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Nimbus 4D - 7 metre

Posted: 14 Jan 2020, 20:01
by harry curzon
This will chronicle my build/restoration of a Nimbus 4D, 7.06 metre span, scale 1:3.75 from a kit by
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.

Re: Nimbus 4D - 7 metre

Posted: 14 Jan 2020, 20:58
by harry curzon
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 -

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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.

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Re: Nimbus 4D - 7 metre

Posted: 15 Jan 2020, 10:53
by harry curzon
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.
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Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 15:16
by harry curzon
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
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The loose hinge pin at the base of the fin/rudder
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My replacement plate for hingeing the base of the rudder, ready to be glued into place
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Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 16:26
by harry curzon
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?

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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.

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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

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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.

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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

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It all runs smoothly now with a well secured mechanism and a servo

Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 17:30
by harry curzon
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
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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

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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

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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

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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 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

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Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 17:48
by harry curzon
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?

Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 18:33
by B Sharp
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. :)

Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 20:50
by harry curzon
That's a fair point Brian, I will see what I can do.

Re: Nimbus 4D - 7 metre

Posted: 16 Jan 2020, 21:14
by harry curzon
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

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