1/24 scale DH Mosquito for rubber power.

[Konrad]

Please, a little more on the lathe. Does the wife just hold the drill motor firmly to the table (on a towel for a bit more contact with the bench). What are you using as a gouge rest (tool rest)?

[Prosper]

Thanks blokes. I'm following your Barracuda Andy - good project.
Konrad, yes, that's it (except for the wife, and I do the turning on the floor - not the best for my creaking frame, but at least the floor is solid and stable). The staged photo (I needed my left hand to hold the camera) indicates approximately the areas of drill in contact with the ground - I press down hard. Tool hand also pressed hard against the floor. The tool, as I mentioned before, can snatch and jerk, but never like in this case. I've never used balsa this hard before.

I thought I'd look at my records to see when I started making spinners this way - I think the two photos show my first . . . 2012 . . . jeez how time races along. I must've made more than a dozen over the years. Of course they're only good for model subjects with bulbous spinners; typically late-piston era - but then that's a big area of interest for me. The whole notion is to get the prop hook as far forward as possible. In the case of this Mosquito I measure the hook at 6mm ahead of the nacelle. Because of the worryingly heavy weight of the spinner, the setup balances well forward of the wing L.E., approx the radiator L.E. It takes the whole wing to balance it! In the pic I'm pinching the nacelle very lightly: it's free to rock back and forth.

More pix on the way.

Stephen. P.S. it occurs to me to mention that this 'homemade' turning method doesn't (unless by happy accident) produce truly round, concentric, symmetrical results in the engineering sense - but for this application I find it good.

[OZPAF]

The whole notion is to get the prop hook as far forward as possible. In the case of this Mosquito I measure the hook at 6mm ahead of the nacelle.

This is interesting and nicely done - is this to maximise the rubber motor length Stephen? The rubber motor would be almost captured on the hook inside the spinner but may be a bit harder to put on and remove.

John

[Prosper]

Yup, that's it, John. I've learned over the years  that the T-hook need have its arms at only a small acute angle to the shaft to keep the rubber on the hook. About 10° seems to do it. Mind you I do tie a loop of thin rubber round the motor, tightly, and push it up to the hook after installation and lubing. Yes, the motor can writhe into a coil inside the cone (depending on motor length V. hook-to-peg length), but I put a loose sleeve over the motor right behind the hook, and this stops any jamming. I mentioned before that I file the ends of the T-hook arms - that's to stop the rubber being 'scratched' by the ends as the motor is installed. Yes, it's less easy to put on with a pre-lubed motor - say you have 4 loops - you get 2 loops on one arm of the T, collect the other 2 loops and press them  over the other arm, by which time the first two have slithered off their arm. That happens sometimes, but I've learned to mitigate such irritations. I'll try to take some photos of this.

It's not so much to do with the length of the motor, but getting the weight forward.

Stephen.

[Prosper]

From yesterday:

These pictures are a bit muddled, some of them relate to one of the spinners or cones; some to the other. I had intended to make the two concurrently, but it didn't work out.

The second of the plastic cones is plunge-moulded from 1mm plastic card. Unfortunately the balsa form is very out of true. I can't remember its provenance - it wasn't made for the test model. From 0.5mm (20 thou) plastic card, a narrow flange is cut using my version of a compass cutter. Here are the cone, the flange with central disc not yet removed, and the second spinner.

I have an offcut of thick ply with a length of 1.2mm piano wire fixed into it and projecting exactly vertically relative to the surface. This has several uses, but here it's ensuring the true fitting of the 1.2mm I.D., 4mm long, brass bush into the top of the cone. The thin card underneath has a circle drawn on it. The fact that in one place the rim of the cone departs from the circle isn't because I've mucked up - it's that the rim isn't circular, due to the inaccurate balsa form that moulded it. There's a similar gap on the other side - the rim is an oval. Funnily enough, I don't think this matters. So long as the bush is perpendicular to the plane of the cone's base, and at the centre of the oval, all should be well.

[Prosper]

Now then; the other brass bush has to be fitted into the spinner. Trouble is, the hole in the spinner made by the spindle/bolt used for turning it, has a diameter greater than the 2mm O.D. brass tube. The quickest, most convenient solution I've come across is to use rolled paper. The very end of a long narrow strip of paper is fixed to the brass bush with thin CA. Then the bush has the tip of a cocktail stick jammed into it, and by turning the stick between fingers and thumb, whilst keeping the paper strip under some tension with the other hand, a nice tight roll builds up around the brass. Then thin CA is soaked into the paper roll. The result is easily a match for hard balsa in terms of strength/stiffness.

The paper-clad brass bush is then pushed home into the spinner and the jig is used to check trueness. There is a circle on the white card (pencil lead from my schoolboy compass) - but the spinner matches it so closely that it's hard to spot. The bush/paper is fixed by dosing around it with thin CA, inside and out, which penetrates the balsa well.

Back to the cone. Light shining through the cone shows that at the tip there's a balsa spacer inside, capped off with another 1mm plastic card disc, so the brass bush is fitted into a plastic/balsa/plastic sandwich. It's a tight fit, and thin CA doesn't penetrate a plastic/brass interface well. So I drilled a very small hole right thru the sandwich, abutting the brass, and ran CA into it. It flows all around. It's funny to watch a sizeable drop of CA quickly vanish into the tiny (c. 0.4mm) hole.

[Prosper]

Light from the other side of the cone shows the last part - a ring of 0.5mm plastic card that is what locates the assembly into the front of the nacelle. This ring of card has one edge chamfered to fit the slope of the cone, so it's CA'd both to the inner edge of the flange, and to the sloping wall of the cone.

Finally, the correct diameter hole is carefully shaved then sanded into the nose former of the nacelle. A socket set provides all sorts of useful diameters around which to wrap abrasive paper, for jobs like this.

Stephen.

[Prosper]

Six propeller blades? I wish someone had told me - I'd never have started this caper.

First, another use for the 'lump of thick ply with a post sticking out' jig. This card has radial lines inked at 120° to mark the spinner for the blade sockets, complete with a nifty tab that folds up, allowing me to draw vertical lines at the relevant spacing. Then there's another card template giving the outline of the socket. I've increased the socket angle - requiring a new template - from around 40° to around 50°. I had a sense from the test model that some more (coarser; higher) pitch might work. The 'steeper' blade sockets should give a blade pitch at 75% radius of about 30°. That's not unreasonable - but now I see the result, it looks pretty radical. We'll see.

Cutting the sockets in these extra-hard spinners was an extra-hard job.

[Prosper]

Blade blanks cut from 0.8mm hard balsa . . . except that this particular sheet is 1mm thick. I bound the blanks three at a time to the former. Usually I use cold water for balsa-forming but given the thicker (1mm), hard wood, I poured boiling water over them and soaked up the excess with a cloth before binding. I suppose in theory I could have done the six blanks in one go, but three at a time seems safer. Because of the thickness of the stack I gave 'em two hours in the hotbox.

A blade former I made. Previously I've used twisted aluminium strip to give blades the twist they need, or I've done it by freehand manipulation - 'in the air'. But for mass-production I thought a proper helical former would be good. It's shaped with a small amount of chordwise curvature - away from the root - to impart some concave undercamber.


Next came the root reinforcements. Balsa is sanded to a wedge, so that the outer, straight edge of the  root stiffener is thin. Then carbon tow (from some carbon cloth) is chivvied into a fan and wetted out with thin CA. The picture shows this perfectly . . . only thing is, I'd glued the carbon to the wrong side of the balsa. I get days like that . . .

A root stiffener is tacked to the back of a blade blank with CA, then the blade is shaped, freehand. The shaping is all on the top of the blade.

[Prosper]

Then the blade is painted with CA. This job is too big for the various metal CA applicators I use, so I turned to cocktail sticks. Before long these build up a jagged crust of dried CA on them, which can scratch or gouge balsa, so they need to be replaced. I think I used two, at both ends, for this job. Also I tend to do this at arm's length, coz the CA can be in a fuming mood. It's okay with older CA but I've just opened a new bottle and it 'goes off' with a puff of smoke. Then a blade is sanded with 120 grit (European grading method) followed by 320 or similar.



I meant to take a shot showing the sharp blade trailing edges but forgot. Last is another turned former, this time for plunge-moulding the spinner caps. Small formers like this can be turned freehand - the drill in one hand and the sanding block in the other. Standing up, thankfully not hunched down on the floor.

These pictures are mostly from Tue 12th and yesterday.

Stephen.

[gravitywell]

What an amazing build.