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Author Topic: 1/18th scale D.H. 85 Leopard Moth  (Read 8540 times)
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Prosper
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« Reply #50 on: June 13, 2018, 10:50:04 AM »

I don't know about the bracing Indoor - would it be less draggy than a single, larger-diameter wire of the same strength, I wonder - or perhaps DH had great spools of the thinner stuff in their stores, and the Director had just given a lecture on the need for economy?

What little modelling time I have just now has mostly been taken up fiddling with the fuselage coachwork. I've tried silver mylar on balsa sheet and now back to al. foil, but thicker foil this time. This example is a bit wrinkly in places but I'm getting the idea of how to do thist.

The thing is, I'm making a silver-finished Leopard Moth, and I really want the coachwork to stand out against the silver paint. Thanks to cvasecuk electronicking me the Eddie Riding article from 1941, I now know that the coachwork was chrome-plated, and not just polished aluminium. Cooking foil on the shiny side has a 'brushed' grainy finish, but I used it shiny side outward anyway, and just polished the bejeezus out of it with metal polish which seems to have got rid of the brushed look.

The whole cabin area presents problems. I think I've got clear how I'm going to make it, but making it strong and light enough will be a task. I never thought it would be easy but the fact is that the test model weighs 5 grams more than my estimate, and the scale model will be similarly overweight. This extra weight changes how strong the model ought to be in order to survive. The test model's extra weight is all noseweight: I don't know how but I cheerfully and hopelessly underestimated the amount that would be needed.

Stephen.
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Indoorflyer
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« Reply #51 on: June 13, 2018, 12:17:58 PM »

The upper bracing looks to be "streamlined", not sure about the ones on the underside.
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Prosper
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« Reply #52 on: June 13, 2018, 01:06:09 PM »

I think you're right. I expect the lower ones are the same.

Stephen.
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« Reply #53 on: June 13, 2018, 03:50:08 PM »

Stephen,
              The photo also shows the amount of travel available for trimming. I have found a whole pile of photos taken at Manderville and will PM you with them.
Ricky
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« Reply #54 on: June 13, 2018, 07:05:14 PM »

... I've tried silver mylar on balsa sheet and now back to al. foil, but thicker foil this time. This example is a bit wrinkly in places but I'm getting the idea of how to do thist.

The thing is, I'm making a silver-finished Leopard Moth, and I really want the coachwork to stand out against the silver paint ...

I have discovered some chrome ink pens that are really impressive provided the surface to which they are applied is sufficiently smooth. They are a type of fibre tip and come in 1mm, 2mm and 4mm diameters. I got them from a mail order craft store in Australia which I think may be an off-shoot of a USA chain. Cannot provide more info at present because I am not at home but expect to be there on Friday. In the meantime a search may find them. I will try now and post if I find them.

Paul
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« Reply #55 on: June 13, 2018, 07:10:42 PM »

Found them. They are called “Molotow Liquid Chrome Marker Pen”
 And are currently advertised on eBay.

Hope that helps

Paul
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« Reply #56 on: June 15, 2018, 04:51:36 AM »

It does help, Paul, thanks. When I looked at these they were stupidly expensive but I've found them at a discount as a result of your 'nudge'. I also watched a YouTube review which seems to suggest they might be what I'm after, so I've ordered one. I think it'll mean using plastic for the coachwork rather than the much lighter balsa/aliphatic sheet, but we'll see.

Talking of which, for this build I'm using 'regular' aliphatic to coat the balsa, that's to say the yellowish woodworkers glue that cures hard, rather than the refined white rubbery stuff. This is because I bought a large bottle of it to make a new garden gate. It was inevitable that I'd give it a try as a balsa coating. In fact someone on HPA suggested that I should try it years ago.

So the model might splinter to pieces in mid air. No, in fact I made a Westland Whirlwind nacelle of this other aliphatic in the winter and that still seems to have its original integrity. The weight of the sheet material produced is the same, but regular aliphatic is much more pleasant to work with and seems to accept heat-scribed panel lines and embossing, and can be soaked and bound to EPS moulds. The picture shows a typical sheet, 300x100x0.3mm or 12"x4"x12 thou in Imperial I think.

Ricky has just sent me a bunch of fantastic close-ups of the Croydon (NZ) restoration, which answer many questions. Thanks Ricky.

Stephen.
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Prosper
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« Reply #57 on: June 18, 2018, 09:39:46 AM »

Very little happening here [take 2].

The Chrome pen arrived thismorning. It seems just the job. I wouldn't have minded a slip of paper with it giving basic specs and instructions. I suppose one's expected to 'go online' nowadays.

The pen does work on aliphatic-coated balsa. My worry was that I'd read that the pen's nib or nozzle or spout, has to be pressed down for the pen to begin feeding paint. This might dent aliphatic/balsa sheet. It turns out though that the nozzle need only be pressed down once, when new (presumably to puncture some membrane or seal), after which it works whenever touched to something, even lightly.

Nevertheless I'm going to use plastic card for the coachwork. Sealing and sanding the curvy edges of the balsa/ali sheet would be a pain, and the chrome pen would show any blemishes right up. You'll see from pic 2 that the pen leaves a slightly uneven or granular surface even when applied to super-smooth plastic card. This may be from microscopic dust particles as found in a working area (errr - especially when someone's just been sandpapering a few inches away).

Now, this pen is supposedly dismantleable and rechargeable, which apparently means the user can break out the cartridge and paint the chrome paint with a brush. It wasn't immediately obvious how the pen came apart tho' and I didn't want to destroy it before even trying it so I don't know about painting yet. I could see that it might give a smoother finish than scribbling the chrome on wih the pen. Anyway, I'm mighty impressed with it. Pics 1 & 2 show a piece of al. foil (shiny side) for comparison. Pic 3 shows the chrome on a conventional silver-painted surface for comparison.

I've also been experimenting with other bits: pic 4 is a wire wing mounting. the two invisible lengths of the wire embedded in the balsa ply are zig-zagged ensuring that they can't be pulled out. Pic 5 shows a couple of bamboo rods (AKA cocktail sticks) reduced to 1mm diameter. These are for the cabin structure and I need to make another six. They're not that stiff really, but when fixed into tripods perhaps they'll do a job of some sort. Pic 6 is a sperimental tail bracing wire.

Stephen.
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cvasecuk
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« Reply #58 on: June 18, 2018, 11:15:31 AM »

Stephen, where did you get the yellow aliphatic and what is it called? Many years ago Flair did a yellow aliphatic and it was really good but it has not been available for some time.
Ron
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« Reply #59 on: June 18, 2018, 12:23:06 PM »

Hullo Ron, I got the glue from http://balsacabin.co.uk . It's Deluxe brand, the same lot that makes the refined white stuff I use. I recall there was at least one other brand stocked by balsa cabin - cheaper, but I've no idea if you gets what you pays for with materials like this. Even the Deluxe brand has the odd fleck or grain of some hard impurity, which is less than ideal for using it to coat balsa sheets with.

Stephen.
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« Reply #60 on: June 22, 2018, 11:25:51 AM »

I've made a few steps. I cut out the fuselage sides and brush-painted them silver. Then came the dreaded batter-pudding hurler of Bexh stencils. . . I'm using 'Frogtape'. I must say I can't see how they call it "Low Tack": I see that it's a large improvement on standard masking tape - it peels off keeping its glue with it, like Post-it notes do - but you really have to tug it. I ended up backing the tape with paper, leaving just a narrow area of gum around the stencil. I peeled the stencils off ve-ee-rr-y slowly.

The G-reg letters need a bit of touching-up by brush; cutting letters like these into the Frogtape isn't easy to do well, at least not the curved bits. By the way, I spent a long time with what photo evidence I have, getting the lettering as close to the original style as I could. Photos show some individual variety and I imagine they were hand-painted not masked and sprayed, but that's just a guess.

As for the green shade, that's just taken from the cigarette card (next post). I wanted a handsome dark green, but mixing greens is most difficult in my experience. I kept feeling the paint in the pot was too light - kept darkening it, and then when I airbrushed it on, it looked almost black. I nearly started throwing furniture and jumping up and down on anything model-related within reach, but when I took the work out into the sunshine I saw that it's actually very close to what I'd hoped for. Panic over.

I also wiped off the chrome paint from the first part of coachwork I made, and re-applied it. The Molotow chrome has very little abrasion-resistance or solvent-resistance. I spent time cleaning up the edges of the windowframe as carefully as I could, and getting it dust-free. The second result is a lot better, and I'll be using this in the build (pic 5).

Stephen.
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« Reply #61 on: June 22, 2018, 11:28:02 AM »

When I was a boy, granddad gave me a book of cigarette cards dating from 1935. He wasn't especially interested in aeroplanes himself but smoked about 6,000,000 Players a day and had many of these cigarette card books on different subjects. There are quite a few 'planes in this book that I've thought of modelling since boyhood, but as static models. This is my first actual start.

The cigarette card example, G-ACHC, was fourth of the initial batch of six Leopard Moths made. Confusingly, the prototype - which De Havilland called "E.1." - was registered after G-ACHC, becoming ACHD. Moths G-ACHB, -C and -D were all entered in the 1933 King's Cup race which Geoffrey de Havilland won, flying the prototype.

The first thirty or so Leopard Moths differed slightly from those following, in having flat fuselage sides not relieved by stringers. They also had what appears to be an external stringer along the rear fuselage spine.

Stephen.
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« Reply #62 on: June 27, 2018, 03:45:24 PM »

A bit more done. The general idea is to build everything structural into the fuselage sides and then join them with cross-members. The reality has proven complex, mainly because of my attempt to make a strong enough central fuselage box with scale-dimensioned structure where possible (or where visible, anyway). I don't know how this'll work out.

Here there's a dilemma: I could choose to make a fancy, detailed interior which would be rendered daft by having a dirty great rubber motor running right through the middle of it - or make a crude and sketchy interior which would be sore-thumb visible from every angle because of all the glazing. I'm trying to steer a decent compromise.

Pic 1 shows the white plastic exterior framing of the rear window and above it the matching balsa interior framing, which is structural. In fact there's a similar device in the full-size.

Visible in Pic 3 are a bottom longeron, 0.8x3mm; a slab of balsa with an arrow on it which is a former for bending the longerons, and a stiffener mounted to the outside of the fuselage side where the wing is mounted. Leopard Moth doors have large gaps round them, with metal scuffing-strips and seals and who knows what, but it's not possible to represent these at all well when the model's skins are only 0.3mm deep. I've had a go anyway.

Pic 4 shows a dry-run of how all these bits fit inside the skin.

Stephen.
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« Reply #63 on: June 27, 2018, 08:44:39 PM »

I'm still amazed at the size of the balsa you work with Stephen and your accurate marking out. It's a real test of skill, concentration and patience. It must be very rewarding trying t duplicate the original structure though - an insight into the original design.

John
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« Reply #64 on: June 28, 2018, 03:36:10 AM »

Hullo John, yes, I'm trying to spend the necessary time making accurate marks. Normally I don't attend to this area well enough which leads to trouble later on. This aspect has actually been driving me nuts in the last modelling sessions, because I made sure to cut the fuselage sides to their true, developed length. This is longer than the length seen in side-elevation, because the fuselage sides are curved near the middle - in plan view they make a very shallow 'V'. All well and good, but when marking this or that line (the edge of a window frame, say), I instinctively and thoughtlessly take the measurement directly from the side-elevation and go to mark it on the balsa and think, that can't be right - that would make the window frame far wider than it obviously is. . . Then I realise that I haven't allowed for the extra length of the developed profile. I'm sure this hardwired error (can't teach an old dog, etc) will catch me out so I'll fix the wing mountings with the wrong spacing so the wings won't fit, or something.

As for the balsa dimensions it's all TLAR. The test mockup is made identically, with two strong frames and four longerons forward of the motor peg. I was worried that this might collapse when stretch-wound because I'd envisaged 3/16 or 7/32" rubber, whereas at one point I was using 1/4". It handled full winds of a 1/4" motor without apparent stress. This one's different though because there's so much load-bearing area cut away for the glazing. I really hope 7/32" will be the most it'll be called upon to carry.

Stephen.
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« Reply #65 on: June 28, 2018, 04:07:29 AM »

Quote
This aspect has actually been driving me nuts in the last modelling sessions, because I made sure to cut the fuselage sides to their true, developed length.
.
This would have been an interesting exercise Stephen. I have never actually produced the developed length off a curved line by hand but imagine it could be done by producing the developed lengths, individually, between a number of points along the curve.
Alternatively one could bend a stick along the curve and mark off points or even on a string running along the curve.
I would probably cheat and just use the 3d programme on my computer.  Smiley
As for marking off wing mounting points onto your developed sides - I think would prefer to make a separate sub assembly for the wing attachment area and then add the sides to it - you may be doing something similar with your 2 frames.

John
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« Reply #66 on: July 20, 2018, 12:31:41 PM »

This project has been derailed by the summer.

Yass, I know I called it my 'summer project' in the first post, but that meant summer in the calendar, seasonal sense. Not for one slim moment did I ever think that we'd atcherly have a summer in the weatherwise sense. But we have. I'd completely forgotten what summers are like, and for sure I don't want to be indoors while one is going on. They're quite an event.

The summer is so ravishing that I've even forgiven the farmer for covering my flying field in corn (maize), though not being able to free-flight in the many calm days and flat evenings makes me a bit sick. To put it mildly.

The last couple of days I have done a bit to keep this project alive - I thought if it lapsed much longer I'd lose interest altogether until next - ahem - 'summer'.

The first pic is a general shot where I'm trying to get my bearings in order to bring the fuselage sides up to the same stage. I noticed a funny blob when I lowered the camera [purple arrow] which turned out to be a fairly snazzy moth, but not a leopard moth I expect.

Pic 4 shows why it's a good idea to let your project idle for weeks: the stretched polythene rigging I was testing has shrunk - those balsa supports were joined at 90° originally. That would have made a mess of the tail surfaces all right. Now I have to test more to see if the shrinkage stops and after how long a period.

Stephen.
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« Reply #67 on: July 20, 2018, 12:34:40 PM »

Pictures 5 and 6 are of a tool for thinning bamboo cocktail sticks. It's a brass tube wrapped round with masking tape to give a better purchase; the bamboo is whittled roughly to the desired diameter then by twizzling it into the brass tube you get a nice round piece of work.

Pic 7 shows another twizzler, this time a drill. It's piano wire set in a bamboo handle and the end of the wire is clipped off with wire cutters at an acute angle. This makes a drill that cuts a hole slightly bigger than the piano wire diameter but which can be centred very accurately and cuts much faster through plastic and thin aluminium than an HSS twist drill. I have quite a few of these which do all kinds of drilling tasks.

Pic 8 shows the cross tube forming the top member of the windscreen, and which holds the wings in place, 1.6mm diamteter. The inner holes in the al. ferrules allow the member to be pinned to the carbon-sandwich uprights visible in other photos, and the outer holes will allow the wings to be held in place. I hope. The whole object has been painted with thin CA for a nice gloss, and CA also bonds the ferrules to the bamboo.

Last pic shows two out of three young kestrels which emerged three weeks ago from the nestbox mentioned earlier in the thread - the picture was taken on July 11th.

Stephen.
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« Reply #68 on: July 20, 2018, 08:13:57 PM »

Those fuselage sides should inspire you to do more work - but not during the day! Smiley Wild looking moth - a real Boris Karlof of the moth family.

The young Kestrels almost look as benign as budgies at that age.

John
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« Reply #69 on: July 22, 2018, 01:46:29 PM »

John, I looked the Karloff moth up: apparently it's called a Silver Y. It sounds like a ranch in Texas to me. Anyway, it was only clicking through this moth database that allowed me immediately to identify this little beauty which I nearly stepped on just outside the kitchen door just now, in broad daylight, on a patch of sunburnt grass. A Leopard moth. I don't believe I've ever seen one before.

I'm not remotely superstitious but this is obviously a sign from the gods: I presume they're just letting me know that my Leopard Moth model will turn out really well and fly for ages.Smiley (Or are they just putting in their vote for a colour scheme?)Cheesy

I've put in a bit more work the last two days  - nearly finished the insides, so the fuselage sides should be joined  soon. Pictures to follow.

Stephen.
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« Reply #70 on: July 22, 2018, 09:09:00 PM »

Silver Y? Smiley Nahh - Boris Karloff  for sure. Smiley That Leopard Moth though is definitely an eye catcher. Agree it's a sign from somewhere - take note of the colour scheme and don't forget the furry mantle around the neck. It should look good just in front of the wing Smiley
Perhaps a new form of turbulator Smiley
What would the judges say? Whadyamean - that's a Leopard Moth Smiley
The brain is rambling.
I'll leave you to your building in Moth Haven.

John
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« Reply #71 on: July 25, 2018, 12:46:02 AM »

Bits of work done over the last few days:

Pic 1 shows a bridge joining two segments of longeron. The carbon upright is in one piece, which means that the longeron has to be broken. This bridge over one side of the carbon upright makes a really stiff solution, what with the fuselage skin on the other side. The longerons are of 0.8mm balsa that I would call medium/hard.

Pic 2 shows some temporary formers to give the longerons their shape. The longerons had been pre-formed on a mould but have relaxed somewhat since then, and these formers aid fixing the longerons to the fuselage skin. They've also made useful handles for general manipulation. They're just fixed with a dot of CA at either end so will be cut away easily.

Pic 3 illustrates a problem with using 'classic' aliphatic glue as a coating as opposed to the thin white rubbery kind: the thin balsa sheets curl up. This may not look like much but the curl is quite stiff, relatively speaking, and had me worried. I think the thin vertical strakes in pic 4 have beaten the curl flat - I mean they have for now, but better wait to see how the sides look after a few heat/humidity changes down the line. In fact this demonstrates a significant fact about boxy fuselages - they're heavier and weaker than curvy monocoques.
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« Reply #72 on: July 25, 2018, 12:47:46 AM »

Pic 5 is a test of another wing hinge style. My original wound-wire idea is in the background. The current favourite is this brass shim job. Two strips of thin brass are set into balsa with CA, and have their ends sticking out: these projecting tabs have holes in them to accept the hinge pin. I've filled the narrow gap between the projecting tabs with white plastic card.

Although the brass strips project only 6mm into the balsa and the balsa is only 1/32" wide, this experiment passed the TLAR tug test.

6 and 7 are the motor peg arrangement. The motor peg is at almost the widest part of the fuselage and weighs a disappointing 0.3g. I thought of a central hook of piano wire but for that to be secure from tearing out, and to pass its load into the fuselage, I guessed the arrangement would probably be as heavy as the humble aluminium tube.

Stephen.
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« Reply #73 on: July 25, 2018, 05:13:05 AM »

Fascinating Stephen. You haven't considered styrene sheet for your hinge bracket material ala George K.?

Would a wood dowel rubber anchor be a bit lighter than your aluminium tube? Perhaps not strong enough at that size.

The delicate engineering really intrigues me. You should have been a surgeon.

John
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« Reply #74 on: July 25, 2018, 06:30:14 AM »

Quote from: OZPAF
You haven't considered styrene sheet for your hinge bracket material ala George K.?

Would a wood dowel rubber anchor be a bit lighter than your aluminium tube?

You should have been a surgeon.

1) Yes, but I discovered a disconcerting thing about styrene sheet (plastic card). I used it for a while in various hinges and linkages in pendulum systems - 15 or 20 thou stuff - then when examining a setup about a year old, a Spiteful I think it was, I found a broken styrene hinge. The loads possibly applicable by the system simply couldn't have caused that (I thought). It turned out that the plastic had gone brittle. All the other links were brittle too. I switched to thin aluminium for those jobs. I never knew that styrene would go brittle - at least not in a year, buried from sunlight and never heated. It could be the CA used to fix it, although the tiny amounts of CA and the distance from the actual hingepin hole seem to make this unlikely. More likely is the fact that I used styrene sheet when I scratchbuilt plastic static models in my yoof, and thought I knew all about it:  but that period of my life was 2-3 years - not long enough to know all about anything!

2) I think about the same, John. I dimly remember making a comparison test once.

3) Too squeamish.

Stephen.
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