A return to model building and flying. Part 26
Hinges, KMD engines and some Free Flight
Having again struggled with elevator hinges and corresponded with someone with the same problem, I though it may help to share my struggles and some of the solutions I have found. It may help someone?
Because I have been forced to use shrink film these days I have also been forced to use plate and pin hinges for reasons of neatness and convenience, the convenience bit is a moot point. I did use them successfully on my F2B stunters years ago but had to relearn a few things I had forgotten in the mean time. The most important one is that I find it almost impossible to cut hinge slots perfectly parallel with the tailplane (or wing) surface. Yes, I have used a slot cutter guide and special blade, which to be honest was a waste of money. Using care I can slot a 3mm sheet tailplane with a conventional disposable knife blade.
My favourite hinges are made by Kavan but seem to crop up with other names ans slightly different variations occasionally. I had to purchase some dubro ones recently and they are made of a harder material and are thicker, which may explain some of the problems you will read about later. The best ones are thin and flexible, take most glues readily, and I have never broken one yet.
The main problem with them is, they are made with tolerances that are a bit too close and needing a bit of reworking to introduce some slop. Contrary to what you may think, this is necessary to allow for any missalignment in the hinge slots, or movement out of parallel when the elevator and tailplane hinges are glued together. As if by magic the sometimes alarmingly sloppy hinges invariably are not loose when the whole thing goes together.
The process of reworking them begins with removing the hinge pin, they can be pushed out quite easily by pressing on a work surface as the plastic is quite flexible. Some makes of hinges just have a piece of wire with a bend in one end, the DuBro ones have a split pin, so there is no effort involved with these types. If the pivot pins are the type with a crimped end, you may have to put a slight bend in the end after reassembly to prevent them working out in use.
Next stage is to drill out the holes to 1.5mm. It's easier to do this in a vice as they tend to roll over if held in the hand.
The last operation before reassembly is to cut thin slivers of the inside edges of the plates to allow some lateral movement before reassembly. the DuBro hinges are a hard enough material to file. Choose a method that is appropriate. If all the above has been done successfully, holding one hinge plate, the other should drop under it's own weight, or at least move with almost no resistance that you can feel; if it doesn't, look to see where the friction is coming from and ease off some material.
This may seem long winded and a lot of work but once mastered takes about half an hour at the most, to do a set of hinges. And the satisfaction of seeing the elevator on the finished model drop under it's own weight is well worth it.
The only other thing to watch out for; no glue gets onto the hinge pivot pin, or edges of the pivot when finally fitting. If it does, there will be big problems. Doing the prior preparation of increasing the clearances on the hinges will help alleviate some accidental contamination, but won't prevent a lockup if too much gets into the wrong place. I found by trial and error that epoxy is the best adhesive as it allows time to correct errors and stays put once set. Cyano will work but can grab the hinge before everything is aligned, That is something I would rather avoid. Peg the hinges if you want a safety blanket, but I have never had a hinge pull loose, Yet! only during crashes when the hinge tends to tear out with a piece of the control surface attached.
The very last important thing to mention is, do not, use too much glue to attach the hinges. it's only necessary to smear a very thin layer of glue in the area I have coloured pink in this image.
The act of pushing the hingeplates in to the slots will spread the glue especially if the surface is is covered with an iron on film. By the way, epoxy sticks like crazy to Profilm/Oracover.
In an attempt to find some way of aligning the slots in the tailplane accurately, I decided to make my latest project tailplane/elevator, out of a sandwich of two half thickness's of wood, in the belief that it would be easier to create a slot on an existing fault line so to speak.
Two problems immediately showed themselves. Not having the luxury of being able to get two sheets of wood of the same density at the local model shop I ended up with the top and bottom sheets being soft and hard. Any attempt to cut a slot always drifted into the soft wood, pushing any hinge off centre. The second problem was the glue I used to stick them together.
I prefer to use Evostick contact adhesive for sheeted areas, but having none to hand, used some 3M Craft Mount spray. Big mistake. As soon as it tried to push a hinge into place the two sheets started to separate. Using backup plan B, I re-glued the halves together with epoxy. Not a good choice as the stuff is almost impossible to work when set and again forced the slot off into the soft wood.
sitting back and having a rethink, I decided to pre slot the two sheets before gluing them together.To a certain extent this works, but judging how much material to remove is not so easy. But, I reasoned, if they are still too small at least it will be easier to open them up once the two haves are joined. This turned out to be true up to a point. any presence of the epoxy in the slot made it extremely difficult.
Some sample pictures of how the slots are made.
The second tailpane unit went together a lot more easily than the first, although due to using an inappropriate glue and two different densities of wood it still took time to get accurate fits. Correcting these choices should make life easier the next time I try, as I still think I am onto a reasonable idea.
Following the reasonable success of the Red Thing design for a 1.5cc engine. I pressed on with a 2.5cc version. I have a couple of KMD Russian team race diesels I wanted to try out and also to effect some minor changes from the original red model. Once a carboard templates have been made for the four parts involved, it's possible to make one of these models in a day. Painting it takes a week.....? I may try Profilm on a future one, just to see how it stands up to the rough and tumble.
One change I made was to mount the bellcrank on the inboard wing, in an attempt to make the construction simpler and and quicker. Whilst this does indeed make it simpler to build, it has the unfortunate side effect of making the inboard wing dip downward on takeoff. Not enough to make nasty things happen but something that i would rather do without.
The paint finish was experimental. I substituted water based polyurethane varnish for the normal dope based processes. This would not work on an open structure as the varnish has no shrinking properties. A coat of varnish was applied to the bare wood and rubbed down when dry then a layer of light weight tissue was varnished on followed by another layer of varnish. Each coat rubbed down with wet and dry paper between coats. Finally a coat of grey car primer, followed by a few (yellow does not cover well) coats of car spray paint, which seems impervious to diesel fuel and glow. This whole process at least cuts down on 50% of the fumes and smells.
The only minor problem was, the polyurethane is quite hard when dry and takes quite a bit of sanding. Thinner coats would probably help. The most surprising thing was that none of the paints reacted with each other.
KMD racing diesels
I must say, I like KMD 2.5's, the one in the pictures was rescued from a batch of Russian engines that cost me £10.00, all in a pretty sorry state and not even sure any of them could be made to run. The KMD felt like the bearings were seized, but after dismantling, it turned out the the crankshaft needed shimming to increase the clearance between the front and read bearings which were being pressed together when the prop nut was tightened. Quite how this situation had come about I have no idea, but after a bit of T.L.C. it now runs nicely.
There is a rumour that the KMD's with a black cylinder head, which are the
originals, have a better performance than the plain sand blasted head variety.
And here is a little bit of history which might explain why.
Having tried the plain head KMD followed by an original black head, I can say that there is an approximate 16kph/10mph difference in airspeed. Even taking into account the unknown state of wear in the engines, this is a considerable difference. What this difference equates to in watts/bhp I have no idea, but it's definitely on the plus side
Even the slower engine can give some good flying as it starts reasonable easily and runs very consistently. As they can be had at various places from £20-£40, even brand new still in box versions don't go for much more, eBay is good place to look, I don't think you can go far wrong with one if you just want to do some none too serious, fun, club team racing without a expensive capital investment. What I spend on control line models and engines is small change to some of the RC flyers in our club. I have ended up with three (don't ask) and would be reluctant to part with any of them.
Whilst on the subject of Russian engines, I finally got one of the MDS 1.5 diesels to work properly. with a 6.5x4 prop it revs quite nicely. The technique for starting seems to be not to move the compression lever from it's running position when cold, but open the needle two full turns. The usual priming and choking will get it to run and warn up, after that the needle can be closed to it's running position. Using a piece of large bore silicon tubing and an alloy restrictor in the end, the noise is reduced considerably, and does not seem to dramatically reduce the power output. It's a cheap and cheerful way of making a silencer for a rear exhaust motor. I have not had any trouble with the pipe coming off the exhaust port, as I was expecting, as you can see it is not even clamped with anything.
My latest project consists of trying to build two (what the hell, if I manage to become competitive I will need a spare.) .40 powered basic Hellcat carrier models. After the Wildcat, some things about this design seem a bit over complicated, but no doubt done for a reason, as I found out when I tried to mount the bellcrank assembly where I thought it ought to go. This lead to the realisation that, it's such a big assembly I would loose a good percentage of the wing area to drag. This led to having to strip the centre section sheeting and replace it where it should have been in the first place, as it acts as a support for the bellcrank mount on one side.
A Bit of Free Flight
A recent Saturday was spent at the Grantham Club's site at Barkson Airfield, anyone that has been to the UK Nat's will know how large this airfield is, RC, CL and FF can coexist and not see each other, no noise restrictions either; all I can say is, lucky sods. I was flying CL which confirmed what I already knew, models need a completely different undercarriage setup to fly on concrete after grass. Bob and Ken Stanley took along some free flight models. Bob's MVVS 1.5 cc powered Mini Super took off to perform an almost perfect loop when the engine leaned out on take off, it survived with only superficial damage. Smaller engine needed I think.
Ken's model, a Tomboy, was powered by an EMBEE 75 diesel. The reason I mention this, is that the original maker of these engines still frequents our club in the guise of Pete Moore. They were only ever made in limited numbers and are really collectors items; to see one actually being used is interesting. It has the ideal power output for a model of this size. It made quiet a few successful flights that day. As I started my aeromodelling with FF models it was quiet a nostalgic feeling watching it lazily circling round.
Proposed Carrier Event
Proposed Carrier-Comp/Workshop/Get-together, call it what you will. Anyone reading last months appeal, will know I would be prepared to hold a carrier 'event' of some kind at my club site.
I have manged to find two people that have expressed an interest, that makes three including myself. Fellow club members are a being a bit vague at present; so whats new? I want a minimum of four people to say, "Yes, I have a model I will come." and I will press ahead. this means I am short of one body. So if you are interested please get in touch.
My club only has one circle so you can appreciate I need some commitment to be able to ask for exclusive use of it for a day. We are having another circle prepared which will alleviate the problem somewhat, but it is lying fallow at present and will be quite a few months before it is remotely usable.