Step 5.5 Wing walk doubler

Yes indeed, the wing walk doubler.  These are exciting times aren't they.  Well, it all has to get done and wining about it doesn't pay very well, so on we go!   The wing walk doubler, in case its purpose is not obvious, is a reinforcement of the wing walk. Which of course is the foot wide section at the wing root (closest to fuselage).  It is actually a 9-3/8" x 26" piece of aluminum sheet that is riveted to the back side of the wing skin "doubling" its thickness.

The wing walk is the only place on the wing where we are allowed to step while entering or exiting the aircraft.  Stepping on any other section will almost certainly result in a dent in the wing.  So no dancing on the aircraft except on the wing walk, and please, no high heels.

The location of the wing walk doubler is at the right of the photo above where the first four ribs are located close together.  It's really important to make sure that the wing frames are sitting level and square, and that there is no twist in the wing.

As far as the squareness goes, the skin's pre-punched holes will pretty much take care of that. Squareness in this case is that the ribs are exactly perpendicular to the spar.  For the twist, we measure from a point at the top of the main spar to the top of the rear spar using a plumb line.  Using digital calipers to measure to the center of the string gives sufficient accuracy to insure a straight wing.  The wing frame is then adjusted such that the measurement is the same at both ends of the wing.  Assuming nothing moves during the match drilling or riveting, the wings should be within a 1/32" of straight.

While I was getting the frames straight, Carol scuffs the right leading edge skin prior to priming.

And the dogs were entertaining themselves...

Removing the blue plastic from the inside of the wing walk skin.  On the outside only the rivet lines are removed.

So the wing walk doubler is cut to length from a piece of sheet aluminum that has already been cut to width by Van's.  All that has to be done here is to snip it off at 26" and debur the edges.  The doubler is then lined up underneath the wing walk skin and offset 9/16" at the forward edge because the doubler does not rest on the main spar.  The skin and the doubler are then match drilled and clecoed together to the wing frame.

And finally, the right wing walk doubler skin on the frame ready for match drilling.

And the left skin in place.

Step 5.4 Leading edge ribs

I couldn't see any reason not to go ahead and fabricate the aileron bracketry.  These are constructed as sub assemblies that attach to the rear spars.  They are an interesting collection of thick (1/8") sheet aluminium and angle.  There are a variety of rivet types and lengths utilized so the plans must be studied carefully.

Once the brackets were riveted on the wing frame, the frame was moved off of the workbench and onto the wing stand.

 Then, I moved on to assembling the left wing frame.

With the wing frames on the stands it was time to level and measure the frames.  Barely visible in the picture below, the plumb bob is set for measuring wing twist.  If the measurement is the same at both ends there is no twist.

The next step after completing the wing frames is to assemble the nose ribs.  This portion of the wing is assembled separately from the wing frames using a cradle to hold the skin closed while the ribs are put in place.  Here is the cradle under construction.

I'm building the left wing nose assembly first and I quickly realized that the stall warning device, if it will be used, should be assembled and installed before the nose ribs are riveted.  I am going to use the device, even though I am also planning an angle of attack meter as well.  Some say having both is redundant.  And I agree. That's exactly why I'm installing both.  Because it is redundant and I like the idea of having a backup to this essential information.  Both instruments actually measure the same thing in different ways.

But what is the Van's stall warning device really measuring?  It measures the angle of the air impinging on the leading edge of the wing via a small paddle stuck out in to the air stream.  When the air moving over the wing blows on the top of the paddle the stall warning is off.  But when the angle of the wing (and paddle) is sufficiently high, the wind can get underneath the paddle and turn the switch the other way, thus turning on the stall warning.

How does the Angle Of Attack meter work?  The AOA measures a pressure difference between two probes (Pito tubes) set at different angles relative to the air stream impinging on the wing.  The difference in air pressure between the two probes is proportional to the angle of the wing, and since a wing will stall at a known angle (or a angle that can be measured), the point at which the wing will stall is determined and the stall warning issued.

So the Van's stall warner is akin to an idiot light in an automobile whereas the AOA meter is more like a temperature gauge since it gives a continuous measure.  Feeling drowsy?  Ok, back to to build:

Stall warner assembly.  Access cover and doubler plate drilled and dimpled.

A slot is cut for the stall warner paddle in the leading edge skin.

The leading edge section is assembled and then clecoed to the wing prior to match drilling.  Notice big orange mallet?  Sometimes micro-adjustments are necessary.  The fuel tank attachment flange is slid into place along the outside perimeter of the W408 end rib and under the wing skin making a very tight fit.

After match drilling the left nose rib skin is dimpled on the DRDT-2 machine.

Meanwhile, Carol comes down to help remove the plastic from the rivet rows on the right leading edge skin. Roxy looks on.  I think she was listening to the radio.

Well it was just a matter of time before I joined the drill-your-finger club.  I bet that not many get through an airplane build without a membership.

I used the pneumatic squeezer for all of the holes it could reach.  The DRDT-2 is a nice machine, but it is not as fast as a squeezer.

With the skin dimpled the left leading edge section is ready to be riveted.  All of the rivets on the leading edge piece can be set by one's self, but I found it easier to have help from Carol while setting the two or three on nose.  Working alone can sometimes be an advantage since no communication is necessary. But when the required reach is excessive, it's good to have help.

The left leading edge section complete, I now move on to finish the stall warning device assembly.

And that completes the left section.  Meanwhile, Carol begins the assembly of the right leading edge.

Step 5.3 Ribs, ribs, ribs!

There are 28 ribs in the RV-7 wing.  That doesn't sound like such a large number, but at this point I'm tempted to leave a few out.  Each one requires a considerable amount of work to prepare.  First off, the rib flanges are not 90 degrees to the rib web as delivered from Van's.  The manual suggests that a hand seamer pliers may be employed to bend the flanges square.  This is the method I used on the horizontal stabilizer ribs, but just by chance I happened across a thread in the Vansairforce forum where a flange straightening tool was pictured.  I'm all about building time saving tools no matter how long it takes!  It turns out that Chuck is too and he wanted to build the labor saving machine.  So off he went and here is the result:

The problem was that none of the pictures I saw on the web showed the tool actually being used, unlike the picture above.  So initially, I was really unsure what the dang thing was for.  All I knew is that if it would save time, I wanted it.  Well once we got it in front of us and had the ribs as well, its utility was clear.  Chuck and I both found it easy to use and a real time saver to boot!  Now that the rib flanges are straight perhaps I can  use it as a high powered nut cracker.

After bending the flanges I still had to flute the ribs.  Unfortunately, there isn't any miracle contrivance that I know of for fluting the ribs, which is a further straightening process which corrects for warping of the ribs during  the manufacturing process (stamping).  So I sorted the ribs between the left wing and right wing and went to work with the fluting pliers.

Here is Carol helping me to flute the time away.  To review: fluting refers to the process of making small bends in the rib flange to affect a curvature along the length of the rib.  In this way, a curved rib can be straightened by the application of an opposite curve with the fluting process.

By-and-by, in the fullness of time, and after many moons had passed, I finally had 28 mostly straight ribs.

The next step is to lay them out in order to prepare for match drilling.  One must really resist the temptation to become hopelessly confused by the plans which only show the left wing.  My big mistake here was starting on the right wing.  Had I started on the left, there would only be half as many parts remaining when it was done.  Hence, there would be less chance of getting a left wing rib into the right wing.

Much of my minor difficulty surrounds the fact that the ribs all come in two types, left and right.  But the leftness or rightness of the rib has nothing whatsoever to do with whether or not that rib goes in the left or right wing.  Let's just say that after much counting and recounting of the seven rib types, and consulting of the plans, and just the right amount of hand wringing, the correct set and order was finally discovered and the right wing frame was nearly ready for the first test assembly.

Once the holes are match drilled at both ends of the rib, the frame is disassembled and the ribs are ready for deburring, scuffing, and priming.  Here Carol is helping me to scuff the ribs while I am off trying to drill holes for the wing conduits and touch up ribs that didn't pass her deburr inspection.

The conduit will be used to carry the wiring for the strobe, position, and landing lights out to the end of the wing,  The conduit holes have to be drilled before the final assembly as it would be difficult to get a drill in between some of the closer ribs. I'll be using a lightweight plastic tube for the conduit, but I'm also installing some plastic snap bushings to protect the conduit from the rib.   Van doesn't give any particular size or location for these holes so we're on our own.

A conduit hole cleaned up and ready for the snap bushing.

As a special favor to you, I'll spare you the obligatory picture of the ribs hanging in the paint booth -- just this one time.  So we'll finish this post with an picture of something actually happening.  

The right rear spar is riveted to the wing frame.

Step 5.2 Rear spar

With the EAA work benches complete, I moved on to the rear spar assembly.  The rear spar is really just a thin piece of sheet aluminum bent into a Z shape to provide flanges to attach the aft edge of the wing skins.  The bends in the spar flanges also provide a great deal of stiffness to the spar.  On the forward face of the rear spar the wing ribs are attached while the aft face provides the aileron and flap bracket attachment.  On the ends of the rear spar are 'doublers' which are additional layers of metal sandwiched together to provide addional strength.  The first task is to cut the doublers to size.

The smooth the plates out with a file and the Scotch-bite wheel.

 Once the individual pieces are looking good, the whole thing is clecoed together.

Chuck came over and lent me a hand getting the rear spars riveted.  Since the rear spar is fairly small, we were able to use the squeezer all the way around.

I put blue tape over the holes that will not be riveted at this time.  I know from past experience, that once I get in the riveting groove, every hole gets a rivet -- It's much better to block off the unused holes now than to have to drill them out later!

With the rear spars riveted, it was time for priming.

Next up:  Rib straightening and riveting the wing frame.

Step 5.1 Spar nutplates

The inventory is finished and I have two wing stands ready.  Time to get cooking.  First up, two heaping teaspoons of nutplates!  The purpose of these first nutplates is to provide an attachment location for the fuel tank skins to the top and bottom of the main spar.  The fuel tanks hang on the forward side of the main spar and form the leading edge of the wing for about half its length.  So first up is counter sinking the wing skin side of the nut plate locations.  When it's time to add the skins, the dimples that allow flush rivets, or screws in this case, will nest within the counter sunk spar to allow the skin to lay flat against the spar.

The tape in the picture above is applied temporarily to prevent aluminum chips from falling into the cracks in between the spar components.  I did have some difficulty counter sinking the first few holes because I didn't understand that the nut plates should be riveted on before the counter sinking.  A quick search on the vansairforce.net forum revealed my error, and I was on my way.

Above, Carol swoops in to vacuum chips during a break in the action.  Below, the finished nut plates are seen below the spar flange.

I quickly realized while working on the spar's nut plates, that the wing assembly would go faster if I had a way to work on either side of the assembly.  Well, one time honored solution is to build the EAA1000 standardized work bench.  No sense in messing with tradition, so I enlisted the help of Chuck to put together two of these very popular work tables.

Finished just in time to trundle off to our favorite pub, The Northridge of Nevada City.