Servo mounting tray/hole repair

In the previous article, I described how to repair a tiny hole in a large, thick part of wood - the kind of wood you expect to see around the landing gear mount. This time, we'll try to do a similar repair in much smaller, thinner wood - such as servo mounting tray, usually made of thin, soft plywood. Or like in this case: 3mm balsa wood.

The case leading to this repair necessity is by itself worth mentioning, so please let me share some background. The plane is several years old Katana Mini, manufactured in Australia by Precision Aerobatics. Just like the Funtana described in the previous article, I found this one on the internet and acquired it for less than $100 with all the electronics minus receiver. You have to agree this sounds like a bargain; but in cases like this, there is always a catch. And there was one, or actually three. First of all, the plane was already repaired at least once, including new LG mount and motor mount rebuild. The second thing was the motor itself: very weak, far from the one recommended by the manufacturer. I can live with both, especially since the motor/prop combo is extremely well balanced and unbelievably quiet.

But the third catch was... the way PA advise to secure servo mounting screws. I do believe there was something changed recently, but the one I've got have balsa servo trays all around, and the manual advises to secure the screws with thin CA glue. This method works ok for short-term exploitation; but the CA glue has a quite nasty habit of seriously degrading the wood over long periods of time. As a result, the plane I bought (advertised as: "immediately ready to fly") had this one surprise waiting for me:

On the picture above, the covering film is already removed around the holes. It was way harder to notice the problem when the holes were still covered; even more with the servos still in place. The previous owner swore he didn't know about the problem, and I can believe him: this can actually happen "by itself" over long enough periods of time sitting on the shelf. That's exactly why you always need to take the used plane apart and check everything before the re-maiden, even if it is sold as "ready to fly". 

Now let's repair the thing. Just like before, we need to remove the material around damaged part. This time, there is much less material to work with and the surrounding structure is much less durable; thus, much more caution was required and no power tools were used.

I cut out a strip approximately 5mm wide and 10mm long. This would be replaced by a soft plywood of the same thickness. But the 10x5mm strip has not enough side surface to securely glue it into the airframe; in such cases, adding another piece of wood is in order to increase the glueing surface. The second strip needs to be longer and glued in below the "replacement" piece. It should also be wider, but there was no room for that in this case. Here's like one pair of new strips of plywood look like just before glueing in (the top one) and the other one already in place (the bottom one).

As you may notice, each strip needs to be manually trimmed/sanded to fit the hole as perfectly as it can; which is not easy when the part is this small (less than 1/4 of a finger nail). The installation itself is also very tricky. I used 5min epoxy to join the strips together, and immediately after that - when the glue was still curing - to fit them in place using very tiny tweezers. Then I held them with my own hands for a couple of minutes. And it worked.

It's time to install the servos again; and to do that you need to drill out new screw holes. And here comes another problem: the hole is supposed to be 1mm in diameter, but I don't have such tiny drill. But there is no need to have a drill when working with soft wood. All you need is a steel wire a tiny bit smaller than you need (in this case: 0.8mm) cut with... dull wire cutter. This leaves sharp tip at the end of wire, which works perfectly fine as a drill when you are careful enough:

Using low-speed setting and applying very gentle pressure, you can use the wire to drill out very nice hole:

 The repair is almost over. All you need to do after that is to install the servos in place. I had to use a file to fit the servo inside: it turned out that the strips were a tiny bit wider than needed and/or the excessive glue made the hole smaller. Both are very probable when working with such small sizes. That's what the files are there for, right?

With all the servos back in place, I finally could take the beauty for a spin. Even though the motor is very weak, she is still a real beast in the air with immediate, crisp and dynamic responses to all control inputs. Without the repair, I believe I would lost her in the very first flight. Now I'm absolutely confident the aileron servos will never fail unless because of my fault.