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 Thursday, October 23 2014 @ 05:14 PM EDT

Degaussing the Roll Bar

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Another builder is having problems with his magnetic compass - very large errors. He asked me if I knew anything about degaussing the roll bar. I have been saving info on degaussing for a number of years, as it is a potential problem that I may need to deal with once my aircraft is finished. I decided to put this info on my web page, as some other builders may find it useful.

First, the Sacramento Sky Ranch has some apparently good info on their web site - Degauss Instructions.

I've collected the following information from a variety of e-mail lists, but I have never actually used any of it. So I have no idea whether any of it is good stuff, or complete garbage. Use at your own risk. If you use any of it, I'd appreciate it if you let me know what worked and what didn't.

Read on for the collection of info.


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Trim wiring, backlighting, etc

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I spent a lot of time in the aft fuselage the week before last, running and securing the wires for the electric trim. It sure is a pain to get all the way back in there. You need both hands above your head to do some of the work, but it is so tight that your elbows hit the bulkhead and won't let you put your arms up there. So, you have to get your arms above your head before moving into the last bay.

I mounted the trim servo in the elevator and puzzled out how to secure the wires as they went from the elevator, into the horizontal stab and then to the aft fuselage. I hooked up the front end of the trim wire and did a complete functional test - front seat trim switch, rear seat trim switch, trim disable switch on stick, rear seat trim disable switch on side console. It all worked.

I had hoped to get the roll trim hooked up that week too, but I ran out of butt splices to hook up the wires. I was going to finish the mechanical installation, but I couldn't find the long piece of 5/16 aluminum tube used to go between the servo and stick. I found two pieces half as long as I needed, but the long guy was missing. I finally remembered what happened - I had screwed up the flap pushrods, and had to use that long piece of tube to make them from. I had planned on replacing it and then forgot about it. Coincidentally I had purchased a long piece of the same tubing a few weeks ago to make some spacers from. I had just made a few spacers earlier that day - I measured how much remained of the tubing and it was a half-inch too short. If I had only noticed that I had those two shorter pieces of tubing I could have used them for the spacers instead. Oh well. One more thing to order from Van's.

I've spent a lot of time over the last two weeks messing around with dimmers for the GNS-430 GPS, GTX-327 transponder, Microair com, Garmin CDI, Van's engine instruments, etc. There are several different types of light technologies involved, and each one has its own curve of light intensity vs input voltage. And the GNS-430 and GTX-327 each has several variables that you can set to customize the intensity vs input voltage. I don't want a separate dimmer knob for each unit, so I need to figure out how to set them up to allow me to use as few dimmer knobs as possible, yet have the lighting on all units remain in balance no matter what intensity I want. I'll have a lot more empathy for the poor designer next time I do a night lighting evaluation of an avionics mod, and note a problem with the lighting balance.

I think I am zeroing in on a solution. The GNS-430 and GTX-327 will get their own separate rheostat, as the solid state dimmer I have for the LED flood lights won't go to a low enough voltage to work with the logic built into those units. They are designed to stop following the dimmer and set the intensity based on a photocell for daylight operations. The Microair com backlighting will work nicely if I feed it from the solid state dimmer for the LEDs. And the CDI, engine instruments, etc will have to be on their own solid state dimmer, as they are all incandescent lights they should have roughly the same curve of intensity vs voltage (I hope - I haven't proven this yet)

I had originally planned to just have the LED flood lights, so many of my instruments do not have internal lighting. But some of them do, and I've decided to use the internal lighting on the CDI and engine instruments. The EFIS is lit, and it has attitude, heading, airspeed and altitude. The CDI and engine instruments will be internally lit. The round dial airspeed and altimeter are of lower importance, so I can accept a bit lower light level from the LED floods.

I also pulled out the avionics rack for a while so I could reroute part of the wiring harness that was too close to some structure.


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Seat Belt Mounts

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I was messing around with the rear seat riser and seat cushions, and decided I should see how the seat belts interfaced with all this. I ordered seat belts from Hooker Harness, with Schroth rotary buckles.

I went to install the front seat lap belts, and noted that there was a very wide bushing for the bolt that attaches the lap belt to the aircraft structure. This bushing required a much wider slot than is provided by Van's design for the belt attachment. You can see how the too long bushing is forcing the two parts of the mount apart.



I pondered simply removing the bushing, but I wasn't sure why it was there, so I didn't want to risk affecting the integrity of the belt attachment. I called Hooker Harness, and explained the situation. They were very aware of the "issue", and recommended using a file to make the bushing narrower. They explained that the bushing was very important, as it ensured that the belt attachment hardware would be free to rotate around the bolt. If the hardware could not rotate around the bolt the loads would be carried by one edge of the belt webbing causing it to be cut and fail if a large load was placed on it.



The bushing looks to have been made from a piece of aluminum tubing, with the ends expanded to keep it from falling out of the hole. I figured out that it was easy to take away material using a ScotchBrite wheel, with a piece of wood holding the other end of the bushing parallel to the seat belt hardware. I removed enough material to make the bushing about 0.020" longer than the attachment hardware.



Now the seat belt hardware fits nicely as Van intended it too.


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Rear Seat Riser Test

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Progress is frustrating slow. Too many other things going on. Last Sunday I spent most of the day with Terry as we haven't had much time together lately. We had company here from Monday until Thursday, so I didn't get much done then either.

Today I made a trial rear seat riser out of scrap plywood from one of the kit shipping crates. I'm not sure who first came up with the rear seat riser idea, but Cleaveland Tools created the plans that I am using (full size plans). The RV-4 and RV-8 rear seat cushions sit right on the floor. The RV-8 does have sunken foot wells for the rear seater to put his feet in, which helps a bit, but the basic seating position does have the butt fairly low. The rear seat riser is a platform that lifts the rear seat cushion about three inches off the floor at the front, and slants back to be about 1.25 inches high at the rear. I wasn't sure those dimensions would be ideal for my aircraft, so I decided to make one out of wood to test out the dimensions before making the real one from aluminum sheet.



I placed the two vertical plywood supports so they would sit on top of the seat ribs that support the floor. I didn't want those plywood pieces to end up a bit off the seat ribs if the riser slide sideways, as that would force all the weight to be supported by the thin aluminum floor. So, I put a short nail in the bottom of each plywood rib so that it would fit into an empty screw hole on the seat ribs and prevent the plywood from moving.



I put the seat cushions in place on the rear seat riser, climbed in and pulled the canopy closed. I found that my head was a bit too close to the canopy, so I pulled the one inch booster cushion off the bottom of the rear seat cushion and tried again. That seemed to work reasonably well, but I'm not a big guy. I'm get my coworker Jim over to try it out. I suspect I may have to lower the top of the seat riser a bit to better fit tall passengers. The RV-8 canopies are free-blown, rather than being made in a mold. So every canopy is a slightly different shape, which affects the amount of head room in the rear seat. My canopy may be slightly lower than average, which would also change the angle that it sits on the frame. This would be consistent with the fit problems I had on the canopy skirt.

There is a cutout in the rear seat bottom cushion to make room for the movement of the control stick. I get the impression that the stick might rub against the seat cushion if the stick is full aft with a large aileron input. I won't be sure on this one until I get the aircraft assembled with the controls hooked up.


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Most Recent Post: 10/31 07:48PM by Kevin Horton

Slow progress

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It's been a busy couple of weeks, but I managed to fit in some building. I finished the wheel pants filling and fitting, but I eventually need to fabricate the fairings that go between the wheel pants and the gear leg fairings. The idea of sinking the Tinnerman washers below the surface of the wheel pants looks good so far.

The canopy skirt is almost ready to be riveted to the canopy. I've just got one more small gap between the skirt and fuselage to sort out.

I've also been working on the flight test plan and a Pilot's Operating Handbook (POH) for the aircraft. There are several nice POHs available on the web, so I am using them as a starting point.


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Canopy Skirt Progress

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The canopy skirt is looking better all the time. I put some more fibreglas at the back to properly tie the left and right halves together, and it seems fairly strong now. I need to get a bit better fit right at the centre, where it fits around the raised canopy track.

When I put the fibreglas layer on to tie the two halves together at the back, I put a layer of wax paper over the canopy track, and tried to form the fibreglas closely over it. It was definitely a mistake to use the wax paper, as some of the wax paper stuck to the fibreglas, and it was a pain in the butt to get all the wax off. I would use electrical tape next time.

The fibreglas didn't fit real tight over the canopy track in the back, so I tried a bit of an experiment. I put some electrical tape over the canpy track and the surrounding fuselage skin, then stuck some West Systems epoxy and microfibre under the back end of the canopy skirt and then closed the canopy tightly. In theory, I should get a nice tight fit over the canopy track.

This canopy skirt was removed when this picture was taken. You are looking at the aft end of the plexiglas canopy, with a bit of electrical tape on it which kept the fibreglas resin from sticking when I bonded the two halves of the canopy skirt together.



Here you see the back end of the canopy skirt, after I tightened it down with the epoxy and microfibres in the gap.




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Wheel Pants Washers

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The plans for the wheel pants call for the flat-head screws that hold the two halves together to have large countersunk Tinnerman washers under them. I have never cared for the look of those washers, so I pondered whether I could get away without using them. I have seen a few RVs without those washers on the wheel pants, so I'm not the only one with this crazy idea. But I worried about whether the fibreglas would stand up without the additional support that those washers provide.

So, I asked a question on the RV-List, and soon had many responses, both on and off list. Several responses said that they had had problems with the fibreglas wearing around the screw heads, so that idea looked like a non-starter. I want a good looking plane, but more than that I want one that doesn't need any more maintenance than necessary.

But an RV-4 builder sent me a good idea:

On my -4 (10 or so years ago) I used the screws with the Tinnerman washers painted to match the plane.

The only problem is keeping up with the washers along with the screws when you take it apart.

During a rebuild of the pants I did what the composite guys do: glue the Tinnerman washers in place, fill the edges with filler, and glass over the washer and filler. Run a counter sink to clean out the washer, then paint. The result is metal in contact with the screws but the washers are captive and hidden. Structurally, this is a good as it gets.

A little more work but easier down the road - you don't have to worry about losing the washers and then wondering if you have any spares that are painted.

I'm doing the same on my almost finished -6a.

Dave

So, I countersunk the holes for the screws, then assembled the wheel pant halves with screws and Tinnerman washers. I used a fine marker to trace around the washers, then disassembled everything. I used a Kutzall carving burr to remove the gel coat around the holes. Those burrs do a wonderful job carving fibreglas - they don't seem to wear, and they don't clog up.

I roughed up the surface of the Tinnerman washers and mixed up some West Systems epoxy with microfibres. I put some epoxy mix under each washer, and assembled the wheel pants, putting a drop of oil on each screw to be sure the epoxy didn't lock them in place. It seems to have turned out OK so far, with the outer face of the Tinnerman washers just below the surface of the gel coat. I put some filler on them, and we'll see how it turns out after it cures and I sand it.


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Most Recent Post: 09/28 07:10PM by Anonymous

Cleveland Brake Issues

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There have been several interesting e-mail list threads about Cleveland brakes in the last few months. I have been meaning to post a note to inform builders for quite a while, but I got distracted. Anyway, here here goes with a few brake gotchas:

  1. Improperly assembled calipers delivered from factory.
  2. Rough finish inside caliper.
  3. O-ring damage due over-heating.

Read on for information on each of these issues.


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Canopy Skirt, Wheel Pants

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I finally managed to get back at the project today.

Last Tuesday night I drilled the new aft part of the canopy skirts to the frame. The right side fits excellently. The left side isn't quite perfect, but it is much, much better than it was before. I'll probably have to make one more small mod to remove a small gap in one spot.

Today I trimmed the new aft skirts a bit more, then put them in place and bonded one ply of fibreglas across the aft end to tie the two skirt halves together. After it cures I'll put the canopy off and turn it upside down on some pillows so I can put a few more plies of fibreglas on the inside at the rear.

I realized I hadn't looked at the wheel pants for awhile, so I pulled them out, intending to countersink the holes where the various screws go that hold them together. But when I looked at them I saw that I needed to put some filler on a bunch of areas. So, I cleaned the surface, roughed it up, mixed the filler and put it on.


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Canopy Skirt - Small Steps

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We've had company visiting for a few days, so I haven't gotten any work done on the project since the middle of last week.

The canopy skirt has been a real bear to fit. The first attempt, many, many months ago, caused me to give up in disgust as I could not get a good fit on the rear portion. The crux of the problem is that there is quite a bit of aircraft to aircraft variation in the canopy shape (it is free blown, so every canopy is a bit different), and how the builder fits the canopy to the aircraft. But the skirt is made in a mold, designed to fit some mythical nominal aircraft. There is an angled flange at the top of the canopy skirt, where it fits against the bottom part of the canopy. On my aircraft, the angle on the flange is too great, which forces the bottom part of the skirt away from the fuselage skin.

I tried Plan B recently, which involved filling in the holes I drilled in the aft part of the skirt, and redrilling them. At first it looked like I had an acceptable fit, but in the end I was still unsatisfied, as there were several areas where I could still slip my fingers between the bottom of the skirt and the fuselage. Plan C involved using a heat gun to try to soften the fibreglas so I could change the angle on the flange at the top of the skirt. This didn't work, as it seems that the material the skirt is made from must get really, really hot before it becomes soft. I ended up turning some parts of the skirt brown from the heat, and causing some delamination, but I still couldn't change the angle of the flange.

So I decided that I needed to make a new aft skirt, that fit my aircraft. I covered the lower part of the right side of the canopy with electrical tape, and put a bunch of wax paper on the fuselage. Then I put some filler on the canopy skirt flange where it met up with the canopy. I taped the canopy skirt in place, making sure it matched up against the fuselage. The filler on the flange hardened, giving me a one-sided mold that had the flange at the right angle. The filler didn't stick to the electrical tape, so I was able to pop the skirt right off and clean it up.





Plan D - I put electrical tape on the inside of the skirt, and laid up one layer of fibreglas cloth on it. This didn't work, as the electrical tape pulled free from the skirt as the fibreglas resin cured, so the new layer had some large, weird lumps in it. Plus the single layer was really too flexible to work with. One for the trash can.



Plan E - I put clear packing tape on the inside of the skirt, and laid up two layers of 2" wide fibreglas cloth. I used the narrow strips of cloth, as I found it too difficult to work with a large area of cloth the first time I did this.

Plan E looks promising. I popped the new skirt off, and it fits very nicely. Tonight I cut the aft portion off the original skirt, and bonded it to the new skirt with a 2" strip of fibreglas. Tomorrow I'll check the result then lay up another couple of layers of cloth on the new skirt to make it stiff enough. If the right side turns out OK, I'll do the same thing on the left side.




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