A return to model building and flying. Part 32
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Tale of a Lathe Chuck
After always being frustrated that I can't bore a hole down a piece of metal rod or alter a venturi, or anything else that frustratingly difficult to do without a lathe, I got hold of an old Unimat SL lathe on eBay, this lathe is nice and small, so it can be lifted off the bench and stored away underneath until needed, very important in the confined space I have to work in. Although the chuck as worn and other things as well it was good enough to turn up a few custom venturies, and latterly make a SC.40 carb fit into a SC.32. Not bad considering what I know about turning can be written on a postage stamp.
Some months later I managed to get a new chuck (a very rare animal indeed) after returning the first one because it was for another Unimat model. The only problem was, I couldn't get the old one off. Truer to say I couldn't get the backing plate off the headstock shaft. All the old tricks of heating it up until red hot and other attempts failed miserably. The shaft is hollow and any attempt to clamp it tightly in a vice could have distorted it. There is hole drilled through the shaft behind the chuck which I assume is for locking the shaft while the chuck is tightened or taken off, any thing I put into this hole just ended up bending because of the extreme force I was having to apply to try and shift the chuck.
Unimat used a 21mm metric size of thread on the shaft which is different on other models. I had a Chinese made replacement Unimat chuck, and could not see or check the thread on the shaft. Destructively removing the backing plate from the shaft, could, leave me with an unusable lathe if the threads did not match the new chuck. So I procrastinated for nearly a year.
Then one day I just got fed up of the problem and attacked the thing with a junior hack saw. This being hard work and me being lazy, soon had me looking for an easier way. It suddenly dawned on my pudding like brain, that thing was a lathe, and the problem was stuck on the end of the headstock shaft, which turns as normal; why don't I just turn it off? After about a solid hour, the motor is not powerful enough to take deep cuts. the bane of my life finally succumbed. After a very tense moment the new chuck was offered up; and fitted! Did I breath a sigh of relief? You can bet your life I did!
And here it is in all it's new shiny glory.
It's Been a Strange Month
The weather is slowly turning into autumn, getting cooler, wetter, and windier. and at times like these I start looking at the models I have sitting around doing nothing. I can't really miss them as they occupy every available cupboard top and fee bit of space and are difficult to ignore in my cramped living space.
My wandering gaze kept falling on two abandoned Goodyear models, the original one I built that felt like it was made of lead, which exhibited some very unpleasant flying charcateristics, and the next one which had and experimental wing with 1.5mm ply mainspar. This was so flexible that it would develop a gracefully curved dihedral in flight. It also had a warp in the wing that caused it to fly outboard wing up. Two likely candidates for modifying to some other use, as they would never be used for their original purpose again.
The other thing that made me look at the models in a new light was chance remark that, "Maybe we should go back to two wheeled undercarriages to cope with the rough ground?" That remark prompted me to think that a two wheeled U/C has other advantages, like being able to support the model while I use a remote release if I am on my own. Something not possible, or difficult at the least, with a monowheel.
The heavy model I had been in the verge of consigning to the waste bin for some time. But thought that with few centimeres sawn of the nose and a different bellcrank setup, it might be useful as a test bed for a few engines that need running in. They all have the same mounting dimensions, which is very convenient.
This model has always been a pig, with some very unpleasant flying characteristics, Mainly caused by it's weight. With an old ASP.25 mounted on it, it was even heavier, 950g at the last weigh-in. I am beginning to think the balsa I used must have been a close relative of Oak. And sure enough it still flies like a pig, just faster. Landings are a case of keeping low enough to hit the ground before the deadstick ballistic curve takes over. Nine time out of ten this results in a flip over on landing as the U/C hits some obstruction in the potholed circle, I am beginning to develop an unhealthy and politically incorrect dislike for nocturnal creatures that dig holes all over the flight circle on a regular basis. These heavy landings inevitably lead to something breaking off the model. So the jury is still out on that one, it may be easier to build a new model than modify this one again.
The second model was a little more successful. I wanted something to try out an old Russian Meteor (Meteop) 2.5cc diesel in. The old flexible wing was removed and a spruce full depth spar let in. The wing span was substantially shortened, which also effectively increases the available tailplane area, and brings it up to a more sensible size. The nose was also shortened as much as possible to accommodate the extra weight of a silencer. It's worth noting that an old Enya silencer, probably off a 2.5 and fitted with a cylinder wrap round strap, will fit with only a minor modification to the strap.
The wing was re-covered with some blue Pro-Film, and what a joy it was to use after my previous experiences with Toughlon. Stripping the old Pro-Film off, proved to me how well it adheres to the wood, and apart from a couple of small patches came off clean, unlike Solarfilm. There was no sign of any fuel seepage, which also confirms that Pro-Film is diesel fuel proof. Some trim was also added as an experiment in the same colour.The model flies as if it is on rails just like it should always have done but never did.
The Meteor however, was an other story. The first time I tried to run it in the model, It just refused to start, I flicked, and flicked, and flicked; and nothing. Priming through the inlet would produce some signs of life, or very hard compression if screwed in too far, and the occasional short burst of running for a second or two. After what seemed like half an hour. I finally twigged that I was using a Russian copy of a an SuperTigre spraybar. The needle has a very fine thread on it. so the normal three turns open for a starting point was nowhere near enough, six was more productive. I bought a batch of these needle assemblies from my favourite eBay seller in Siberia at a bargain price. They are so useful I keep praying that some more will turn up at some point. One side effect of all this unacustomed flicking was a aching arm during the night that made it difficult to sleep.
The engine has a disappointing power output, but then it is an old engine, and I have no idea how the silencer is affecting it's output. However, after a couple of flying sessions on different days it seems to be getting easier to start and I am beginning to find the settings. The compression and needle adjustments feel very woolly and the engine will keep chugging away over a wide range of settings. This engine would be ideal for a freeflight sport model or a bit of vintage radio. It has a curious tendency if set too lean on take off, to trundle round until the tank is almost empty then slowly loose power until it makes a powered landing and the engine finally stops. I found by experiment that if set richer, this does not happen.
In spite of the engine being sold as used, and it looks used. I can't help feeling that it is slowly loosening up as if it is running in. It is certainly a little easier to start now and the needle seems a bit more sensitive. If nothing else the engine plane combination looks pretty and is a pussy cat to fly, if not fast at present. The addition of a pair of KeilKraft streamlined rubber wheels I happened to have looking for a home, makes it look very vintage, and reminds me of my early days learning to fly CL. I wish someones still made that style of wheel.
One thing I am growing very curious about, is all the stories I keep hearing about people having trouble with engines of Chinese/Taiwanese/Russian origin. The only ones I have had trouble with are ones that where not right to start with, and as nearly all my engines are second hand, most problems are probably down to the previous owners. The story that keeps cropping up most and involving RC models is, they just stop running in the air after a short time and won't run out the tank. Mine do keep running. The only thing I can think of that is different, is that I pay an inordinate amount of attention to fuel tanks, tank vents and feeds; I have to with CL models as the physical behaviour is a little different in a permanent circulatory world. I suspect most RC models just use a standard clunk tank and exhaust pressure. As I know from past personal experience with RC models, this setup sometimes needs tinkering with to get it to run right. I'm not going to tell you how, do some research for yourself, the principals have been written about often enough for many years; and remember, "Google is your friend."
Now I wrote this piece some time ago but other things kept cropping up to put on these pages. The gods however move in mysterious ways, as in the mean time I found this message on a Usenet nntp newsgroup for RC. The guy had said he was having trouble with an RMX.40 stopping in flight. There followed a string of replies, of which the general tone was get a more expensive engine and throw the that heap of C**p! away.
I have a theory that RMX engines (pictured left) were badge engineered by MDS for Ripmax. If I am right they will be far from C**p. So if anyone wants to throw any of these C**p engines away, throw them my way.
The original message read thus:
I am rapidly becoming our club's deadstick champion! Aborted takeoffs, 6 ft in the air, a speciality. We have a most unforgiving hedge to get over when the wind is from the NW!.
My new RMX 40 ( fitted upright) powers my World Models Superstunt 40 consuming a Model Technics 18% lub 5% Nitro fuel & using a 10x6 Master prop.
With supervision from club experts I have tried every variation of needle setting. Have installed a taller Tank higher in the nose, redone the clunk and fitted an inspection hatch (Major surgery!) All ground checks ( both H&V) up to max revs seem OK, there is just something in the air causes it to cut out.! Only now after consuming a full gallon of fuel am I able to complete a 15/20 minute flight, but only if I run rich & don't get too enthusiastic with the throttle.
I can only assume that at operating temperatures she is still tightening up and stalling, but I can see no seizing symptoms after landing. I actually landed, for the first time last week , still ticking over, so perhaps things are at last easing up?
If this is an engine weakness I would be reassured, but perhaps I just have a 'Friday afternoon' motor!
Any comments welcome. TIA P
Some time later the original sender of the message sent this:
This is by way of an apology to those nice guys at Ripmax. My RMX 40 now sings a sweet song and seems happy with an 11x6 prop.
As I am from the generation who carefully ran in a new Mk II Cortina progressively at 40/50 then 60 MPH maximum for the first 500 miles, I cringe when I hear a new engine screaming its head off at max revs.
I have now learned that you cannot get the correct setting on that mixture screw if the throttle is not fully open, otherwise she will almost certainly lean out in flight. That is why I spent a couple of months practising deadstick landings.
My next mistake! In order to keep my model as clean as possible I simply fixed a 1" plastic pipe as muffler extension. Don't even think about it, the extra back pressure, ( varying with revs) pressurised the fuel tank and this fuel injected 2 stroke became very erratic. But that only gave me one apoplexy for one evening.
I like to share, it might save someones model. Cheers P
I was so perplexed when I read this, that am I am really beginning to worry about the plight of aeromodelling in general. Have the general modelling populace in a few short years completely forgotton how to set a needle valve so the engine runs properly, or in the case of RC, a carburetor? Or is it that the circles I have moved in (no pun intended) have always known how engines and fuel systems work? I have actually stood and watched some RC fliers in our club adjusting the main needle on a carb with the engine ticking over.....! I find it amazing that the things survive as well as they do when handled this way, just a quick Internet search will throw up the correct way to set a carb. With regard to the first message, if a club had 'experts' that could not sus out that particular problem, it's time to seriously think about changing clubs.
The last comment about exhaust pressure (which I am very dubious about) also shows certain lack of grasp of principals. It's very simple: more pressure in fuel tank = more fuel for the same amount of air = rich mixture. Screw in the main Bloody! needle and lean it out....! And adjust the idle jet as well if necessary
Also, if people don't pay due attention to the effects a changing fuel head can have on a running engine, the bigger the tank the worse this effect will become, they certainly will have problems. A simple exhaust pressure system will only partially help this, and will still be subject to fuel head changes. This can also lead to over stressing an engine if it ends up running lean after half a tank. With a new engine this could possibly drastically shorten its life, without the engine necessarily stopping. This effect is probably not noticed on most RC planes in flight, because the throttle is adjustable at will and probably being changed throughout the flight. If a large tank is run nearly empty, then the throttle opened up full and kept there, there could be a nasty surprise waiting.
From what I have learned about lubricating oils recently, the increased use of synthetic oils and lower concentrations of lubricants may not exactly help this situation, and may in themselves cause a sudden death situation under the right circumstances. Synthetic oils break down at specific high temperatures, and loose any lubricating properties. Perfectly OK if you run the engine properly within the temperature tolerances of the lubricant, most of the time this is probably what happens. But in those odd unpredictable lean run situations; maybe not so good. Castor oil does not share this characteristic and will keep changing it's properties as it breaks down, but still remains a lubricant. The reduced or removed Castor content may explain some of the sudden death scenarios we have all heard, "It was going great on the last flight, but it has never run right since."
When I found the following on the O.S. website I was quite surprised at the proportion of oil recommended and the page is quite informative. And there is even more information oil and fuel here and here
I have personal experience of the quite remarkable properties of Castor oil. I had an original Merco 35 plain bearing lapped piston stunt motor literally cook on a lean run just after it took off. For some unknown reason, maybe it was the weather conditions, the engine went into a very lean run and developed that sagging revs sound that you can't mistake. The engine was closely cowled in a stunt model, and there was no long grass or other high crop to att empt to dich the model into, so there was no way I could stop it for six minutes without trying to clip the prop on the ground, with a .35 powered stunter still flying fast inspite of the the engines obviouse distress would not be good thing to attempt. I was on 18 metre lines in still air, and I could smell the burning Castor oil in middle if the circle for the whole flight. When it finally stopped and landed, it had not seized, just run out of fuel, the smell resembled a chip pan fire with the engine making those clacking noises that overheated engines make as the different bits cool down at different rates. All that combined with the smoke emanating from the cowl and the visible heat haze, I honestly thought that it was as scrap job. But after it cooled down sufficiently for me to be able to touch it without burning my fingers, or the neat fuel boil as I primed the choke, it started up as normal. This was the same engine I used in my first club open class racer over thirty years later, with no obvious signs of excessive wear.
When my model club used to fly at Wymeswold Airfield, I also saw a 2.5cc speed model clip the runway and take off both prop blades. The engine did not stop however, and did a shaft run in flight and on the ground for quiet a few seconds until it could be stopped. The pipe being taken off tune by the unexpected increase in revs probably held the revs down, but it certainly didn't sound like it. A new prop fitted and it made another flight. I wonder if the result would have been the same with a synthetic oil?