A return to model building and flying. Part 4

Frustration with a screw

June 2004

The next step in this saga was to make sure the engine could be securely mounted. In the past I have seen engines come loose in flight and even fall out; not only damned inconvenient, but dangerous as well. I also lost one of my competition stunters because the wing bolts worked loose, so you can probably understand why I take this matter seriously.

OK I thought, lets get some shiny new nuts and screws, my existing collection consisted of lots of old, recycled, and oily, BA sizes. Every DIY store and corner shop carries packets of metric M3 screws and nuts, so I purchase some. What followed next was both annoying and frustrating.

To understand the complexity of this seemingly simple task, I had better explain the method I had devised over the years that was pretty foolproof.

fixing boltsThe mounting consisted of the screw heads being soldered onto a thing steel or tin plate the screws passing though the bearers with two lock nuts tightening down the engine from the engine side, Nylock nuts work as well as two conventional nuts locked together, as long as they are replaced if removed a few times. They probably would work fine a quite a few times, but is it really worth the risk.

Now, for some unknown reason, I had it stuck in my head that the sides of the fuselage covered the engine mounting lugs, so I would not be able to get conventional spanners in to lock the nuts. No problem, epoxy the nylock nuts on the far side of the bearers, Soldering to a plate is not an option as the heat can destroy the nylon ring. Now whoever made these damned nuts, made them so tight I was stripping the screw slot in the head trying to get the screw into the nut. I doesn't take much imagination to see what happened next. After the epoxy had set and I tried to remove the screws from the nuts they just tore off the wood.

By running a taper tap though the nut I could have eased the pressure a little and made them usable. Unfortunately I only have BA taps and dies and one M4 tap.

M3 is a very common size of screw, so lets look round for small size metric taps and dies, and also a box spanner to fit said nuts, very useful for getting at nuts in narrow recesses. After an intensive search of the all the UK suppliers I could find, I discovered that trying to find a small metric box spanner is something of a joke; I wonder if any exist at all. M3 taps and dies are equally rare, and when I did manage to track some down; expensive!

So here I am stuck with the most common size of screw imaginable that I can't use.

nutsIn the end I gave up the struggle and sorted out some of the least grubby 4BA screws and found a packet of blind square nuts which have sharp corners that dig into wood to prevent turning. OK., so now I screw the screws through from the engine side; but that is still not vibration resistant. After another fifteen minutes of searching through a pile of nuts and washers I finally found four spring washers of the correct size, and that combination will have to do. A totally unsatisfactory solution as far as I am concerned. Hence my frustration.

The final annoyance, was when I found that, I can, get at the sides of the engine mounting lugs on the finished model, so I could have soldered the the screw heads to a plate and used two locknuts after all. By this time I had wasted a full afternoon on this problem, and was so fed up, I just blocked in the front of the fuselage covering everything up and tried to forget about the whole episode.

fuselage
The state of the fuselage at present.



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