After building a second coax and still getting unacceptable levels of vibration I decided to shelve the whole project until winter time and it was too cold to go out and fly. Well it's the first week of January and while the rest of America is under snow, Florida is merely freezing, so now its time to get to work.
I decided to rebuild the first machine and during the summer I had been organising various items which I knew I would need. The most important thing was perfectly straight shafts, and that wasn't easy. Volker sent me one which he said was perfectly straight. It had 3 thou runout, which is very low. I had some more centerless ground and of the 4 I had done, one was 2.5 thou and the other was 0. Perfectly true. The other two were 6 and 8 thou out so they were put to one side.
The first job as to strip the mechanics apart and clean and check everything. When I was doing this, the main gearbox felt very rough which was worrying, but when I pulled the 20mm mainshaft, I found the bearing, which was a little notchy to start with, had turned into a pock marked nightmare. So that got replaced.The gears were mounted on the shafts and the bottom one had a 3rd fixing hole drilled in so it could be perfectly centered on the shaft. Green loctite was liberally applied and the whole lot left to set overnight.
Next morning I had these parts ready to assemble
Another hours work and it was coming together nicely
I decided while I was on a roll, I would clear most of the stuff out of the way so I finished the basic assembly
Progress tomorrow will not be so quick. I have to remake the long push rods with 4mm stock and metal ball links, tear apart each rotor head and check it very carefully and then set it all up again.
That was an interesting couple of days work. I had noticed that some blade holders would turn 360 degrees, others would not as the pitch arm would catch the hub. This meant that there were variations somewhere so the first thing I did was strip both heads down and clean up the axles ready for the micrometer. I found 2 axles bent, so that was an immediate source of vibration problems. One of the pitch arms was a bit battered and all of the thrust bearings were notchy. I had a new pitch arm, so that was no problem. I am trying to find bearings and that may be a problem, and I had some Hirobo 10mm main shafts which were easy machining material. So I machined up 2 new axles and rebuilt the lower head without the blade holders. Another part I was not happy about was the 0.1mm shim placed between the inner bearing and the o rings. These were all deformed into a cone shape where they had been pushed against the o rings
I found the blade holder mounts were very variable in the way they turned. Some were stiff, one was loose. I checked with a Robbe head which was made in the same manner and the blade holder on that head was very easy to turn. I measured the distance between the hub and the outer edge of the thrust bearing and found the loosest one was also the shortest by 1mm. That was a surprise as the friction is caused by compressing the whole assembly against some o rings which did the feathering and flapping damping so in fact that one should have been the tightest. Examining the o rings showed one was worn where the pressure had been too high and the shim had rotated against it and worn it down. Replacing the o rings with some new harder ones Volker had sent me made all the blade holders very difficult to turn
So I made up some new axles 1mm longer than the originals which allowed me to use some thicker 0.5mm shims Now, when I reassembled everything the distances between the hub and the thrust bearing was less than 0.1mm variation between the three and the friction level was very low, in fact, one blade holder would fall under the weight of the pitch arm, but it had a good thrust bearing. So now, I have to wait for more Hirobo main shafts and try to find some good bearings. I do have some with a slightly larger bore and if I cant get the 8mm ones I will machine up some new shafts with 9mm bore and use the other thrust bearings I have in the bottomless junk box. This is how it looks without the blade holder mounted
Well thanks to Jon Tanner of Model Helicopter World and a good friend in the UK, some of the correct bearings were sourced there and shipped over here quickly. All of the blade holders have been reassembled and they are all reasonably similar in friction. Of course, one had to be different from the others and it took a while to find the problem, but it turned out to be one of the thrust bearings slightly too thick. When I measured the distances between the centers of the hub and the edge of the bolt hole for the blades, they were within 0.1mm of each other.
Meanwhile I had machined up some Trex 700 flybars to make pushrods. The difference is obvious when compared to a stock one. I also fitted some metal ball links for extra security.
Today I put everything back together and fitted all the parts. Needless to say something isn't quite right so during tomorrows 80 degree sunshine, I will be trying to figure out what went wrong this time. However, it's starting to look real and I am sure it wont take too much sorting as everything has already been built once.
After a lot of messing around getting all of the blade holders at exactly the same angle at mid stick, I took it out for a test flight. It was 65 degrees flat calm and blue skies. Perfect testing weather. On spool up I got a few vibrations in the tail boom in various different directions as the blades settled into their correct location and then on lift off I was amazed to see the tracking was spot on on both disks. There were no waves in fuel tanks but the tail boom was going up and down about an inch. The tail boom is quite thin and not well supported and it occurred to me that maybe the blasts of wind coming off the blades were causing the up and down movement by blowing on the very large horizontal stab. So, I took it off. Now the tail boom moves less than 1/4" and the heli is very smooth. It's the best it's ever been.
So what did it take? A ground main shaft with 0 runout. New axles machined to tight tolerances and slightly bigger to allow the washer to work on the dampers smoothly. New thrust bearings, machined blade holders to accurately place the pitch arms and 4mm push rods with metal ball links. Now it needs the tail boom sorting and its good to fly. Meanwhile, there's another one waiting to be done so it can go into the Kamov fuse, but first there is a 212 with a PHT2 calling to me.