(Page 16-02) revisited: stall warning tab

The stall-warning tab, which protrudes from the leading edge of the left wing (second pic), is to be adjusted such that the normally open switch which is actuated by the tab closes with minimum upward travel of the tab a few knots prior to stall, thereby actuating the stall horn.  This adjustment is accomplished by trial-and-error, using the continuity function of a volt meter.  The problem is that the wing skin panel with with the slot for the tab has to be temporarily installed for the adjustment to be

made, since the tab rests on the lower edge of the slot when the airplane is not at the critical angle of attack.  At this point, the tab is supposed to be perpendicular to a tangent to the leading-edge curvature.  If this condition doesn't exist, it is allowable to bend the tab (which I didn't do).  I found it difficult to judge this.  I ended up holding a straight edge at what appeared to my eye to be tangent, then eyeballing the perpendicular.  The leading-edge curvature is not a circular arc, which would be much easier to judge, but appears to have a constantly changing radius of curvature.  Compounding the problem is that the slot cut into the leading edge is much bigger that it needs to be if the builder does as instructed and minimizes movement of the tab.

I ended up doing three trial installations of the skin to get what seemed to be the correct angular

position of the switch (correct meaning the tab was perpendicular to the tangent).  I started with the screw in the center of the housing slot, and that was surprisingly close to what I ended with.  I suppose the adjustments could be made through the access hole (shown in the first pic) thereby avoiding all but one trial fit, but I found this difficult.

With the Skyview providing angle-of-attack information with programmable stall warning (and a female voice saying "angle, angle, push" just prior to stall -- I can't wait to hear this!), some builders are electing to not use the tab stall warning system at all.  I plan to use it.

Kerosene heater is back

Yesterday was the first day of the year that I had to fire up a kerosene heater.  During the coldest nights of the year I use as many as three at once in my uninsulated garage, the fumes from which usually give me a headache.  I did buy a carbon monoxide detector last winter, and according to it, the CO levels are OK.  It can't be good for me, however.  I'm probably being saved by all the air leaks.

Spar bend -- OK or not?

When I unpacked the wing kit many moons ago (way too many) I noticed that the inboard end of the right wing spar bent aft a degree or so at the point where the ribs end and the tapered part of the spar, the part which passes through the fuselage, begins.  Examination of the left spar showed a similar but lesser bend in the opposite direction.  I convinced myself that this had to be by design.  The spars have

individual serial numbers and, being one of the most critical components of the entire airplane, were surely subjected to tight quality control.  As I proceeded through construction of the wing kit, preparing and installing all the ribs, I periodically pondered the very noticeable bends and became increasingly troubled by them.  Before skinning the wings, I figured I'd better get a ruling from the Mother Ship.  In retrospect, of course, I should have done this much earlier, probably before attaching ribs.  I carefully quantified the bends with measurements and sent the results along with the above pic to support at Van's.  The quick reply from Ken Scott was "The ends of the spars always go slightly one way or another, and it doesn't matter at all.  
Once they're installed in the airplane and the pins are through the holes, all will be well."  Instant relief from my self-inflicted worry.

Service Bulletin 13-12-12

Apparently, several flying aircraft have experienced electrical failure caused by water entering the avionics bay by seeping between the upper edge of the firewall and the underside of the fuselage skin.  This Service Bulletin was issued to correct this.

After putting the SB off for six months due to my dread of removing the Upper Forward Fuselage Skin (with all its machine screws), I hit a stopping point with the main build (space to work) and could delay no longer. The instructions call for applying Boelube as a release agent to the underside of Upper Forward Fuselage Skin, then applying a 1/16th-inch bead of Permatex 16BR Black Silicone Adhesive Sealant (#81158) to the opposing face of the Upper Forward Fuselage

Doubler (yellow arrow in pic at right).  After reading the opinions of those who had done this procedure already, I stuck with the Permatex but used O-Ring lubricant as a release agent.  I found this stuff on the aviation isle at WalMart near the pool and spa supplies.

It turned out to be impossible (for me, at least)  to make a nice, uniform 1/16th-inch bead as called for, so I instead made a more or less constant-thickness ribbon between the aft edge of the doubler and the nutplate holes.  I then re-installed the fuselage skin and tightened all the machine screws, the idea being to squeeze the silicone into a continuous, uniform water barrier before it sets up, while preserving the ability to remove the fuselage skin without affecting the water barrier.  I tested this with some scrap aluminum, and it seemed to work after curing for about a week.  No way to know what will happen after it has been cinched up for longer periods.  I'll definitely have to remove the skin at least one more time (and probably multiple times) to make the modifications required for the switch to a Garmin radio.

Pilgrimage to Mecca complete for 2014

It's hard to believe I just completed my 26th trip to Oshkosh.  It's still the high point of my year.  After all this time it's still a sensory overload, at once too much and not enough. Over 10,000 airplanes flew in and over 500,000 people attended.  As I've frequently said, Oshkosh is like sex in that the only way to get enough is to get too much.  After a week of showing up at the flight line at 6:00 am each morning and leaving at dusk I'm energized, inspired, and tired to the bone.  I did manage to get my fourth flight in an RV-12, this time in an S-LSA.  Of course it flew just like my three previous flights in

RV-12s registered as E-LSA.  This store-bought version has been adopted by several flight schools as a primary trainer, but I'm not at all sure it's well suited for that role.  It's too easy to fly.  Student pilots who learn to fly in an RV-12 will have no idea what those pedals on the floor are for.  Even rapid roll inputs leave the ball centered with absolutely no rudder input.  I've never seen anything like it.  The roll rate is astonishingly high (45 degree left to 45 degree right) for a non-aerobatic airplane.  I think it will be very difficult to resist the urge to do aileron rolls in this airplane (my history of resisting temptation is not good).

The picture shows a factory S-LSA with a Ford Trimotor and a Mustang (Bud Anderson's Old Crow, no less) in the background.

Summer school and alien abduction

Teaching summer school in mechanical engineering is like having five weeks surgically removed from my life without benefit of anesthetic.  Each day goes like this: prepare three hours of lecture, deliver the lectures, meet with students, grade papers, eat, sleep, repeat.  No time for building airplanes or much of anything else.  The last time I worked on the RV-12 was about five weeks ago and I can barely remember what I was working on.  The sole reason for doing it, of course, is to be able to afford things like building airplanes.  The pay is good but, at this stage of my life, time is my most precious commodity.  The strange thing, however, is that with one week left, it seems that only about a week has elapsed.  This leaves three weeks unaccounted for.
People who claim to have been abducted by aliens often speak of "lost time."  Inexplicably, the clock has of its own accord sprung forward.  That's how I feel.

The good news is that in about a week and a half I will, at least theoretically, have four uninterrupted weeks to work on the project.  Oshkosh will then consume a week (the high point of my year), then I'll have a couple of weeks before Fall semester begins, which is like a vacation compared to summer school.

(page 16-04) Stall warning tab

With the stall warning switch assembly ready for installation, luck would have it that the wing hanging from the rafters (left) is the one which needs the switch installed.  Also, the construction manual calls for the left wing to be skinned first, meaning I have to get the one on the table up to the rafters and the one in the rafters

on the table.  This is going to require carrying the right wing out to the driveway, lowering the left wing to the table, carrying it to the driveway, placing the the right wing back on the table, raising it to the rafters, then, finally, placing the left wing on the table.  Problem is, there's a construction dumpster (house remodel) blocking the way.  Haven't solved this one yet.

Moving the un-skinned wings around makes me nervous since the ribs are quite frail and floppy in their current configuration, most of them attached only at the spar.  As with the rest of the airplane, once the skins are riveted in place, the structure feels quite solid and substantial.  The skins come next and I'm looking forward to that.  I'm hoping to continue my "one in the rafters, one on the table" procedure after

the wings are complete, since I have nowhere to put them.  Most builders fabricate a roll-around cradle for the wings in which they can be stored nose down.  Maybe I'll do this when I rent a hangar and move the project into it.

(page 15-06) Space crises postponed

By hoisting the left wing spar with all ribs attached up near the rafters I temporarily put off running out of build space and am proceeding with the right wing.  Almost all of the rivets attaching the ribs to the spar can be pulled with the pneumatic puller, which makes things proceed quite rapidly.  The closely spaced ribs near

the inboard end require the hand squeezer because of the lack of space between ribs.

I discovered that once the ribs are attached to the spar any curvature due to inadequate fluting is easily corrected, verifying my earlier decision to not be overly anal about absolute flatness in the fluting stage.

In their current configuration, the spar-rib structure is quite flimsy and I'm looking forward to attaching the skins.  Once skinned, I'm thinking I can continue to store the wings as they are now -- one on the work table and one suspended from the rafters.  The suspended wing will be turned upside down to allow use of the tie-down ring.  As noted earlier, there's a tapped hole on top but this would require drilling a hole in the skin to expose it.  As a full-fledged aero-nerd I could never bring myself to drill the hole in the top skin.

(page 15-05) Mistake in the build manual

Page 15-02 step 9 shows one nutplate and one W00028 doubler plate installed on a nose rib, yet page 15-05 and 15-07 refer to the nose rib with three nutplates.  After puzzling over this a bit  and studying all the

revisions, I contacted the Mothership and they confirmed that there are no longer any nose ribs with three nutplates.  These pages refer to a previous configuration and are now in error.  I'm RV-12 builder 593 and I'm sure hundreds of builders have encountered this, yet no revision has been issued.  I don't understand it.  This explains why the Homebuilt Help video didn't match what I'm doing.  It referred to the earlier configuration.  Those guys at Homebuilt Help need to keep up a bit better as well, but considering how inexpensive their product is, I forgive them.

RIP my favorite tool, the pneumatic rivet puller which came with my Isham RV-12 tool kit.  I never knew who actually made it since Isham re-branded it.  It became necessary to refill the hydraulic reservoir frequently, so I ordered a replacement from the aviation division of Harbor Freight for a mere $40.  It works great.  Can't beat the price.

It's a bit tricky figuring out which ribs go where on the spar, but the actual installation is easy.  As seen in the picture, almost all the ribs for the left wing are installed.  Space is a major problem.  Don't know the solution unless I evict the car from the garage.

(page 15-04) Wing ribs

Attaching ribs to wing spars seemed fairly painless and straight forward until I discovered that I had somehow managed to attach the rib-attach angles to only one side of the spar.  When page 15-04 called for clecoing and then riveting nose ribs to the rib-attach angles starting at the outboard end of the spar I got that familiar sinking feeling that accompanies the realization that I had made a mistake.  Looking back at page

13-03, I saw clearly that attach angles go on both sides of the spar.  I have no idea how I missed it.  The good part is that everything that needs to be redone is completely accessible and involves pulled rivets which are much easier to drill out than squeezed rivets.  The bad part is that there are 50 rivets that must be drilled out.  Fortunately, I have acquired great skill at drilling out rivets, so it didn't take long to accomplish.

The real pressing problem is where I'm going to store the left wing  spar with ribs attached when it's time to do the right.  I put a 3/8-16 eye-bolt into the previously-drilled-and-tapped extrusion intended for the tie down and used it to hoist the right spar up to the rafters until I'm ready for it.  I'm hoping the wings can be stored in a similar fashion.

(page 15-02) Wing OCD (obsessive-compulsive deburring)

Deburring all the wing ribs is a bigger job than I previously suspected.  The lightening holes require use of the little curved, freely-rotating deburring blades (I don't know what they're officially called), the small flange cut-outs require a tiny rat-tail file, and I'm doing the flange edges on a Scotch-Brite wheel mounted to a drill

press (visible in the previous post).  The build manual calls for flattening all the ribs via fluting, and I quickly learned that it's easier to debur before the fluting process.  The edges of the flanges are cut in a scalloped pattern so the flanges are scalloped in two dimensions after fluting.  The picture at right shows the fluting tool and the tool I used to remove fluting.  After fluting the first 10 or so nose ribs, I settled on this procedure which went more quickly: lightly flute every bay between holes, then decrease or increase the fluting where needed as indicated by laying each rib flat on the work bench.  The main ribs require a lot less fluting.  I found that lightly fluting the seven bays between the embossed stiffeners in the web was near perfect with no correction.

Since the fluting didn't require and solvents or lubricants and didn't produce any aluminum chips, I was able to escape the horrendous cold that has descended on the sunny south by moving the fluting operation into the house.  The Spousal Unit (my beautiful and patient wife Karen) was none too pleased but allowed it considering the temperature in the garage.

The description in the build manual of the various operations to be performed on the ribs is somewhat confusing, and the situation was not helped by viewing the Homebuilt Help videos, which don't agree with the manual.  I couldn't find a revision on Van's site which would explain this discrepancy, so I went with my best interpretation of the manual.

The bare ribs come in two flavors labeled W-1208-R and W-1208-L.  I assumed, of course, that the L referred to ribs for the left wing and the R referred to ribs for the right wing.  Wrong.  All the nose ribs for the left wing are labeled R with one exception.

Off topic  The engine for my antique car recently came off the dyno: 544 hp @ 6600 rpm, 486 ft-lbf at 5000 rpm.  I was a bit disappointed in the torque number, but it'll probably be hard to get it to hook up anyway given the limited rim width.

(sections 14 and 15) Where's the global warming when I need it?

Meanwhile, here in the frozen North (Carolina), the temperature has been close to single digits several mornings in the last couple of weeks, and more of the same is predicted before sane weather returns.  This time of year, on average, the low should be around 31 and the high should be around 50.  I can live with that.  I'm not happy about it, but I can survive in it.  This brings to mind the dire predictions from the pundits

back in the sixties and early seventies, when they solemnly proclaimed that we should be preparing for the coming ice age.  I am not making this up.  Maybe they were right.  This is all made worse by the fact that we just returned from our yearly week in the Florida Keys (81 degrees, kayaking in the ocean).

The result: With three kerosene heaters in the shop, I'm still too much of a wuss to work much on the airplane.  I have finished with the stub spars and rear spars.  The 1/8th inch solid rivets all got squeezed with nary a drill-out.  The nose ribs required a huge amount of de-burring, and I'm close to installing them on the main spars.  This requires supporting the spars with saw horses while riveting the ribs.  I simply don't have room without evicting my car from the garage.  I don't have a solution yet.