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Scratch Build Twin

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G-JIMG01/04/2020 13:05:55
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145 forum posts
108 photos

I retrieved my Son's Lego from the attic and made up a box large enough to take the cowling. I lined it with cling film (to stop leaks) and made a silicon mould of the cowl.

After thoroughly mixing the silicon I liberally coated the cowling with the mixture to minimise air bubbles against the surface. I then suspended it within the Lego box and slowly poured in the remaining silicon mixture from a height (the idea is to get as thin a stream as possible to reduce the amount of air in the mould).

The mould was left to cure overnight before the plastic cowling was removed.

cowling mould.jpg

Jim.

Chris Freeman 301/04/2020 14:13:40
353 forum posts
525 photos

Very nice project and you have some great equipment to help with the build. I am following the progress with interest.

Nigel R01/04/2020 14:30:14
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3756 forum posts
587 photos

"Not all BECs will do this happily, I understand."

I'd be surprised if you can find a single hobby BEC that will be happy when paralleled up.

Chris Walby02/04/2020 10:30:23
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1200 forum posts
299 photos

Jim, messaged you a couple of days ago, hope you are well. Chris

G-JIMG02/04/2020 11:56:05
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145 forum posts
108 photos

To make the first engine cowl I left the mould in the Lego box, so it wouldn't deform, and started by coating the inside of the mould with a Finishing Gel Coat.

Once the gel coat had gone tacky I applied resin and started laying in strips of fibre glass. (Sorry, forgot to take pictures).

The cowl was allowed to cure overnight before removing it from the mould.

The upper edge was trimmed using the drill press and the spinner ring trimmed using a Dremmel.

32 cowl trimming.jpg

32a engine cowl.jpg

32b cooling air.jpg

33 cowling internal.jpg

Jim.

G-JIMG03/04/2020 18:35:10
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145 forum posts
108 photos

No progress today, swmbo decided there were gardening jobs to do frown.

However, I have had an opportunity to think about Elevator control. Being a T-Tail there is a potential issue with Elevator operation but fortunately the aircraft in question has a Radome mounted on the tail. This means I can conveniently hide the linkage within the hollowed out structure.

51 elevator hardware.jpg

51b elevator control.jpg

Jim.

G-JIMG04/04/2020 20:24:20
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145 forum posts
108 photos

Managed to do some work on the wing today.

The model will have lights fitted, including wing tip Nav lights, so I needed to make the covers.

I shaped the wing tip and then carefully cut out the relevant section. This was then stuck on a piece of dowel and fibre glassed to strengthen and harden it. To make the cover I made a wooden frame and attached a piece of acetate sheet. I used a heat gun to warm the acetate to the point it became flexible and then pressed the mould into it. While pressing the mould in with one hand I used the other (gloved) hand to press the acetate against the sides to maintain the correct shape.

7 wing lights mould.jpg

acetate frame.jpg

wing tip cover.jpg

wint tip light.jpg

Jim.

G-JIMG05/04/2020 15:07:53
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145 forum posts
108 photos

I can't speak for anyone else but personally, once the model is built and flyable, I tend to stop working on it and move on to other projects. As a result the model rarely gets the scale finish I originally planned. To prevent this from happening this time around I decided to make the fiddly bits now.

As you can see from the pictures earlier in this thread, the aircraft has a host of antenna. Fortunately the antenna manufacturer's web sites provide dimensioned drawings of their products. All I had to do was divide by 10 and copy the shape laugh.

I also made the two pitot probes from some brass tubing and rod. The chrome spray finish is not perfect but looks better in the flesh than it does in the picture.

antenna.jpg

All I have to do now is put these somewhere safe and remember where they are when it comes time to fit them frown.

Jim.

G-JIMG05/04/2020 15:24:56
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145 forum posts
108 photos

The King Air has quite distinctive propellers, so I decided to do those now as well.

First I trimmed the 11" APC props down to the scale 10.5" by removing 0.25" from each tip., shaping it to match the King Air tip at the same time. I then balanced the props in the normal way. I then spray painted the props using rattle cans and placing masking tape in the relevant areas.

The real props are made by Hartzell and each blade carries their logo. I managed to scale the logo down and printed it out on some transfer paper. After spraying the prints with some clear varnish they were transferred to the prop. You obviously can't read them at that scale but I'm pleased with the overall effect.

prop.jpg

prop logo.jpg

Denis Watkins05/04/2020 15:30:35
4335 forum posts
104 photos

Every single build Jim, I would put something away safe and mislay them but now I use those 4 for a £1 boxes with lids

And a label.

The chrome spray looks chrome by the way.

You are spot on with finishing the job as you do end up flying an unfinished model in the future.

We all don't like knocking bits off in the long grass though.

Those props look real

Edited By Denis Watkins on 05/04/2020 15:31:49

Martian05/04/2020 16:50:46
2485 forum posts
1163 photos

Loving your work Jim

G-JIMG06/04/2020 12:37:23
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145 forum posts
108 photos

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

wired port wing.jpg

Jim.

G-JIMG06/04/2020 12:56:26
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145 forum posts
108 photos

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.

34 retract door mould.jpg

35 retract mould filled & smoothed.jpg

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.

36 retract mould release agent.jpg

37 retract mould gel coat.jpg

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

38 retract mould fibre glassed.jpg

Jim.

Nigel R06/04/2020 13:25:36
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3756 forum posts
587 photos

fine work

puts my efforts to shame

G-JIMG07/04/2020 10:00:24
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145 forum posts
108 photos

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

39 retract door mould.jpg

39b retract door gel coat.jpg

39c retract door fibre glass.jpg

Jim.

G-JIMG07/04/2020 15:34:04
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145 forum posts
108 photos

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.

nose leg preparation.jpg

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.

main gear 2.jpg

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.

main gear.jpg

Jim.

G-JIMG07/04/2020 15:43:53
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145 forum posts
108 photos

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 Elen07/04/2020 17:47:54
522 forum posts
1021 photos

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 Watkins07/04/2020 18:43:37
4335 forum posts
104 photos
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-JIMG08/04/2020 10:35:11
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145 forum posts
108 photos

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

39d retract door.jpg

39e retract doors trimmed.jpg

Jim.

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