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RV-7 N8ZG
Empennage
Horizontal Stab
Vertical Stab
Rudder
Elevators
Fiberglass
Wings
Spars
Ribs
Wing Skins
Fuel Tanks
Ailerons
Flaps
Fuselage
Cowling
Engine
Paint
I was all psyched up to mount dual flop tubes and fuel return ports, then started thinking critically about what I really expect to do in the way of aerobatics. Van urges us to limit the tail-twisting to gentleman's aero, and quite frankly the heavier stuff makes my head hurt. Simple, positive-G maneuvers will be just fine. I will not intentionally tumble an RV, nor will I run down the centerline upside down.
So, I'm not spending big bucks on a Christen oil system, and I'm not complicating the fuel system with unnecessary parts. In fact, my tanks are about as simple as they can be. No internal pick-up tubes, no flapper doors, no 6-inch access hole in the inboard rib. The fuel pick-up is a simple threaded flange in the lowest corner with a finger strainer that threads in from the outside, stolen from the RV-10 design. Thanks to Evan Johnson for the idea.
The fuel tanks are attached to the spar with screws thru the top and bottom flanges and bolts thru the spar webs. The gap between the rear baffle and the spar web is spanned by a double-flanged angle extrusion called a Z-Bracket. These brackets are fitted by first bolting them to the spars, then match-drilling them to the rear baffles.
Begin by marking a vertical center-line on both faces of each angle, then mark the center point on each face. Drill a 1/8-inch hole (#30) in the center of one flange of each bracket. Set two (2) brackets aside, and drill a #12 hole in the center point of the opposing flange of each of the remaining 12 brackets.
Lay
the brackets out on the wing in their respective positions and correctly oriented, flanges pointing in the right direction and
top end up. This
is gonna seem strange, but hang with me... Hold the bracket in position, then roll it edge-for-edge 180 degrees so that
the flange with the #12 hole in it is now up (forward, actually) and the top end is still pointing at the top side of the spar.
Without changing the orientation relative to the spar, move the bracket to the aft side of the spar and cleco it in place.
The bracket should now be clamped to the spar backwards, but not upside down - that is, if the wing were in it's "normal" orientation, the top end of the z-bracket should be up and the flange that will mate to the forward side of the spar web is temporarily mated to the AFT side of the spar web. Align the marked center-line with the pre-punched holes in the spar. Clamp the bracket in place and back-drill, using the holes in the spar web as a guide. A drill guide block will help you keep the drill perpendicular to the spar. Repeat for the remaining brackets.
Remove
the brackets. Prep and prime as desired - I chose to Alodine. Attach the
#10-32 nutplates. You might want to
mark them for position in some way - I used very light punch marks.
Bolt
the brackets in place on the front side of the spar. A little BoeLube
or anti-seize on the bolt threads will make it a little easier and reduce the
wear on threads.
Then
cleco the rear baffles to the Z-brackets. Verify that the z-brackets are reasonably centered on the holes in the baffle.
Retrieve the two brackets that you set aside earlier and cleco them in place in the inboard position. Align the
centerline on the brackets with the holes in the baffles and back-drill the z-brackets thru the spar web. The four
remaining holes in the z-bracket will be match-drilled with the inboard ribs later.
Remove the baffles and install the three nutplates on the aft face of each spar web.
Find
the bundle of tank stiffeners and make little pieces out of big pieces. Cut the individual stiffeners from long angle
stock. Trim the excess, round the corners and de-burr all edges.
Cleco
the stiffeners in place and ream to #40. Remove the stiffeners and
de-burr the holes. I didn't bother marking the stiffeners for position.
Dimple the stiffeners and set them aside.
Cleco the tank skins to the
baffles and spars. If necessary, adjust the holes in the baffles to allow a perfect match between the skins, the spars
and the LEs. Match-drill the tank skins to the rear baffles.
Remove the skins and de-burr the holes.
Cleco
the ribs to the baffles and Z-brackets and ream the holes to #30.
Beginning at the forward holes, cleco the skins to the ribs, alternating
between the top and bottom of the tank.
Remove
the bolts from the Z-brackets. Pull the tank assemblies off the spars
and set them in the cradles.
Ream
the skin-to-rib holes to #40. Take it all apart and de-burr the parts.
Mark
the profile of the forward tank attach brackets on the heavy chunk of
aluminum angle. Hack the two blanks from the angle stock and trim them
up. It's much easier if you have a bandsaw. I did it the old
fashioned way in about 10 minutes.
Clean
it up with the file and finish on the wheel. Why file on it when you
have a perfectly good grinder? Because, 1) it gets HOT quick and then
you have to either a) put it down and wait for it to cool or b) dunk it; 2)
the grinder throws a lot of dust everywhere; 3) those wheels are
expensive; 4) I can file and still hear Bill O'Reily castigating bureaucrats;
and 5) filing counts as aerobic activity, and I'd rather take a beating than go
the the gym.
Whittle
on the brackets until you're happy with the fit in the nose of the inboard
ribs. Relieve the mating surface edges to prevent stress risers.
Clamp and drill the brackets to the ribs.
Mark
and drill the holes for the tank vent fittings. If you're planning a
fuel system that requires a fuel return, this would be a good time to drill
for it, too. I seriously considered installing a -4 fitting for return
fuel, but in the end left it out. If I need a return line, I'll tee it
in somewhere between the pick-up bung and the selector valve. If you're using capacitive fuel level senders, you also
need to drill for the BNC connector.
Put
the skins back in the cradle and use a few clecos in each rib to hold the
tank's shape. Use plenty of clecos in the two ribs that form the outboard
bay. Put clear packing tape over the filler hole and secure the fuel cap in the filler neck. Note the formed curve
in the filler neck flange and stick the filler cap and neck in place.
Hold it as best you can and match-drill the flange thru the tank skin, adding
clecos as you go. Mark the flange for left/right and fore/aft before you take the clecos out.
In retrospect, I think I'd have gotten better results if I had used more
clecos, left the outboard bulkhead rib out and turned the tank over, rather
than standing on my head to drill the filler flanges.
This is as good a time as any to work up the capacitive fuel senders. I don't think I have any photos of the prep work on the senders, but it's pretty simple. Measure and mark the plate positions on the four indicated ribs and drill the pilot holes for the screws. Mount three nutplates on each of the sender plates and make the relief cuts necessary to provide clearance around the stiffeners. You can make up the pigtails for the wiring now, or do it when you mount the plates for good.
Might
as well make the clips that hold the free end of the vent
tube in place. I stole this design from the RV-10. (And it's a fuzzy
picture - sorry. I have a better one - somewhere. I'll change it
out when I find it.)
Take
it all apart again and de-burr any holes or edges that need it. I used a nylon abrasive wheel in my cordless drill to
rough up the skins, stiffeners and rib flanges for the sealant. (Another fuzzy picture - sorry. I guess I need
to read up on the timer function...) Dimple all the holes for the ribs, stiffeners and flanges. Dimple the
stiffeners and rib flanges, and countersink the filler flanges.
I carted all the tank parts up to Hickory for the Thanksgiving holiday and talked my dad into helping me build them up. If your arms are long enough to reach the four rivets in the nose of each rib, you could do the tanks alone. Mine won't reach and I'm not comfortable enough with the rivet gun to hold it backwards, work the trigger with my thumb and trust that it won't bounce all over the sharp curve of the leading edge. A second person to buck rivets and wipe tools really helps speed things along.
You may be tempted to use leftover bits of sealant to mount the vent fitting and the BNC connector. I'd recommend you wait until the bulkhead ribs are riveted to the tanks. The fitting and the BNC will be in the way when you try to buck or squeeze the rivets that hold the rib to the skin. It's better to mount these parts just before you close the tanks with the rear baffle.
You might also be tempted to rivet the forward tank attach bracket to the inboard rib while it's loose and you still have easy access to the rivets. Don't. If you do, you'll be taking it off. With the tank attach bracket riveted to the rib, there is not enough room to buck or squeeze the rib-to-skin rivets.
Clean
the skins, stiffeners and rivets with your favorite solvent. Then insert and tape the rivets in place for the
stiffeners. I put electrical tape on the bottoms of the skins to keep the sealant off of the areas that mate with the
ribs.
I mixed my first batch of sealant and tried scooping it into a sandwich bag and piping it onto the tank skins. It worked, but not well, and I ended up tearing the bag trying to keep the sealant in it's assigned corner. Disposable cake decorating bags are a little tougher and would probably work better.
I
squirted a thin ribbon of sealant around each row of rivets and set the stiffeners in place. I don't remember
back-buttering the stiffeners. Position the stiffener over the back-rivet plate and set the rivets. Repeat for
the remaining stiffeners.
A
popsicle stick makes a good disposable tool for spreading sealant, forming the fillets around the edges of the stiffeners
and dabbing the shop heads of the rivets. When you're happy with the fillets, pull the tape out of the rib positions
and set the skins aside for a few hours. If you want the insides of your tanks to be pretty and clean, I found that
the inevitable strings, dobs and smears were easier to clean up after the
sealant had set up. You can scrape the majority of the hardened sealant
off the aluminum with a fingernail or a plastic scraper, leaving a lot less
to get up with solvent. After most of the work was finished, I also
found that denatured alcohol cut the hardened sealant much better than MEK.
I haven't tried it yet on un-cured sealant.
Mix
a batch of sealant and set the drain flanges and filler necks. Butter the mating surfaces, cleco and clamp the flanges to the
skins and rivet. Butter the shop heads and clean up the outsides. I left the bead that squeezed out inside the
drain hole. The next morning, a fingernail removed the bead with no mess.
Here's
the fuel pick-up modification. The flange is a Van's part from the
RV-10. A finger strainer screws in from the outside, and the fuel
line fitting screws into the strainer. I put the pick-up flange as
far aft and as low as I could get it, but should have moved it forward
about 3/4". I wanted to use flush sump drains, but by shear dumb
luck, the strainer screen and the long body of the flush drain interfere
with each other.
Decipher
the marks you made earlier, clean the mating surfaces and cleco the filler
neck flanges to the tank skins. Re-check the alignment, orientation and
the fit to the curve of the skin. Smear on some sealant and rivet the filler neck flanges to the
tank skins with AN426AD4 rivets. Don't forget to include the vent line
retaining clip. Smooth the squeeze-out into a small fillet and butter
the shop heads
Decide how you're going to close the tooling holes
in the bulkhead ribs. I had some short AD5 rivets, and was able to
pre-squeeze them enough to get a descent fit in the holes. A dab of
sealant left over from one of the many batches and one more bite from the
squeezer - no more hole. Butter the shop head and set it aside to cure
a bit. Don't mess with the forward hole in the inboard rib - it will be
covered when you mount the attach bracket.
Cleco
all the
ribs in place. Begin in the center of the tank and work out to
either end, one rib at a time. My dad has an OLD digital scale that is a pain to use, but it's
paid for, so we used it. It defaults to grams, so I mixed 110g batches of sealant. That turned out to be about
all we could work before it started to harden. One 110g batch of
sealant was enough for two ribs with a little left for fillets and other
touch-ups.
UPDATE: When I got to building Gene McNeely's RV-8 tanks, I worked alone and used Wilton disposable cake decorating bags. The piping bags are thicker than Zip-locks and seem relatively unaffected by the solvents. They also provide a lot more control than a smearing stick and I had a lot less waste. 55-gram batches is enough to do two ribs.
Butter a rib flange up with a liberal coat
of sealant and stick it in place. Add a few clecos and stick rivets in the open holes. Set the forward rivets
and work aft. Keeping the bucking bar and mushroom set clean will improve your results. Dab the shop heads with
sealant, scoop out the excess and dress the fillets on both sides of the rib flange to suit you.
Once all the interior ribs are in, you can work up the vent lines and fuel level senders. Here's a great opportunity to cut your thumb wide open. The wire for the capacitive fuel level senders runs down the outside of the vent tube. The vent tube pretty well fills the hole in the snap bushings, so you need to cut a relief notch in the inside rim of each bushing to clear the wire. I used an X-acto knife because I couldn't find my rilfler files. Put snap bushings in the holes indicated for the vent line, slip the aluminum tube in place and check the notches for size with a scrap of wire.
Bend
the inboard end of the vent tube to meet the vent fitting in the rib and
flare the inboard end of the tube. Use the outboard rib to locate the
open end of the vent tube, mark it and cut it off. I put a flare on the
outboard end to keep me from pulling it out of the clip when I was working on
something else.
With
the vent tube in, you can seal and rivet the bulkhead ribs to the tank skins.
Connect the vent tube to the vent fitting and torque the nut. If the
outboard end of the vent line is a little long and rubs the rib, a gentle
bend in one or more inboard bays will shorten it enough to prevent future
chafing. Solder the wire stubs to the BNC center conductors, then mount the BNC connectors in the inboard bulkhead ribs
and cover the assembly with sealant.
Time
to install the capacitive plates. Make up the stand-off insulators
using the poly tube and washers and mount the plates on the ribs. Using
socket-head screws would have been mush easier. Ensure that you have
the proper clearance between the plates and the stiffeners. Cut a
length of wire long enough to run from the terminal hole in the outboard
plate to the inboard plate routed down the vent tube. Strip one end and
crimp on the ring terminal.
Use
the 4-40 bolt, washer and nylock nut to secure the terminal to the outboard
plate. Thread the wire thru the notches in the snap bushings to the
inboard plate, wrapping it around the vent tube along the way. Trim the
wire to length for the terminal hole on the inboard plate and strip the end
for the connector. Cut another short wire to reach from the inboard
plate to the BNC connector and strip one end. Crimp the two wires
together in a ring terminal and bolt the terminal to the plate.
Alternative: Crimping two wires that have already been cut to length inside the tank is hard enough. The screw-nut-ring terminal assembly is downright painful to fasten to the capacitor plate. When I built the tanks for Gene McNeely's RV-8, I riveted 4-40 nutplates to the capacitor plates. Socket head cap screws and a ball-end Allen wrench made this task MUCH easier.
Thread
the loose end of the wire thru the last interior rib and wrap it around the
vent tube. Trim the wire to length, strip and solder it to the BNC
connector's center terminal. Grab your multimeter and check for
continuity between the BNC center conductor and the shell. There should
be none. Also check to ensure that you DO have continuity between the
shell and the rib. On the inside of the bulkhead rib, smear sealant around the
BNC connector and vent tube, making sure to cover all the joints. Use
dabs of sealant to bond the wire to the vent tube in each bay to prevent
chaffing.
Now you can seal and rivet the tank attach angle and it's reinforcement plate to the inboard bulkhead.
Attaching the rear baffles to close the tanks is a big job; you'll appreciate some help. It's not any harder than setting the ribs, but there's a lot to do in the short pot-life of the sealant. I had a 6-oz tube that I bought for the rudder TE and then didn't use, so I broke it out and started mixing. It was enough to do the job and was dead easy to use with the air-powered dispenser - mixing by hand was a challenge. I was working alone and really pushing the limit on time.
Sweep or vacuum out any trash and clean the inside of the tank to suit you. Clean the rear flanges of the ribs, the tank skin flanges, the z-brackets and the rear baffle with your favorite solvent. Clean the two sizes of pop-rivets and the solid rivets with solvent.
All your tools laid out where you can reach them? Plenty of solvent and rags handy? Did you check your rivet puller to ensure that it will pull a rivet in the tight space against the z-bracket web? Ready?
Mix a generous batch of sealant. Butter the rib flanges and tank skin flanges with sealant and set the baffle in place. Make sure you put a generous pile in each outside corner - with the relief cuts in the bulkhead rib flanges, this is the only thing sealing the corners. Use a scribe, pin punch or some other type of probe to get all the hole aligned and start setting the pop-rivets. Dab each rivet in the left-over sealant before you stick it in a hole. The two short rivets go in the top and bottom holes in each rib, while the three long rivets go in the center, thru the z-bracket. Squeeze the solid rivets that connect the tank skin and the rear bulkhead flanges to the rear baffle.
Not a single picture of the operations to close the tanks. It was dark, I was by myself and pressed for time. Sorry.
Sidebar: Some of our peers seem to have taken great pains to clean all the goo out of their clecos. I didn't worry about it and just wiped off the worst of it when we quit for day. I had good results using ten or twelve clecos per rib, and used the same clecos in all the places that had fresh sealant. I tossed them in a coffee can with a little solvent in the bottom when they weren't being used. Months later, whatever sealant was left inside the bodies doesn't seem to be causing any problems.