Hi All,
I thought you all may be interested in the following
- We have finally got our replacement prop from Hoffmann (10 mths) and was rather expensive so when I was told a forklift fork had gone thru the box in transit my reaction was rather predictable. Fortunately it was not damaged
- Before this happened, I had contact with Airmaster props here in New Zealand. They could produce a really nice little constant speed prop (electrically operated) which would be a real plus for the L2000 Dimona”s. However I got a call from the head honcho of NZ CAA airworthiness who totally rejected the idea. (being a type certified aircraft) Hence the Hoffmann prop. However now the Limbach engines no longer have EASA approval, that may change the H36 type certificate to enable “special Category” or even “experimental” so if there is anybody interested in installing an Electric Airmaster prop that will fit on an L2000 Limbach (correct rotation) let me know as I have organised 2 blades (Made by Sensnich).
- I think the cracked ferrells of the props has been caused by excessive temperature and humidity levels in our uninsulated, shingle floor and wet hangar over a long period of time. So I am thinking of a controlled heated insulated blanket over the prop and engine. Raychem Autotrace heating cable seems a good suggestion or a slightly modified electric blanket.
- Currently I have almost finished an annual inspection on H36 S/No 3667 ZK-GPH It has been a real mission. I found a turnbuckle fork end of one of the thrust cables under the engine was broken .. and had been so for a long time so was held only by lock wire. Quite strong lock wire I reckon
- I have ferried it around NZ for subsequent owners and the aileron control was unacceptably stiff, to a point of “sticking” when operated. Making it a really unpleasant little rocket to fly.
This H36 was not a Thai one, but was imported to NZ from Italy (tho German registered D-KEBA) in 2010. However it does have the early “aileron push rod in compression when both ailerons are forced upwards. This was a design problem which was corrected with later serial No’s which had the rods in a tension mode. The aileron bell crank for the older Dimona’s is fabricated from tube while the later (tension) bellcranks are made up of flat plate and there is only 3 bearing sets in the wings as opposed to 5.
Now for the jucy bit ..
We made a detailed inspection of each of the 10 bearing sets (5 per wing) with a small camera and found some of the bearing sets (3 ball bearings pointing inwards on a 3 piece metal “holder”) mounted on wooden mounts ie the control rod passed thru the bearing sets and through a hole in the mount. We found that some bearing sets were not installed dead centre of the mounting bracket hole The over all clearance being about 1 to 2 mm.
The upward load on the ailerons of the H36 when doing say 90 Kts is REALLY high. And I think is underestimated. Hence the compressive load into the pushrods is also VERY high forcing the rod to bend like a snake. Putting a high lateral load to each bearing set.
So with misaligned bearing sets some of the bearings were not touching the rod hence “rubbing” on the wooden bearing set mounts. So that is why the ailerons were really stiff in flight but seemed OK on the ground
To repair was a mission. We didn’t want to put holes in the wing so designed a cutter system which we made from a short length of brass turned it down to 18mm, put an 8mm thread in one end, rounded off the other end and drilled a hole thru the thing used a 6mm drill cut off and a cutting edge ground so it protruded from the brass”slug” about 3 -4 mm. we then used an old push rod with the “slug” on one end and a battery drill at the other and with great care, removed the mounting bracket material as close as possible to the metal bearing fittings.
So it still needs a test flight but problem solved. I think Hoffmann has issues because prior to changing the push rod configuration, there were about 5 different variations of bearing sets in the wings.
If any of you want to see some pictures of the procedure let me know.
Our Dimona (ex Thai) bearing sets had a solid nylon outer including the outer ball race, so we had no choice than to make big holes and replace them. As they all were seized Another Ex Thai (currently being restored) didn’t have ball type bearings, just Teflon (or nylon rings) S/no 3667 fortunately had proper bearings (ie with a metal outside ball race) and a nylon “tyre” against the push rod.
So I thought you all may be interested. I am astounded that the aircraft has been flying for over 40 years like this and would be interested in your comments
Best regards
Ian Williams
Drury
Auckland
New Zealand
