A return to model building and flying. Part 36

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February 2007

My run of bad luck still isn't over, with lots of personal trauma to deal with at present. The weather being B****y! awful for the last few weeks,rapidly turning the CL circle into a swamp, is not helping either, making this cartoon seem very appropriate at present. As a result of all this my scribblings may become a little thin for a while.

wet



Vertigo

Back in July 2006 I wrote about my attempt at making a VTOL CL model, the initial design was based on an article by Ian Barret in an February 1970 Areromodeller. You can read my piece here. Recently I recieved a very nice email from Collin Barret the son of Ian Barret, with the news that Ian had sadly died recently. But the story he related adds a nice historic perspective to the models, and I hope he will not mind me quoting part of it here.




My dad, Ian Barrett, designed the Vertigo model as your Web page reports.

vertigo bipeWhat was never reported about the original Vertigo was its maiden flight (1968?). I must have been about 7 years old and the plane was tethered to the ground in the back garden with me holding it down and my dad at the other end of the garden in a Deck-chair, holding the controls. On signal from him I let go of the plane, with the idea that he'd control the throttle and hover the model. Bad plan. The plane keeled over and the propeller slashed into my left bicep. Blood every where, Mum and Dad in a panic, tourniquet applied, dashed to hospital, stitches in arm, scar still visible to this day. Powerful diesel engines and kids don't mix!.

You are probably aware that my dad was quite a prolific designer of models. All the back issues of magazines with his designs in are in the loft. Is that a copy of the vertigo article on your webpage? I remember the Vertigo being a biplane - perhaps he developed the design. I wish I'd had done the web search a couple of months ago before my dad died - he'd have been proud to see himself in cyberspace. He never really got into computers but wasted hours on his Microsoft Flight Simulator.

Dad packed in making planes around the early 80's. He was commissioned by the Aeromodeller magazine to review a load of kits (Keil craft etc) and I think having to build 20 odd planes to a publishers deadline took the enjoyment out of it.




I hope my future attempts to make a VTOL CL model can do Ians work some justice, I think it would make a fitting tribute.


Crusader Nears Completion

crusader

One main wheel being lower than the other makes the anhedral look far more severe than it is.

The one thing that has taken me more time than all the rest together is fitting the double bellcrank. These devices need vertical room and quite a bit of radial clearance. This wing root has neither in abundance, so an inordinate amount of time had to be spent making sure nothing fowled on the structure. Once the wing root is sheeted in I will not be able to adjust anything without cutting holes in the top wing sheeting, which I would rather not do once it's covered.

Another mild annoyance is that no one seems to make a 3oz tank clunk tank, 2 or 4 is all you get. Everything I have built recently could have done with a 3. Making tin tanks is one answer, but tin plate is not so easy to come by, and before you say it, yes I do use tin cans, but straitening a curved bit of plate cut from a round tin is not so strait forward. Also the model shop is having difficulty in obtaining supplies of brass and copper tubing, so making metal tanks is a bit of a problem at present.

crusader covered

Not much left to do now, the covering is on and things just need tidying up a bit.


Vindication of a Theory

Some time ago I obtained a nearly new MDS.25 for my father, who had demolished his previous one by piling it into a very hard runway. He had been pleased enough with the original to want another one. Incidentally, my father only flies RC models, and also seems to have a marked reluctance to clean engines. I also found out for the first time, that the original now deceased engine, was given to him by someone that could not get it to run properly.

Recently however, he had been experiencing difficulties with the new engine running lean then cutting out after a short time in the air if the throttle was closed and opened. All the classic symptoms I keep reading about; that I have never experienced.

Finally I persuaded him to let me have the engine so I could run it in a test stand and try to get to the bottom of what was going on, and the oily blob was duly handed over. As you will well know If you have been following my exploits, I do not subscribe to the idea that there is anything wrong with the original Mk1 MDS carb's, and that most problems are down to adjustment. So this would be an opportunity to prove a point; after I had restored the engine to something that didn't resemble a sticky toffee.

The first time I set the engine up and ran it, there was obviously no problem with the engine itself, at full throttle it is a little belter, just like my own. However, it suffers from the same reluctance to start with the choke wide open as my own, and requires flooding to achieve it. Nothing strange there them. With the throttled closed it's a first flick starter, but had a tendency to loose revs and die after 10 seconds in this condition.

Opening the throttle also caused the engine to die before the revs would pick up, and it was possible to see a veritable waterfall of fuel in the carb at half throttle. This led me to the assumption that in the mid range the engine was getting too much fuel and flooding. The obvious solution was to screw in the mid range needle. However this made very little difference in practice. and no amount of adjustment would produce the right effect.

As I was trying to sort out another model/engine combination at the same time, and it was cold and wet, my attention was not exactly 100%; so I decided to go home and think things through, with the hope that I was not going to have to publicly eat my own words if I couldn't solve the problem.

Back in the warm and dry I stripped the carb and checked for any obvious air leaks, even though I don't believe that would cause any problems anyway. I found none, or anything out of the ordinary.

Another day dawned and I tried again. This time the weather was mild, which aids concentration a little. I sat looking at the engine and decided that it was pointless going through the same procedures that I knew had not worked the first time, so I decided to approach the problem from a different angle. I would set the mid range needle as far out as possible and see how the carb responded as a pure airbleed carb, and start from there. To my surprise it actually worked, the pick up was ragged in mid range as you would expect, after all the engine is being flooded on tickover.

From then on it was a case of leaning out the mid range jet a quarter of turn at a time and seeing how engine responded. Within one turn full turn the response was transformed to almost instantaneous The engine would tickover for prolonged periods of minutes, yet still give an almost instantaneous pickup. Also noticeable was the lack of fuel spitting back out of the carb at half throttle.

I need to point out that this method is not what is recommended in the engine instruction sheet. Using the recommended method will probably end in failure, as the instructions are generic and cover all the carb's and engines. Also a lot of people will just open the midrange jet to the recommended starting point of 1.5 turns, then start wildly screwing it in and out when it doesn't work, and seeing how some people try to set carb's, in all sorts of weird and unpredictable ways, will almost certainly compounding the impression that the carb does not work properly. It does actually state in the text that this is a starting point and will probably have to be altered, but not really a clear and concise explanation of what to look for if things are not going so well.

For completeness, I will state that I was was running the engine on a 20% oil 5% nitro mix, and that changing the Enya 3 plug for a Firepower F7 produced a slight but noticeable improvement in throttle responce and tickover. The prop was an APC 9x6. Once the carb is set, changing any of these parameters may mean altering the carb to compensate, but should be within, at most, half a turn of the main needle and the midrange needle.

So there you have it. I feel vindicated in my theory that there is nothing wrong with MDS carb's that a little patience and know how will not sort out. I also suspect the same is true of stories I have been picking recently about later model Super Tigre carb's being unreliable.


The Six Steps To Happiness

Or how to set up a carb from scratch


Although not part of the carb setting sequence, it is important to get this part right first.

With the throttle open and the mainneedle set to just under peak revs, if the revs drop, or the engine cuts when the glow clip is removed, change to a hotter grade of plug. Failure to get this right will lead to false conclusions when trying to set the carb.

The following steps require patience and can be tedious, but once set the carb should stay that way, and only occasionaly need some fine tuning. I also suggest you do not try setting up a carb on a new engine until it has had a least 30 minutes running, preferably in a test stand.

Please try to get into the habit of making adjustments standing somewhere behind the prop arc. That way if something goes wrong or the prop sheds a blade you will be in safest place. Maybe not any spectators, when you swear at the pain and let go of the model, but you at least, will be. Even I don't always do this, but if I get hurt, then I know it's my own stupid fault. Any remarks in red, I think are important; far more important than the, "Don't start the engine by flicking the prop by hand under any circumstances." advice printed on every instruction sheet these days; which should read, "Don't stupidly put you hand through the prop arch when the engine is running." The last is much less likely to happen if you do what is printed in red.

One last point before we begin. If you have a pressure feed connected to the tank when going through this sequence, you must fly the model with it connected, and vice versa, or you will have to go through the whole equence again.

(1) ALWAYS adjust the main needle with the throttle wide open for peak revs, then unscrew the main needle a small amount so the revs drop slightly. This is to make sure the engine is not running in a borderline lean condition which could do damage.

If the engine will not run like this, there is something seriously wrong, and most likely nothing to do with carb. If the engine, WILL, start and run like this, then it is in good health mechanically and the basic fuel plumbing is working. Worn engines (bad piston seal) will not start readily by flicking, especially when hot; something that an electric starter will disguise.

(2) Close the throttle, tick over for ten seconds, open the throttle to full

(3) if the engine dies, unscrew the midrange needle 1/4 turn and repeat from (2).

For safetys sake, stop the engine each time, before adjusting the mid range jet. Some have very fiddly small screws and it will only take one carless move or slip to do a lot of damage to yourself. So please, use some common sense.

(4) If the engine splutters then picks up, screw the midrange needle in 1/4 turn and repeat from (2).

(5) Aim for a clean pickup from tickover when the throttle is opened quickly to full throttle.

When close to the desired settings reduce the midrange needle turns to 1/8 of a turn at a time to fine tune.

(6) In a dire situation where nothing seems to work, try unscrewing the mid range needle out as far as it will go, then start afresh from (1)

And most importantly. Ignore all the rumours you have heard about engine/carb X,Y, or Z, and even the club expert that leans over your shoulder and says, "Try this." Work through this list first. I will be very surprised if you don't have some success.

Now you know as much about setting carb's as I do.



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