Saturday, June 16, 2018

(Page 12A-01) Stabilator tip fairings

The stabilator on the RV-12 as built to plans has squared-off tips, giving it, in the opinion of most builders it seems, an unfinished look.  I always sort of liked the look, but very much disliked the tooling holes (12 or so can be seen in the pic below) in the end ribs as well as the many holes in the wing tips where the tabs are bent and riveted and other holes in various places on the fuselage.  I'm convinced that all these holes were left
there in order to dirty up the aerodynamics to keep the speed below the LSA limit of 120 knots.  My plan all along has been to embark on an extensive aerodynamic clean up as soon as the FAA hands me the airworthiness certificate.  The stabilator also was limited to exactly an eight foot span to keep it legal for trailering on the highway.  Tips put it over the limit.

A couple of after-market companies make and sell rounded tip fairings and I've seen quite a few RV-12s with them installed (legal only after the airworthiness certificate has been awarded), but I couldn't decide if I liked them, other than the fact that they close all the offending holes.

Then, fairly recently, the Mothership got in the game and offered an officially-blessed kit.  What actually pushed me over the edge was doing the recent service bulletin 18-02-02 (covered in the previous post).  Complying with this SB involved drilling out a lot of rivets (punching their steel mandrils into the interior of the spar box) and drilling a lot of new holes in the spar box.  All this
resulted in quite a bit of debris in the spar box where it couldn't be reached.  It occurred to me that removing the end ribs as required to install the new tips might allow me to shake out a lot of the debris.  It still had to exit the few holes in the sides of the spar box, then make it through the lightening holes in all the ribs.  I could hear all the crap rattling around in there and I couldn't stand the thought of flying that way.  Tips it would be!

Installation of the tip kit involved drilling out 94 rivets to remove the existing end ribs (remember, I have achieved world-class status as a rivet remover), then replacing the end ribs with new ones reversed so that the flanges are out rather than in.  With the end ribs out, the shaking commenced in the driveway outside the shop.  I may have resembled a sign-spinner on a street corner in LA.  Along with a small amount of metal, I started seeing bits of shredded fabric, and I knew immediately
what that meant: mice had built a nest (shown in the pic above) in the spar box.  Visions of metal severely corroded by mouse urine danced through my head.  Inspection with a super-bright flashlight showed the nest to be not far from one of the few holes in the spar and I was able to hook most of it with a bent coat hanger and extract it (shown in the picture above).  A sudden epiphany lead me to tape a flexible piece of PEX tubing to the Shop Vac hose.  With this rig (pic at left) I was able to suck out not only the remaining bits of the mouse nest but all the metal debris which started all this.

At this point I inspected the entire interior of the stabilator with a bore scope to look for mouse urine corrosion.  Happily, the only spots I found were on the end ribs which were removed and replaced with the new ones in the kit.  A few small corrosion spots were found on the interior skin adjacent to the end ribs.  These were sanded and primed.

I keep live-capture traps set all the time in the shop (yeah, I'm that tender hearted), and had trapped and relocated a couple of mice a year or so ago.  Nothing since.  They had to be the culprits.

The new end ribs have to be straightened by fluting the flanges, just like all the other ribs in the
airplane.  The kit instructions say to modify the fiberglass flanges which slide underneath the rib flanges, making appropriate notches to clear parts of the spar, shop heads on rivets, and the humps produced by the fluting.  Lots of trial fitting here.

The instructions say to trim the fiberglass flanges to 15/32nd of an inch, which really means something less than 1/2 inch.  If this isn't done, the flange won't slide all the way in.  I used a Dremel tool with a sanding drum to "scallop" the fiberglass flange to
clear the fluting humps.  Files were used to make the rest of the notches.  I was not completely pleased with the cosmetics of the finished product, but I keep telling myself that perfection is the enemy of flying your airplane.

Saturday, June 2, 2018

Service Bulletin SB 18-02-02: Cracks in the stabilator spar box

A high-time RV-12 was discovered to have a crack in the front spar box in the vicinity of where the stabilator horn is connected.  I don't know how the hell anyone discovered this crack considering that
dye penetrant was used to make it visible.  After dye penetrant was applied, the part was sanded to bare metal and application of a bright light revealed the crack.  Amazing!  I'm trying to imagine what prompted them to suspect that a crack was there. Anyway, the Mothership came up with a fix consisting of doubler plates which go between the horn bracket and the top and bottom of the box spar, doubler plates which go between the stabilator hinge brackets and the front of the box spar, and gussets which connect the hinge brackets to the stabilator skin and the spar, top and bottom.

The result of all this is an upgrade which is massively stronger than the original part.  I don't mean simply 50% stronger. I mean massively stronger.  The original stabilator hinge brackets simply bolted to the spar web, and the stabilator horns bolted to the spar box flange.  The new system rivets doubler plates to the stabilator horns, which then get bolted and riveted to the spar, top and bottom.

New, two-piece hinge brackets are riveted together and then bolted and riveted to the spar.  New doubler plates go between the hinge brackets and the spar.  Gussets tie the new hinge brackets to the stabilator skins and the spar, top and bottom.  The six #40 holes seen in the top of the spar (also on the bottom) are left open.

In the finished product (below), the original hinge brackets (green pieces on the table) are shown for comparison.

Quite a few rivets had to be drilled out, with their mandrels punched into the center of the box spar.  When I'm moving the stabilator around, I can hear them rattling around in there.  This tipped the scales in favor of installing the new fiberglass tips on the stabilator (what's another $130 at this point?).  Installation of the tips requires drilling out all the rivets in the end ribs and reversing them.  When they're out I can dump out all the debris.  Ordered the kit today.

Monday, May 28, 2018

Service Bulletin SB 18-03-06: Cracks in the anti-servo tab skin.

On three high-time RV-12s (hours > 900) cracks were discovered in the skin of the anti-servo tab where the control horn attaches to the inboard-most rib.  All of the internal ribs in the AST have angled tabs at their leading edges which attach the rib to the spar and the skin.  The inboard end ribs,
to which the control horn attaches, did not have these tabs.  Makes sense, right?  The one rib which transmits all the load from the pitch input from the stick didn't, in the original design, connect to the spar in the leading edge of the anti-servo tab.  This original is shown in the picture.

The fix for the poor blokes with already-flying airplanes involved stop drilling any cracks, adding various doublers and replacing the end ribs.  For those of us with airplanes still under construction, all that was required was end-rib replacement.
One of the few benefits to working as slowly as I have is having an easier time with service bulletins.  The fix involved drilling out a total (both ASTs) of 10 solid rivets and 16 pulled rivets, match drilling some holes (my 273rd favorite thing), countersinking holes for the solid rivets, and re-installing everything.  I have become an absolute expert at drilling out rivets.

I appreciate the concept of service bulletins.  As the fleet ages, problem areas appear and a re-design of various parts ensures safety.  As I implement the changes, however, I feel that I'm simply treading water, re-doing something I've already done and making no progress toward that first flight.  The worst of these was the landing gear beef-up, but it's clearly a more solid airplane now.  I guess I'm just tired of building and ready to fly.  Building this airplane has been one of the most interesting things I've ever done, but I'm really ready to "slip the surly bonds of earth" (thanks again, John Magee, for the poem that has inspired me for most of life).  My new goal is to fly the airplane to my home in Colorado next summer.  I'm still on the waiting list for a hangar at KLMO, and have been called several times as one became available there, only to turn it down.  My big fear is that nothing will be available there when I'm ready to go.  That airport is 1.5 miles from my home, so no other airport makes sense.  I will, of course, have to have a hangar here in NC at KVUJ (three miles from my NC home) for the flight test period.  I think it's a fundamental law of nature that RVs can't live outside in the elements as so many store-bought airplanes do.

Sunday, May 20, 2018

(page 30-02) Trial-fitting the wings (or dumb-assery on display)

So with the gear mounted and the fuselage easy to move out of the shop (and back in quickly if it started raining) I decided the time had come to trial fit the wings.  I was somewhat apprehensive after having read other blogs with descriptions of this, but there was no reason to wait.  Moving the wings around is a two-person job, so I enlisted the help of two of my former star students (thanks Sam and
Kristin!).  The procedure involved moving a wing from the wing rack onto some blankets near the fuselage, then with one person holding the wing-tip handle and another person handling the spar and attempt was made to slide the spar in all the way.  Notice I said attempt.

The first problem involved interference between the spar and the skin surrounding the slot previously cut into the skin.  In retrospect the slot should have been at least 1/16th inch larger in both directions, probably a bit more.  There's no downside to making the slot larger.  It's completely hidden by the wing root.  The Vixen file made enlarging the slot fairly easy.

The second problem involved rivet heads in the spar interfering with the fuselage skin.  Easy fix with a Dremel tool.  The spars still wouldn't slide in all the way.  At this point we had tried both wings with similar frustrating results, leading me to try wiggling the wing a bit too vigorously.  The result
was a broken electrical connector at the wing root.  The connectors had been installed in the wing roots and in the fuselage at a much earlier date and it didn't occur to me that I could have made a mistake at that point.  One of the locating "pegs" on the male (they're actually kind of hermaphroditic) plug broke off.

Turns out I had installed the connectors upside down in the wing roots.  The locator pegs would slide into their receptacles and move in a half-inch or so, but the part making the electrical connection wouldn't mate.  This was discovered, of course, after I broke the peg.

A new connector was ordered from the Mothership for only a few dollars, but that wasn't the problem.  The problem was extracting the pins crimped onto the wires from the connectors.  The really maddening thing was that I had extracted these pins months (years?) ago after inadvertently putting them into the wrong holes in the connector.  This time, I couldn't figure out how to get them out.  I have acquired a collection of pin-extraction tools, but nothing worked.  I ended up having to cut/break the connector apart (it was being replaced anyway) to get the pins out.

The really bad part is this:  I still don't have the wings fitted.

The new connectors are installed and a new wing-fitting session is imminent.

Although I swore I'd never teach summer school again, classes start tomorrow.  I need the money to continue pursuing this madness called "building your own airplane."  $40k down and $40k to go.  Hope the Spousal Unit (my beautiful and brainy wife who is now known as Dr. KTH to her students) doesn't read this blog.

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.