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Bob's PSS Nimrod MRA4


Bob Jennings 1
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I've been talking about building my own Nimrod for a while now. About 2 years and its taken 12 to finally get a plan together. So now it's drawn up, here we go!

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It's very much a prototype test, so bear with me as I work stuff out and tweak the plan as I go.

The model is designed to break down so I can get it in my car without too much hassle. The nose and tail section are separate, locking into the centre section. The wings, plus nacelles, are a slide fit and it should all come in around 14-16 pounds at 2.66m span.

First job... cut some formers.

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Planking continues apace as you'll see I need to invest in some more modelling pins!

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I'm always amazed the extent ammonia will allow me to torture balsa wood into shape. The skin in this section is 1/8"

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Planking 'best practice' is to chamfer the edges. Later you'll see how this can be really useful in tight spots.

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Here, the juncture between curves needs tackling first with some tailored, chamfered balsa strips. Those spruce stringers tie the formers in nicely resisting any twist. There's a lot of pressure on some of those clamps.

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My entire stash of pins was employed to tackle this! Work stopped... Once dry, I could tackle some of the lovely curved panels on the roof. See how the grain points across the formers at an angle, not straight down the spine. This allows the balsa to follow the curve nicely.

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The overlapping edge can then be trimmed to accept the next panel.

This is the opposite side.
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Edited By Bob Jennings 1 on 30/07/2019 11:42:32

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Getting there with the underside
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This is one of those tight spaces I was talking about.
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Chamfered edges and a tailored piece that bends the correct way needs to fit here.

Here, the edge is chamfered under the next panel
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Presenting a rough piece, marking it up (measuring twice) and cutting slivers off at near enough the correct chamfer angle.
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Applying pressure across a spare piece of balsa uses fewer pins in this case
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And its really starting to look like a Nimrod.
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Edited By Bob Jennings 1 on 30/07/2019 11:59:48

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It's been a while since the last post. More glueing and pinning to get the Nose section to a near enough state prior to starting the tail section. I want to get all the big bits done together and then glass the whole lot once i'm sure the fuselage will fit together.

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I'm leaving the structure open here so I have access to the formers when I install the bracing tubes across the holes you can see inside.

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Just got to ammonia some more balsa to fit the upper cockpit area. It may need a sub-former the support the curve. Another addition to the plan!

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Hi Chris, it’s really simple. I’d wear a mask and gloves... brush ammonia onto the balsa, leave it for 10 minutes and you’ll see how pliable the wood fibres become. The bending can be enhanced by carefully assessing the best grain direction. Hold the piece in place until the balsa dries and it will have ‘set’ in shape. An additional coat of ammonia will allow further distortion. It is amazing how the balsa returns to its original rigidity once the ammonia has evaporated off. Try it, it saves hours of planking.

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Chris,

I used a 10% ammonia/warm water mix and totally immersed the balsa in the fluid for about 15 minutes. The balsa was then strapped/banded/taped to a shaped foam block. After 24 hours of drying the balsa has adopted the required shape.

The photo below is a 1/8" balsa shell for a Hind helicopter that I did many years ago:

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I hit a profile snag on the nose, so I'm leaving it to germinate!

The tail section is next and the formers essentially create a cone that needs reinforcement for the tailplane. There also needs to be access for the elevator linkages as the tail is a slide fit in two sections with the strakes added as flexible plasticard. That MAD* boom needs to be as light as humanly possible, way out at the back! (*Magnetic Anomaly Detector)

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I seemed to get carried away and Here the topcracked on attaching the tail end formers to the balsa spine without taking any photos! Here the upper skin has been attached with the stringers in place.

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I reduced the skin thickness to 3/32 in the hope that it will keep the weight down... It certainly bends better. No ammonia used here. The back end is open so I can install some reinforcement for the tailplane seat and rods.

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Used most of my pins and found my stash of binder clips!

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This is by no means an easy build. The structure is a beautiful shape, but there's so much to think of that I hadn't considered on my very 'general arrangement' plan. The tail section may well be the first section completed with servos, wiring and flying surfaces attached.

More later.

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