Scale Soaring UK

This will chronicle my build/restoration of a Nimbus 4D, 7.06 metre span, scale 1:3.75 from a kit by glidersparadise.cz
I get the impression that some glidersparadise models are what were formerly KV models, (Karl Vagenknecht ?)
I cannot find any info about this model on any forums.

It has a glassfibre fuselage, tailplane and rudder. The wings, in 4 parts, are veneered foam with spars and carbon reinforcement.

I will not be attempting to fly this from any of the slopes I use, it is intended for aerotowing only.

I believe I am its 3rd owner, I don't think it has flown, but Paul Watkins will be able to correct me if I have got its history wrong.
The first owner made some errors installing the radio such as cutting holes for servo boxes right through the wing - yes, top and bottom skins. The servos installed in the fuselage are very old, weak, their wires have at least one solder joint where the plugs have been changed for the old Multiplex servo plugs that MPX stopped doing 10 or 15 years ago, and so on. The Futaba servo used on the retract is so old it isn't on servodatabase and googling for it comes up with "vintage Futaba 1/8 car servo from the 1980s" !!
It then came via the factory to Paul Watkins to undo the damage. He has done a lovely job of letting in a diamond of ply into the top skins over the servo box holes, removed the film covering and glassed the wings, painted the top surface and vinyl covered the bottom. He then decided that it is not his type of model, at which point I bought it from Paul.

There are no instructions but by email I managed to get the CG location and some control surface travels from glidersparadise. Therefore the build will be to my experience and personal tastes - and any advice that you might offer!

So, let's head out to the workshop, load up Classic Rock on Spotify, turn up the volume, turn up the heater, and get started.

I made a start on the wings. Although the undersides had been covered with vinyl I just have a preference for a painted finish so I peeled off the tape hinges and the vinyl and spent many hours with white spirit removing adhesive residue. The control surfaces are in 4 parts per side which I shall name as flap 1, flap 2, aileron 1, aileron 2. Aileron 2 does not have a servo and is driven by a pin between it and aileron 1. There is no change to dihedral at aileron 2 but the hinge line sweep changes, hence the separation. The pin is very loose, being between the foam cores of ail 1 and 2, so will need a better fixing before I hinge it. I am hoping to make silicon hinges for the wings but aileron 1 and 2 are very thin hence flexible and I think the springiness of a silicon hinge at such at distance from the servo driving the ailerons would just add to the airflow trying to twist them to lie flat, so I might use silicon for flap 1 and 2, and revert to tape hinges for the ailerons.

Although the wings are glassed top and bottom, the bottom of the control surfaces is bare wood which had been vinyl covered, and as I intend to paint them I went ahead and glassed them which may add a tiny bit more torsional rigidity, using 50g cloth and one coat of the excellent epoxy from fighteraces.

The 8 elements of the trailing edge controls -



Slim wing servos are used for aileron and flap 2, and 15mm servos for airbrake and flap 1. I made mounts from ply, then used double sided tape to hold hardwood rails to the outer surface of the servo to act as a jig to position the servo as close to the wing surface as possible while the glue holding the mount cured.

The spoilers were rough and notchy when operated so that needed fixing. I think they are Graupner units. The ends of the plastic arms are C clip shaped rather than full circle O shaped, so they can be popped off the brass bearing tubes at both ends and the blades removed from the frame which stays in the wing. However that could not be done because the pushrod from the servo had been attached to the brass pushrod in the spoilers by a clevis, and a clevis is too big to pull through the plastic end plates of the spoilers. They must have been attached with the whole assembly on the bench and then installed into the model and veneered over. The plastic end plates are quite a neat piece of design with an H shaped slot for the brass pushrod in the verticals, and the horizontal part of the H is deep enough for a Z bend but not a clevis. So the only way to lift the blades with their pushrod out of the wing was to use pliers to wiggle the plastic end plates out of their crimping in the aluminium frame.

Note for the future - always use a Z bend to connect the pushrod from the servo to the brass pushrod of the spoilers!

One spoiler had general dust and grit in the mechanism, the other had a lot of resin and paint in it. All contamination had managed to get into the bearing surfaces between the plastic arms and the brass bearings, at both ends of the arms. I spent a while cleaning and scraping them with a scalpel or cotton bud soaked in meths as required. Also, some of the tiny brass studs that hold the lower blade to the plastic arms were falling out which allowed the blade to flop around, so these were re inserted and fixed. Finally all rubbing surfaces were given a coat of dry ptfe lubricant. The C clip shape of the ends of the plastic arms then allows everything to be popped back together.
The plastic end plates with the H slot were reinstalled but since I had to loosen the crimping of the aluminium frames to get them out and there is no way to get at the crimps down inside the wing, I secured the end plates with a fillet of Hysol 9462.
I made new pushrods with Z bends and put the blade mechanism back into the wing. Now it all works beautifully smoothly and with little force required, as if new.

I will give the epoxy glass on the wing control surfaces a few days to really cure whilst I work on the fuselage.

The rudder is hinged at just 2 points - a plastic C clip in the fin which pops onto a rod in the rudder, and a wire glued into the base of the fuz with a 90 degree bend up into a hole in the bottom of the rudder. The mechanism was stiff due to some misalignment of the angle of the bottom wire and the hole in the rudder. Also the bottom wire was loose in its bed of epoxy and one arm of the the C clip was partly broken, it did hold the rudder but only just.

I drilled out the C clip from the tail fin post and Hysoled in a replacement. When that had cured I decided that rather than re-epoxy the bottom pin I would glue the pin into the hole in the rudder and make an aluminium plate with a hole in it for the pin, this would require no fiddling to get the angle of the pin and hole in the rudder to match, and the plate provides a bit better support/bearing surface for the base of the rudder. I used a dremel and permagrit tool to grind out the old epoxy fairing from the base of the fuz, made several glue holes in the aluminium plate, curved the plate to fit better down into the base of the fuz, and glued it in with plenty of Hysol 9462

Now the rudder is firmly attached and runs max travel side to side freely and smoothly.

The elevator servo is in the top of the fin with a short pushrod to the elevator. All I had to do was slightly lengthen the hole for the servo I am using, glue in some wood support for the mounting screws underneath the thin glass mount, and make a short pushrod which I did with a bolt-through ball link at the servo end, and clevis at the elevator end for easy removal of the tailplane.

The damaged C clip hinge

The loose hinge pin at the base of the fin/rudder

My replacement plate for hingeing the base of the rudder, ready to be glued into place

Moving right up to the other end, time to deal with the installed towhook and replace the towhook servo. The servo must have been screwed into a ply plate and then the ply plate glued into the fuz. How do you get to screws where there is poor access and no room for even a short screwdriver?



What is needed is this wonderful little tool, a mini ratchet wrench. Without this I could not have removed the towhook and rudder servos, the retract and wheel brake servos, or the entire retract unit.




I was expecting a fight to get the towhook out but I had barely gripped it with the pliers and it just dropped out! It had barely any contact area with a blob of epoxy glue, and since epoxy does not grip well to aluminium anyway....
Dismantling the mechanism showed the hook was rotating on the threads of a bolt and the nut was secured to the bolt with superglue. That can be done better




I used a longer bolt with an unthreaded portion for the hook to rotate on, and a nyloc nut so no glue required. I also used a dremel with a diamond burr tool to round the sharp edges of the forward part of the hook where the towline pulls.




I roughened the aluminium surface thoroughly and then glued the towhook back into place with lashings of Hysol.
And finally the hole in the mounting plate needed lengthening about 1mm for the new servo, which I was able to do with a dremel with a 90 degree attachment, and permagrit tool




It all runs smoothly now with a well secured mechanism and a 20kg.cm servo

The retract mechanism was mostly smooth running but once the mechanism got into the part of rotation that would provide a geometric lock with the wheel up, it started grinding and would not achieve a lock. That gave me no choice but to remove it and take it apart. Once again I had to use the mini ratchet wrench shown in the previous post to be able to get at the 4 bolts that hold it to the fuselage.

The unit removed from the model and on the bench ready for some maintenance

All parts had a good coating of general sticky dust and grime so I cleaned them with meths. The large rotating part that has pins that run in the tracks of the side frames, has a bit of side to side float, and was grinding on the side frames. That was blocking the mechanism from its full rotation and geometric lock. I used a trick that I developed many years ago when sorting retracts on jet models - I made washers from mylar hinge material! It is very slippery and hard wearing. These sit between the rotating block and the side frames, on the pins that run in the side frame tracks, the rotating block still has a little float but it can no longer contact the side frames, the thickness of the mylar keeps them apart and makes for a very free rubbing surface. I gave all rubbing surfaces a sprinkling of locksmith's graphite powder. Reassembled, the mechanism runs very smoothly from full up lock to full down lock, and when unlocked from the down position the spring snaps it up all the way to the up lock (without the wheel). Cannot ask for better than that! Photo shows the mylar washers between the rotating block and the side frames


The wheel has an aluminium hub running on a steel axle, with short bits of tightish fitting brass tube on the axle either side of the wheel to stop side float. The brass tubes were grinding into the wheel hub and could be pushed sideways anyway so were doing no good. I wanted to improve the bearing surface by drilling out the axle hole in the hub and installing an oilite bronze bushing but when I took the hub apart I found that there was insufficient diameter of material to enable me to drill it out, so it just has to stay as it is


I put it all back together but binned the bits of brass tube and pushed on some large tygon tube over the axle on either side of the wheel and cut them flush with the ends of the axle, thus holding the wheel in the centre of the axle and not cutting into the hub like the brass did if the wheel presses against the tygon


Now I have to make it all go up and down. The old servo that was in the fuselage (which turned out to be a 1/8 car steering servo from decades ago!) was fractionally larger than a standard servo and it occurred to me that a big servo was a good idea because of the big chunky gears, so I ordered a Savox jumbo, high-voltage servo. I should have read its specification and compared it to the servo that was in the model, for when it arrived I thought it was a house brick!! It's a whopper!! Oh well, it fits under the scale cockpit tub, has 40kg.cm of torque, big gears and a servo arm several times as thick as normal. I set a 1 second travel time for it in my Tx, and it operates the retract and doors without even noticing that they are there. Photo shows it with a standard size servo for comparison

A couple of thoughts on the wheel -

1. There is no suspension which surprises me having come from building and flying mostly jet models for the last 15 years. This model will be as heavy as some of my jets and the entire load on touchdown is on 1 wheel, not shared between 2. Any shock not absorbed by the tyre will be taken by the weakest point in the retract or the parts of the fuselage to which it is bolted. Nothing I can do about it except land as softly as possible every time! Just surprised by it.

2. The retract unit has a wheel brake. At this moment I am not planning to use it and so am not fitting a servo for it, but it can be done if required. I will only ever be flying the Nimbus at aerotow meetings, I don't envisage ever flying and landing it on hillsides. The model will weigh something like 13kg to 14kg. Will I regret not fitting a wheel brake?

Harry, I would get rid of that plastic control arm on the side of the retract if I was you. It will only give you grief in the future. Replace it with a good metal horn.
Brian.

That's a fair point Brian, I will see what I can do.

To paraphrase Police Chief Brody in the film Jaws, "I'm going to need a bigger workshop!"

Here's a taster of what is to come, this is Nimbus with the inner wing panels installed, the outer panels are almost the same length again. The fuselage is 2.3 metres long

Harry,
I saw a link on another forum to a diy retract with a springing option provided by RC car rear shock mount springs.
Whilst these may not cope with a model of this weight, the design concept may be of interest.
The design was done some years ago by a German engineer Ingo Seibert & published in a German magazine at the time.
http://www.rc-network.de/magazin/artike ... QZ0BqT0KYE

harry curzon wrote: ↑16 Jan 2020, 21:14 To paraphrase Police Chief Brody in the film Jaws, "I'm going to need a bigger workshop!"

Here's a taster of what is to come, this is Nimbus with the inner wing panels installed, the outer panels are almost the same length again. The fuselage is 2.3 metres long


20200116_134347.jpg

Harry , great work . May I suggest rigging of a big model like the Nimbus is done in the garden to save on "Hangar Rash" if the weather allows. Regards John.

Harry, I think you need to invest in a gym membership and get on the weights 3 times a week. You are going to need some muscle to launch that machine off the side of a hill.
Brian

B Sharp wrote: ↑17 Jan 2020, 11:07 Harry, I think you need to invest in a gym membership and get on the weights 3 times a week. You are going to need some muscle to launch that machine off the side of a hill.
Brian

This model is strictly for aerotow and thermalling, none of the slopes I frequent are suited to launching and landing this beast.

B Sharp wrote: ↑16 Jan 2020, 18:33 Harry, I would get rid of that plastic control arm on the side of the retract if I was you. It will only give you grief in the future. Replace it with a good metal horn.
Brian.

I had a hunt through my extensive parts store and found this aluminium arm, I think it was from the tailplane of an F-4 Phantom I had many years ago.


It needed a new hole for the grubscrew to match the location of the flat on the retract drive shaft, so I drilled and tapped it for M3. I prefer to use proper socket head bolts instead of grubscrews as the much larger socket or hex driver is less likely to get rounded if the bolt is tight.
The hole for the shaft was too large so I bushed it with a suitable size of brass tube and now it is a snug fit over the retract drive shaft.

It had some minor issues interfering with other things so I removed the clevis at the retract end of the pushrod and fitted a bolt through ball link which is probably better anyway, an M3 HT bolt being a bit stronger than a clevis's little pin

That looks the part Harry

Recycling at its finest for the modern world we now live in !

Opinions please -

Problem - I want to fit 2 x batteries of 5200mAh, and also an airspeed pitot tube sensor (I would like to have airspeed compensation for the vario). The best place for the batteries is right up in the nose but the plate that the towhook servo is mounted on leaves room for only one battery. I could fit two if I use the 2600mAh batteries, but the 5200s would be nicer. There is nowhere for the pitot tube, the hole in the nose being occupied by the towhook.

Possible solution - remove the towhook and servo plate, fit the pitot into the hole in the nose and fit 2 x 5200mAh batteries in the nose. For a towhook, do what I have successfully done on my 4 metre ASH 26 and 4 metre K-18, but suitably beefed up for this glider. Run a piano wire in a plastic tube along the bottom of the fuselage. Epoxy the tube down and when cured remove the piano wire and mill a slot from the outside of the fuz right through the tube. Re-insert the piano wire and hook it up to a servo. For this glider I was thinking of 2.5mm to 3mm piano wire, I could reinforce the fuz around the slot with a large patch of light carbon fibre cloth and then a disc of 1mm aluminium, say 2 inch diameter with the tube epoxied to that.

Your thoughts?

Real gliders have it in the fin.


Keith

Harry

How about leaving the nose release as is.
Fit the two 2600mAh batteries in the nose wired in series, put another 5200mAh (or 2 more 2600's) battery/s close behind them (it'll still be in front of the CG.
The TEC probe for the vario energy compensation 90% of the way up the fin, where the full size one is?
Bernie Jones does the small bore tubing for connecting to the vario or you could use compressed air U/C retract tubing.

John M

SP250 wrote: ↑24 Jan 2020, 20:31 Harry

How about leaving the nose release as is.
Fit the two 2600mAh batteries in the nose wired in series, put another 5200mAh (or 2 more 2600's) battery/s close behind them (it'll still be in front of the CG.
The TEC probe for the vario energy compensation 90% of the way up the fin, where the full size one is?
Bernie Jones does the small bore tubing for connecting to the vario or you could use compressed air U/C retract tubing.

John M

Do you mean two 2600mAh batteries in parallel?

Yes Peter I do - well spotted (its been a long week)!

No longer than last week John............................

Its all relative Robbie.................

You're a tough audience...... but you're right. I have left the original towhook and servo mount in place and am working on other ways to install the batteries and pitot. The batteries are more important to me than the pitot, as the 2600s will be in my K-18 and I want the 5200s in this Nimbus but for the moment I have gone ahead with installing the pitot in the fin. Options are very limited as near the top of the fin is a wall to wall thin glassfibre "floor" which acts as the elevator servo mount and the only access to the fin is the small space between that and the top of the fin. So, that's where the pitot has to fit and has to be pushed well out so that its two air tubes at the back do not collide with the elevator servo.


The distance from the pitot to the receiver is 1.9metres, so do I put the pressure sensor beside the pitot with short air tubes and a long electrical data wire to the Rx, or fit the sensor near the Rx and extend the air tubes all that way? Advice I received is to extend the air tubes which I did with Robart 1/16" i.d. air tube. In order to avoid any sharp change of direction and possible kinking of the air tubes I have routed them past the servo and then down the fin through a hole in the servo mount plate. To avoid the thin gf plate from chafing and cutting the tube I drill the hole larger than required and glue on a piece of 1/8" wood with the correct size of hole cut in it and countersunk - this acts like a grommet, protecting the tube. To keep the tube well controlled and away from the servo I glued a tube tidy to the side of the fin, it is meant for 3mm air tube and this tube is slightly loose in it so I glued on a piece of balsa over the top to keep the tubes in.



Then the real frustration started, as I could not get the pull through to turn from the fin into the fuselage and drop forward. it took me a long time to realise that there is another gf floor at the base of the fin. It has a tiny hole in it near the front for the elevator servo wire, how that was ever installed I do not know. The only access to this "floor" is from a small opening at the bottom rear of the fuselage underneath the fin post, so I had to drill a hole almost right at the back and again install a wooden "grommet". Then by taping the tubes to a length of thin piano wire I was able to route them from the top of the fin down into the fuz, and pull out the piano wire. At last I could attach a small weight and drop the tubes all the way down to the front of the fuselage to where the Rx will be. Plugging the tubes into the sensor and blowing gently on the pitot showed an airspeed registering on the transmitter so that system is now ready for final installation of the radio.

Back to the underside of the wings, iirc Paul had used poly-c varnish to attach the glass cloth which was fine when covered with vinyl but not good for painting. A trial with a thick coat of two-pack filler primer could not fill the weave of the cloth, but even the grain of the veneer was showing through the cloth into the primer! I put on a coat of finishing epoxy (from Fighteraces, best one of a few brands that I have tried using over many years) and after it was cured I wet sanded it down smooth. I spray on a dusting of cheap grey primer before sanding epoxy coats, then I can see what areas have and have not been sanded, and little spots of grey sitting down in the low points shows areas that still need sanding until all the grey has been removed.

After cleaning up the mess, I gave the wings a thorough wash with panel wipe (also known as pre-paint) and set them out for priming. I wipe over with a fresh tack cloth immediately before spraying. My first choice of paint is always Klass-Kote 2-pack epoxy, it is a paint that I get on well with and have been using since it was first imported into Britain about 15 years ago. My favourite primer is white with the "fast white" catalyst and I am applying it with a mini-HVLP gun with a 0.8mm needle and 2 bar pressure. A very thin coat so that it is minimum possible weight, not enough to be opaque but enough to even out the colour for the top coat.


The mix ratio is 1:1 volume which I measure by syringes. Many years ago I had terrible trouble with fish-eye contamination on a paint job, which I had to strip off and clean several times, I degreased everything over and over but the fish-eye kept occurring. Eventually I realised that the rubber plunger in syringes has a tiny bit of lubricant on it which I cleaned off with soapy water and the problem was cured. To make life easier I now use special syringes with no rubber and no lubricant and these are also much in use for injecting Hysol or other epoxies around fillets, or with extra nozzles to get all sorts of glues into tight spots etc.


One of the primed wing panels

While the primer on the wing cures, back to the fuselage and I can start work on the cockpit and the radio installation.

This may be sacrilege to some of you, but I am just not into scale cockpits and dolls, especially when the dolls cost a 3 figure sum of money. My aim is that the model looks real if someone takes a photo of it in flight, so it needs someone in the cockpit, but going beyond that does not excite me enough to justify much work or cost. I appreciate the beautiful work and lots of details that some people add, it just doesn't fire me up enough for me to put the effort in.

The model came with a cockpit tub, two instrument binnacles, and two seat backs. To that I will add just one pilot, in the front seat, and I have got a head and body kit from "scale-me-down". Scale-me-down attend some of the model shows where they take a laser scan of your head and can then 3D print you at any scale required, I already have mini-me in my Pilatus B4 and ASK-18, so I have bought another mini-me for the Nimbus.

The cockpit tub is a single gf moulding, covering the entire length of the canopy opening. It has been painted with stone effect paint, I might try stripping that off. I cannot find any decent on-line photos of full-size Nimbus cockpits and am not sure that they would have a separate inner skin as implied by this tub - why add weight for cosmetic reasons? The fuselage has 4 mounting flanges, each with a nut on the underside, but the tub has no corresponding holes for bolts. Before I go chopping up the tub I thought it would be a good idea to get the locations for the bolt holes in the tub. I put a bolt through the nut in each flange but from the underneath, protruding a couple of mm above the flange, put a drop of white paint on the tip of the bolt, then put the tub down into position so the paint transferred to the tub and marked the required location for the holes, which I then drilled.

The tub blocks all possible access to the radio, switches etc., plus it leaves too thin a space between it and the fuselage to fit some of the radio and plugs. My plan therefore is to separate it into front and rear sections with a few inches of what is basically the back seater's foot well removed altogether. I will have to remove the pilot, front instrument binnacle and front tub to get the front battery in and out, but during a flying day I will have access to the switches in that gap in the middle and there is no depth limit for fitting some of the radio equipment in the gap.


The tub in place


Bolts installed from underneath to mark the tub


the underneath of the tub with the 4 white paint dots showing location for drilling the holes

I cut out this section of the cockpit tub and then put a coat of a mild paint remover on the stone effect paint on the inside, but that only partly removed it so I applied another coat and left it overnight.

I can get one of the 5200mAh batteries plus a little mount for it, sideways in the nose where the front seater's feet would be, immediately in front of the tub and the nosewheel cover. I made the mount from 1/8" ply with velcro straps epoxied to the underside and a piece of non-slip mat cyanoed on the top, then the assembly was glued to the fuz with Hysol.

I 3D printed mounts for the satellite receivers, drawing obtained from thingiverse website.

To locate the ends of aerials, I cyano a bit of plastic tube to a section of wooden coffee stirrer (free at your fav coffee shop!!) and that bit of wood is then easy to stick to any material, I find a dab of Zap-Goo is excellent for a quick stick to an uneven surface such as the inside of a glassfibre fuz

I have now completed the fuselage radio installation and wiring, only the wiring in the wings remains which will be done after painting.
I need extension leads between the Jeti distribution box (a Central Box 200) and the two batteries. Jeti CBs use Mpx sockets. When I make leads with Mpx plugs I add on the plastic insulation grip, made with a hot melt glue gun and a Winkler mould (available for several plug types, from esoaringadgets and others).


Each wing has 4 servos so I would like to connect them using a 12 pin Ashlok but the hole in the fuselage is a reinforced square not long enough for a 12 pin plug to pass through and I do not want to enlarge it so I am using 2 x 6 pin Ashloks on each side. I fit them as 1 male and 1 female so that when connecting the wing there is no doubt that the correct pairs are connected with no need to mark which is which.

I have applied the colour coats to the underside of the wing and am not happy with the result, but will just learn to live with it.

I chose to put indigo blue on the inner panels and fluorescent orange on the outer panels. It will not win any awards for aesthetics but all I am concerned about is visibility of the relatively very narrow chord wing at the distances that I hope to be able to use this large model. I would not have predicted that fluoro orange would work on the unlit underside of the wing, but I tried it on my 2.6 metre Mpx Cularis and it does work, showing up at the model's max distance in any conditions.

Applying the colour coat shortly after the primer was not an option so I left it for a few days to thoroughly cure then scrubbed it with a 3M grey pad to break the skin and key the surface. Paints used are Klass Kote indigo blue with gloss catalyst applied as per the primer with mini-HVLP gun 0.8mm needle at 2 bar, and an aerosol can of fluoro orange.
The problem is a very large number of pinholes which are extremely visible due to the contrast of white primer and dark blue top coat. Had I used grey primer or white top coat they would be pretty invisible. The coat of epoxy that I applied just could not cope with the huge number of deep holes through the cloth into the pores of the veneer that the acrylic lacquer leaves. If I had used a grey primer I would have seen them and dealt with them before applying the colour coat, but they just don't show up against a thin translucent coat of white primer.

The ailerons and flaps which I glassed from bare wood have no pinholes as the epoxy resin fills everything first time round so they look perfect.

It would not be unusual to get 2 or 3 pinholes if I had glassed the wing from scratch, so in hindsight it would always have been a better decision to use a grey primer on the panels that I intended to paint dark blue, as the grey is much better at showing up problems to be dealt with before the colour coat, and any that are missed are not so glaringly obvious due to less colour contrast. Live and learn!

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.

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

The new seats

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

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.

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.


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.

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.

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.

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




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.

The last job of painting is completed, the red nose

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.

Well done Harry. She looks very nice indeed.

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?

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.

Thanks for that info John, it confirms what I suspected so it will keep my fingers busy during launch!

The last cosmetic job is done and the Nimbus now awaits an opportunity to fly. The pilot is mini-me, by "scale-me-down" who go to some of the model shows, take a laser scan of your head, and can then 3D print you to whatever scale you want. I have shaved off my beard now so I will have to get them to re-scan me at some time. Shirt is a kit by "scale-me-down", stitched together by my wife, and then filled by me with stuffing from one of our dog's toys! Harnesses by aerocockpit. Yes I know there should be a parachute harness too!

That's it all done, nothing more to report until the maiden flight.