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Nimbus 4D - 7 metre

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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

If there is a national shortage of servo wire, you can blame me! I have been making the wiring looms for the wings. Paul had thoughtfully left in the wires laid by the original builder so that they could be used as pull-throughs, so all the wires have been made and installed with no problems.

I was curious about there being only two original wires sticking out at the root, it turned out that there was one for the spoiler and one for all 3 trailing edge controls, the original builder had soldered in spurs at the 2 flap servo locations. With further home-made Y leads in the fuz linking the wheel brake and tow-release, it looks like the original builder was running 13 servos from an 8 channel radio. Other clues such as "vintage" servos and the yellowing of the glassfibre suggest the model was started a long time ago, before channel expanding powerboxes became available. How easy it is for us nowadays with so many channels in our radios and an abundance of power supply boxes.
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

The cockpit tub and the two seat backs had a brown fabric seat covering with what looks like felt tip pen stripes drawn on it, so I peeled the fabric off.
Two applications of mild paint stripper failed to remove the stone effect paint from the cockpit tub so I went at it with Gun Wash which is a particularly vicious thinner for cleaning spray guns and that dissolved it instantly - if Gun Wash doesn't remove something the next step up in power is a flamethrower! I have experienced Gun Wash dissolving plastic mixing cups and other hard plastics so it has to be used very carefully and quickly on glassfibre and then immediately washed off. I think the blueish greenish colour of the cockpit tub is more likely to be scale than the stone effect paint.

I got some nice blue felt to use as seat covers, and applied copydex contact adhesive to the felt and the seats. I did not know that copydex is so thin it would seep straight through the felt and show on the outside. I hoped it would be okay since copydex dries clear, but it left ugly dark stains so some pieces had to be peeled off and binned. I cut new pieces and attached them with double sided sellotape and that seems to have worked fine. The base of the front seat is badly stained but as that will have a pilot figure sitting on top of it I could not be bothered to replace it.

I decided that I would never learn to live with the rashes of very visible pinholes in the blue paint on the underside of the inner wing panels. A fantastic model like this deserves better. The pinholes are there on the outer panels too but are not visible against the fluorescent orange colour. So, I got my tube of 3M Red Acryl stopper and set about spreading it over the pinholes on the blue inner panels, then wet sanded the entire bottom of the blue to flatten down the stopper, remove much of the existing blue and key it for another thin colour coat. The wing is now a faint blue with rashes of red dots, I hope to get the new colour coat on it today.

The original seat fabric
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The new seats
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

I attached flaps 1 and 2 to the wing with silicon, but used tape for ailerons 1 and 2. This was my first time using silicon as a hinge and it has worked ok, but the results are very variable, some being stiff and some not. I expected silicon to be stiffer than tape but was prepared to try it on the flaps which have very little travel. The ailerons however are very long, very thin with little torsional rigidity, so I feared the stiffness of silicon would cause the ends to stay still when the end with the servo moved! I went with the method shown at http://smsagliding.weebly.com/make-silicon-hinges.html

With that all done, I moved on to installing the servos in the mounts that I put in some time ago, and making the pushrods. Then I was able to program the radio for the wing to the travels told to me by glidersparadise, so the radio system is now complete and ready to fly.

My first attempt at a silicon hinge, done with white silicon
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The outer panels are held to the inner panels by Multiplex Multilocks and need the little red plastic wedge that comes in the Multilock packet in order to prise them apart. I have a couple of the wedges but they are in gliders that need them - I find ways of keeping all the special tools required for a glider in its cockpit so that I can pick up a glider from the hangar and go flying without forgetting to take an essential part or tool with me. The solution to this was easy, in a couple of minutes I had drawn the concave wedge in 3D CAD and then set about printing a few of them. They need a bit of fettling because the layering of 3D printing makes an angled surface stepped rather than smooth, but a couple of minutes with filler primer and sandpaper does the job.
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

The canopy is hinged on the starboard side so I needed some way of latching the left side when it is closed. In amongst the bits that Paul gave me are these three little metal posts with holes through the tapered end. I did not know what they were for until now, when I noticed that the canopy's left edge and fuselage had three holes and to began to work out what the system is meant to be. The posts are fixed in the fuselage so that when the canopy is closed their tops protrude through the canopy frame and a locking wire can be pushed through the hole in the top of the post.

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There were also 2 small holes in the canopy frame close to the vent windows, which seemed to be for pivots for bellcranks. There will be a single rod crank to the front post, and a double rod crank in between the middle and rear post. I made tiny bellcranks from 1.6mm epoxyglass and sprayed them with aluminium colour.
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I cut the heads off 2mm bolts and glued the threaded section into the small holes in the frames. With the bellcrank on the bolt it is topped off with a small O ring and a nyloc nut. By tightening down the nyloc nut and compressing the O ring I can control the friction against the bellcrank and stop it from coming open on its own.
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

The old glassfibre of the fuselage and tailplane were yellowing, so as well as painting the top of the wing, Paul had painted the glassfibre parts as well with a one-pack white paint then a 2-pack clear coat. I set to work on them with 2000 grit wet-or-dry to make the surface of the clear coat flat, and then T-cut the surface back to a shine. The result is spectacular, the surface is utterly smooth and shiny as if it is fresh glassfibre just out of the mould. A few spots are a bit dull, I am hoping that a car polish will bring them up or at least make them less noticeable.

I like a bit of colour to break up the white but could not find a photo on-line of a Nimbus 4 which had everything on the one airframe. I found individual airframes with an element that I wanted so mine is a composite of many. Somewhere there are, or were, many different Nimbuses with the wing tips, tail codes, registrations, logos, airbrakes, wheel doors, rudder etc in red, just not all on one airframe!

Much as I love Klass-Kote epoxy 2-pack paint, it demands a complete stripping to clean the airbrush/spray-gun immediately every use and that guzzles a lot of time for what might have been a few seconds of spraying. Being 2-pack it is constantly trying to cure inside the gun and so a spray through with thinners just does not remove the paint that has part-cured against the walls of the cup and gun. For that reason I use an aerosol can of car paint for many little jobs. I used such a can successfully on the wing tips, rudder and wheel door. A friend with a vinyl cutter made paint masks for me for the tail code, fuselage registration, and the lovely Nimbus 4D shadow logo on the front of the fuselage. On spraying the Nimbus logo the red reacted with the paint underneath and pickled up. I ripped off the mask and wiped the area with thinner to remove the red but it also removed the white, grey primer and yellow filler primer right down to the original glassfibre and left a softened stodgy mess. I said "oh bother" and walked away and left it overnight to harden up again so that the next day I could wet-or-dry it all down smooth.

Why did that bit pickle up but large areas of the wings and rudder etc did not not? My guess is that where I was painting the logo, I had cut through the 2-pack clear coat and exposed some white paint which reacted with the red. I have had this happen before, where supposedly environmentally friendly water based acrylics for cars seem to have some very reactive chemicals in them. It took a lot to repair the paint on that whole area, in the end I found that Klass-Kote epoxy white primer with a top coat of aerosol clear (no top colour!) blended in nicely.
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

A friend with a vinyl cutter made paint masks for the logo, registration and tail codes, after the disaster with an aerosol can of paint I went back to Klass-Kote and had no problems

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I put the model together for the first time with all 4 wing panels, with one wingtip against the far wall about 1/4 of the span is out the door of the workshop. The tip is red, it looks orange as I had to overexpose the photo to see inside the workshop. A trial balance showed that it needs a huge amount of noseweight, I hung about one pound of lead at the nose and it made little difference and will need a lot more so I have gone ahead with installing a pound of fine lead shot before I try again at balancing it.
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harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

The last job of painting is completed, the red nose
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The last of the airworthiness jobs to be done, as opposed to cosmetic jobs, is balancing. Glidersparadise say to start with a CG at 45mm from the leading edge. I did a CG calculation, not the easiest due to it being a 4 panel wing, which said that a sensible CG range is 45mm to 55mm from leading edge, so that works out nicely. I used around 600g of fine lead shot mixed into epoxy, poured into the nose to get it to balance at 55mm, and then 200g of self adhesive weights to get it to balance at 45mm. Then once I am flying the model it will be easy to remove small amounts of the self adhesive noseweight until I am happy with the balance point, but with no chance of accidentally going further back than the 55mm point.

The final weight of the model is 11.2kg which is well under the 12kg quoted by the manufacturer.
Last edited by harry curzon on 15 Mar 2020, 15:38, edited 1 time in total.

User avatar
Peter Balcombe
Posts: 1072
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: Nimbus 4D - 7 metre

Post by Peter Balcombe »

Well done Harry. She looks very nice indeed. :)

harry curzon
Posts: 100
Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Nimbus 4D - 7 metre

Post by harry curzon »

Thanks Peter, just the dreaded maiden flight to do now!

Flying it doesn't worry me, I think the most likely threat to it is on the ground in the early stages of the tow before the wing is going fast enough to not drop on one side. With such a span it only takes a small angle of bank for a tip to touch the ground, and then there is lots of leverage to make it yaw. The full-size Nimbus has killed pilots in such accidents. My finger will be on the tow release switch and ready for the slightest hint of trouble, (though it always is anyway).
I also found out that in the Nimbus 2 the launch is started with negative flap to reduce the lift and stop the wing from flying before the tailplane does! Perhaps that applies to the other variants of the Nimbus as well?

John Vella
Posts: 234
Joined: 20 Mar 2017, 22:09
Location: UK

Re: Nimbus 4D - 7 metre

Post by John Vella »

Harry you have done a great job on your Nimbus. A friend of mine flies the one at Egdehill . It is a great xcountry machine, but can be abit tricky on the ground. By the way on the start of take off negative flap is used to help the aileron control at low speed then the pilot selects small positive flap on the ground roll for lift off. Best of luck with the flying. Regards John.

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