A real good, full and productive day today at the shop.
I'm pulling together the last of the details about the elevator. Started off today by laying out the access hole for the trim servo:
The hole turned out well and deburred nicely. I think I may have to enlarge the hole some, but I've already cut my cover plate as per the plans:
The easiset way to make sure the cover plate is centred over the access hole is to extend the layout lines far enough, that layout lines copies to the cover can be lined up:
The plans call for A4 pulled rivets to secure the cover - I think I'll add riv-nuts like I did on the fuel tank cover of the 701 wing repair. I want access to the servo during routine maintenance inspections and drilling out rivets each time doesn't make sense to me.
Next up was figuring out how the elevator connects to the stabilizer. According to the plans, the centre elevator hinge requires a bushing between the attach bolt and elevator horn and the specs are described in the drawings below.
I cut a small length of bushing tube material and confirmed the diameter is 1/4 inch:
Initial width was brought close to 2.5mm using the bench grinder. Pro tip - hold on tight with a pair of pliers, the busing tends to go flying across the shop if you don't (this was my 3rd attempt!)
Careful hand sanding brings it down to the correct width of 2.5mm
This is where things got a bit confusing. The plans clearly show the bushing extending on either side of the elevator horn - 2.5mm is barely wide enough to protrude out each side of the horn? Should the bushing rotate in the elevator horn? Right now, my bushing is almost a press fit in the horn with very little to no movement. The bolt rotates freely in the bushing. I'm guessing the bolt rotating in the bushing is better than the bushing rotating in the horn - easier to replace a worn bolt/bushing than a horn?
Also, I've been told that any bolt involved in a moving assembly should be drilled and cotter pinned or safety wired. The plans call for a nylon lock-nut here, but I'm kind of leaning towards a drilled AN3-5 and appropriate castle-nut and pin.
An email to Roger at Zenith cleared things up nicely. The bolt should clamp the bushing and the horn rotates around the bushing, not the busing around the bolt. Roger was kind enough to send a reference page that explains all this:
So, as counterintuative as this is, now I know what the correct assembly should look like (just like the plans). One difference is that the reference page explains the dimensions better. If you look closely the bushing should protrude a minimum of 0.5mm on each side of the horn when the nut is tightened.
To make the bushing rotate freely, I ran the bushing tube on the belt of the bench sander to very slightly reduce the diameter, only enough so that it would rotate freely in the elevator horn, and then cut it to the correct width:
I test fit the hinge and everything looks good. I used a non-locking nut temporarily to hold everything together, the proper nylon lock nut will be added at final assembly. Just enough clearance and no slop in the hinge - very good.
Next up, I had to devise a way to accurately place the hole for the outboard hinge pin in the wing fences. To do this I made a template for each side using card-stock paper. To make a clean hole that doesn't tear open and also matches the 3/16 diameter of the outboard hinge pin, I placed the card-stock between to thin scraps of wood, clamped it together and drilled through:
Placing the template over the pin, I now have a convenient place to mark a reference line (the wing fence rear rivet holes) - sorry, took a pic from each side as I was doing this:
Black reference marks on the card-stock where the rivet holes are:
Connect the reference marks and voila - when laid across the wing fence, reference marks line up with rivet holes and the drilled hole in the card-stock shows where the hinge pin hole will be. Easy! From here, I drilled out the hole to 1/4 inch, in anticipation of adding a bushing:
The plans don't call for a bushing on the pin, but there is room for one and I'd prefer that for smooth rotation and protecting the aluminum of the wing fence.
To make the require bushing I added the inside washer then measured out the pin to where the other washer will be inboard of the cotter pin hole - turns out to be about 6mm between the washers when assembled:
It's hard to position this completely as the clecos protrude out from the hinge pin plate, but this is close enough for now. Unlike the centre hinge, this bushing is free to rotate in the wing fence and around the outboard hinge pin. It will be well lubricated and I may add some more washers on final assembly to tighten up any slack:
With the Arduino trim control pretty close to final set up, I wanted to double check the power source as it will be in the plane.
The aircraft will have 12v DC nominal power from the Corvair engine, so I need a way to regulate this down to 5V for the Arduino board and the servo. Welcome to the "Buck Converter" - it takes up to 48V DC input and brings it down to a flat regulated output of the users choice.
The brass set screw on the potentiometer allows the user to set the output. The beauty is the output remains a constant flat voltage, regardless of changes in the input (say when the alternator is charging the battery circuit). Once set, I'll add a drop of LocTite to prevent it from changing due to vibration.
For testing purposes, I borrowed a 12V utility battery from Ron and hooked it up to the converter, and the converter to the Arduino test board:
The Arduino trim system runs exactly as it would powered by the USB cable - perfect!
I might use the converter below for powering the trim system. The trim system doesn't draw a tone of current (very little actually). This converter does the exact same thing as the one above, just not as bulky as the other one - reducing the footprint even further. The converter above might be better suited for powering LED landing/nav/strobe lights. Arduino will be used for lighting control too.
Next up, I started working on the mounting bracket for the trim servo. I grabbed a previous incorrectly bent bracket to re-purpose as my mount:
Once cut down to size, I laid out the lines for the servo mounting hole:
A cutoff wheel in the Dremel and the air-saw made cutting the mount hole easy:
With the bracket built, I can work on getting it mounted inside the elevator. I haven't got the push-rod or connectors yet, so I'll wait until I have those to mount the servo. I'm not sure this will be enough support for the servo either. The plans call for the Ray Allen trim servo to be riveted directly to the elevator skin - I'm thinking I might do a rework on my mount to attach it to the elevator spar.
What I could do in the meantime is fabricate and mount the trim control horn. I cut the piece a long time ago and bending such a small part was challenging but I got it done fairly quickly:
The control horn will be riveted to the trim corner. The horn end will be drilled once I know the size/type of push rod connector I'm using.
It took a long time to get to this stage and I'm almost ready to.... pull it all apart for deburring, priming and reassembly! It's nice to be able to move the elevator up and down now that it's attached to the stab - no binding, no grabbing. Nice and smooth! Really like how the fences turned out too!
With the horizontal tail group getting close to completion, it will soon head to storage. Next up will be the slats and flaps - both parts of the control surface systems on the wings.
I've been spending a lot of time considering creating a 3D digital model of the slat and flapperon tips. Normally these are blown plastic inserts or made from fiberglass and way to expensive to buy from Zenith. My theory is that I should be able to print them on a 3D printer - possibly even out of carbon fiber!!
To create a 3D model easily, it should be possible to scan an original (which are the same as the 701, which Ron has several in stock). Wouldn't that be cool!
There are many commercial grade (read expensive) 3D scanning tools available to buy. But what if I could hack a 3D camera sensor that used to belong to a Microsoft XBox gaming console? Yup, we can do that thanks to YouTube tutorials.
I picked up an almost brand new XBox 360 Kinect sensor bar for $15 dollars in Facebook Marketplace! Next up, hacking it to interface with my Windows laptop, scan example items, and print them! <Insert evil laugh here>
Thanks for following along, more to come!
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.