A return to model building and flying. Part 39

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May 2007

Frustration

This month has been a frustrating one for lots of reasons. Having to get get to grips with being a single woman again after splitting up with Paul, still trying to get an ongoing extremely dire financial state sorted out, that has gone from can't possibly live like this to, just surviving if nothing goes wrong, an ongoing legal affair, and car breakdowns, have not been conducive to a relaxed frame of mind. Even the weather has conspired to be pretty dire for most of the time, robbing me of the will to try and get out to fly something. The few times I have managed to fly has thrown up one problem after another regarding models and engines.

Having broken my KMD 2.5 powered team race model in a the teeth of a gale, a hunt round for a second model threw up an older model fitted with a Russian Meteor 2.5 diesel. The engine is reliable and starts reasonably well, but lacks power. trying an MVVS 2.5 in the same airframe caused even more frustration, as the engine seems to have a complete inability to handle changes in fuel head, making optimum settings almost impossible to achieve. The KMD would not fit in this airframe because of it's rear induction system. After three separate days of fiddling with the MVVS I was really beginning to appreciate the KMD 2.5's handling; it might not stand up in anything like a real race, or against a half decent PAW, but for club none too serious stuff, I can't think of anything better, it has an added bonus of having a side exhaust port, which makes fitting a silencer much more practical. An old Irvine 20 silencer fits without too much work.

The 1.5cc mds diesels I have been playing with for a long time, I finally gave up on, as I want to fly not fiddle with engines all the time. After the last race it developed a firm refusal to start at all, so it has been retired. This means that I am back to very unpredictable PAW 1.5.

Even the carrier models have reach a state where I can't get them to fly any better without either some radical surgery, or building new models. Except for the Wildcat (thanks to Jan Odeyn's plan and advice) which is just a shear joy to fly.

So apart from being very inventive with food to save money, it's been a stay in and build month so far.


Buster

Progress on the Buster continues. I lengthend the fuselage by 2.5cm and increased the elevator chord by 2.0cm. I hope this is sufficient as I don't want to make changes that are too radical, then end up with a totally unknown setup.

buster airframe

Covering always gets me oscillating between being annoyed that I have to use iron on film and being glad of the speed I can complete a model because of it. Covering a profile fuselage is not the easiest thing to do with iron on film especially round the nose area. If I had the space and facilities I would sooner spray paint models, or even brush paint at a pinch, even though it takes longer.

buster finished

The rather bilious green colour of the wings is a result of using some Profilm/Oracover I won on eBay at a cheap price. As beggars can't be choosers and it was the nearest colour to yellow that did not clash, this was the only real choice.

A thing I did notice about this translucent covering, it seems to take a lot more heat to shrink it than the opaque colours.

Powerplant is my Leo.28 that I had so much carb trouble with in a carrier model, let see if it will perform as a stunt engine? All I need to do now, is fly it......!


The first sunny calm Sunday for a few weeks dawned and high hopes of doing some aerobatics for a change where soon dashed after the first flight. I have flow sensitive models before but this was ridiculous! A bit of up elevator to get it off the grass resulted in model turning through 90° and heading for the clouds. A similar situation occurred when I tried to correct it's attitude, except it was heading for the ground. The following rollercoaster ride was not pleasant. I finally got it under control, but it was almost impossible to fly strait and level. On a windy day I would have been in deep trouble. The Leo.28 also insisted on going flat out and was way too powerful.

With a larger prop and the RC throttle set at half choke in an attempt to tame the engine, I had left the RC carb on hoping that I might be able to find the choke area that worked before I make venturi. The Leo carb is one of the worst I have come across, and I would place it on step below an Enya carb which is pretty bad, so maybe that was not such a good idea. I tried again. Whilst trying to start the engine, fuel had siphoned out of the tank into the muffler and was steadily dripping out of the tail pipe onto the grass. The engine had decide to be stubborn and was taking longer than usual to start. When it did finally fire there was a Wump! sound followed by a smell of burning grass. I had the presence of mind to move the model off the bonfire that had started under the wing, but not before a hole had been burned in the lower wing covering. For once I was glad it was covered in plastic film; tissue and dope, and things may not have ended the way they did.

The hole was not big enough to prevent the model flying so I took it up to see if the engine mod's would work, which they did up to a point. The engine still wants to race and is way to fast. Trying an experimental loop the thing seemed to have an incredibly small turning radius, much too small for confort. I can see why it is marketed with one option as a combat model

As fuel was beginning to enter the hole in the wing I decided to call it a day. There where just too many things to alter at once, I would stand more chance at home. Why the model is so sensitive I can't work out. My increase of the elevator area should not have made that much difference, and the CG is much further forward than it should be. If cutting the elevator to it's original size doesn't reduce the lively flight performance. I can always substitute a heavier smaller engine to get the CG even further forward. If turning up a new small bore venturi for the Leo doesn't tame it, I will do this anyway.


On a whim, and because I still don't really trust the Leo (a probably unjustified feeling), I decided to utilise an old APS.25. like nearly all of my Chinese engines it had quite nice handling characteristics the last time I used it. It is also heavier than the Leo which I hope will drag the CG forward a little, plus he fact that I could do with loosing some of the power of the Leo.

Turning up a plain venturi on my lathe I made a stab at guessing what the size of the bore should be. With an an old Enya spraybar sitting in the middle I ended up with 6.5mm.

buster tank 1buster tank 2I realised I had made a rather stupid design blunder with the tank, the cause of the pyrotechics the last time I attempted to fly the model; I had taken the pressure feed into the tank in the middle of the wedge and pointed it downwards to match up with the muffler nipple. With this configuration as soon as the tank is full it will start to drain into the silencer. For a long time I have been trying to remember why I never had this problem with my F2B models all those years ago, until finally it all came back to me. I did not use exhaust pressure, which meant the vents could be placed in a rather more sensible and convienient place. In light of this recollection, the uniflow vent was turned round and jury rigged to feed from a better position above the tank, thus avoiding syphoning to where it is not wanted.

The elevator was also cut down to near it's original design size temporarily; no point in making it permanent if it subsequently has to be altered again. And as a precaution I added an elevator horn and some small bolts to my field kit, another idea I had picked up from messing with the carrier models, oversensitivity can be reduced by screwing another horn onto the old one to extend it.

A visit to the field proved that a lot of my modifications had worked. The engine can be set to a nice fourstroke and the tank run seems to be a lot more consistent. This all makes me feel a little more in control of the things.

horn extensionThe reduced elevator area had tamed the twitchiness a little, but not much. I was still having difficulty flying strait and level, upright or inverted. As it is difficult to hang weight on the nose, I screwed the extra horn onto the old one. This definitely reduced the twitchiness to a point that I can put up with, but was far from good, the twichiness still makes the whole experience a bit of a trial. No way could this model be flown with any sort of precision in its present trim. I find this quite surprising behaviour as the CG is only 4cm behind the LE. I would expect it to be nearer 7-8cm, somewhere around the mainspar.

Another thing that struck me on the first flight was the fact that I could visibly see downthrust on the engine in flight, this was inspite of checking everything many times whilst assembling the model. This was still visible even though I had tried to adjust thust line as much as practical. Quite how this has come about I can only guess, in all the altering of the bearer spacing and easing out the wing slot in the fuselage something must had moved more than I expected. Using the ASP had another advantage in that the previous owner had bored out the mounting lug holes to 4mm, a practice I frown on, but in this case it worked to my advantage. But downthrust was still visible.


Back at home, some brutal opening up of the bearer holes allowed more play to alter the thrust line as close to zero possible, and a large piece of lead was epoxied to the nose space under the engine. This has moved the CG forward to 3cm behind the LE. I'm beginning to wonder if this lively behavior is anything to due to the model weighing in at 0.9kg.


When a calm day presented itself, I tried again. Thankfully my modifications had made a difference. Level flight was now tolerable, and more to the point controlable. The new venturi keeps the engine withing a tolerable rev range and is choked enough to stop it going crazy if given it's head; I felt no panic when I got the settings wrong and the engine ended up flat out, infact it is now at the stage where it would be possible to choose a the way you like to fly, quick and steady, or a 4.2.4. The Elevator is still very sensitive and it takes a lot of work to not over control the model. Moving the CG even more forward should help and also dampen down the level flight a little.

The tank is also too big, so a tin bashing session is looming. It's no good wishing I could buy tanks ready made as mine tend to be a bespoke design for each model in question and not of a very conventional design; this will be even more so in future, as I have some ideas to try out for the carrier models. My only gripe about building my own tanks are lack suitable tools and of readily availability tinplate, discarded tin cans are not as easy to use as you would imagine (although they can look colourful). I really ought or start making some bending bars and jigs.

I may have plenty of time for the above, as my cars automatic clutch has failed yet again, just at the time I have hardly any money. So the prospect of ending up with no transport, hence no model flying, is VERY real.

Whatever happens I can still scan the magazines and maintain this website, but with no transport this diary will become a little sparse.


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