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Author Topic: 1/24 scale 'Birdcage' F4U-1 for rubber power.  (Read 1738 times)
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Prosper
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« Reply #25 on: July 28, 2022, 04:30:34 AM »

Dave, apologies. I knew my post might not shed sufficient light or be well-written enough - but it was such a long post I thought I'd try to get away with it. Here's some more about the general principle.

Rubber motors are famous for hook-climbing, even when they're under tension, and when this tension is reduced by having a long motor sit in a short distance 'tween hook and peg, the climbing tendency increases. Braiding allows a long motor to sit between prop hook and rear peg without contacting any fuselage structure, when unwound, and this residual tension created by braiding keeps the noseblock in place. But when this long motor, relative to the HtP distance, is wound up and 'walked back' into the fuselage, it folds up into clumps of knots rather than the orderly, hierarchical layering of knots seen in a motor that's still under considerable tension due to a long HtP distance. Taken to the extreme, the wound motor becomes more like a ball of rubber, with the hooks front and back actually obscured by the mass of knotted rubber. This precludes any kind of viable motor run - the rubber has to be kept from fouling the rotating bits at the front end, and from jamming itself in the fuselage structure. A 'T' prop hook means that the motor can only fold itself perpendicular to the hook (or forward of the hook) in two planes - this side or that side, you might say. but that's still two planes too many. Constraining the motor from folding up by using a greased hoop, say, suggests too much friction loss, and vibrations transmitted to the airframe. But since I've always used a sleeve around the rubber motor right at the front, I thought that a thin disc around that sleeve, attached to the airframe, should prevent the motor from folding in any way. I didn't act on this because I was still worried about friction losses, but in the event I had to try the idea through necessity, and found that the side-loads the motor imposes in its eagerness to fold up, are minimal - negligible. So the rubber side-loads are ultimately reacted to the balsa nose bulkhead. The wound, bunched-up motor can still want to wedge itself into the nose (this depends on motor length, cross-section, amount of braiding, lubrication, number of turns wound . . .And that's the purpose of the endplate on the sleeve - it just keeps an over-eager motor away from the nose.

When I ditched plastic drinking straws I could make paper sleeves to match a given motor cross-section, and the sleeves are a loose fit - there are no turns lost by being trapped in the sleeve.

Greg - my pleasure. I've always thought that getting the rubber right up to the propeller is a sine qua non. That's mainly due to my choice of modelling subjects I suppose. With a 'T' hook the rubber commences about 10-12mm behind the prop in a model like this F4U.

As you say Greg,  there must be other ways to go at this, and improvements. Regarding the tension, there must be a scientific principle here somewhere - instinct would suggest that the internal friction of a balled-up mass of wound rubber would be greater than that of a stretched wound motor with its orderly system of knots. I've played with long motors in short HtP distances on and off. I have a breadboard test rig which was last modified to test the setup shown above. I remember some many years ago testing the extreme 'ball of rubber' idea, and while the motor was unwinding and the prop turning, grabbing the noseblock and stretching the motor way out in front of the rig, then stepping it back into the rig. I seem to remember that it was hard to judge any change in the prop RPM. That suggested no discernible loss of efficiency in the balled-up motor, but then the whole thing was too subjective to mean much. I take lubrication to be even more important in these bunched-up motors.

Even if the setup is less efficient, it's better than a project that fails due to the sheer amount of noseweight needed to balance a motor going into the rear fuselage.

Stephen.

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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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cvasecuk
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« Reply #26 on: July 28, 2022, 04:59:06 AM »

How do yuou make your "T" hook?
Ron
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Prosper
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« Reply #27 on: July 28, 2022, 05:14:16 AM »

Hullo Ron,

For both the T-hook and the freewheel I have to revert to soldering. I've used  soldering  for donkey's, but tbh never quite understood it and its mysteries,  despite professionals giving me the old 'cleanliness is everything' type spiels, and never trusted it mechanically - but finally it was aeromodelling that forced me to crack it - I did so, and now I'm happy to use soldering mechanically. Incidentally, with the T-hook, which has one arm soldered onto and L-shaped length of wire, if the joint does break, then nothing much happens. The broken stub-arm is held by the turns and/or the braiding of the rubber motor. The motor will have only half its strength left and will vibrate somewhat as it winds down, but that's it.

Stephen.
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strat-o
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« Reply #28 on: July 29, 2022, 08:53:38 PM »

Stephen, have you any advice for would-be true-to-scale modelers?  For you I know that you try to be faithful to outlines, airfoils and dihedral.  Do you have advice for others, especially for WW-II scale fighter type aircraft when it comes to trimming for flight?
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MarkM
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« Reply #29 on: July 30, 2022, 04:31:09 PM »

Apologies if this sounds a stupid question Prosper, but am I correct in thinking that the motor isn’t braided when using the long motor with short HtP?

If so, how do you deal with the ‘slack’ in the motor when it’s run down? My understanding is the nose block is held secure, but I’m intrigued about the rest of it.

I always enjoy reading about your builds as I find the ingenuity intriguing!
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Prosper
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« Reply #30 on: July 31, 2022, 03:52:43 AM »

Hullo MarkM, thanks. No, the motor is indeed braided. Just enough to keep it suspended between hook and peg when fully run down. The nosecone is free to pop out if forced slightly, in order to protect the propeller on landing. As you can see in the static motor run video linked to in reply No.22, the motor hangs there even when the turns are exhausted.

Blimey, that's a big question Marlin. Regarding the Corsair, I have no idea whether it will be trimmable, so perhaps I'd better not hold forth about what to do here and what to do there, only to have to report that I can't trim my new model Cheesy.

I suppose, very broadly, it comes down to having a belief that the design changes made to ensure a 'scale' model flies, are usually overdone. But to make a true-to-scale model (excepting the list of usual suspects that can be built true-scale without compromising performance) does require an acceptance that things mightn't work out.

Oh, and making light propellers.

A couple of days ago I sprayed some white paint on the model - today I may get so far as to spray some blue paint on the model.

Stephen.
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Prosper
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« Reply #31 on: August 02, 2022, 02:45:22 AM »

Hmmm. Things looking bad. I got the main airbrushing job done yesterday. The underside white was already on. I was upset about the registration of the various parts of the national insignia on the fuselage. The star and the dark blue disc within the red border came out way out of alignment. Will require large and clumsy touching-up. Plenty big bodge. The wing insignia came out okay - easier to align the stencils because of the relatively flatter work surface.

Anyway, this morning I find that the tissue covering is still baggy and wrinkled from the painting. Now, this always happens with the acrylic inks I use . . . but sorts itself out over a matter of hours. In this case though, the pics were taken about 12hrs after the final airbrush work, and the main large-area spraying was done maybe 18hrs previously. The atmosphere is humid - but then it nearly always is, in the UK. I'm getting a bit nervous about this.

I plugged the bits together to get my first true indication of the weight. It weighs 31.5g and balances at the mainspar. That's everything except rubber, some final details, and additional noseweight (if required).

I'm hoping more noseweight won't be necessary, because considerably more of the rubber motor is ahead of the nominal CG than behind it.

Stephen.
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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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Prosper
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« Reply #32 on: August 03, 2022, 11:07:11 AM »

This morning the F4U's covering was as baggy and wrinkled as yesterday, so I decided to cut my losses: suspend all detailing and painting tasks, glue the damn thing together and see if it would fly. I had intended to do a full marking and weathering job on this one, since pictures from the earlier stages of the 'island-hopping' South Pacific war so often show really worn-looking aircraft - including the only picture I have of '435'. All that gritty coral dust I expect. Never mind - since I had abandoned the covering, I didn't have any precious feelings about getting it gashed or holed. Convenient, because I only have my small patch to fly on, and apart from the trees and sheds, the weed stalks, thistles etc, can be pretty harsh.

It was far too windy for first flights; gusting 22kt, and plenty of gusts. Never mind, I didn't care about dinging the model (within reason, that is).

But . . . it's unusually sunny and dry (dry as in humidity) here, and within a couple of minutes outdoors the tissue covering had smoothed out entirely. By the way, I still have to figure out just why the covering went so wrong.

I didn't learn anything solid from the glide and short hops in the wind, but I musta learned something because I reduced the prop blade pitch to 30° at 75% radius, and put a Gurney strip, 60mm x 1mm x 0.3mm under the port wing. Maybe the wind will abate this evening.

I've decided to see if it will fly properly, and if so, strip the tissue and try re-covering the model. Unfortunately I have no access to my bigger flying field, probably for several months, so this plan might not come off for a good while. Meantime I can see if it can be trimmed to fly in my little area of weeds.

The blue of the topsides looks too pure to me - ironic because when I first mixed the colour I decided it looked too grey, and too dark, so I added more white and blue!

As flown the model weighs a fraction under 40g, and has a 26" motor of 7/32" rubber - that's one loop of 3/32 and two loops of 1/16". Balance is at one third of the root chord.

Stephen.
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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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Greg Langelius
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« Reply #33 on: August 03, 2022, 11:59:52 AM »

When/(if) I use acrylics, I do not depend on them to tighten the covering. I use thinned Minwax Clear Gloss Brushing Lacquer; about two parts thinner to each part lacquer as my initial coat. This keeps the weight addition minimal, allows the tissue to shrink tight, toughens it somewhat, and provides a base shine over which to airbrush the acrylic. For the most part, I employ colored tissues for the main colors, and reserve the acrylics for detail work. Lacquer is just lighter, the tissue color is more WYSIWYG, and yes, your upper surfaces do look a bit lighter in shade than my internet artwork prints out. If I'm contemplating buying a bigger bottle of acrylic paint, it's time to reconsider my color application method.

But no biggie; the model you have is excellent practice for a second attempt. No doubt you've already encountered points of interest upon which you would improve during the original work and with all the initial questions already answered, the second plane should just go faster and better.

Not to worry...

Greg
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Don McLellan
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« Reply #34 on: August 03, 2022, 03:18:57 PM »

Beautiful model Stephen.  I especially like the second pic.  I too have had tissue go quite baggy after spraying (with Krylon in my case) but every time, with time, the tissue retightens. 
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Greg Langelius
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« Reply #35 on: August 03, 2022, 04:06:50 PM »

This link https://duckduckgo.com/?t=ffsb&q=f4u-1+colors&iax=images&ia=images demonstrates the effects of age, light, film, and maintenance on aircraft colors. IMHO, your shades fall within a rather broad range.

Greg
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« Reply #36 on: August 04, 2022, 04:02:16 AM »

Hi Don, same here. I've used these same acrylic inks since the dawn of time. They are neutral in the sense that they don't induce extra shrinkage, but just leave the covering tautness where they found it, so to speak. They do take a long time to dry fully, so an extended spell of mild worry can ensue - but as you say, retighten in time.  So, a mystery.

That panel of thumbnails  just shows how subjective this camo business always is Greg.. Indoors (see pic), the colours are about where I'd like 'em, maybe a bit duller would be good - it's outdoors in bright midday sun where they start yelling. But this colour shift would be true of the full-size too, presumably. In fact, the picture of '435' is so light in tone that I was sure it must be one of the early blue-top grey-bottom Corsairs. However there's a colour artwork of it in the same book showing the three-tone scheme. I'll take their word for it, although colour artwork is oftentimes impressively fanciful.

I got my first indication of real flight this morning. Last evening the wind hadn't dropped much but a few lulls were enough to convince me that the CG needed to be further forward - no surprise really. But the Gurney strip wasn't holding the left wing up. I had built a significant amount of washout into the right wing, but this had evidently disappeared over time. With the strip removed from under the left wing, a new twist steamed in the right wing, plus 2g of noseweight, today the model could fly, but with unpredictable turning behaviour. It seems underpowered. I might fine the prop pitch some more, but in the past I have found coarse pitch and a bigger motor to work well.

Stephen.
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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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Prosper
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« Reply #37 on: August 05, 2022, 05:40:07 AM »

A static motor run showed an average RPM of only 800. that's after reducing the prop pitch. That's a really low value for any motor/prop combination I'm at all used to. Maybe my judgement of what 'seems about right' is way out of calibration. Anyway, I decided to move the motor peg forwards by 28mm (just over an inch), and use a 1/4" motor 25" long. With the new peg position this motor is now 5.7 x HtP distance. Some calculations showed that this change should mean I could shed the 2g of noseweight - and so it turned out; the model now weighs 39.1g despite the heavier motor, with the CG unchanged. With this new motor a static run with 0.75 of theoretical full turns gave an average of nearly 1,000 RPM. Veeeeeeeeeeeeery roughly, that static RPM might translate to about 1300 RPM airborne (averaged through the flight). 0.75 of max turns is 1400, so the potential for a minute's flight is there. *Cough* . . .in theory . . . *cough*.

I did get in some short hops yesterday evening and they were rather concerning, in terms of stability. There was a constantly shifting breeze, perhaps that was the problem - but the model showed no interest in pursuing the same flightpath from one flight to the next. Not good for small spaces . . . I believe the wind is set to drop in coming days so I hope to be able to make sense of things soon.

Stephen.
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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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« Reply #38 on: August 05, 2022, 07:25:41 AM »

I did get in some short hops yesterday evening and they were rather concerning, in terms of stability. There was a constantly shifting breeze, perhaps that was the problem - but the model showed no interest in pursuing the same flightpath from one flight to the next. Not good for small spaces . . . I believe the wind is set to drop in coming days so I hope to be able to make sense of things soon.

Is she trimmed for a particular flight pattern?

(I would assume yes ... upon which point I would invoke the old saying about "ass" - "you" - "me" ...)

Best-
Dave
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Greg Langelius
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« Reply #39 on: August 05, 2022, 12:22:08 PM »

I second your intention to wait for calmer weather before engaging in heroic acts to conquer any stability issues. Once you have them down, building in a modicum of gust stability should be easier. I suspect that subjects like dihedral height and tail volume could be at the core of these issues.

One trick I've resorted to is to append a clear celluloid tab to the rear flying surfaces; to allow some increase in tail volume, while allowing a wider, more positive range of adjustment, as well. In flight I becomes less obvious. But to me, it seams like "cheating".

"435" appearing differently in different studies could have an easier and simpler explanation. The plane/squadron may have been repainted between the studies.

Greg
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Prosper
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« Reply #40 on: August 07, 2022, 02:46:28 AM »

Quote from: dosco
Is she trimmed for a particular flight pattern?
I think first and foremost it's trimmed not to crash, with some 'left circles would be nice' additions . The trouble is, the left circle additions tend to make it crash, so . . .

It often performs an 'S', but not the slow kind that comes of torque reduction at the end of a full flight - this S takes place in the few-second flights it's been making. I think about ten seconds is its best, and this in a straight line or very shallow curve. The picture shows where it ended one of these flights. It had threaded itself past more stone, brick, wood and metal obstacles than I can bear to think about. 

Quote from: Greg Langelius
"435" appearing differently in different studies could have an easier and simpler explanation. . .
I see what you mean, but in this case the captions to both photo and artwork are the same; BuAer 02435 Vella LaVella 3 Nov 1943.

That does it for now. It looks to me as if it will need to fly in very wide orbits. It's going to need a bigger field, and this mayn't be available for 9 months or so unless I fly it over dirt. This is probably not a good idea as the model still rolls off and digs a wingtip in - hard - on some flights. Poor spiral stabilty. I tried flying it with the rudder removed, but saw little difference. Incidentally, it has a phenomenal flick at the full stall - just like the full-size - same wing too! The model's on its back pointing earthwards in the blink of an eye. I'd almost think it could spin. Vought ameliorated this by putting a stall strip on the right wing leading-edge, to encourage the right wing to stall earlier, but judging by the USN WWII training film the flick was still dangerous.

Stephen.
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Re: 1/24 scale 'Birdcage' F4U-1 for rubber power.
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