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Saturday, April 12, 2014

Yak EXP Motor Box Repair


 
Recently I had a really nasty prop strike that took the motor box out of my test Yak.  Nothing else was damaged, and since a motorbox kit is only something like $16, I thought it was worth taking a stab at fixing.

The motor box kit comes with a lot of stuff, including the first former and enough pieces to repair a broken landing gear block. I didn't use all the pieces because mine was not damaged that badly. I only needed the two sides and top piece. I was lucky it was not that bad. In fact, I was pretty lucky through this whole project.

The first step was removing the broken motor box. For getting the box apart, once again it's Golden West Super Solvent, which is available at Tower Hobbies and most wood working stores. Some hobby shops even carry it and I love the stuff. Nothing even comes close for cleaning nasty CA spills off of covering or just getting CA to let loose so you can get things apart. The big surprise is that it completely destroyed the glue that the factory uses on the motor box. I think it is some sort of alphetic resin kind of white glue or something and I was surprised that CA debonder would work. It just ripped right through it and I had the pieces I wanted out in about 20 minutes.

Click Pictures To Enlarge
 
I was really fortunate because the bottom piece was undamaged. Just the side and top were broken,. This looks like the hardest piece to get out so I lucked out here. I had to do a little cleaning, scraping with an Xacto and sanding with an emery board, but it cleaned up real nice.
 
You may have to peel the covering back a bit around the top of the fuselage so you can get the top piece off without tearing the covering too. Be careful not to get any debond on the covering because then it won't iron back down as well. Get it all ironed back into place and flattened down and sealed before you start fitting the new parts.
 
Next I dry fit everything together one piece at a time. Work slowly and be patient here. You may have to trim a tab or open up a slot here and there, but the parts fit pretty nice. I started with the side pieces and saved the top as the last part.
 
 
 Here is the motor box all mocked up except for the top piece, which I'll show next. Don't glue anything yet.
 
 
I did not yet have the motor box kit when I took the old box out of the Yak. On the planes that come out of the factory, the top piece of the motor box is one piece of plywood all the way back to where the turtle deck begins. However, the top piece of the motor box kit only goes back a few formers. This is why it's a good idea to have the motor box kit before you start taking the old one out ....so you can mark the top piece and only remove what you need to remove.

Of course I didn't know this, so I took the top off to a certain point and then cut it so I would not have to remove the whole thing. I accidentally guessed pretty close to right. Again, I was pretty lucky. I trimmed about 1/2 off the rear of the new top piece and it fit fine,

Here I compare how the plane finished up to how the motorbox top piece looked before I cut it to fit.


Sorry about the blurry photo, but here's the box totally mocked up. I used the old firewall because it wasn't hurt and it saved me the work of fitting the blind nuts. Basically the thing snaps together like a jigsaw puzzle. Any trouble I had with fitting was just old glue that I hadn't gotten completely off the tabs in the wood, so a little scraping with an Xacto #11 fixed that. I suppose it took me maybe 20 minutes to get it all locked together and then I soaked all the joints with thin CA. After that set up I used mediun CA to fill any gaps and form a bead around joints.


Finally, just because summer is coming I wanted to be wary of engine and ESC temperatures. I've never had any overheating problems, but the baffles do give you some additional cooling.


I flew the Yak a few times today and it trimmed out and tracked just like before. I really can't tell any difference in her performance, so I guess it was a pretty good job!

I love the Yak. I can't wait for the 60" version.






Here's a bonus for those of you who made it this far into the article.......