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Author Topic: 1/24 scale Mew Gull  (Read 2982 times)
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Prosper
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« on: June 15, 2012, 01:29:47 PM »

Hullo all,

I was enjoying Pit's thread about his Peanut Mew Gull build when it slowly dawned on me that this might be a good candidate for some experiments with all-sheet models.

I've never been that taken with the Mew Gulls. However, the Mew has lots of straight lines and few compound curves - good for sheet construction.

I've been gnawing at this sheet construction idea for years - long before I ever cut wood on an actual model. Various tests with carbon/epoxy, tissue doped direct to balsa etc all petered out. Too heavy, surprise surprise.

I had some joy with small areas of sheeting in other models (successful in the sense that the result was as light as trad. construction), and it seemed to me that a Peanut of the tiny Mew Gull would actually consist of nothing but small areas of sheeting. 1/24 scale gives a 12" model BTW.

The pics are where I've got to so far. It's beginning to advance quickly now, so more to follow. Target weight without rubber and noseweight is 11g. That's a beeeg ask for a sheet aeroplane this size, especially with a relatively huge canopy. However, component weight so far shows I haven't entirely fallen off the rails, so I'm keeping calm and carrying on.

Stephen.
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1/24 scale Mew Gull
1/24 scale Mew Gull
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Don McLellan
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« Reply #1 on: June 15, 2012, 02:07:34 PM »

Hi Stephen,

Interesting project.  What is the thickness of the sheeting?

Don
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« Reply #2 on: June 15, 2012, 02:46:10 PM »

Very nice work! Looks very delicate.

I know it's not quite as refined a method as what you are doing here but have you seen the Frog Junior method of sheet construction? http://www.houseoffrog.co.uk/junior_plans.htm

My Speedy came in at 11g including rubber and noseweight and is one of my favourite little models. She'll do 30 seconds reliably.

Jon
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« Reply #3 on: June 15, 2012, 04:19:47 PM »

Hi Don,

The sheet is 0.2mm  nominally. It's surfaced with cyanoacrylate. There are several ways I've stumbled on to do this, each with pros and cons. What you have is a composite sheet material effectively. After much experiment I can now produce small areas of sheet with a hard, eggshell/gloss smooth surface of a fairly regular weight, 0.0065 to 0.0075 g/cm2. There are difficulties in its use of course, but after a slow start (new territory) this model is building very rapidly. My main concern is how the material changes with age - and whether it will distort or warp or somesuch with changes of temp/humidity.

The attraction of this method that's kept me nagging at it is that you have a kind of monocoque construction available. In theory it's in roughly the same league as trad. methods in terms of weight, but in practice probably not, even at the smallest scales like this Mew Gull. That's because the sheet does need interior support to stop it deforming, unless it has a good deal of curvature.

Jon, as you might guess from the mention of cyano above, the result is actually very tough and resilient. In fact most bits (rear fuse, tail surfaces etc) are far stronger than need be, but I can't think of a way to "delicatize" them, since 0.2mm balsa is as about as thin as this mortal can handle, and anyway, it seems to be the point at which the cyano becomes so much stronger than the balsa that the wood just rucks up into grooves and creases. The leading edge of the wing in the pic has no support whatsoever except for the root rib and a tip rib which are there to remind it what shape it's supposed to be. If this model can fly (aerodynamically speaking) then its portly weight and presumably high airspeed may be an issue in terms of crash momentum - but I'm rooting for that L.E. because if you look at it, it just stares right back at you.

I've done quite a bit since the posted photos so more tomorrow I hope.

Love that Frog site. I've bookmarked it for future luxuriating. My brother was a big aeromodeller (me static models) when we were kids, but I can't remember Frog flying models, only KK and Veron.

Stephen.

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« Reply #4 on: June 16, 2012, 06:40:28 AM »

...the result is actually very tough and resilient.

Stephen,

I just meant delicate in the sense of needing steady hands Shocked rather than not tough. Very interesting to hear about the monocoque/composite structure...

Jon
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« Reply #5 on: June 16, 2012, 10:59:02 AM »

Hi Stephen
Mike Stuart has a really nice all sheet Hawker Fury on his site...flies great in the vids.
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« Reply #6 on: June 16, 2012, 01:39:33 PM »

S-ss-steady h-h-hands, did you s-say Jon? Gosh, I wish I still had a set of those. I'd go for three next time, given the choice Grin

So far, the job is less demanding than a rib-and-stringer type airframe in the delicacy respect. It's tricky, but I think that's because I'm making it up as I go along. The mistakes are accumulating, unfortunately, but I kind of expected that. This leads me to think that I won't spend much time on details and twiddly bits. The important issue is whether it weighs less than 1 ton when finished. I had supposed that problems and errors would creep into the build, and that I'd make another one when I knew what I was doing. However, when I tacked it together for the photos below, I found that I just can't work up any enthusiasm for the Mew Gull, so maybe there won't be another from me. The Percival Gull IV - now there's an aeroplane.

Anyway, the pics show it at a weight of 7.2g, and a shot of the wing structure before sheeting. I'm afraid there's a lot of weight to be added yet, not least that huge canopy well behind the C.G. I'm going to have to build a canopy from soap bubbles, methinks.

Hi Mooney, thanks for pointing that out. I've seen that Fury on Mike's site. It''s a lovely job, but frankly it seems odd to me to use sheet construction to replicate an aircraft that was fabric-covered in the first place. I'm very pedantic about this I know, but tissue models emulate fabric covered originals extremely well (far better than any static plastic kit can do f'rexample) so I have to say if I were making a Fury it would be a "stick and tissue" job every time. That's not to detract from the builder, who made a good flying machine. It just seems an odd route to take aesthetically.
Attached files Thumbnail(s):
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Re: 1/24 scale Mew Gull
Re: 1/24 scale Mew Gull
Re: 1/24 scale Mew Gull
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« Reply #7 on: June 16, 2012, 05:18:18 PM »

I'll have to look if I noted the weight of the canopy that I drew.  It DID come out a lot lighter than I anticipated on my peanut one.  The AUW w/o rubber is 12.05 grams.

I just looked thru some of my notes, but didn't find the weight of the dome Sad.

One thing I'm curious about.  Are you modeling the CURRENT iteration of G-AEXF - the one with the low-profile canopy and STRAIGHT cowl and rear spine?  The earlier versions had a slightly curved cowl and a noticable one in the rear spine.  IF you are doing the original configuration, I could draw a pot-top for you (the plug is in good condition).  The slightly larger size should trim down OK.
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« Reply #8 on: June 16, 2012, 05:54:06 PM »

Hi Stephen,

Thanks for the sheet balsa info.  How did you apply the cyanoacrylate?

Looking forward to more progress and pics.

Don
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« Reply #9 on: June 16, 2012, 08:01:39 PM »

Stephen

This is a very interesting project. I am very keen to learn what your final model airframe weight will be, ie ignoring rubber and the propeller.

So far I have been experimenting with 0.4mm balsa to make two ply laminates to replace conventional sheet balsa formers etc. and also a bit of work using starch type fillers on blue foam. Sometime soon I will see what I can do with starch type fillers and acrylic resins on thin balsa. My attempts at sanding balsa down to 0.3mm were marginal at best so I am impressed with your 0.2mm effort.

Largish amounts cyano like what you appear to be using and very small amounts of dope play havoc with my asthma so I am looking for alternative technologies.

Paul
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« Reply #10 on: June 17, 2012, 04:18:38 PM »

Hullo Pit, thanks for stopping by. It was your Mew Gull thread that got me started on this. I had intended to finish this model as G-AEKL in red and (I think) gold. I saw the link in your thread of the guy's model, and have a couple of photos which suggest gold. I may paint it white though as have no red'n gold at home. As for the shapes you mention, I reckon the spine is flat, or as good as (see pics). I estimate that the first canopy tops were flat too, and the curves came later. Perspex moulding was considered very difficult/expensive/fancy back then, as I understand it. The thing is, all these a/c were hand-built to order, and all of them were constantly modified by racers (including Percival himself) unencumbered by the dead weight of beaurocracy that prevents such endeavour nowadays. G-AFAA particularly was very different as I think you said in your thread.

If by pot-top you mean the top cowling, I made one yesterday and managed to get some double-curvature into it (a bit too much possibly). Thanks all the same for your very generous offer. I appreciate it. I'm glad yur canopy turned out lighter than expectation - mine needs to!

Hi Paul. I found that turning out thin balsa was fairly easy. I started hoping to turn 1/32 into 1/64" and just kept going. It's a matter of rhythm. I use MDF as a base, and use silicon carbide (wet'n'dry) paper attached to a very flat block with that aggressive double-sided tape that carpet-layers use. I only use new sharp abrasive whatever the grit.

I start with 120 grit, holding one end of the sheet with fingertips and roughly counting strokes - 20, then turn it round and 20 the flip it over and 20, turn and 20, flip and 20 etc. Early on you can sand cross-grain and the material flies off (do it outdoors!). As it gets thin get progressively more careful and light-handed. Only make outward strokes (i.e. keeping the blasa in tension) and switch to 240/320 grit when it's really thin. All the time check the edges to gauge progress. Each panel is different but normally the centre needs more work than the edges, and the corners thin quickest. Making an incautious stroke now can be exasperating - the balsa will shatter if put under compression (i.e. a stroke towards the end your fingers are anchoring). When it's about 0.2mm you can fold it in two along the grain and rub thumb and finger firmly down the fold (like you were going to pop it in an envelope) and it won't split. You can read newsprint through it. I judge the thickness by looking at the sky through it to see how translucent it is, and confirm with a Vernier calipers. It's jus' practice.

Re. asthsma, the technique I currently use for putting cyano onto sheets (see below) is done under polythene. I don't smell any fumes so this may work for you? I know it's down to the sensitivity of the individual mind you, and I've never had allergies AFAIK.

Don, I use several ways to get cyano onto balsa. The pic below shows the best 'batch production' method for flat sheet, but it's still hit and miss and requires practice. There's an MDF sheet covered with adhesive-backed plastic film. There's a strip of double-sided tape holding a sheet of thickish polythene. Put the sheet of balsa on the MDF, squirt lashings of thin cyano onto the near end, drop the polythene sheet on top and squeegee with a smooth large radius thing like a bottle or tube as shown. It has to be done coolly and very rapidly (seconds). The polythene can be peeled back in a few seconds (you'll feel the heat of the exothermic cure) and dry areas covered if necessary. There's too much to this for a regular post but ask if you want to find out more.

The pics if I get the order right are my estimations of Mew Gull spines, my 1st cyano/balsa experiment from around 18 months back, which was a tapering tube that would suit a Peanut flat-backed Spitfire for example, 14cm long, 1g, extremely torsion-resistant, puncture-resistant, water-resi. . .you get the idea.

Then the rig as described above, then the top cowling+one side panel of the Mew Gull model, attempting to show the double curved top, then the model tacked together today, weighs 8.8g now - how did that happen??

Stephen.

Edit - picture order wrong - I guess you can work it out. . .
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Don McLellan
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« Reply #11 on: June 17, 2012, 06:32:12 PM »

Thanks Stephen.  I'll have to try making some panels using your sanding and surfacing methods.  Very interesting that you use thin cyanoacrylate.  I would have guessed thick so there is more time to spread etc.

Don
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« Reply #12 on: June 17, 2012, 10:48:01 PM »

 Embarrassed Grin I hit the wrong key... I MEANT to type PIT (as in cocpit) cover Grin Grin!  Durn keys are too close together.  I actualy did the cowling very similar to your method, but left it at 0.4mm and wet wrapped the very front piece.
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« Reply #13 on: June 18, 2012, 01:10:02 AM »

...
Hi Paul. I found that turning out thin balsa was fairly easy. I started hoping to turn 1/32 into 1/64" and just kept going. It's a matter of rhythm. I use MDF as a base, and use silicon carbide (wet'n'dry) paper attached to a very flat block with that aggressive double-sided tape that carpet-layers use. I only use new sharp abrasive whatever the grit.
...

Stephen

Your methodology for sanding balsa sheet is similar to mine. See http://www.hippocketaeronautics.com/hpa_forum/index.php/topic,5715.0.html

For sheet balsa wider than 32mm or for sanding blue foam, I use an 85mm wide block of aluminium as my sanding block. I milled one of the faces flat with a slight ripple. I think the ripple helps the sanding action because multiple edges and hollows are presented to the balsa surface; 120grit 'open coat' (or similar) paper is attached using double sided tape.

The block has adjustable plastic slides on two facing edges to cater for different sanded thicknesses and the whole contraption slides over a sheet of plate glass. I hold down balsa wood as you describe and plastic foam is sanded wet. Water surface tension helps to hold the foam down and prevent it failing in tension. Wet'n'dry paper is used on the foam.

About white paint

White paint or ink is generally heavier that darker colours such as red. It may be better to use the red scheme you mentioned if your Mew Gull turns out a bit too heavy.

Paul
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« Reply #14 on: June 18, 2012, 02:06:34 AM »

Hi Stephen,

I know this is a stupid question, but just to be clear, did you first sand the balsa to thickness then apply the CA?

I find your project very interesting and might give your CA'ed sheet balsa a try (a little later).

Thanks again,

Don
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« Reply #15 on: June 19, 2012, 12:56:14 PM »

Embarrassed Grin I hit the wrong key... I MEANT to type PIT (as in cocpit) cover Grin Grin!

Ha ha! Now I get it. I thought, well, pot is slang for cylinder, so maybe pot-top is some new slang for top cowling??

Thanks for the paint suggestion Paul - this is something I know in other contexts, but it hadn't entered my mind in this one, so it's a good nudge. You're obviously way ahead of me with balsa reduction strategies, what with rippled blocks, but I'm glad to realise the similarity of our methods - makes a bloke think he's on the right track.

Don, it's actually the single most important question. The reasons I didn't make it clear in earlier posts were that it would require a long exposition, which I wasn't up to as I was tired - and also because the answer is "both".

Briefly, there are lots of wrinkles to this method - and some of them are in the balsa, unfortunately.

I started off by thinning tha balsa first, purely because I remembered from misty memory that to sand wood down on one side only leaves it warped and unusable. As I hinted in an above post, when the balsa gets really thin it actually becomes less vulnerable because it can flex rather like paper if mishandled. This makes it bearable to deal with, but there's a hitch: this thin, the wood has rifts in it. The rifts are where the darker, gold threads in the grain (presumably sap-bearing veins in the live wood?) are abraded through. When you apply the cyano of course some gets through these gaps. This could be good in the respect that you now have a genuine sandwich composite, but really its a nuisance, as the bleed-through is just dead weight for the modeller, and even worse it creates random inflexible patches in the sheet, making it difficult to bend round L.E.s for example, and causing undulations in what should be a smooth surface.

Recently then, I tried applying the cyano to the thick sheet and reducing it after. This does seem a better bet. It's easier, because the composite sheet is less vulnerable to damage during sanding. And the result is more uniform, and it seems somewhat lighter. I'm cautious about this last point because I tend to use any half-decent balsa for these experiments, not selecting the best. Some of the panels I've made (and incorporated in the Mew Gull) are pretty manky. There is a temptation to sand the wood even thinner, now it has a firm cyano coating giving it extra strength. At some point though the sheet will become so thin it rucks up (I mean, not while sanding, but over time due to internal stresses, temp and humidity changes). Since the major weight of this composite is in the cyano, I'm wondering about keeping the balsa slightly thicker and reducing internal structure even more.

There are different ways of achieving a finished part. For instance the test section pictured above somewhere is still exactly as it was made after 18 months or more, whereas flat panels I've curved over formers on the Mew Gull have dished somewhat. The original experiment involved steaming 0.2mm balsa over a wooden mandrel (protected by plastic film) and applying the cyano after drying it thoroughly. This is evidently a more stable method that bending flat, pre-formed sheet, which seems to retain stresses which can distort it. However, it's obviously a far more involved and time consuming business. Lots more experiment required.

Stephen.
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« Reply #16 on: June 19, 2012, 06:40:12 PM »

Thanks Stephen.
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« Reply #17 on: June 22, 2012, 12:12:01 PM »

Here's some more news on this model.

It turns out that the weight of the basic airframe, without paint, without noseblock/prop, without canopy, without motor peg, is 9.3g.

I measured this particular value because the important thing with this experiment is to get an idea of sheet construction v. "stick and tissue". The other items don't seem germane to this, and what's more I don't have a conopy yet to weigh! I hacked a crude 5" prop from a broken 6 1/2" one, just for first tests. This and the noseblock weigh about 3g. The tailplane is a temporary one, in place until trimming is explored further. The weight of the permanent tailplane will differ very little as far as I can estimate. My original target for airframe/noseblock/prop/canopy/paint was 11g so I'll be way over that when it's finished - but maybe it was a somewhat ambitious target.

All the control surfaces on the original were wood/fabric. I've made the rudder to emulate this because you can see the ribs in some photographs of the original - not so with the elevators and ailerons. BTW the rudder's in the cockpit in the weighing pic below.

The canopy isn't finished. The hood's OK but the windscreen is difficult. I don't have any traditional thermoforming plastic like acetate or styrene. I tried moulding stuff that looks the same but turns out to be some modern super-polymer that's more or less indestructible. Layering CA on a mould looked hopeful but bloomed in the very humid atmosphere. Anyway, I think I'm getting there.

I got into knots with the finishing procedure. I couldn't add wing fillets nor finish the u/c until I knew what dihedral I could get away with. This required test glides, which I couldn't do until I'd fixed the engine cowlings down (these are structural in this style of construction of course). BUT, I couldn't fix the cowlings 'til the wing fillets were in place, because the Mew's side cowlings have a saucy way of bending round the front of the wing fillets, and I wanted to copy this if poss. In the end I tacked things together very precariously, and tried some glides.

The immediate fact to emerge was that the model had absolutely no idea that it was an aeroplane. It seemed to think it was a scrap of newsprint in the wind, or that it was a gannet, because it would flip over and dive with great acceleration straight into the ground (long grass luckily). I'd made the tailplane fractionally bigger than scale size, but still it may as well not have been there.

I moved the balance point well forward, which helped but not much. Because the weather was really unsuitable and getting worse, I hastily tried some power, which of course added to the general instability. In the end I rigged a much bigger, temporary tailplane, about 18% wing area. I also moved the motor peg well forward to save the 0.75g noseweight and to reduce pitch inertia. The thing then showed promise but "rain (and wind) stopped play". There was some slight damage from these shenanigans, because in my rush I'd tacked part of the u/c and part of the wingroots to non-structural areas of skin, which split when said components were knocked off. Soon fixed, at the penalty of a tiny weight gain.

I plan to finish the model to some basic standard (too many errors and flaws have crept in to make it worth going for a slick and detailed finish). As soon as I'd bought some red and gold enamel I realised it's just too rich a colour scheme for an aeroplane. Fine in a Chinese restaurant, but nah, too lush for a plane. I've found that G-AEKL was blue before this last red and gold showing, so if I build another it'll be white or blue.

Stephen.
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« Reply #18 on: June 24, 2012, 03:02:07 AM »

Lawks, guv'nor, that paint's heavy. . .

I was hoping that on this small model,  paint would add less than half a gramme. I sprayed a coat of red enamel thinned with cellulose thinners yesterday, after adding an airscoop and crude louvres to the cowling. It was obvious as soon as I started spraying that hiding the dark grain of the balsa was going to be difficult, and would need another coat. However, it ain't getting one, because when more or less dry that coat added 0.4g. That's without trim, reg. markings etc. In fact the paint did become considerably more opaque as it dried, so the dark threads of grain have to be looked for, now. I knew from a mis-spent yoof of static plastic modelling, that painting brings out every last flaw, but I wasn't shocked by this, as the flaws were so obvious in this case that I could see 'em without paint anyway. . .

Going to try some gold paint now. This is going to look like a Christmas tree bauble.

Stephen.
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« Reply #19 on: June 24, 2012, 03:28:40 AM »

Yep, paint = weight

That is one of the reasons I have been experimenting with super light Tengujo tissue (6gsm) but the downside is that 6gsm Tengujo is VERY porous, unlike Esaki, but Esaki weighs 12-13gsm.

My ambition is to get a fully decorated covering (eg. base colours plus markings etc.) that is no heavier than raw Esaki, preferable lighter. Experiments with various combinations of acrylic ink, artist grade paint, mediums and water has shown that a coloured finish, eg, blue, red, yellow or orange etc. based on 6gsm Tengujo can be had with a weight of 8-10 gsm but it is transparent or translucent. An attempt to increase opacity with a coating of white easily resulted in a weight of 28gsm. At that rate, white Esaki would be better for a white finish except that Esaki tends to not stay white.

Paul
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« Reply #20 on: June 25, 2012, 10:26:31 AM »

I slapped on some gold paint. The mock-up expanded polystyrene canopy should weigh much the same as the final one, which gives it a flying weight of 15g with 12" loop of 3/32 rubber. This adds up to a very high wing loading, but I'm hoping for some kind of a flight because the first Chilton DW1 I built had a similar loading and flew OK. However I tried some flights with the Mew Gull with a new prop and wasn't rewarded. It's too windy to gauge what's going on, but the phrase 'unstable in all axes' comes to mind. I'll play some more if the wind ever diminishes. The main point of all this is the sheet build though, and I'll definitely be pursuing this method.

Let us know if you reach any conclusions with your colour/covering trials, Paul.

Stephen.
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« Reply #21 on: June 25, 2012, 11:01:58 AM »

It looks really nice! I'm sure you'll get it going even at that weight  Cool It is a racer after all  Grin
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« Reply #22 on: July 14, 2012, 04:02:06 AM »

I'm still mucking about with this model. Flight-test weather is either 'unflyable' or 'completely and utterly unflyable', that's the choice.

Although the Mew Gull has a very small fin, I'd hoped that the deep, slab-sided rear fuselage would provide the necessary keel area for stability. Not so, at least in windy air. I haven't coaxed the thing to fly a full circle yet, despite offering it a nice gold spinner if it does so.

I made the main gear to slide up and down over stub fairings, to give the model 'ground' and 'flight' u/c positions. This enabled me to remove the main gear (1.6 gram) and this improved the flight pattern a lot, even though I was reducing pendulum stability by omitting the gear. The weight saving suddenly gave it zesty performance, and I realised that it's jus' too dam heavy. Not only was the paint heavy but of course it's mostly behind the balance point, so needs noseweight to balance it. The model balanced without noseweight before I painted it. Anyway, it flies very fast on 3/32 rubber, and benefits from very coarse pitch (45 deg at 0.75 blade length). This seems to bear out one of my original hopes - that the high weight would be mitigated to some extent, by low drag. It skates around wagging its tail till the wind knocks it over.

I wondered if I could make a lighter airframe, and I realised that this sheet-construction business needs much more experiment and practice, so I started another one. Its design is almost identical with the first one, but it is quite a bit lighter so far, due to better-produced sheet, I think.

The pix show the fuselage 'bones' with integral fin and sprung tailskid, and details of wing and fuse skinning.

Stephen.
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« Reply #23 on: July 14, 2012, 04:34:11 AM »

This is such beautiful work Stephen, really impressive.

Wow, 45 degrees is a Pitch/Diameter of about 2.3, that's a lot!

I was thinking about weight saving: what balsa density are you using? I'd imagine you could use super light stuff because with the CA impreg, the balsa acting as the 'distance spacer' rather bearing a load itself. Although perhaps the amount of CA it absorbs is another factor.

I also wondered if you could get rid of some of the internal structure, I know its already minimal but you have what is effectively a stressed skin monocoque so again the structure could be very light balsa and still add a lot of stiffness.

What about tissue covering? It would be a different route (maybe change how you use the CA) but it would add strength and colour for reasonable weight penalty compared to paint...

Very cool project  Cool


Jon
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« Reply #24 on: July 14, 2012, 07:45:43 AM »

Hi Jon, these are all pertinent points you raise.

The internal structure is a nuisance for sure. My original foray into this, a small test section pictured in reply #10 above, is far stronger and more rigid than need be, and has just a 1/32 ring former fore and aft, connected by two 1/32 x 1/16 stringers. Youc'n just see the forward former in the pic. This is about as minimal as can be I guess, but the MG has flat side panels which need more support. The longerons and spacer in the pic above are necessary to give a strong point to hold the model by.

Other than that, it might be possible to eliminate more structure. I chose the MG because of all the straight lines and flat bits, but maybe they offer no advantage in this style of construction. As for the wing shown above, I think I've probably approached a practical minimum there too. There need be no spars theoretically as the skins do this work, but in practice having no spars makes 'zipping up' the wing skins very awkward. The spars are really just summat to attach the ribs to. Importantly, the skin is so heavy that the weight of some internal structure seems allowable - the internals of the wing shown above weigh 0.25g whereas the skin weighs about 1 gram. I'm expending most of my effort in trying to minimize skin weight at the mo.

The density and role of the balsa is another vexed question. It looks to me as if essentially the CA forms just a thin film over the wood except where it infiltrates the gold coloured pores in the wood which I take to be sap-bearing veins in the live wood. Thus, the balsa is still providing the core rigidity of the material, the CA being a thin flexible film. I believe this is so from removing as much CA and then alternatively as much balsa as possible from prepared sheets of the material, and assessing the resulting properties. Ultimately you get a very thin skin that will need internal support and with poor bending characteristics caused by stiff spots where the CA has filled the wood's pores. I'm still a'workin' on every permutation in the hope of finding a satisfactorily light optimum.

Another interesting thing which you've latched onto is the CA absorption: I assumed lighter balsa would be better but have seen repeatedly that light balsa makes a heavier composite. Presumably due to its mopping up more cyano. I naturally tried hard balsa then, and found that heavy too. My best results consistently come from medium weight quarter-grain wood. Too early to make an absolute conclusion, but I'm beginning to be convinced. C-grain makes good wing panels too, and can be coaxed round leading edges most times as well.

The lightest sheet I've made was 0.0055g/cm2. However it's really hard (impossible?) to predict how a sheet will come out just by looking at the balsa. I base my model design calcs on 0.006g/cm2, and even this is rather optimistic in practice.

As for tissue, my incompetence at handling the stuff is one reason I started dabbling with balsa sheets to start with! Strength; none required as the cyano/balsa sheet is more than adequately strong in most areas of small models anyway - and also it offers the promise of giving a really hard-looking smooth/sheeny/glossy finish, which tissue can't do IMO, unless perhaps half a ton of dope is larded onto it. I suppose the idea is that you might get a truly scale-looking ship that flies somewhat, compared to a less-scale thing that will undoubtedly soar across the world. The old Scaleness v. Duration tug 'o war which is the essence of this arm of aeromodelling.

The prop pitch is a bit uncanny. The weather is so unforgiving this year that it's hard to assess what's going on - the model always comes to ground through instability in gusty air, not motor exhaustion, so maybe a finer pitch would be better, but after all, I only coarsened the pitch to start with because I seemed to see an improvement from doing so.

Stephen.

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