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.

Here's what comes in the motorbox kit. It's all dry fir together in the picture, but I'm sure you get the idea how complete the package is. If you've torn up much more than this, the plane probably isn't worth fixing anyway. The way mine worked out, not I've got spare wood for other repairs down the road, which I hopefully won't need but probably will.

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.......

Yak EXP Pardon My Yak from Doc Austin on Vimeo.

 

Yak EXP Pardon My Yak II from Doc Austin on Vimeo.

The Zonker Rides Again!

I've been flying my Screaming Yellow Zonker 64" MXS so hard for the past two years that the checkerboard finally blew off of the tail! I lived with it for awhile by putting a big Extreme Flight roundel on one elevator, but the whole time it kind of drove me crazy. As a result, I was fully intending to kill the plane so I could buy another one!

 

Of course, I just couldn't bring myself to fly her that carelessly. I have always loved this plane and often refer to her as "the best plane ever." That description is not off the mark, either.

 

This one doesn't owe me a nickel for a lot of reasons. First, I have been hammering the snot out of it for about two years, and we have also shot a ton of video with it. It's been a most awesome plane, and I think this is probably the best flying plane I have ever owned or even flown. It's been simply perfect, except, that is, for the peeling Ultrakote.

 

Here is the last video we shot with this plane, and you can see the checkerboard is missing from the left side. She's still a good looking plane, but the asymmetry makes me a bit crazy. When the checkerboard on the other side started to peel, that was when it became time to do something one way or the other. 

 

MXS EXP__Just Fly from Doc Austin on Vimeo.

In a break with our traditional format, video of the restored plane will be toward the end of the article. If you want to get straight to the flying, just scroll down to the bottom.

 

 

I stole the power system from the yellow Zonker to get my 60" Edge EXP flying, and then hung her in the garage. There was just no more room for her in the house, and I suppose I was going to sell her if I ever got up the heart to let her go. Whenever I would be out there working, I would catch myself stopping to admire her, and that's when I realized that I either needed to get another one or recondition the current one.

 

Obviously, I wanted to fix the checkerboard, but there were also a few other things I wanted to tighten up. She's had a hard life needed a thorough clean up and inspection.

 

 Click To Enlarge

 

Control System

Everything will wear out eventually, and since this plane has been flown so hard I was expecting to find a few things that would need replacing. I was actually mildly surprised that there was so little to do though. The plane has held up remarkably well.

 

However, I found an alarming amount of sloppiness in the control surfaces. Checking the hinges, linkages, ball links and control horns, everything seemed tight, so all that was left was to incorrectly blame the problem on excessive gear lash in the servos.

 

This turned out to be completely wrong, because in reality all of the looseness was coming from the aftermarket servo arms, which were not Hi Tec arms. Apparently the splines in the arms had spread out under load and now we had a sloppy mess with the arms wobbling around on the servo output shafts. In 35 years of RC I had never seen anything like this, so it kind of blindsided me. I lived with the problem for awhile thinking it was the servos, but eventually the plane started to fly badly.

 

The last thing in the world you need on a high speed precision/3D ship like the MXS is sloppy control response, and I was starting to get enough of that I was not enjoying flying the plane. Previously the MXS was always super locked in, so when it started wandering off course and losing trim,  I became considerably less happy with it. The plane didn't fly well and lost it's precision edge. It just took the fun out of flying it because I was always fighting the plane.

 

Fortunately it turns out the fix was easy and inexpensive enough. I switched to the  Hi Tec PN55709 servo arm set. I have used these types of arms before flying a 65" plane with HS985MG servos. The splines on the two servos are the same, so it was a perfect fit. Since these arms were no problem on a bigger, heavier plane, they will work just fine on this one. I also tested them on my 60" Edge and they have so far stayed tight.

 

 

 

A nice side benefit is that the set comes with extra servo hardware such as grommets and screws .... stuff I am always scratching around for anyway. Most importantly included is the bolt that holds the servo arm on....you know, the one that you're always losing. It's great to have spares.

Servos
While I already had the plane apart, it was a great time to send the servos off to Hi Tec for a good cleaning and inspection. They have had a hard life, and are nearly two years old. A good cleaning and service is simply inexpensive insurance. Hi Tec service is top rate and they usually don't charge me for anything (and did not this time either) other than whatever parts the servos need.

All of the servos were still operating properly and centering fine. Three of my four servos had reasonably tight gear trains. The rudder servo has some slop in it, and this turned out to be a broken case, presumably from the several ground strikes it has suffered. That was a $16 repair. Considering the usage I've gotten out of these servos, I don't think that's too bad.

Servo Deadbands
The servo's deadband adjustment controls how tightly the servo centers. A high number gives you poor centering and a low number gives you better centering. That's as technical as I'm going to get because that's all I know about it!

When I got my HS5245MGs, some of them were coming with the deadbands set improperly. That's how I found out about deadbands because mine were set high and centering poorly. Knowledge is everything, and once I learned about this it was a simple matter to adjust them.

Hi Tec assures me this problem has been addressed, and 5245MGs I have bought since have been fine. Still, I always check the programming on any new digital servo I get. Since there are so many different brands of servos, it is probably pointless to do a how-to article, so instead below is the process I used for programming mine. First I clear the programming, which sets everything to default. Sure enough, as you can see, the deadband is reset to the 5245MG's default setting of 2. In the last picture you can see I turned it down to 1 and saved it.

While this might seem unnecessary, it is. I just check he servos for my own peace of mind, and doing things like this in advance can ward off potential problems in the field.

As you can see, I bought the HFP-25 Digital Servo Programmer & Tester. I like this unit because unlike most programmers, it is completely independent from a PC or laptop and has it's own power source. I keep it under the seat in my car so I can use it at the field or lend it to friends who might need it.

I still need to tinker with the unit and learn how to use all it's other features. It tests analog servos too, so I am sure once I learn how to use it properly it's going to be an even more useful tool.

The Unfair Bad Rap 

The HS5245MGs have unfairly gotten a bad rap lately for being sloppy and centering poorly, but I attribute that to the aftermarket servo arms being so sloppy. If the arms are flopping around loose on the servo, you're damm right the control surface isn't going to center well.

 

While there's some less complimentary talk on the Internet forums about these servos, mine have been pretty close to golden. I always check the dead bands on the servos when I get them, and they center very nicely.

I've also never gotten any stalling or blowback with one of these servos. You can see for yourself on any of my videos how hard I push the plane and how the servos never let me down. In the future I do plan to try some of the much more expensive HS7254MG servos, because expensive servos are generally worth it. However, the HS5245MGs have served me very, very well in these 60" planes and they have been worth every penny.

I take great pains to make sure there is no drag or binding in my linkages and I always seal all my control surface gaps. I also use an Airboss ESC so I am sure that the servos are being fed on a full 6 volts.

In the end, the truth about these servos is that if you set them up properly, set the plane up properly, and try to take care of them, they will serve you extremely well.
 

CONTROL SET UP 

Before I took her apart, this plane was set up exactly the way I wanted her, so I tried to be careful not to lose that when I changed the arms out. Fortunately, the hole spacing on the new arms is exactly identical to the old arms. That just made the whole process easy. 

 

By contrast, a pack of the arms that were causing the problem cost about $12, whereas the Hi Tec arms, which work, only costs about $4.95.  Talk about a no brainer.

 

Of course, I deleted the plane's memory from my TX, so I have to start over, but this plane takes so little mixing, and I use the set up from the manual anyway.

 

Ailerons

Using the outermost hole on the smallest arm from the pack, I'm getting exactly the same throw as I was getting from the aftermarket arm. The only difference is the aftermarket arm was one hole longer, but I always used the second hole anyway. I am going to get the same amount of control deflection. 

 

  

 

Elevator

Elevator throw is unchanged because the single large Hi Tec arm is the exact same size as the aftermarket arm that I was using before, only again, one hole shorter. Again, I use the second from the outside hole anyway, so the Hi Tec arm duplicates what I had before. I'm getting about 65 degrees of throw, which on a 60" plane is considerable. We are so used to seeing 88 degrees on the 48" EXPs that 65 degrees doesn't look like much, but it is. The pitch authority feels about the same as a 48" Extra, which to me is perfect.

 

 

I don't think I could get any more throw anyway because the elevator pushrod is almost hitting the bottom of the stab at full deflection.

 

 

 

Rudder

Rudder is a little different. There was only one long single arm in the pack, so I cut one side off the shorter of the double sided servo arm. 

 

 

 

 With these arms, nothing really changes except they fit tight and stay tight on the servo output shaft. There is still a minute amount of slop, but the new arms are so much better that this was worth doing. Basically I just saved myself the cost of four new servos by spending less than five dollars for good servo arms.  

 

Eventually I will put an SWB aluminum arm on the elevator because those take out every little bit of play. They have a cinch bolt that tightens the spline down on the servo output shaft. The only problem with those arms is that my LHS doesn't carry them and I need to order some. 

 

I initially got five if them, but used them all up on my 48" planes. The spline is the same as the HS85MG I use on those plane's elevator servos. I was getting really bad slop there too, but the SWB arms took all of that out.    

 

Checkerboard Restoration

One of the great discoveries I have made in the last couple of years is 3M High Performance Vinyl.  Places like B and E Graphics can make you almost anything you want. They just print the pattern on it and all you have to do is apply it. It can be a little tricky if you've never worked with it, but once you learn a trick or two it's easy.

 

 3M High Performance vinyl is also very sweet stuff to work with, so it made sense to go with this for putting the checkerboards back on. I peeled off the bad Ultrakote checkerboard and then cleaned up the mess. Once the clear covering peels off, all that's left is colored goo, and it cleans off easy enough with lacquer thinner, which won't hurt the covering. It just stinks.

 

 

 

There was still good checkerboard on the right stab, so I laid a strip of the material over the elevator and lined it up with the existing checkerboard. Here I used plenty of Rapid Tac decal application fluid so I could float the decal into perfect position.

 

 

 

I measured the right side when I was finished, and then tried to duplicate that on the left. From there I used a straight edge to cut the decal at the hingeline.

 

 

It's not exactly perfect, but I think it's pretty close. It was certainly less expensive and less work than building a new plane!

 

From there it was a matter of checking, cleaning and tightening up everything. Everything on the plane is in surprisingly good shape, so all it took get her looking nearly new was a lot of scrubbing and a new set of wheel pants. The originals had taken a few knocks, but are still good enough to repaint. I'll get to those down the road, but since I had a new set in my parts box it seemed like a good time to use them.

 

 

 Click To Enlarge 

 

 

 

POWER SYSTEM

Part of what put this plane on the sidelines was that I needed the power system to get my new 60" Edge flying. Once financing stabilized from that project I ordered a new Torque 4016/500 V2 motor and Airboss 80 ESC. I've been running the Torque 4016 and it's predecessor, the  Torque 4014, in all my 55" to 64" planes since I got into electrics and have been exceptionally pleased with all of them.   

 

You can see in any of my MXS videos how much crazy power I've got, and the reliability speaks for itself.

 

Everyone has their own way of mounting the ESC, but early on I got in the habit of putting them on the left side of the motorbox. I like the on/off switch to be on the left hand side of the plane because in the old nitro days, the exhaust was usually on the right. Mounting the switch on the opposite side kept it out of the goo coming from the engine and it's just a habit I never got out of. I also like that this places the deans plug in a most convenient and out of the way position.

 

 

I still have to shrink wrap the bullet connectors and tuck all the wires away,  but outside of that, this is what the motor/ESC installation will look like.  

 

BATTERIES

Previously I had flown this plane with Thunder Power 6s 3850 65C Pro Power batteries with great success.  However, they were a bit  large, and therefore heavy for the application, making balancing the plane a bit difficult. I had to add a bit of lead to the tail wheel bracket to get the CG where I wanted it. Weight is bad.

 

When I built my new red 64" MXS, I made an effort to get rid of the extra battery weight and also the tail weight by buying some Thunder Power 6s 3300 70C Pro Force packs. I put Pro Force in italics because I have been saying Pro Power for so long that Pro Force still doesn't sound right to me!

 

With the battery all the way against the wing tube and 1/2 ounce of lead in the tail, the CG is just right. I still hate having that lead there, but it hurts the way the plane flies a lot less than having it be nose heavy.

 

My initial impression flying the 3300 packs is that they are even more powerful than the 65C packs. Of course, my 65C packs are over two years and 400 or so flights old, so it's probably not a fair comparison. Still, the 70c packs give me blistering, frightening power, especially on a fresh charge. That first downwind, full throttle pass is always breathtaking.

 

 

Now the plane feels lighter. I know this sounds a bit nuts considering that we only dropped 550mah worth of weight, but it shows up handsomely when the MXS will just hang there in the air and rotate around her yaw axis in a pop top. The first time I tried it I thought I had hit a monster thermal or something because the plane just sat there, spun and hung in the air. It's probably not quite that dramatic, but I am getting a full turn more with the lighter packs.

 

Later this summer I plan to try some lighter 40C Pro Force packs. I've got so much power now that I can't even use it all. I can give up some and never even miss it, and the lower C packs will get some more weight out of the plane. If I can lose enough battery weight to ditch the tail weight, that will be pretty sweet.

 

Flying

I got my servos today,  put everything back together and took her out for six flights. She's the same MXS I've been in love with from the beginning. With the new servo arms she tracks beautifully, just like I remember when she was new. At the end of the day the arms were still tight and I never touched the trims except to put in a notch of up on the remaiden.

 

After flying out the day my conclusion is that this plane was definitely worth all the effort and a tiny expense. It was a lot of work, but this has been such a special airplane and I am really glad to be flying her again.

 

 

MXS EXP Reborn from Doc Austin on Vimeo.

 

I flew the plane Friday through the weekend every day and fell more and more back in love with it. On Monday my friend Luv3D came to town and we shot two videos with the MXS, plus one with his brand new 64" MXS. We always have a blast and usually tear up everything we bring, but this time we got away with no damage in spite of flying pretty hard in horrific winds conditions.

 

It's tough to make good video in high wind not only because the flying is difficult, but also because the wind usually sounds much worse than it really is, and it blows the soundtrack completely out. I get around this by turning the music way up, but I generally like to have the motor loud enough that you can at least hear it when the plane is in close.

MXS EXP Reborn II from Doc Austin on Vimeo.

 

I have not been flying either MXS much lately because I have been so preoccupied with the Yak. I truly believe the Yak s making me a better pilot, and as such, I am better able to enjoy pushing the MXS harder and with more confidence than before. I think I am just now beginning to understand just how good this airframe really is.

 

It also helps that I have a new attitude about this one. I still dearly love this plane, but I'm no longer afraid to lose it. I got my money out of it a long time ago, and I believe I've shot more videos with this individual airframe than any I have ever owned. When the end comes it will be sad, but I know I can build an even better one next time, simply because I learned so much from this one.

 

Nail: Best plane ever.

Yak EXP__The UTube Files

I've had a few requests to start publishing on UTube so people can watch on older computers and on their phones, Ipads, whatever. I've had so much fun with the Yak lately that I want everyone in on the fun, so please enjoy.

While we're at it, here's My UTube Channel