A return to model building and flying. Part 31

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September 2006

Verigo Puzzling Problem

Flying the vertigo on several different occasions, I have never had a problem with the engine (AP.15) before, but the last time I did.

The only engine related change ince the last session was to make the fuel tank into a uniflow vent system to try and alleviate one of the minor fuel feed problems.

The weather was cool and blustery, but certainly flyable. I was on my own but that is no problem with this model as the engine will fall into an idle in the vertical position (see previous episode) first flight was the usual hairy take off and a few laps collecting my wits. Nothing out of the ordinary so far.

On the second attempted take off the problems started. As soon as I opened the throttle the engine died. After several attempts with identical results. I was beginning to curse in ever stronger language.

In an attempt to duplicate the effect without trekking backwards and forward to and from the handle. I started the engine with the model in its normal takeoff position waited for ten seconds then opened the throttle, perfect pick up every time I tried it. So far so good, but at fifteen seconds the engine would die every time the throttle was opened. It takes me a few seconds longer than this to start the engine walk to the handle and sort myself out before opening the throttle.

The only thing that I changed after the first flight was the pressure feed pipe, which was small bore silicon, the sort of thing found in clunk tanks, to a standard 3mm thick walled fuel pipe; this was done because it was a tight fit on the pressure nipple and was short, making the tank a little awkward to fill.

The only other thing I can think off is the plug/engine cooling down, with the simple carb is flooding the engine with prolonged slow running. On all but one of the previous successful fights, the weather was hot (for the UK) 30+ degrees C. This day it was in the low twenties.

The AP.15 throttle is well thought out in some ways and crude in others, basically a rotating barrel that controls the air flow and a stop screw to set the tickover. This is unlike the O.S. airbleed carbs that have an extra small hole through which a small bleed of air can be controlled with a restrictor screw. Whether the O.S. type is better is a matter for conjecture.

The AP arrangement works fine most of the time, but I seem to have found it's Achilles Heel. I suspect more nitro would would help keep the head temperature up, or a hotter plug. I'm already using an Enya No3 adapted to fit a short reach head by the addition of an extra copper washer, and have never even seen an O.S. A3 hot plug on sale within a 50 mile radius. let alone a short reach one. All I can think of is to try and get hold of a Taylor short reach idle bar plug. I had no higher nitro content fuel with me at the time, so couldn't try that option.

The only other thing is a twin needle carb, but I am not sure one is available for engines this size. Another crude method would be to lag the cylinder head to keep it hot, this could however, do damage in unexpected ways, so it's something I would rather avoid.

The fuel tank is also a very important part of the set up as it has by it's nature to sit below the engine in the vertical position producing a long fuel draw. The only compromise I can come up with is to make it into a cube shape, this is the next job.

All in all a frustrating hour not being able to fly. and I completely forgot about the change of pressure feed pipe, so I can't put it back until I get another chance to fly.


The next time I managed am attempt at flying it, was very brief. I spotted a couple of O.S. A3 plugs at the nationals and promptly handed over the money. The first thing I noticed once ftted was what felt like a reduction in compression, but no sign of the proper plug size curing the problem, with the engine still cutting when the throttle is opened after about 15 seconds of idle.

Having someone around to assist this time, I was able to at least have the engine running flat out until I could pick up the handle, this getting round the 15 second barrier. After a couple of backflips into the ground on full throttle, the fuselage broke away from the wing. I was spending so much time wondering if the throttle would work I was not consentrating on the elevator position, which is extremely critical on takeoff. I would have liked to have tried more nitro in the fuel, but that will have to wait until I can repair the damage.

Something changed quite dramatically after the first flight of the session before and not being able to identify what it was, is irritating to say the least, as everything apart from the engine cutting when the throttle is opened seem to be the same.


After some atrociously windy weather for a few weeks I managed to give it another airing. Everything had been epoxied back together, and a smaller tank fitted. I decided to go right back to the beginning and not use a pressure feed, it could be added later if necessary.

To my utter surprise the throttle worked perfectly and would open up on cue no matter how long it had been idling. So the problem, was, fuel supply related. Unfortunately after a couple of flips into the ground, the fuelage broke again ending experiments for that day. Somehow I have to reduce the sensitivity of the elevator and still have enough control for a hover. As take off is extremely difficult, and my success rate so far means it's impractical for regular use. I also now have to decide how whether to make a new model. somthing ultra light would help over come the inertia on take off. How that woul affect hovering and everthing else is unknown.




The rest of the day made up for it by learning how the Hellcat handles in a reasonable blustery wind; entertaining in it's own right. The MDS.28 appears to be being strangled by the Enya silencer I had to fit (see last episode), and I am also being hampered by the lack of a full range of Kavan yellow nylon props to experiment with, a 10x6 is slightly over propped and a 9x6 under propped, probably because the silencer is restricting the revs. I have no 4 inch pitch props in these sizes; unfortunately Kavan do not seem to make any 5 inch pitch props in the nylon range of props. I don't know why, but the Kavan props just seem to work better for the sort of flying I am trying to do, maybe they flex to a lower pitch when the blades are near to a stall as in prop hanging or near to the stall attitudes. Whatever the reason I like them, but availability is sporadic. They also have the added bonus of being able to soak up a lot of punishment that would destroy all the popular brands at the slightest provaction. Whilst wandering around the the numerous trade tents at the nationals, I found a few Kaven props and was surprised to see an new verion of the yellow nylon variety, it was shape like an APC, with scimiter shaped blades. At this point I can't help wondering how much of this prop design thing is about fashion, and how much is based on on actual performance? Unfortunately I had just spent the last of my cash, so couldn't get my grubby little hands on it to evaluate it. I bet this design becomes like hens teeth to get hold of.

After checking the price of a replacement silencer for the MDS.28 to get round the shape and restricted volume of the standard, older, MDS silencer and the Enya one that had been pressganged into use, It would cost around £17.00 inc. postage. As complete engines go for around this price on eBay I kept a look out for something useful. It was not long before I came across a fairly new engine that had worn out the piston liner and conrod which had been removed. As I was only after the silencer I put in fairly low bid, after all, if I paid £17.00 for it I would be getting more than just a silencer. I won the whole lot for £11.00 inc. postage, a considerable saving.

mds silencers

Top is the standard original MDS type silencer. Below is the newer type.

When it arrived the engine turned out to be in remarkably almost new condition, and also fitted with C2 carburetor, the supposedly better one introduced after complaints about the original versions being unreliable, although I have found no evidence of this personally, at least not yet. So the engine is now in my ENGINE JOBS TO DO box until I can find a good or new, piston/liner and conrod for it.

The C2 carb. has also highlighted another trend that is raising a potential irritating problem for me. It seems that most if not all modern carb's are made, without, a throttle stop screw. No problem if it is used in an RC model where the servo can be set to any position, but a pain in the neck for a three line control line setup, where it could be be quite easy to accidentally stop the engine in flight, or even before I can take off. I can't see this trend reversing, as it is of no consequence to the RC world; and that drives the industry.

mds carb

Above is an early MDS carb. Below is a later MDS C2 without a stop screw.

mds C2 carb

So I am faced with either using old carb's, or buying replacement ones. Fortunately Just Engines range of replacements, all have throttle stop screws; at least for the present. The other alternative is to drill and tap the body, and grind a flat on the barrel on the newer carb's and convert them myself; not quite a five minute job, and I could ruin a few trying. Plus the fact that having just dismantled a C2, and a S,C. carb to see if it is possible, the internal design of both is different from the throttle stop types. Suffice to say, without going into details, it would not be worth the inordinate amount of time and effort required to modify them.

RC! I have to love it for the cheap products it has produced, and at the same time hate it for the disregard it can engender for older forms of modelling.


After trying the Hellcat again with a the proper silenced on the MDS.28 I came to the conclusion that the it is just underpowered for the the size and weight of model,(heavy airframes are pain in the ****!) and that the Enya silencer was not strangling the engines as much as I had assumed. The new MDS silencer was remarkable, the exhaust note being reduced to muted hum.

Next engine to try was the SC.32 which has about 30 minutes progressive running-in in a teststand, and has been showing great promise. As mentioned previously about carbs with no throttle stop, this is engine has just such a problem. I had ordered a replacement from Just Engines on line, received the automated confirmation and patiently waited, I knew that there may be delays after the sad death of Paul Landels, as there must be a period of readjustment. After six days I was beginning to wonder what had happened. After an email query, I received an email explanation 24 hours later, the business had been closed for ten days for staff holidays. Although I have every sympathy for the upheaval Paul's death must have caused, there is no reason a note could not have been left on the web site pointing this out, or the auto mated response email being altered to give some indication of this. It is attention to detail like this that can make or break a business, or at the least, alter peoples perceptions about how well they treat customers. I don't mind a delay, as long as I know why things are being delayed and I am kept informed.

With a weekend looming and everything ready to go except the SC.32 carb I had a rummage round and unearthed a SC.40 carb (with stop screw) which on measurement was all the same dimensions internal and external, except for the spigot part that fits into the engine, which was slightly shorter and larger in diameter. In a, what the hell, moment, I got out my old Unimat SL lathe and proceeded to carfully turn it down. If you knew how worn the chuck on this lathe is, you would have some idea of how reluctant I was to do this.

I must have done something right as on the Sunday the throttle worked perfectly. This is probably a lot to do with the engine, which is wonderfully powerful and seems to have an very precise almost instant throttle response. It is still not run in yet as I could hear and feel a slight unevenness at peak revs; yes, you can physically feel what an engine is doing with a control line model; but as I spend most of the time with it throttled back this is not a problem, at least for the near future.

The only real problem I have, is having to use the Enya silencer again. The SC.32 becomes a bit raucous at peak revs and I have no way of fitting the silencer that came with the engine until I can find a manifold extension that will allow it to clear the leading edge of the wing, and not foul the undercarriage. All my SC engines seen to share the same characteristic of becoming disproportionately loud at peak revs; something that my MDS engines don't seem to suffer from, at least not as obviously. In light of this I this I tried the later MDS.28 silencer on the SC.32, after having it to pieces to see what if any baffles where inside that might explain the difference in noise reduction. The MDS has a cone shaped baffle with a central hole that is bigger than the outlet pipe, similar in concept to OS silencers. If this baffle is the cause of my MDS engines running quieter, I may try to fit the SC silencers with a similar baffle.

I love this interchangeability of engine parts. It was something that was rare when I was involved in the hobby the last time.

On the first flight test it was obvious there was a difference. The harsh bark at peak revs was much reduced, not eliminated but much more acceptable, and so far any drop in power has not been noticeable. So the MDS silencer stays in place. It needs some gasket sealer as it tends to leak around the baffle joint, but that's a minor problem, and the more I run the SC.32, the more I like it.



UK Nationals Visit

You can read about it here.



LMAC annual Stunt Comp

Ever since I rejoined the Leicester Club I have wanted to at least attempt to fly in the annual stunt comp, but lack of space to build suitable models, and getting side tracked into trying to make suitable racing models to cope with our grass circle, and experimenting with carrier models and recently VTOL has not left me with a lot of time.

I had always had the Hallam Swift in the back of my mind ease me back into stunt flying again; it's small and almost like a combat wing with a fuselage and tailplane and should be capable of flying the F2B schedule given half a chance, and a lot of practice.

swiftDetermined to at least attempt to fly in the club comp this year, I modified the Swift to a two wheel undercarriage, and tried to get some practice in on a rare calm weekday before the comp.

I have a Thunder Tiger GP.15 mounted in the nose with a home made plain venturi for carburation. It's a nice little engine with plenty of power for the job. It also carries a very effective standard silencer.

After lurching around the sky for a couple of tankfulls, and feeling really ashamed that I couldn't remember all the manoeuvres and was making a hash of the ones I could, some weight added to the wing tip and tail helped with the line tension and made the turns a bit snappier.

When I tried to slow the engine a little, I was getting slightly dizzy during the six laps of inverted flight, the problems started. The TT.15 is surprisingly spritely on a 8x4 prop at peak revs and does not hang about. The venturi bore must be a little on the large size, as the power seems to drop off dramatically the richer the engine is set. The end result was the model literally dropping out of the sky in the reverse wing over, and plummeting vertically downwards until the ground stopped play.

This model is tough. Especially impressive after grafting on a longer nose taking it back to the original length after a miscalculation on my part left the original one short.The result of this encounter with Terra firma, was one sprung wingtip trailing edge joint which was not in some vital stressed area. The second time this happened the metal fuel tank shifted forward and pushed the fuel feed pipe back into the tank, and the tailplane came loose. Enough lessons for one day, I took the hint and went back home for repairs.


Arrives the day of the comp, not exactly a large scale affair, there were only four of us entered, I return with a repaired Swift with some more rudder offset to help with the high manoeuvres. Both class 2 and F2b where flown. But I decided it was s*** or b*** time, and if I was going to make a fool of myself or fall out of the sky it may as well be during the full schedule.

Apart for completely forgetting the reverse wingover, messing up a couple of manoeuvres, and running out of fuel before I could finish the schedule, I was quite pleased. This is the first time I flown the schedule in any sort of comp for a few decades. The extra rudder offset also helped a great deal.

Second flight, the engine just refused to start, so an attempt was called. Third flight I managed to get the engine running, but it left the ground running lean and cut after a couple of manoeuvres. Adding insult to injury, I remembered the reverse wingover, but my sun classes, which vaugely resemble a pair of welding glasses, Sod it! they were the only thing I could find that would fit over my vari focals and were cheap, decided to slip down my nose at the top of the wingover obscuring my view of what the model was doing . There followed an unseemly high wobbly pullout.

The engine starting problem affected my MDS.28 in the same way on the same day and I have had this happen before with quite a few glow engines. All glow engines can be finicky when it comes to hot starting but these engines where lukewarm to the touch, so not in the range where I would expect trouble. Around early afternoon on a hot day the engines will not start without being set richer, which brings with it the increased chance of flooding the engine, making things worse. It could be something to do with humidity, but as the club does not have a weather station handy, I have no way of checking this out.

All in all it was well worth the effort of trying to fly the schedule. I am in the process of buying a Sterling Ringmaster kit from Bob Stanley; is there some unwritten universal law that says, spoken in a deep rumbling God like voice, "Thou shalt always be offered bargains when thou hast no money."? So I should be a little better prepared next time. Whether I will be able to build and manipulate a wing of this size in my cramped workspace I will find out once I start? In the immortal words of the Governor of California, "I'll be Back."


Second Circle Shows Signs of Progress

new circle

This shot taken from the centre of the new LMAC controlline circle showing signs of it nearing completion. The whole field is on a slope, so it has been lowered one side and raised the other. It was only when I stood in the new levelled circle that I could appreciate the slope of the original one, which can make it a swine to get some sense of level flight, especially in a wind; one minute you are skimming the grass next flying higher than the hedge that can be seen in the back ground. Then there is short decent if you land up hill, or longer than expected if landing down hill.

The offer of a carrier deck is getting a little more certain with a provisional date of November for it to turn up. What state it's in or what work will have to done to it remains to be seen, but the new circle will take a long while to settle down and grass to grow any way. So it will be next year before anything can be even tentatively organised.


The Plot Thickens?

magnum xl 36A

Above is a picture of a Magnum .36 XL series. And this time I have not rearranged the remote needle, this is as it was in the advertisement. Referring back to part 30; again, can you tell the difference?

What this suggests to me is that manufactureres are concentrating their production in China. Logical, but the thing that surprises me is that they all seem to be the same design of engine, badge engineered for each client.

And this next on is a real rarity, a Kyosho GX 40, which seems indistinguishable from my SC 40A Mk II.

Kyosho engine

If anyone else has come across another weird makes that looks like these I would love to see any pictures?

One curious benefit of all this, could be that if you own one of the these engines all the other brand parts should fit. Is this design of engine going to become the Volkswagen of the model world?



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