A return to model building and flying. Part 41

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July 2007

Three Line Bellcranks

The model I built for my former partner to learn to fly CL, for which incidentally, it was very successful, was beginning to look a bit tatty around the nose area. I had also always been annoyed that the throttle on the Roberts handle worked the wrong way round, i.e. push the open the throttle. There was also the fact that I am fed up with the Solarfilm covering. The last time it was flown I was witness to the bizarre slackening of the film when the sun came out and it immediately tautening again when a cloud obscured it. So as it has rained and blown consistently for over a month at the time of writing, and parts of England been flooded under considerable depths of water, sometimes repeatedly, I decided it might be a good a time as any to attempt a bit of destructive modification.

Removing the nose which looked like it was falling appart was not as easy as I thought, Epoxy is a very strong adhesive, and despite it's appearance the nose was quite soundl, but crushed and worn around the engine mounting holes caused by many changes of engine. Digging into the sheet covering the bellcrank was not quite so arduous. I had originally intended to just invert the bellcrank assembly to reverse the cockeyed setup, but like all things that seem simple at first sight, this tuned out to be several days of frustration and head scratching, which at time had me wanting to junk the whole thing and build another model. But patience prevailed in the end, I want to keep this model in the air as it was built as a trainer and is very fit for that purpose. It would also be very easy to add a hook and use it as a carrier trainer, so it's worth persevering with.

For those unfamiliar with the Roberts Bellcrank, it's one irritating feature (for me) is the very close proximity to each other of the various components, chief of which is the vertical clearance between the bellcrank and the throttle actuating arm. Unless very careful attention is payed to the how the push rods are attached, and the leadout attachment, things can easily foul each other at extreme throws. This can manifest itself in various ways, if you are lucky, on the ground, with the elevator suddenly locking at one extreme position or the throttle not achieving it's full range of movement. Once the bellcrank is installed and covered in, this is a royal pain in the rear to attempt to correct, as it means digging into the fabric of the airframe.

However I arranged the bellcrank assembly, there was always something a bit dodgy about the clearances, which left me with the distinct impression that as soon as everthing was finished and covered something would bind, a situation I am very familiar with and something I want to avoid like the plague.

Feeling like I was being backed into a corner, I started to look for alternatives. So I pulled up a file I have of scans of the Mick Reeves Bellcranks taken from a 1976 Aeromodeller Annual. It never ceases to surprise me that I can still learn things from these books, as old as they are. The action of the various designs and how they work is not always obvious, or maybe I am just getting senile, but one that I have looked at over the years and could never see the point of, suddeny became very attractive. To describe this assembly in words is not easy, as is trying to interpret the description accompanying the diagram. Suffice to say the centre of the connecting bar always moves across the fuselage without any forward or rearward movement of the elevator bellcrank pivot relative to the fuselage, thus eliminating the throttle elevator interaction I get on my simple two bellcrank existing setups.

The big plus is that the connecting bar can be as long as is required, I could have one bellcrank on one side of the fuselage and the other on the opposite side, getting round a problem that had caused frustration in the past; that of having two holes on the same side of the fuselage accessible to a lot of engine muck, or having some sort of translator to carry movement throttle from one side of the fuselage to the other. Now I can reduce vulnerable holes to one.

Once I had grasped the basic principle of how it all worked, I found that the bellcranks and levers could be arranged in a surprising number of different ways to fit into the available space. Because I was building it myself I could adjust the various clearances at will using nuts or washers as spacers. It still requires care, but is a much more flexible system than the Roberts.

reeves bellcrank
My Reeves bellcrank assembly

In the above picture the elevator crank is on the left, throttle on right. The only points that are fixed to the plywood plate I have tinted red, this may help in understanding how it all works.

The end result is an eerily smooth action, which I suppose illustrates the point that if you want anything to work properly, there is no substitute for making it yourself.

There now follows a before, during, and after sequence.

Tutor original
In it's original pristine state my own design 'Tutor'
trainer, with Roberts system and O.S.20

Tutor with reeves crank
All the worn stuff removed,
and how my home made bellcrank assembly fits, relative to the fuselage

tutor modded
The rebuilt finished model with shortened fuselage.


It's strange how my mind works, but as soon as wrote the title word I went all literary and had the words of Robert Burns and one of my favourite Jethro Tull songs jumped into my head. Both seem poignant

For anyone with a native language other than Scots, don't be surprised if you can't make sense of the Burns quote, it would sound very clumsy and awkward translated into English, loosing all it's grace.

Wee, sleekit, cowrin, tim'rous beastie,
O, what a panic's in thy breastie!
Thou need na start awa sae hasty
Wi bickering brattle!
I wad be laith to rin an' chase thee,
Wi' murdering pattle.

(Robert Burns, 'To a Mouse')


From early days of infancy, through trembling years
of youth, long murky middle-age and final hours
long in the tooth, he is the hundred names of terror ---
creature you love the least. Picture his name before
you and exorcise the beast.

He roved up and down through history --- spectre
with tales to tell. In the darkness when the
campfire's dead --- to each his private hell. If you look
behind your shoulder as you feel his eyes to feast, you
can witness now the everchanging nature of the beast.


If you wear a warmer sporran, you can keep the foe at
bay. You can pop those pills and visit some
psychiatrist who'll say --- There's nothing I can do
for you, everywhere's a danger zone. I'd love to help
get rid of it, but I've got one of my own.

There's a beast upon my shoulder and a fiend upon
my back. Feel his burning breath a heaving, smoke
oozing from his stack. And he moves beneath the
covers or he lies below the bed. He's the beast upon
your shoulder. He's the price upon your head. He's
the lonely fear of dying, and for some, of living too.
He's your private nightmare pricking. He'd just love
to turn the screw. So stand as one defiant --- yes, and
let your voices swell. Stare that beastie in the face
and really give him hell.

(Jethro Tull, 'Beastie')

Told you before; you can't come here and leave without learning something.

Flying the Beastie was a surprisingly pleasant experience, I had shortened the fuselage and nose slightly, more for aesthetic appearance sake than anything else, and also strengthend up the undercarriage, they do take a pounding on uneven grass/ground when doing repeated touch and goes, and other unfriendly contact when trying something unusual. Judging from past experience I was not expecting wonders with it's flight characteristics; but it all worked.

The only thing that didn't work quite as I would like was the Thunder Tiger.15 GP throttle. This model started life with an old OS.20 Max, followed by an Irvine.20, then an Irvine.25, so it is quite versatile.

I can nearly always get my own designs to work well, but never seem to get the same results with other peoples. One of life's great mysteries I suppose. Or could it be that I actually know what I am doing sometimes?

The TT.15 is a remarkably powerful for a cooking plain bearing engine, when it's propped and choked properly; it also comes with a very effective baffled silencer; unusual on a .15. It's only Achilles Heel is the throttle. It's well made, with a nice raked backwards needle assembly to keep my fingers out of the prop, but a really naff old fashioned concept (use by a lot of manufactures) of just using an air bleed hole to control the tickover. For my purposes this is just not good enough. It produces a throttle response that is a bit like an on-off switch, and makes the tickover a bit uncertain with the engine slowly flooding and eventually cutting out no matter how the settings are juggled about.

sc carb tt engineThe SC/ASP/Magnum carbs (I bet they are all made at the same Shenzhen Sanye plant) are all twin needle carbs; that I know work well. So with a bit of work on the spigot of an SC.15 carb, it fitted the TT.15. The only thing I don't like about this is the fact that the needle is a conventional strait affair, and quite long, which brings it a bit too close to the prop for comfort on small engines, and also leaves it vulnerable to accident damage on a side mounted engine because of it's length. Picture right is the TT with SC carb fitted, illustrating the problem

This design produced a very peculiar problem the first time I tried to run the engine. I could not understand why the engine would only run on a prime unless the needle was opened out a long way, and every time I checked the needle setting it was closed tight; until the penny finally dropped that the close proximity of the needle to the prop, meant that every time I flicked I was inadvertently caching the needle and closing it. Unfortunately a rear needle assembly for these engines is not available until over the .20 size. Looks like another home made project is looming.

After the flick starting problem was identified, a bit of flight testing and tweaking the carb settings had everything humming as it should, and I could enjoy myself. I cant prop hang it (yet), it was never designed for that purpose, the elevator is too small for a start, but I am sure with a few modifications I could. I am quite pleased with the results, as it's transformed a good basic trainer into an even better model that I enjoy flying, unlike some I have built that I have to make do with as best I can. I am also sure the basic design could be adapted very easily for carrier operation (with a vaguely scale profile) or a stunt trainer (three line or two).

A digression. I can't let an observation of the SC carb pass without mentioning the fact that it was leaking like a sieve around the needle assembly carb body joint, fuel leaking from the joint was clearly visible when the engine was running under pressure, the carb spigot was not an airtight fit and the carb fuel feed nipple was not tight. In spite of all this it functioned quite normally. So why do we get so many complaints about early MDS carbs? I have never seen one being operated in a worse condition than I was running with this combination. I would love to get to the bottom of the mystery.

I am beginning to wonder if it is something to do with a characteristic of Schnuerle ported ABC engines that I have noticed when running on a plain venturi. They will run happily in a lean state, but sometimes when started will sound as if they are set rich and not pick up. If the choke is blocked with a finger briefly, the engine will skip the slow run and pick up to full revs. If this is happening in the air, it might explain why some throttled engines appear to die after being throttled back and opened again. But what causes this phenomenon is a mystery to me as I have only had it happen a handful of times, and just richening up the main needle a little usually cures the problem.

After I wrote the above, I was taught a salutary lesson. On a day that was NOT pelting down with rain or trees looking like they are about to up root and walk about because they are angry at something, I zipped up to field strait into an oppressive hot and very humid day.

For the first time ever, the SC.40 that had been so reliable for more than a year, suddenly would not throttle correctly. It would run quite happily flat out and also tickover, even if it was with a slight misfire. The problem was, if run at low revs in flight for any length of time, it was extremely reluctant to open up again. I also noticed the usual one or two flick starts had become a, when it felt like it, affair.

There followed an hour of frustration trying to narrow down the cause. The first thing that was tracked down was a failed O.S. No3 plug. Replacing the plug cured the starting problem, or at least made it sensible, but had no effect on the throttling.

In desperation I went through my own throttle setting system again, but to no avail. By the end of the day I had managed to get it to work reasonably well, but nothing like the previous Swiss watch type response from the engine I had enjoyed up to now. The only conclusion I could come to, was that the high humidity was producing this effect, as nothing else had changed.

And sure enough, the next time I flew it on a normal day, everthing was back as it should be. For me, this is pretty conclusive proof that humidity can have a striking effect on the carburation of glow engines. I had a suspicion of this from previous experience with glows with a plain venturi, not a carb, where if the humidity goes up they become more difficult to start.

I have no idea how to get around this problem except by changing the fuel, but what to do to the fuel is a mystery that will only be solved by experiment and a lot of luck. If anyone out there has any suggestions I would love to hear them.

Oh well, it's yet another challenge.

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