before priming.

In preparation for painting with primer I use 3M Putty on the surface:

It's very easy to apply (the hotel key card again), dries extremely fast, is very easy to sand down and fills the fibre glass weave, and any surface irregularities, perfectly.

It looks a lot worse than it is, the putty is not as thick as the picture makes it appear.

After sanding with fine glass paper the surface is very smooth, flat and ready for paint.

Jim.

[G-JIMG]

Still doing the tedious filling and sanding but took time out to make some cooling slots.

The real aircraft has cooling slots either side of the nose and a scoop on the starboard side.

I figured it would be easier to make panels that I can let in to the fuselage, rather than try and cut them into the skin itself. The panels are LitePly cut on the CNC machine. The starboard one has the scoop attached so I can let the entire assembly into the fuselage.

The observant ones will have noticed the real aircraft has 6 slots whereas mine have 7. I tried it with 6 but it just didn't look right. Using a 1mm Bit and 6 slots leaves too much material between the slots. Using a 1.5mm Bit (my only other option) means you can only get 5 slots and not enough material between them.

Anyway, as Platt's 4th law states, "How right it looks matters more than how right it is".

Jim.

[Denis Watkins]

In brightness and "brain deception" terms Jim

The Original has 6 black curves stripes ( maybe an optical illusion)

But your build has 7 whites stripes or 8 wood stripes, which appear straight and narrow.

The Romans played with lines on pillars and builds and appearance can alter perception.

Flipping heck, I never meant to go on so much

But car makers too know that curved black lines tell a different story than straight lines.

Am watching keenly

[G-JIMG]

Still fibre glassing, filling, sanding and basically prepping the flying surfaces for paint.

The aircraft elevators have a reinforced leading edge.

I have simulated this by affixing metal tape to the model's LE.

I have also affixed acrylic 'dots' to simulate the rivet heads. Once spray painted the overall effect should be more realistic.

The black line is 1/64" Chart Tape. I will eventually spray some primer over the tape, gently rub it down until the tape is just visible before removing it. The resultant indentation will simulate the Elevator Trim Tab once the model is finally spray painted.

Jim.