Broken motor mount repair

Almost half a year ago I described a very simple repair of my first balsa/plywood airplane. I had a balst flying her since then, and she taught me a lot; but this could not last forever. I was practicing slow, high-alpha flights feets above the deck, when the wind gust brought her back to the ground in less-than-perfect manner. At first it didn't look bad; but it turned out that the propeller hit the tall grass at high power setting and - to my very surprise - broke the motor mount off.

First thing to do in cases like this is - obviously - damage asssment. After removing the motor cowling (which was apparently not damaged at all), I carefully inspected the motor mount from all angles. Here's what I've found:

1. both side plates broke in exactly the same locations, where the mounting holes and the weight reduction cutout are the closest to each other. Additionally, sharp corners of the cutout contributed to the stress and caused premature structural failure.

2. glue joints between top plate and side plates were apparently very weak and broke off relatively cleanly

3. the motor mounting plate showed no noticeable damage; except for one missing mounting screw

4.the top plate delaminated in two locations.

5. bottom plate's mounting tabs broke off completely; but no other significant damage was noticed in this part of the structure.

Removing the motor revealed that the whole front and top part actually holds on to the rest of the plane by friction fit and a couple of wooden fibers. After considering all pros and cons of this move, I decided to break it off completely. It required very little additional damage, in return providing way better access to all the parts that needed to be glued back together - especailly the delaminated part of the top plate. To laminate it back together, I used a mix of white wood glue and water (1:1). This mix penetrates the cracks very well and leaves very thin layer of glue after drying out; which is perfect for repairing parts that did not break completely. It works best when the large bonding surface is available - just like in case of delaminated plywood - but can be sometimes used to fill in and glue togehter smaller cracks, especially in soft types of wood, where the crack is too small to use any other solution. You can see the examples of this technique right below:

1. top plate laminated back together

2. small pieces of plywood glued back into place.

While working on the repair like this, it is always good to use a bit of force on the remaining part of the plane and check if everything still holds together nicely. This additional precaution revealed damage location that was missed during initial assesment: the top part of the firewall detached from the top part of the fuselage. It's actually very easy to miss this type of damage since balsa wood is pretty good in hiding small cracks within it's irregular structure. The best way to reveal then is to use some force and check if nothing is moving - at least nothing that is not supposed to move.

The damage itself was fixed using the same 1:1 water/glue solution

Now it was the time to prepare the parts to be joined back together. It starts with inspecting the existing glue joints to check if any of them would not need re-doing. In this case most of the glue joints were fine, except some of them had a lot of excessive glue dried off around them that had to be removed to make the parts fit back together.

After that, I tried to dry-fit the parts toghether, and it looked pretty nice.

All the break lines came together without problems, and with just few tiny parts of wood missing. Due to limited bonding surface and the expected excessive loads, it was best to use two pats epoxy rather than any other type of glue. In my case I used 5min epoxy, because the clean fit allowed the work to be done quickly and smoothly; but in some cases you may need a litle bit more time to put the glue in all places where it needs to go, and join the parts togheter before it cures. I also added two pieces of balsa wood at the corners between side and top plates - the ones where the glue failed without breaking any parts. You can see them on both sides of the motor on the picture below, on both sides of the motor (as viewed from the bottom of the plane):

After installing the motor, I decided to fix the main problem of this structure - all the holes in the side plates located close to each other in a very tight area. I did that by cutting out additional, strenghtening plates - with gently curved lines where the original construction have a lot of sharp angles - and glueing them on the both sides of the motor mount as shown below. I used epoxy for this part of the repair; however it was a bit of overkill. The bonding surface was large enough to use wood glue. I didn't do that solely because of a hurry - I was planning to fly the airplane later the same day.

When the last gue join cured, I finished installing the motor, reconnected the leads, installed the cowling and prop back in their places. The only thing left to do was to conclude the repair to be completed, take the first photo of completely assembled airplane, and take the beauty to the flying field for a spin. This time I was even lucky enough to have a camera crew with me; please enjoy the results of our joint efforts in a form of the video linked below.

Here's a direct link for those who cannot see the video in embeded player: Funtana Passion

and some pictures for those who can't use video streaming at all:

I have to admit, now the plane flies even better than before. I would not bet on that actually; but I'm still going to believe in that anyway. I hope you found this article helpful and encouraging. Please don't forget to rate it accordingly; and if you want to add anything to the topic, share any advice or just simply say "hello", please don't be affraid to use a comment section below.

Good Luck, and Have Fun!