Saturday, December 2, 2017

Page 35: She's got legs!

Installing the landing gear

The urge to install the landing gear has been almost overpowering lately, but doing that causes me to lose the ability of turn the fuselage on its side on the saw horses in order to work on the bits on the fuselage floor in relative comfort.  Since the controls are essentially done, I finally gave in.  I can't put
off trial fitting the wings any longer, so having the ability to easily roll the airplane out of the shop to provide room for the wings sealed the deal.  I'm showing the last picture first to reveal the installed gear since I'm so happy to see it.  Note that I also installed the entry steps to make it easier for me to get in, sit in the pilot's seat, grasp the stick and make airplane noises (yeah, I've done that a few times).

The Mothership emphasizes that the main gear legs must be primed with two-part epoxy and painted, I opted for SprayMax 2K rattle can primer.  It seems strange to have two-part paint in a spray can, but it works great.  The catalyst is in a separate bulb which is punctured to release it into the primer.  The can is then shaken for an appropriate length of time to mix in the catalyst and the spraying can begin.  It's expensive, but one can did both main gear legs and the nose fork.

The need for the new beefed-up nose fork has been hotly debated on the forum, but considering that I will have spent 80 large on this airplane, what's another $300 for peace of mind.  I primed and painted the fork at the same time as the main gear.  I used Rustoleum automotive rattle-can paint for the top coat.

I approached the gear installation with a bit of trepidation since I had earlier done Service Bulletin 12-11-09 (detailed in the post from 1-20-17) which involved drilling holes in the center channel which is the main structural member in the entire airplane.  The holes in all the pieces then had to line up to allow insertion of the bolts.  With a bit of
effort it all worked out.  This is fortunate since, considering the center channel can't be replaced, I would have been totally screwed if it hadn't worked.

The build manual says to hold each leg along with the seven other pieces shown in the picture in place while inserting the bolts.  Easier said than done.  I put a rope through the inboard center holes in all the pieces and looped it around the outboard end of the leg as shown in the picture.  The other bolts could then be inserted and torqued to spec.  The later version of the airframe comes with all the holes pre-drilled, which would make the assembly go together much easier.

At this point in the build, I've been pondering how many hours I've spent on this project.  Some builders faithfully record build time in a log.  I'm really glad I made the decision from the outset to not do this.  I'm glad I don't know.

The instructions at this point say to measure the toe in (or out) using blocks of wood held against the axles and a string stretched in the proximity of the wood blocks, affording a visual check of the toe.  Shims can be ordered which will adjust this.  Though not mentioned, allowance must be made for the taper of the axels.  The bigger concern for me, however, is that without the weight of the engine, wings, etc., there is considerable positive camber (clearly seen in the first pic).  As with most suspensions, even sophisticated automotive ones,
there will no doubt be a toe change associated with suspension compression and its attendant camber change.  Although it will be a much bigger PITA to do this later, my plan is to wait until all the weight is on the gear.

Sunday, November 26, 2017

Is my torque wrench telling me lies?

Well, yes and no: A tale of two torque wrenches.

In addition to the approximately 12,500 rivets in the airplane, there are many hundreds of MS, NAS and AN fasteners, all of which are supposed to be torqued to spec using a torque wrench.  Over a lifetime of turning wrenches, I have managed to acquire five different torque wrenches, ranging in
size from a small beam-type, 1/8th inch drive wrench I use on bicycles to half-inch drive jobs I use on cars and trucks.  The two I have used exclusively for the airplane build are a Craftsman snap-type 3/8th-inch drive, where the desired torque value is set on the handle and a distinct snap is felt when the torque is reached, and a 3/8th-inch-drive Snap-On dial-type wrench (shown in the picture connected so as to compare torque readings).  The literature always advises that the wrenches must be calibrated in order to ensure accuracy.  I had always (erroneously) assumed that the accuracy would be "close enough" considering that all my tools are name brand and the torque specs for each size fastener are fairly broad.  Since the airplane is going to be hauling me and my braver (or more foolish) friends around in the sky, I figured the time had come to check this.

Considering how much Snap-On tools cost, I assumed (correctly) that it would be the more accurate.  I bought an electronic fish scale on Amazon and also bought a "certified" weight (accurate to +- 0.5%) to check it with.  Amazingly, the rather cheap fish scale was accurate to within 0.9%.  I used the fish scale to apply a torque to the Snap-On wrench (shown in the pic) and it seemed to be dead-nuts accurate, at least to within the error band of this rig.

The shocker came when I checked one wrench against the other: When the Snap-On read 100 inch-lbf, the Craftsman indicated 84 inch-lbf.  This is unfortunate since many of the torque values being measured are essentially impossible to read with the Snap-On.  Henceforth, I will apply a correction when using the Craftsman.

A dial-type wrench is required since the prevailing torque (the torque required to turn the fastener in the self-locking mechanism) can't be measured any other way.  Measured torque minus the prevailing torque equals the actual torque applied to the fastener.  Maybe I'm worrying needlessly considering the error which results from the varying amounts of lubrication on the threads due to handling the bolts with hands which are no doubt far from clean.  Clamping force is all that really matters, and can only be determined through measuring bolt stretch.

Sunday, July 30, 2017

Page 31B-12 + 29th trip to Mecca

Page 31B-12 says to temporarily install the control sticks in order to install the push-to-talk switch in the top of each column.  A bolt and a bushing are called out for the side-to-side pivoting motion at the base of each stick.  Both sticks connect to a shaft which was previously installed (shown in the second picture).  Problem is, the bushings wouldn't fit into the tube welded to the base of the stick (near the right-angle turn in the picture) and the bolts wouldn't come close to
fitting into the bushings.  No mention of this was made in the build manual.  The OD of the bushings was fairly easy to reduce slightly using the Scotch-Brite wheel, steady rotation of the part and lots of trial-and-error fitting.

The ID of the bushing, where the actual pivoting takes place, was another story.  This is a place where the fit should be near perfect with no slop. The OD of the bolts measured about 0.3754 with a micrometer, so a 3/8th reamer (which I had) didn't work.  I could buy a 0.3755 reamer for about $75, which seemed ridiculous for a single use.  Instead, I took a 5/16th bolt, cut the head off, sawed an axial slot in it about 1 inch deep, wrapped some very fine sandpaper around the bolt (and through the slot), chucked it up in a drill and ran it carefully in and out of the hole until the bolt fit in the bushing with no play.  Lots more trial and error.

The next issue was getting the two wires through the sticks.  As shown in the pic, the wires have to make a right-angle turn and then squeeze past the aforementioned tube and bushing.  Trying to push them through didn't work.  To accomplish this, I used the ShopVac to suck a string through (the yellow string in the pic), then used the string to pull the wires through.

The sticks on the new RV-12is, shown below, have three buttons per stick (PTT, trim and autopilot disconnect) and six wires.  I asked the reps from the Mothership at the recently completed Oshkosh how they got that many wires through.  Turns out it's now OK to drill a hole in the sticks at the right-angle bend.  Now they tell me!  I'd like to have the three-button sticks, but I doubt I'll retrofit it.  I had always assumed that if the injected engine were offered, I'd get it.  Turns out it's not possible for people who have already finished the fuselage.  I'm consoling myself by thinking about the money I'm saving by sticking with carburetors.  The new firewall-forward kit costs about $5000 more with the injected engine.

Speaking of Oshkosh, I just returned from my 29th trip.  Hard to  believe.  Where did the years go?  A bucket-list item is to fly my RV to OSH.  Maybe next year.

Sunday, May 28, 2017

(page 31B) A mechanical engineer doing wiring -- what could go wrong?

The wires in my hand are just for effect.  They actually came out of my antique car when I foolishly decided that the warmed-up LT1 engine I had put in it didn't have near enough horsepower.  With the
money I proceeded to spend on the followup engine I could have easily bought the firewall-forward kit for the airplane.  We live and we learn (I hope).  Money is shaping up to be a major player in how soon I can "slip the surly bonds of earth."  The avionics and engine, which is what I will have left to buy after completion of the "finish" kit, represent more than half the total cost of the airplane.

Given that the airplane will have sophisticated avionics, autopilot, etc., I've always contemplated doing the wiring with a sense of dread.  So far, the dread has been justified.  I immediately discovered that some of the holes shown in the pictures in the build manual through which the wire bundles must pass were not present in my kit, owing, I suppose, to the slow pace of my build.  During the time
lapse from when I order the sub-kit to when I actually get around to working on it, the kit has evolved.  No mention of this is made in the build manual, so step one was to drill the missing 3/4-inch holes in the instrument shelf and three fuselage bulkheads.  The left arrow in the picture shows an array of cooling holes that also aren't there, but I'll put off drilling those until I figure out what goes there.

The step drill shown worked well for these holes.  BNC connectors and two fat wire bundles go through these, so the required snap bushings will be slit, placed over the wires, and snapped into the holes later.

The cooling fans were installed at this point.  After hooking them up to 12 volts, I discovered that they blow in opposite directions relative to the fan housings, and one runs at a higher RPM than the other.
The Mothership said it's OK.  Simply flip one over to get the correct direction and the slow one will probably come up to speed after it "loosens up."  It would be so easy to replace one now rather than later, but I'll trust Van's.

The wire bundles and the three coax cables go over the rudder tube.  A wire tray will be added later. The bolts holding the rudder tubes up must be loosened in order to allow the BNC connectors to pass above the tube.  They still have to be forced through the gap.

In the picture at right, both wire bundles have been run.  Various wires are identified and pulled out of the bundles as the bundles make their way aft through the various snap bushings, with some wires going all the way back to the tail cone area, others stopping at various locations.
The build manual stresses repeatedly to run each wire individually from the instrument shelf to its final destination.  I didn't do it this way.  Instead, I laid out each bundle on the shop floor and taped the end of each shorter wire to the main bundle, then ran the whole thing through the various Adel clamps and snap bushings.  The sticks have been installed in order to run the wires for the push-to-talk switches.  Issues with the sticks will be addressed in a separate post.

Much time was wasted removing snap bushings which the build manual had me install in various bulkheads and seat ribs.  With the bushings in place, various wires with their already-installed connecters won't come close to fitting through the bushings, requiring that the bushings be removed and the wires run through the bare holes.  The slit bushings can then be placed over the wires and finally snapped into the holes.  The problem is that the already-installed bushings are difficult to reach and remove in the tight spaces.  If I were building another airplane I wouldn't install any of the bushings in the fuselage until all wires are run.  The build manual warns of this, but way too late.

Most of the builders who have gone before me agree that delaying the attachment of the tail cone is the smart thing to do (also not suggested in the build manual).  This allows easy access to the rear
bulkhead and also makes it easy to turn the fuselage on its side. Having the fuselage on its side allows me to comfortably sit while routing the wires.

One of my students dropped by and offered to help with the fine adjustments.

Friday, January 20, 2017

Service Bulletin 12-11-09 Part 2: Landing Gear Beef-up

After the RV-12 fleet started to accumulate a significant number of hours, several suffered wrinkled side skin following hard landings.  Close inspection revealed cracks propagating from the bolt holes in the  center-section channel which allow attachment of the landing gear legs.  The Mothership
developed a beef-up kit for the side skins (previously covered in a blog post) and a kit to substantially strengthen the landing gear attachment.  The kit consists of thicker aluminum and steel plates (shown in the pic at left) which go between the U-shaped bracket (U-1202) and the center section.  The T-shaped aluminum piece (T-1204V) on top of the center-section web is a drill guide with two #30 holes, which the builder was to use to match-drill through the web and the steel and aluminum plates below the web (U-1202C-B and U-1202D-B).  These #30 holes were then to be enlarged to 1/4 inch.  Happily, since I purchased my finish kit in 2016, the U-1202 parts already had the 1/4-inch holes drilled (my experience with match drilling holes has been less than enjoyable).  I only needed to drill the holes in the web.  The problem was that the holes I drilled in the web had to be perfectly aligned with the already-drilled holes in the other parts.  This wasn't an issue with the original beef-up kit since all the holes were drilled together and were automatically aligned.  Past experience using thin aluminum sheet with #30 holes as a drill guide showed that it's quite easy to "hog out" the original holes, causing the holes which are supposed to match the original to be off center.  I had visions of screwing up the center-section channel, which is the main structural member in the fuselage.  These 1/4-inch holes needed to be dead-nuts perfect.

To reduce the chance of misalignment, I ordered additional drill guides from Van's (they're cheap) so I could use stacks of three.  This allowed me to use a #30 transfer punch to accurately center punch the hole in the web and reduce the tendency for the #30 drill to walk off center during the drilling of the #30 pilot hole.  As instructed in the build manual, I used the reflection of the drill bit in the shiny drill guide to keep the #30 bit perpendicular to the web.  I then repeated the process with the 1/4-inch bit.

I won't know how well the holes align until I bolt on the gear.

Saturday, December 31, 2016

Page 30-02: Fuselage pin stopper assembly

The fuselage pins are the heavy-duty steel cylinders which secure the wing spars to the fuselage in a fashion similar to that of glider wings, allowing easy and quick removal of the wings.  The fuselage pin stopper is the mechanism which keeps these pins from backing out in flight, an important task in
my estimation.  The stopper is a spring-loaded aluminum cylinder which must fit within the small steel tube welded to the main pin.  The spring is inserted, followed by the stopper, a hole within which has been tapped to allow the machine screw assembly to be threaded in, holding the whole thing together.  Problem was, the stopper was not even close to being able to fit into the said tube, the OD of the stopper being significantly larger than the ID of the tube.

I first tried fixing this by using  a pistol bore cleaning rod with a strip of ScotchBrite inserted in the slot rather than a cleaning pad.  The rod was turned in the ID of the tube with a drill.  This didn't remove anywhere near enough metal to make it fit.  I next created a poor-man's reamer by cutting the head off a 4-inch-long 5/16 bolt, then cutting a 2-inch-long lengthwise slot in the bolt
(shown in the third pic).  Into the slot I threaded a length of fine-grit 2-inch-wide sandpaper from my belt sander, wrapping enough around the bolt to make a tight fit in the ID of the tube (shown in the second pic).  I chucked the bolt in the drill.  The sandpaper looks too big in the picture because it unrolls when you're not holding it.  After lots of trial fitting and reaming it fits perfectly.  I cleaned the ID with the bore-cleaning rod (to the left in the picture) and swabs, then applied some Boelube and assembled the whole thing.

All of the other builder blogs I could find which discussed this spoke about the earlier system which required that a magnet be inserted in the stopper and it then be filled with epoxy.  The magnet was somehow used to tell the Skyview that the pins were secure.  That system apparently proved
problematic and was replaced.  Not sure how this new arrangement lets the Skyview know that everything is secure.  I suppose I'll find out in the near future.

The finished assembly is shown at left.  When the wings are installed, the machine screws allow the stoppers to be retracted and then snapped into a hole in a plate in the fuselage.  I doubt the pins would ever back out in flight even without the stoppers, but it would ruin my whole day if they did.

Friday, October 21, 2016

Service Bulletin SB 16-08-24 : Loose engine mount bracket rivets

Each one of the two upper engine mount brackets has 22 LP4-3 rivets attaching it to both the panel base and the upper firewall.  On two high-time RV-12s the aft-most six rivets on each bracket have been observed to be "smoking."  Two solid surfaces which are mechanically held in contact with each other and which are subjected to vibration can experience fretting corrosion.  Fretting corrosion
occurs when the two contacting surfaces experience relative motion, often of microscopic magnitude.  In the case of aluminum, the small-scale rubbing between mating surfaces produces fine aluminum power which quickly oxidizes, causing a smokey appearance around the rivet head.

Van's fix is to replace the offending LP4-3s with much stronger Cherry rivets.  This, of course, required drilling out the 12 LP4s in the engine mounts.

Some time back, Van's switched from the Garmin SL-40 com radio with separate intercom to the Garmin GTR-200 com radio with built in intercom.  Naturally, I had just installed the radio tray towers (the grey objects shown in the pictures) for the old radio when the announcement came out about the replacement, so those rivets had to come out as well (6 LP-4s total), so I did all the rivet removed at once.  The manufactured heads on the rivets were on the bottom of the of the panel base, so I originally started the process by awkwardly lying on my back in the fuselage drilling upward.  The drilling process in rivet removal is quite delicate, since enlargement of the #30 hole is to be avoided.  A clear view of the rivet head and a steady hand are required, both difficult to achieve in that position.  It occurred to me, somewhat belatedly, that this process would be much easier with the fuselage on its side, allowing me to sit upright to do the drilling.  Once again, I'm reminded of what a great decision it was to put off installing the tail cone as long as possible.  Without the tail cone, I have great access to the area between the aft bulkhead and the seats.  There's still lots of stuff to do there.

As much as I'd like to install the landing gear, I'm resisting that urge as well.  With the gear on, even without the tail cone, it'll seem much more like a real airplane.  I would no doubt be unable to resist the urge to sit in the cockpit and make airplane noises.  The downside, of course, is a lack of options for positioning the fuselage on its side while installing the many systems still to go in the fuselage.