Plywood/balsa screw hole repair

Judging by my articles and/or forum presence, you may get the feeling that I'm an almost exclusively the depron guy. And I do in fact love this material: it's light, strong and stiff (as a foam at least), easy to work with and (at least in my local area) relatively cheap. But it's not good for everything, nothing really is; and I really wanted to go ahead and try something more advanced - both in terms of construction and flying qualities.

The most logical next step for me was balsa/plywood aerobatic airplane; however I do agree that doing two new things at once may not be the best idea. It may work with foam planes because making mistakes at any point is really cheap and easy to fix, but it's not the same for balsa/plywood planes. That's why I decided to buy my first balsa plane, learn to fly it, study how it's built and then try to build new plane on my own. It took me a long time to decide what plane to buy: I didn't want to go very basic (you always need to challenge yourself a bit, right?); but had enough common sense (and not enough money) to rule out "going big". Well, that didn't play out in 100%. The story is a bit long, but I ended up buying 5 years old, one-of-a-kind (literally!) 1,15m (45in) Funtana in a bit dusty, but flyable condition.

The beauty was laying on the shelf for 2 years collecting dust when the previous owner decided to sell her via i-net auction. I couldn't believe when I won it for just above $50. Another "fifty bucks" for motor and ESC and she was ready to fly. I have to admit, I was a bit scared when taking her for the first flight, but She did not disappoint me. The flight was a blast, and even being surprised by the maneuverability, I had no problems flying some basic aerobatics and bringing her down safely. Everything went well. Almost. One of the landing gear legs collapsed after quite gentle landing. Brief examination revealed that one of the mounting screws (the metal one) was torn out of the wood, and the remaining nylon bolt was just not strong enough and broke. Fortunately, nothing else was damaged, but she couldn't fly again without a repair. 

The image below shows the original screw hole (the small one, inside a rectangle of covering film removed, slightly to the side), new desired location of this hole (plus it's intended size) and the nylon bolt hole (the big one on the top).

Even though I never built an airplane like this, I know this actually is not a big deal. Landing Gear leg was not damaged, and the fuselage part (the one the LG is actually screwed into) was not broken or even cracked. The only two damaged things were the nylon bolt (which obviously needed to be replaced) and the metal screw hole. The hole itself is tiny (even after the damage) compared to the surrounding material, thus the easiest and the most simple repair would be to "replace" just a tiny bit of the wood around the old hole and drill out new one.

To "replace a piece of wood", one needs to remove the old (damaged) one first. It is wise to remove it in the way that will make inserting new piece as easy as possible. In this case, I started by drilling out the damaged hole with larger drill size. This was done solely to be able to insert the next tool inside. The next tool I used was a Dremmel with small milling bit. I used the bit to make the hole more rectangular and big enough to accept the smallest replacement block I could actually build.

While milling out the hole, I foud out that the material used for this part of the airplane is not aviation grade; it's a "furniture plywood" made of three very thin layers of hard, high quality wood and two thick layers of soft wood. I figured out it's just a matter of time for the other LG leg to break off the same way; so I "proactively fixed" it as well.

The new piece of wood is made of 3 layers of 1.5mm av-grade plywood (3 layers each) glued together. Since the milling tool is actually round, the hole it created have rounded corners. To fit the repair block inside (and tight), I had to sand it down to proper size and shape.

As you can see, the replacement block fits perfectly into the hole. It was just a tiny bit too thick, so I had to sand it down a little from the top. It was definitely easier than trying to make the hole just a tiny bit deeper.

After that, it was just a matter of some two part epoxy, fitting the LG legs back to their place, drilling new screw holes and installing the screws; and the One-of-a-Kind scratch-built, wooden Funtana was back in perfect shape, ready for her long-deserved re-maiden.