Page 47-03 Spinner

The spinner comes as a fiberglass cone with no cutouts for the prop blades and a bit too long.  The first task is to trim the length and cut out the slots for the blades.  A previously discussed bushing for the Pitot must be glued into the tip.  

After all the fiberglass cutting I've done for the project I've reached the conclusion that nothing beats a Dremel tool with a fiber cutting wheel.  With all the cutting for the cowling (coming soon to a browser near you) I bought Dremel's knockoff of a Fein tool (vibrating blade rather than rotating or translating), thinking it might put less dust in the air (and in my lungs -- I think facial hair prevents a good seal with the dust mask).  The Fein tool was overkill.  If there were a smaller, battery-operated version of this tool it might be perfect.

After that trimming, the spinner was trial fit as shown, being careful that the prop cutouts allow the inside of the spinner to press firmly against the forward and aft bulkheads. It was immediately obvious that the original cuts to the scribe lines were not enough.   Clamps held the spinner in this position for drilling.  As received, there are pilot holes in the front bulkhead for the nut plate center holes to be drilled in the spinner. 

At this point I checked the runout in the Pitot tube referenced to the sharpened nail pressed into the stationary board shown.  The point was centered immediately in front of the Pitot and the prop was rotated 360 degrees while checking for movement of the Pitot.  I did this with considerable trepidation, not knowing how I would fix it if the runout were unacceptable (>1/16th inch).  I could see no movement.

The center holes for the nut plates attached to the aft bulkhead were drilled using the drill guide that I cobbled together (shown below).  The lower plate rests against the aft edge of the bulk head flange, the middle plate is a spacer for the fiberglass, and the upper plate has the guide hole for the drill.  I made these out of scrap 4130 and held the sandwich together with vice grips.  This assured that all the holes were drilled in the correct axial location.  The aft edge of the spinner fiberglass extends a bit beyond the aft edge of the bulkhead flange and will be sanded flush once the nut plates are installed and the spinner is attached.

The holes for the solid rivets which secure the nut plates to the aft bulkhead required use of my TiteFit drill kit (shown below).  As I've said before, there are quite a few holes I couldn't have drilled without this thing.

The nut plates are held in place temporarily with a #8 screw while the #40 holes are drilled and attached with solid, counter-sunk rivets.

The final step is to construct the "gap fillers" which go behind the prop roots, ostensibly reducing drag.  These are made from the fiberglass pieces cut out in the first step.  I didn't realize this so I didn't make any attempt to make the cut with the Dremel tool as thin as possible.   This lack of reading ahead caused me to end up with a much wider gap than I'd like.  I'll probably fill it with micro later.

I hate setting solid flush rivets in fiberglass.  The countersink has to be made with the rivet head sitting a little proud of the fiberglass surface before setting.  Otherwise you'll crush the fiberglass a bit.  (Side note: When a part which should be flush with a surface instead protrudes a bit above the surface it is said to be a bit proud of the surface.  I'm trying to introduce a new term to the lexicon of aircraft fabrication: If a part which should be flush ends up a bit under the mating surface, it will henceforth be described as being a bit ashamed of the surface).  

There's definitely an art to getting solid rivets squeezed perfectly, even in metal.  With several of the solid rivets attaching the gap fillers to the flange I had the opportunity to exercise my considerable skill at drilling out solid rivets and re-doing them.  This hard-won skill is a testament to the number of times I've screwed up a rivet.

Off topic: Another one of my new Colorado neighbors.

page 47-07: setting propeller pitch

The propeller pitch is set by loosening the six bolts shown going through the front prop bulkhead into the hub, twisting the blades by hand, then re-torquing the bolts.  This obviously is an imprecise, trial-and-error process, made worse by the fact that the act of bringing the bolts up to the proper torque causes a very slight rotation of the blades.  The build manual says to set each blade to a specified angle within plus or minus 0.1 degrees (!?).  This is to be accomplished using a supplied steel bracket placed at a particular radial position on each blade and a digital level with a magnetic base.  The prop is leveled horizontally and the level is referenced to (zeroed on) the longeron.  Sensenich supplies different diameter metal "pegs" that are to be inserted into a hole in the hub to set the pitch to three different common angles but Van's says these can be off 0.7 degrees and therefore shouldn't be used.  

Here's the problem: The commonly available digital levels, most of which advertise accuracy of plus or minus 0.1 degrees, actually deliver this accuracy (if you read the fine print) only near 0, 90, 180 and 270 degrees.  Otherwise, it's 0.2 degrees, and I doubt it's actually that good.  

Since having each blade at the same angle is far more important than the actual angle itself (as long as you're close to the desired angle), I decided to use the laser built into my level to project a line on the floor (hard to see on the blue tape).  

My procedure went like this:  Use Van's procedure to get as close as the digital level will allow to the prescribed pitch angle for the first blade, mark the laser line on the floor, rotate the prop 180 degrees, transfer the bracket and laser to the second blade, twist the blade until the laser line is on top of the mark for the first blade.  Tighten. Tweak. The blades are at the same angle.

Off topic:  For the past nine months I've been having a house built in the hills west of Loveland.  There's lots of wildlife there so I've had a trail cam set up in the back yard (you can see the house in the distance).  During the cold months elk come down from the high country and hang around until spring.  Here's one pic.  My neighbor who lives about 1/4 mile away trumped it by showing me several video clips of two mountain lions at once in his yard.  Another neighbor had two moose in his yard and a bear rummaging through his trash can,  Ah, life in Colorado.

page 47-02: Pitot tube

The pitot tube on the RV-12 goes through the hollow propeller shaft and protrudes out the front of the spinner.  The tube itself is stationary while the prop and spinner turn around it.  This somewhat unusual arrangement was done to accommodate easy wing removal, making it unnecessary to disconnect the tubing for the Pitot each time the wings are removed.  

The thought behind the removable wings was that people could save money by storing the airplane at home and trailering it to the airport, avoiding the cost of a hangar.  I may have heard of one person who actually tried this while everyone else keeps the airplane at an airport in the usual fashion.

The Pitot tube as supplied has male threads on one end and is threaded into a rigid nylon block which is bolted to existing threaded holes in the gearbox housing.  If all the flat surfaces on the nylon block were at right angles to each other the forward end of the Pitot would be perfectly centered in the hole through the propeller hub.

As can be clearly seen in the second pic, it's way off.  I took everything apart and cleaned all the mating surfaces with no effect.  Somewhere a surface wasn't square with its neighbors.

After passing through this hole, the tube goes 
through a hole with bushing in the forward tip of the prop spinner.  Left like this, a constant radial load would be applied to the bushing, no doubt contributing to wear as some builders have documented on the forums.  I explained all this to the Mothership and was told it's fine.  Let the bushing handle it.

A few measurements and calculations showed that placing a 0.008 inch shim between the upper forward surface of the nylon block and the mating machined surface on the gearbox housing would bring everything into alignment.  The pic at left shows what that might look like if it were done.  If I were building Experimental-Amateur Build (E-AB) rather than Experimental Light Sport (E-LSA) I could do this sort of thing.  Since I am E-LSA I must build my airplane exactly like the ASTM Conforming Prototype, at least until I get my airworthiness certificate.  Then, oddly, I can do pretty much anything I want as long as I don't take it out of light sport parameters.

If I had used the shim, the Pitot tube would have been centered perfectly as shown in the pic at right, eliminating the radial load on the bushing.

Off topic:

I just got back from my 33rd trip to Mecca (a.k.a. Oshkosh).  After swearing last year I wouldn't do this drive again, I drove from Longmont, CO to KOSH and back, this year in a rental car (a little over 1000 miles each way).  Rental cars in Milwaukee were hard to get and expensive, not to mention the hassle and cost of airfare.  Turned out to be a good decision and I'll probably keep doing this even though my airplane will be ready for subsequent years.  I'm becoming increasingly nervous about parking my airplane outside in the elements for a week.  Maybe I'll change my mind.

In 2020 when I quit splitting my time between NC and CO and moved permanently to CO (selling my beautiful home in NC😢), I kept hearing local pilots talk about flying "over the hill" for breakfast or lunch.  Turns out they meant flying over the high mountains along the continental divide (Colorado has 53 peaks higher than 14,000 feet and 600 higher than 13,000 feet).  To a country boy from the Carolinas, this seemed like a serious undertaking.  I got my chance to try it a month or so ago when we attended an EAA fly-in breakfast in Granby, CO.  I got to go back seat in my hangar-neighbor Tom Book's RV-4 (got some stick time including pointers on formation flying).  This is a pic I took with my iPhone of my other hangar-neighbor Chad Rennicke's beautiful RV-6.  The building which contains my T-hangar has a total of 11 hangars.  In this building alone we have an RV-4, RV-6, RV-7, RV-10 under construction and my RV-12.

Pages 46-1 through 46-22. Engine prep and hanging

When the time came to actually hang the engine it was pretty exciting.  For the first time I would not need a stand under the tail tie down ring to keep the airplane from tipping over backward.  It could stand on its own three feet, proud and unsupported.  No crutch!  A quick perusal of the build manual, however, damped my excitement a bit.  A surprising amount of work was required to prepare the engine before hanging.

Since everything I was about to do required the use of a torque wrench, sometimes in awkward places where I couldn't see the dial on the Snap-On, I decided to calibrate the click-type Craftsman so I could use it if required.  In an earlier post I discussed calibrating the Snap-On with a known mass (not a bar bell weight!) and found it dead-nuts accurate.  A fresh calibration showed the same.  In the torque ranges required for engine installation the Craftsman was off by as much as 18%.  Glad I checked.

The first task requiring some thought was to check and set the five crank triggers shown in the picture below (five because one of them is a rev counter).  The gap between the tooth on the flywheel and the sensor is specified to be between 0.012 and 0.016 inches.  But wait a minute, I'm thinking, these Rotax

engines are broken in at the factory by being run on a dyno for about an hour.  Wouldn't that mean that the crank triggers are already set?  One  respected guy on the VAF forums just left them as is.  They're all tighter than the Van's spec, so I went ahead and set them.  This requires rotating the crank, which is accomplished by putting bolts in some threaded holes in the flywheel, removing a spark plug from each cylinder, and using a big screw driver as a lever.  When the prop is installed, turning the engine is much easier and has to be done a lot when fitting the spinner.  I still left the plugs out, however, with rags stuffed in the spark plug holes to keep trash and insects out.

The next challenging step required disconnecting four coolant hoses from the water pump in order to install the engine mount.  I've disconnected many a coolant hose on many a car over the years without any problem.  These were different.  Nothing I tried would budge them.  I finally fabricated the piece of wood shown and drove them off, still needing a little assist from the padded vise grips to supply a little rotation.  There was some coolant still in the engine which had to be caught in a shallow pan.

The engine mount is attached to the engine with four 10mm cap screws which must be accurately torqued to a hefty value which now escapes my mind but is obtained from Rotax Heavy Maintanence manual.  I was glad I checked the manual because it's a substantially higher value than is given in the standard bolt torque table.  The problem arises because it's hard to put a torque wrench on the upper

port-side bolt.  I've read where other builders removed the intake manifold to do this, something I really wanted to avoid.  These bolts must be rechecked at every annual condition inspection giving added incentive to find another way.  I ended up using a double box end wrench as a "torque adapter" as they're known commercially.  Keeping a 90-degree angle between the handle of the torque wrench and the box-end wrench means the torque reading in unaffected by the wrench (the position vector crossed with the force vector gives the torque).  There's a little segment of an Allen wrench in the end of the wrench not attached to the torque wrench which goes into the cap screw.

With the engine attached to the engine mount I was ready to move it to the airplane.  An engine hoist on rollers made this a simple, one-person operation.  Before attaching the mount to the firewall a couple of 3/8th inch holes had to be drilled through firewall, easy for me because I could do it from the front since I'd already installed the upgraded nose gear leg.  The poor souls who had to install the new leg to an already-flying airplane had to drill the holes from the inside, a much more involved task.

Service Bulletin 00053: I had my head in my tail cone!

Recently, for several RV-12s, cracks in the skin of the tail cone have been discovered at the lowermost point where each of the three bulkheads attaches to the skin.  A structural member with a J-shaped cross section about an inch tall runs the length of the bottom of the tail cone but is discontinuous at the bulkheads, leaving the skin itself as the load path.  There are eight more of these stringers arrayed around the circumference and running the length of the tail cone, but each of them is a single piece of shaped aluminum (no discontinuity).  The Mothership ran an FEA analysis and decided that stiffening brackets needed to be riveted through the front and back of each bulkhead (except the aft most, which gets only the front) and to the skin, providing a load path and relieving the skin itself of some of its burden.  Sounds easy, and it would be if it were being done as the tail cone was originally built.  For an airplane with the tail cone already attached (with hundreds of rivets) to the forward portion of the fuselage, it's a massive problem, far and away the most onerous Service Bulletin to come out in the entire build.  A Service Bulletin for an airplane licensed in the Experimental category is roughly equivalent to an Airworthiness Directive for a store-bought airplane.  The picture shows the completed installation so you can get an idea what I'm describing.  If you click the pic and then expand it you can see the forward brackets installed at the bottom center of each bulkhead.  The aft brackets are hidden.

First off, the six rivets joining the skin to the stringers (remember, it's discontinuous) fore and aft of the bulkheads must be drilled out.  This is trivial since it can be done from outside the tail cone.  Then the three rivets joining the two halves of each bulkhead must be drilled out.  This, of course, must be done from inside the tail cone.  The five stiffening brackets are then clecoed in place (front and back except for the aft most bulkhead which gets only the front) and riveted.  This means a human being must somehow go inside the tail cone for its entire length and do all this.  The pic makes the inside of the tail cone look much bigger than it is.  Trust me.  Van's suggested, somewhat tongue-in-cheek I suppose, that enlisting the help of a smallish teenager might be appropriate.  The problem is that whoever goes back there has to have the skill to drill out the rivets without enlarging the holes and then set the new rivets.  I can't imagine trusting someone else to do this, especially someone who hasn't built an aluminum airplane before.

The instructions said to cut pieces of 5/16th plywood to place in each bay to support the load without damaging the skin.  I used pieces of 1-inch-thick foam insulation from the aviation aisle at Home Depot, doubled to give a stiff but soft place to put my knees and elbows.  I also used moving blankets and bits of carpet (which gave me carpet burns on my elbows, reminding me of my highschool years).  All this stuff and I were restricted to the left side of the tail cone because the tensioned elevator cables are on the right.

The third pic shows my work space (crawl space?) with all this stuff in place.  A bit of bad luck, completely by chance, manifested itself when I realized that when I built the tail cone I put the shop heads on the rivets forward rather than aft.  This meant that the rivets had to be drilled out from the aft side of the bulkheads.  When I made it to the aft-most bulkhead, I discovered that my head wouldn't fit past the stabilator counterweight, meaning I couldn't see to drill out the rivets, necessitating the use of a mirror, all while painfully perched on my elbows and

knees.  The gas tank on the lower left made it hard to get in and out, which had to happen several times per bulkhead.  If I had had an assistant to insert clecos from outside, the whole process would have been much easier.

I hear other builders saying this seems impossible.  The moral of this post is this:  If an old gimp like me can do this, any builder can.

Coming soon to a browser near you:  The Engine.

Page 25A-04 Bonding the rear Plexiglas window to the aluminum

Back in September of the miserable year that was 2020 (and now that I've seen the entirety of 2021 I think it was worse) I described the expensive and tedious process of fitting and drilling the rear window. It was expensive because I had to buy another window.  The last task in the window installation was to bond using ProSeal (my 431st favorite substance) the PlexiGlas to the aluminum turtle deck.  In order to allow easier access, I put this permanent attachment off until everything aft of the seat backs

(ADAHRS, ELT and fuel tank) had been installed.  The forward edge of the window screws into the previously drilled and tapped holes in the roll bar, and this attachment is done first.  The aft and side portions of the window attach with screws and nuts to the aluminum turtle deck through previously drilled holes.  All this is done before bonding with all fasteners loosely attached.

ProSeal has a six month shelf life and I had bought this batch to seal the fuel tank back before the move from NC approximately two years ago.  I would definitely not use expired stuff on the tank, but for this application I simply mixed up and observed a test batch to make sure that it would set up.  I'm never confident with this stuff.  It has to be mixed 10:1 by weight, ProSeal to catalyzer using a postal scale.  It took longer to set up, partly owing to low temperatures, but worked.

As shown in the first picture, I used ice cream sticks to create a space for the ProSeal between the aluminum and the PlexiGlas.  A gob of catalyzed ProSeal about the volume of two golf balls was put into a zip lock baggie, a corner was cut off the baggie about 1/16th inch, and I squeezed the stuff into the gap as if I were decorating a cake, removing spacers as I went.  All fasteners were then given their final low torque and the squeezed-out ProSeal was scraped up using the rounded end on the smallest ice cream stick.  These words don't come close to conveying how messy this process was.  If ProSeal gets onto any fabric, it's there forever.  I have a particular pair of pants, shirt, socks, etc that I wear for any operation involving ProSeal.

A close inspection of the first picture will reveal tape following the contour of the aluminum and tape spaced 1/16th inch from the aluminum on the PlexiGlas.  This, in theory,  prevents ProSeal from getting on anything it shouldn't be on.  The problem with this is that cutouts must be made in the tape on the aluminum around every screw head, leaving little room for error.  Acetone can be used to clean up errant ProSeal from aluminum.  The ProSeal simply can't be allowed onto the Plexi outside the 1/16th inch strip.  This process was not fun and I was not pleased with the result.

The real problem, or so I thought, was that I'd foolishly applied the tape to the Plexi back in NC two years ago.  As a result, when I attempted to remove the tape the adhesive remained on the Plexi while the tape backing peeled off.  A search ensued for a chemical which would remove the adhesive while not damaging the Plexi.  Opinions on the forums and internet in general were all over the map, often contradicting one another.  With fear and trepidation I settled on naphtha and applied it to the offending adhesive.  Worked like a charm.

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