A return to model building and flying. Part 25
Redwing and an MDS
March has been a frustrating month for many reasons, not least that I have never had so many very large utility bills and threatening final demands in such a short space of time in my life. So much so that I have no savings left. Entering a new financial year with no reserve and only a credit card for support is not a nice prospect. Something is very wrong with this country. Fortunately most of my hobby related hardware is purchased by trading on eBay. I manage on the whole to sell enough to be able to buy what I want, or life would be very restrictive to say the least. The other frustration has been the weather, characterised by, bitterly cold, windy, or both at the same time; not exactly conducive to enjoyable flying.
Most of the time has been spent trying to design/build a new Dwing racer to get round the problems with my green one. Please bear in mind if you are reading this for the first time, I have a specific goal of making the damned things work, with silenced engines, reliably, on our less than perfect grass flying site to our own club rules, not for any recognised national or any other competition rules. And usually with the cheapest engines I can find. Sound like a mess of contradictions? it probably is, but a very good mental challenge. So far I have solved the undercarriage takeoffs, that at least gets round the problems of dodgy hand launches. Landing is another matter. There seems to be a minimun size of model that can be landed reasonably well on the uneven grass surface and it's around the 2.5cc size. with a 62mm balloon wheel. This means a much bigger model for a 1.5cc engine than I would have expected. However that does mean I can design one model for both classes.
Finally I seem to have found a paint that works. The red colour is ordinary car spray paint, and seems impervious to diesel fuel at least. I can just about spray small models but bigger ones are a no no, so it's heat shrink film for those. The best I can say about heatshink film is, I can tolerate it, but I still think it's far from ideal
My biggest problem engine wise has been 1.5cc diesels, or lack of cheap easily available ones that can be fitted with a silencer. I always have trouble starting glows, especially smaller ones, they are not one flick starters, so any advantage gained with speed in the air is lost on trying to start the engine and during pit stops. PAW's are OK but relatively expensive for what you get. Mine is old and although still powerful is a pig to start and very unreliable when it comes to getting two runs the same. Not an ideal combination when it comes to trying to do any sort of race. The ideal is a later MVVS 1.5 but these are no longer made and fetch quite high prices second hand.
Just recently there has been a spate of Russian MDS 1.5 engines for sale on eBay, made in both diesel and glow versions, but with a front facing exhaust. My guess is they were meant for boat use. However it is possible to swap the front and rear housings round to make a rear exhaust engine.
These engines have a very short crankshaft and a prop driver with a sleeve. This means drilling large holes in prop hubs but with a brand new in box engine costing around £19 on average including postage (I have three now), this is a small price to pay. The main problem with all of them has been a super tight contra-piston, in fact so tight that the only way I could move them was with a large pair of pliers on the adjusting screw and the head in a vice. Clearly there was no way the compression could be adjusted, at all, in this state.
The the design is not what I am used to, the contra-piston is a small brass (or what looks like brass?) slug in a steel cylinder head. The only thing I could attempt to do was lap the head as the contra-piston is too small to grip easily. To cut a long story short, I eased out the head until I could hear the contra-piston back off if the engine fired.
This picture of the cylinder head is misleading. The contra-piston is upside down the correct side has a depression which I suppose acts as a squish chamber. The picture will give some idea of the size of the contra piston.
This is not the end of the story though. Once running the contra-piston again tightens up to the point where it cannot be adjusted. On one engine I took it to the extreme. The contra piston was loose but stayed in place. What I had forgotten was that on a freezing cold day, the thing would be so loose it would fall out into the cylinder. A crude clout with a hammer followed by a week making an external lap for it got it back into some sort of running order.
One of the engines I now have in a workable state, The contra-piston is loose and needs a locking lever to stop anyone, or thing, from screwing it out altogether. But still, unless the compression is set within the first few seconds of firing. it will be impossible to back off. How tight it becomes was brought home when the comp screw fell out in flight and the engine just carried on running as if nothing had happened.
So far in spite of looking like they should go fast they are not developing anything like the power one would expect. Quite why this is I have not worked out, As far as I know, changing the front housing and backplate round should not alter the port timing. There are two drive pin locating slots in the induction drum. I tried to be careful to note which slot was in use when I took them apart so they could go back the same way. What the 180 degree opposed slot is for haven't worked out yet. I still have to find the right prop combination which should help. Their saving grace so far, they are extremely easy to start hot or cold.
I have also managed, after much head scratching and working out how my Linux KDE printer poster print system works, to scale up a plan for my next carrier project, a .40 powered Grumman Hellcat.
The wing I will attempt to cut from foam. Local supplies of balsa here are none too consistent in quality, which means making two models the same a bit problematic. At least this project will give me plenty to write about.
The idea of writing this series originally was to put in print a warts and all look at all the problems I would possibly incur whilst trying to take up the hobby again. It's certainly been that, and still is, but if it helps someone avoid a problem it will all be worth it.
And on a final note:
if there is anyone reading this and living with striking distance of Leicester/Mkt Harborough that fancies having a none too serious attempt at a VERY beginners carrier comp, please get in touch.
So far I am the only one with with a workable model. I have one club member building one and another possible. But a couple of others would be nice. That would at least justify setting a date, sorting out the wires drag weights and marking out the deck on the ground.
Non club menbers only need BMFA insurance and an EFFECTIVE silencer on the engine, You can be signed in as a guest for the day. Engine size is open, any type of model is permissible.
In essence, takeoff, 7 laps as fast as you can, 7 slow as you can, land on the deck. And be able to put up with the cat calls and insults which we are all bound to get.
Go on; you know you want to.