Carl Goldberg Piper Cub

[Pete B]

I splashed out on Ebay a short time ago and ended up with three traditional kits which should keep me fully occupied over this winter - and probably the next one as well!

 

Together with a DB Cirrus Moth and a Porterfield Collegiate, this CG Cub turned up at my door and, after some ruminating, I've decided to start the Cub first.

 

The kit seems to have had mixed reactions on the net when compared with other manufacturers' Cubs but most builders suggest it flies well and is a good model, so let's see what happens.....

 

It's a standard J3 Cub, 76.5" span, making it about 1/5th scale, with a clipped-wing option, which many people prefer as the end result is more aerobatic. Having a Decathlon already, I wasn't looking for an aerobat so I've gone for the full-span build.

 

Here's the box - and some of the bits inside:

Now, being an all-leccy flier, my mantra is 'add lighness', so it came as a bit of a shock to find that, wings and tail excepted, the whole model is built up from 1/8th ply

 

Perhaps I should have done a bit more research before bidding.....but the general consensus is that it flies well despite the weight, whioch should be in the region of 7 - 7.5lb.

 

I'm planning to power it with an Emax BL4020 520Kv motor on a 5S Li-Po. Whether that is overkill on this model I'm not sure but if I can get a comfortable 80-90W/lb, that will be plenty for this model.

 

I'm not going to document every stage of the build, but put up a few pictures and comment on changes, observations etc, as I go.

 

I started with the wings and they are pretty conventional in construction. The leading edge sheeting provided was rather too hard for my liking so I substituted some rather lighter stock which coped with the curve much better:

 

This kit departs from scale in that the scale 'barn door' ailerons have been replaced with strip ailerons, and I couldn't cope with watching it fly like that, so a bit of extra work on each wing has resulted in a more scale appearance. The area is the same as for the strip ailerons so I'm hoping this will work OK:

 

Once again, the T/E stock for the ailerons was quite dense, so a built-up construction should save a few grams:

Right, more later, Spooks is just starting.......

Pete

 

 

 

 

[Terry Walters]

Yep - read it all with interest Pete but Spooks is beckoning!

 

Well done so far!

 

Terry

[Ben B]

Very nice. I have a soft spot for Carl Goldberg planes after building an Electra and a Gentle Lady back in the day. The GL wing was initially built to replace the Electra wing which I inevitably crashed into a thosand pieces whilst learning....

[Pete B]

Ah, Spooks isn't the same without Ros.........

Back to the Cub......next step was the tailplane and fin/rudder. I've forgotten to mention that the laser cutting on this model is a revelation (not having had the pleasure of a laser-cut kit before) compared to the old die-cut kits. This must have been one of the later versions of the kit and it's a joy to just have to nick the joints and push the parts out

Good quality, clean-cut ply and balsa does make assembly a lot easier. The instructions suggest using CA to bond just about everything except the wing joiners and F1 but I'm not happy with CA and ply, so I'm using aliphatic on those joints. Balsa joints are a mix of ali with CA on the non-stressed areas.

Laying out the tailplane pieces, it became clear that all was not well. The cut pieces were too big for the plan.....

 

My immediate thought was that I'd have to trim the parts to fit - but then commonsense kicked in. Given that the cut materials came from a carefully-measured source, it could only be the plan that was either printed under-size or it had shrunk, so with a bit of shuffling and evening-up, all was glued to produce a fair representation of a Cub tail assembly:

 

Conventional nylon pinned hinges have been used on the rudder as I'm planning to use a closed-loop system and they'll cope with any tailwheel shocks. The elevators have been fitted with hinge points, which I also plan to use on the ailerons.

 

CG include the ply parts to make up some very useful little sanding tools which have made a very nice neat job of the control surface bevels and neatly rounding-off the edges (I'll upload a piccie of the tools if anyone is interested).

 

Right, on to the fuselage. Now, one of the unfavourable comparisons made between this and other Cub kits was that this one doesn't have an opening door/window on the starboard side. I've been thinking through, and researching, a way of incorporating this feature but have come to the conclusion that the construction method doesn't lend itself to doing it. Other kits, such as the Sig, have the centre section of the wing as an integral part of the fuselage and hinging the window is relatively easy. The CG version, as you can see, has the wing seated on top of the fus and any attempt to cut away the top frame would compromise the strength of the assembly, so I've reluctantly given up that idea:

Going back to the plan sizing, you see here just how far out it is. With the fuselage side placed accurately at the F1 former, the plan is 23mm under length at the tail!:

That means the plan is effectively only a working guide, really () so I'm relying on the accuracy of the cutting, not that I have any concerns so far.

 

The cabin framing having been glued, the ply doublers have now been added and it's drying overnight:

 

That's it for this evening. I've still to decide on battery access, probably through a hatch in the floor behind Fl - but that can be sorted out tomorrow.....

 

To be continued.....

 

Pete

 

[Wiltshire Flyer]

Looking good! i love the Cub's chunky looks always have, i have a soft spot for the Auster as well.

[Pete B]

Ok, I've made a fair bit of progress with the Cub and it's been a bit of an eye-opener in some respects. With so much of it being lite-ply there are a few differences in building technique.

The motor mount, made from 4mm threaded rod, was pre-fitted to the bulkhead. The mount offset looks quite extreme but, now all is in place, it has worked out to be OK, with about 3mm right thrust and, being set above the thrust line, a couple of degrees of downthrust too:

Being mainly of lite-ply construction, and laser cut, assembling the fuselage consists of slotting all the bits together and holding them with elastic bands, tape and clamps to maintain symmetry and contact:

 

I'd planned to use aliphatic for most of the construction but I could not work out a way of applying the adhesive to all the surfaces, then locking them all together, without getting into a heck of a mess.

 

Reluctantly, therefore, I used thin CA on most of the tightly-closed joints and aliphatic where it was practical. It all seems to be holding together at present......

 

Before assembling the fuselage, some thought was given to thrust lines, etc. The motor bulkhead is shown as square on the plan but to assist with sidethrust, I set the right-hand side about 4mm aft of the designed position on the fuselage side.

 

Sometimes you just have to use what tools you have in the garage. The doubled ply took some persuading, even after dampening the outer faces!:

 

Once everything was dry, I turned to the issue of battery placement and established that a hatch below the nose would allow access for a 4S and, if necessary, a 5S pack. A hatch was marked out:

Duly cut out, the access hatch allowed supports for the battery tray to be fixed within:

 

 

Fearful of the page size limit, I'll continue in the next post..........

 

Pete

[Tom T]

Very nice! Looks very well built so far! I've gotta agree Spooks is not the same, they need to bring Malcolm back! Next weeks looks good, i wonder what'll happen to go old Harry.

[Pete B]

To continue with the fuselage, it seems the undercarriage fitting is considered a weak point. My landings, and runway, don't allow for weakness in that respect.........

 

With the u/c wires being held with #2 x 3/8 sheet metal screws (about 3mm dia in sensible terms), I'm not surprised, so, making use of the supplied engine mounting bolts which I didn't need, I epoxied the blind nuts to the inside face and the u/c straps are now secured with #6 x 1" socket head bolts (about 3.3mm x 25mm in metric terms):

 

Ready to receive the socket head bolts, it all looks a bit more serviceable. The battery tray is approaching completion:

Turning to the cockpit area, the rudder and elevator servos have been fitted towards the front end of the bay. Rather than using the prescribed pushrods, the elevator is controlled using a snake, with a closed-loop system for the rudder.

 

With the rudder cables on a separate arm, I feel there is less strain on the servo bearings and fewer direct shock loads, as well as allowing the servo to be located forward of the C of G. I have left the gap to allow a pilot to be fitted in the notional rear seat. Given the size and AUW of the model, I've used Towerpro SG5010 servos for these controls:

 

On to the cowling now. With the prop and airflow cutouts made in the cowling (but not yet having sorted out the dummy engine cylinders), the motor was fitted and the cowling offered up.

 

In this case, I've taped a piece of plastic to each side of the fuselage, making pilot holes for the cowl mounting screws, ensuring they drive into a doubled part of the fuselage side.The plastic stips are folded back, the cowl slid into place and the plastic slips are folded back over the cowling, to give an accurate location for the screw hole in the cowling:

One retaining screw fitted (I will fit rubber grommets on the finished product to minimise splitting):

Cockpit glazing being trimmed to size:

 

Moving to the back end, I am using a Dubro Semi-Scale Tailwheel rather than the supplied rudimentary fitting. I've inset 1/64" ply either side of the rudder leading edge to beef up the fitting, with a hinge pin now located below the control horn assemby. An M2 nut and bolt will secure this after covering. The cut-out above is for the dowel elevator joiner:

 

That's enough for this post, I think...........

 

Pete

 

[Pete B]

Thanks Tom! Well-built, I'll say - you could knock fence posts in with this fuselage.........

 

I fear things will not go well for Harry in the final two episodes.......

Pete

Edited By Pete B on 21/10/2011 23:44:07

[Pete B]

Returning to the wings, the scale barndoor, rather than strip, ailerons meant that individual servos was the way to go. A sub-frame was built between two ribs:

 

DYS0207 servos are entirely adequate for the ailerons and they were screwed to beech blocks mounted on a cover plate made from a laminate of 1/16" balsa and 1/64" ply. The misaligned screws have now been rectified :

 

The servo mount fitted:

Time has come to join the wings. Rather than having a dihedral brace extending well into each wing, this system relies on bonding a number of ply formers to each other and the basswood mainspars. I used 30min epoxy and, after sheeting the upper wing, a 2" glass bandage was epoxied around the root:

There do not appear to have been any reports of wing-folding on this model, so I'm accepting that it works.........

 

It's always nice when things start coming together so here she is today, looking very much like a Cub:

Having despatched my fast-running coolie, Terry Walters, to the UK for supplies a couple of weeks ago, I'm pleased to have heard from him this morning that he has completed his mission, coming back laden with Solartex and runny stuff etc.

 

I'll be making some more progress soon!

 

Pete

 

[Pete B]

Just a short update on the Cub.

 

It's often the case that the most time-consuming components of the build can be the least significant, in terms of apparent progress, and this was the case with the engine parts.

The kit is provided only with a port-side ABS dummy engine assembly as it was intended that the cylinder head of the IC engine would take the place of the starboard side. With a leccy motor, I needed to make up a matching pair.

Looking at lots of walk-round pictures, I came up with an idea which, at binocular scale, doesn't look too bad, IMHO. Not having the fine skills of many of my fellow forumites, I went about it like this.....

Two sets of cylinder head fins were cut from 1.5mm balsa sandwich, the nearer of each set being made a little smaller:

These were duly glued together:

Ignition covers were carved from balsa.......

.....and exhaust pipes made from beech dowel:

The cowling had openings cut and balsa inlaid behind. Cylinder head cowlings were fabricated from litho:

The assemblies were painted black and await installation:

Pinned in place for now......

.....and a view of the pair:

I reckon they'll look fine from a few feet away.........

 

Thanks to my erstwhile courier, Terry, I've been able to get on with a bit of painting and a lot of covering but she's still a little way from completion and I need to take progress photos, so that's all for now, folks...........

 

Pete

 

 

[Tony Bennett]

nice cub.

[Andy Shailer]

Looking good Pete, how the cub doing.

Andy

[Pete B]

Cheers, Andy. I've had a forced separation from it whilst I was doing my duty trip around the UK for the past couple of weeks but I'm back in deepest France now and I hope to be making some progress this week.

 

As it stands, all the main components are covered and ailerons fitted and working. I was a little disappointed that the Cub Yellow Solartex didn't give quite the depth of colour I was expecting and the fuselage in particular is more translucent than I would have like.

 

I went to B & Q and tried out their new Valspar paint bar to get a couple of matchpots for the Solartex. You get a matt finish emulsion (at £1.98 a tin of 214ml) so I thought a couple of tins would do.

 

Unfortunately, when I got back, a test paint of some Solartex showed that the match wasn't that good, with the paint shade being too pale, so that idea has been shelved and I'll settle for the Solartex finish. To be fair, the guy said that the colour was so near their base colour that matching may be difficult.

 

At present, I'm torn between spending a lot of time making a more scale u/c or just going with the kit version, as I'm keen to get it off the board and get the Black Magic started for the Vintage Build. I'm tending towards just getting it done, TBH.

 

I have some thoughts about using some 'O' ring neoprene material to simulate the suspension legs, rather than go down the fully working, piano wire route. I'll see how it goes.

 

Blimey, I have gone on a bit - but you did ask!!!

I'll get some piccies up in the week.

Pete

 

 

 

[Pete B]

Time to update my progress on the Cub. Since mid-November I've pressed on and the finishing-post is now in sight!

 

Prior to covering, the cockpit area was primed and painted with Solarlac Dark Yellow, which is the matching paint (allegedly!) for Solartex Cub Yellow. Pity it's a shade darker.........

 

Loads of room in the cockpit so the AR6200 Rx is fitted towards the front with an HK extension lead to the satellite, mounted with plenty of separation at the rear of the cockpit. The upper servo has a pushrod leading to the closed-loop pivot and the lower, the elevator operated by a snake:

 

Further view of the front end of the cockpit with extension leads permanently connected to the aileron channels of the Rx:

At the rear, the closed-loop system can be seen, as well as the Multiplex plug for the aileron connections - much easier than fiddling with separate servo plugs each time:

The battery bay, capable of holding a 5S 4000mAh Li-Po, is ready:

 

 

At the tail, the closed-loop rudder system and elevator have been installed. The tailwheel is the Du-Bro product:

 

 

When it came to the pilot, I've made use of an Action Man I picked up at a car boot sale,or vide-grenier, as they're known here. For under 50p, he was a good deal cheaper than the commercial product! Unfortunately, AM's are quite heavy as supplied, so I'm afraid I had to resort to surgery/butchery, involved amputation at the waist, a very big drill making holes all over him and slicing away most of his muscly arms. No photos - can't be upsetting the children, can we?

 

At 1/6th scale for a 1/5th scale model, I've had to call him Tom Crew............

Back to the modelling in the next post...........

 

Pete

 

 

[Pete B]

My original plan involved using an Emax 2832/05, which is variously described, it seems, as either 805 or 960Kv, depending where you buy it!

 

On 4S, propping it for about 800W was pulling the best part of 60A, which was more than I had hoped for. I therefore retrieved the 700Kv 4250 from my Beaver, which is currently, err, indisposed........ Honestly, these two motors have been on and off this Cub more times than you could imagine...........

 

I didn't want to go up to an 80A ESC, so went for a 5S pack, which helps with the C of G.

 

This gave a much better set of figures. On a 12 x 6 prop, I'm now getting 720W for 39A, which I'm quite happy with.

 

Another issue I've pondered is the undercarriage and, whilst a scale-type assembly was my original intention, I don't believe the basic model is close enough to scale to justify the effort, so, for now, I've gone with the rather simple kit version. I might change my mind after it's flown - if I'm that impressed, I might just go that extra yard!:

 

Having covered and decorated the model (I found a basic Cub scheme on a UK registration on Airliners.net) one of the last jobs has been to fit the struts and that has kept me busy this evening. As you can see, the wheels are far from scale. The Hangar 9 1/5th scale wheels are now difficult to source in the UK and, nice though the Du-Bro wheels are, almost £30 a pair is too much - for now at least!:

 

That's it for tonight. I'll get some more piccies of the completed model up in the next few days - and then pick a suitable time for the maiden!

 

Pete

[Steve W-O]

I have one in the loft (It's in the loft as I don't have the plans to make new parts) it doesn't seem that heavy.

 

I'm told by the person I got it from that you should not let the speed drop to much on landing, as it drops a wing quite viciously, which is the reason I have it

 

As I have been unable to repair it, I don't know if that is correct, but it may be worth checking.

[Terry Walters]

Looking really nice Pete! Home straight now. I'm still approaching the final turn with the DB Tiger - want to spray fuz and fittings but still too cold n wet and I'm not allowed to do it indoors!

 

You bringing it with you on Thursday?

 

If you get your video camera back before the maiden - I'll record it for you!

 

 

Terry

[Pete B]

Just hope the forum holds together long enough to make a post this evening..........

Finishing touches put to the Cub yesterday. AUW 3260g or 7lb 4oz. Using a Tipple 5S 4000mAh Li-Po, the C of G, 100mm from l/edge +-10mm was easily achieved by setting the pack partway through the bulkhead into the motor bay, therefore no additional noseweight was needed.

 

I like not adding weight..........

I set the C of G at about 90mm to be on the safe side. As the control throws listed are for the strip ailerons, I looked up the recommendations for the Sig 1/5 version which has the scale inset ailerons and I set them at 18mm up, 12mm down, with 25mm each way on the elevator and 38mm on the rudder. I reduced all these to 70% on the rate switches (with 40% expo all round) as I like a soft response on takeoff - particularly on maidens. Porpoising a/c don't do the heart-rate any good whatsoever..........

Nothing left but to kick the tyres and light the fires () - and I woke up this morning to thick fog.......

Forecast was improving so it was off to Terry W's for a nice lunch, a drool over his DB Tiger Moth (soon to maiden....) and BT Hurricane plans, then off to his club site in much brighter weather.

 

She looked very good on the tarmac whilst the pre-flight checks were carried out:

 

Opening the throttle she set off, tail lifting very quickly (I'd read that the secret to keeping a Cub straight is to resist the temptation to hold the tail down with up elevator) and she tracked straight down the runway with just a dab of right rudder. A touch of up after 20 yards and she unstuck, climbing away strongly on little more than half-throttle. This is where it started to get exciting. I reduced the elevator and she immediately headed for the ground.............

 

A chorus of oohs and unmuffled screams from the small gallery brought an immediate reaction from me in the form of rather more than a a tad of up elevator and she resumed her climb out.

 

"Just joshing....." I murmured to the gallery - but began to wind in as much up-trim as I could find. That, together with bringing it back to a cruise speed and by holding in a bit of up elevator, enabled me to complete a circuit or two whilst I pondered a landing......

 

Round onto finals, cut the motor and she held off for much longer than I expected but made a very safe wheeler landing, running to the end of the strip and onto the grass. Phew!

 

Just before it became exciting. A tad of right rudder to hold her against the torque:

 

Back in the pits, I wound about 6mm of thread off the elevator clevis, reset the trim to zero and moved the Li-Po back about 15mm, balancing on the mark at 100mm.

 

Plenty of juice in the battery, so with a cry of "Once more unto the breach, dear friends........" it was off for the second flight.........

 

(Just learnt my post is '725 characters too long' which is a distinct improvement on losing it all, so more to come soon.........

 

Pete

 

[Pete B]

Well, that's a real improvemt in the forum software - at least we know when to split a post now!

 

This time, she tracked true once again and, with little more than half-throttle, she eased off the runway in about 25 yards and climbed out 'as if she was on rails', as all good reviewers say..........

 

I didn't feel the need for any trim corrections at all and had about 7 minutes of circuits, passes and loops. I took her up high and try as I might, couldn't get her to stall - rather she just mushed and turned away from the wind - so I suspect there's a bit more available with the C of G position.

Looking good on a low pass in the wintry sunshine:

Once again, I set her up on a long approach and cut the power over the threshold at about 10 feet. Despite the fresh breeze, she justed floated along - and along - and along, finally touching down smoothly some yards past me, tracking dead straight and again running off the end of the strip. She has certainly got some momentum..........

 

Well, the glow of satisfaction after a successfulmaiden flight is there again this evening - always more so with a kit-built rather than an ARTF, I feel.

 

Conclusions? She flies beautifully and the C4250 on a 12 x 8 with 5S provides bags of
power. Much of the flight was between idle and a little more than half-throttle - even for looping. Despite her weight, she is a pleasure to fly and a bit more relaxing than my Decathlon!

 

Whilst I enjoy flying my smaller, lighter models, there is a noticeable smoothness once you get over about 5lbs in the air, that's for sure.

Next time, I'll explore her speed range a little more and play with the C of G. I'm now thinking a little more work to make the u/c more scale-like and perhaps an investment in pukka Cub wheels will be well worth while...............

 

She's great!

Ta for the pics, Terry....

Pete

 

 

Edited By Pete B on 22/12/2011 22:40:27

[Terry Walters]

Posted by Pete B on 22/12/2011 22:37:34:

Ta for the pics, Terry....

Pete

 

No probs Pete - My pleasure!

 

It was great that the weather just gave you the chance you needed! You have done a great job with the building and mods. My heart has just about calmed down after that exciting first take off!! Until I read your blog I thought that you were bravely displaying your barnstormer flying skills (or it was the effect of the sherry and wine at lunchtime?) to impress us but now I know it was just a lack of elevator authority!

 

Onwards and upwards!

 

Terry

[Barrie Dav 2]

Well done Pete. Lovely job. I like the quality and quantity of the photos. They should be of help to anyone building a Cub. I like the rug too!

 

Barrie

[ken anderson.]

well done pete...nice models by goldberg----- i had a ultimate bi plane.....was also bee's knee's.....now i'm told goldberg have ceased trading......

 

ken anderson ne..1.. ultimate dept...

[Steve W-O]

Very nice.

 

Noted your comment that you could not get it to stall, that makes me a little more enthusiastic to try and repair mine, as the previous owner told me that it dropped a wing easily, and that was how the hedge hit the plane!

 

My Mentor used to land like that, even with instructors flying it, but now I have discovered that it can be brought in much more slowly, it doesn't float, and the landing roll is short.

 

On the Cub I have, I did notice that the ends of the torque rods at the end that attaches to the servo, were angled backwards, which would give differential in the wrong direction, ie more down than up, I wonder if that was a building error, and the reason for the tip stalling?

[Myron Beaumont]

Steve

The worst possible scenario for a Cub unless you fly inverted all the time ? (On second thoughts maybe not !)