Step 8.7, Center section seat ribs

This step prepares the ribs under the pilot an passenger seats.  On one forward end the ribs attach to the center section spar carry-through and on the aft end, the ribs are attached to the F705 bulkhead which, as you know, sits just behind the seats.  The four center ribs are modified in two ways to accommodate the control column linkage. 

First, the lightening hole is enlarged using a fly cutter.  If you've read any of my previous posts on the usage of this most dangerous tool you'll understand the I'm not a big fan.  In retrospect, I now realize that I could have just used a hole saw and been done with it.  Alas, it is my lot in life to observe, ex post facto, the extant solution of least resistance and think, if only.

So the fly cutter is a kind of whirling dervish of tool steel and sharpness that would just as soon kill you as cut  a hole for you.  The first thing I notice as I set up the drill press is that most of the fly cutter's sharpness had somehow leaked away.  Well, I knew at once that it had departed the tool whilst I was in the throes of the conflict now known as the battle of the firewall.  That was quite a match-up, when we pitted the wily and very mean fly cutter vs. the nearly impenetrable stainless steel.  Anyway, to make a long story longer, the cutting tool seriously needed some sharpening.

Now with the fly cutter sharpened, the ductile aluminum ribs are no match for tool steel.

Then the all of the edges are deburred.  Now here are those center section ribs again this time showing the start of the second modification.  The flanges are cut out to permit the control column assembly to drop down into place.  

Here's a quick reminder of the control column assembly.  The seat ribs attach to the face of the gold colored spar carry-through.  It should be readily apparent how very difficult it would be to get the control column in place without the flange cut outs in the ribs.

In order to replace the flanges on the ribs these splice plates are fabricated.  Two come with the kit, the other two are made from sheet stock.  Can you see the difference?  Only their fabricator knows for sure!

The cut out flanges are riveted to the center two holes and the end holes match nut plates on the ribs.  Thus the flanges can be replaced after the control column assembly is installed.

And here is the result.  The seat ribs are match drilled to the spar carry-through (gold) and are ready for match drilling to the center section bottom skin.  In the next installment, we will build up the baggage floor ribs and assemble the center section.

Step 8.6, Tail cone assembly

Now for the fun part:

And so the aft fuselage assembly begins...  The tail cone is the tapering part of the fuselage between the rear of the cockpit going all the way back to the rudder.  The frame for the tail cone is made from the bulkheads previously constructed, the longerons, and J stringers which are a kind of longeron that are shorter.  Here's a look at the end of a J stringer.  

Before getting too far along on the assembly we are instructed to setup three saw horses.  I didn't want to use my ratty old saw horses for my new plane so I found these in the Home Depot aviation department.  I added the pine feet to preserve my floor. Notches in the wood and a screw in each leg will help prevent an accidental fold up.  Although the horses are rated for 500 pounds each, I wouldn't trust them with more than a couple hundred.

The first part of the assembly is the test fitting and match drilling.

At right the F779 tail cone skin is mated to the F711 and F712 bulkheads.  This skin is very thick and doesn't bend easily.  Unfortunately it doesn't exactly fit either.  It takes considerable muscle to massage it into a shape that approximates the rounded bottom of the bulkheads.  Fortunately, I ate my Wheaties.  Care must be taken here or the resulting holes from the match drilling will not be placed properly or may be elongated if the drill goes through at an angle.

Eventually, the fit is sufficiently good that move on the the big stuff.

Here the bottom skin is set on the horses and some of the J stringers are in place.  The fore and aft bulkheads are checked with plumb bobs to guard against twist in the fuselage before the stringers are and  side skins are clecoed.   

At this point the tail cone is assembled and the match drilling can begin.

Hey where did that blood come from?

Note to self:  Remember to keep fingers clear of drill bit on back side of skins whilst drilling.

There are a lot of holes to debur and dimple.  While I see to that chore, here is my puppy Trina who is more interested in chewing than aviation.

I'm using the angle drill and a piloted counter sink for most of the deburing.

Along the overlapping edges the skin is given a slight bend downward using this tool.  The sheet is clamped between these two roller dies and then pulled along the edge to be treated.  

 Here the treated edge is visible at right.  One can just see the downward bend at the edge of the sheet that keeps the seam tight with the underlapping sheet.

When all the holes are deburred the dimpling begins.  Most of the dimples are made with the DRDT-2 machine below.

Here is a finished dimple.

I wouldn't be surprised if there were more than a thousand holes to be dimpled on the just the tail cone.  Eventually, the last hole is deburred and dimple.  Hooray!  Here are the skins ready to be assembled after I've primed those areas that will come into contact with the structure.

All that remains is to reassemble the pieces and rivet.  Lots and lots of rivets to drive.

Self portrait.  I hope it makes a better airplane than a mirror.  Before calling it good, I go back and inspect each rivet.  I find six or eight that are voted off of the airplane.  Out comes the drill and punch and very soon these structurally sound, but aesthetically unappealing little boogers are gone.

And with no fanfare whatsoever, the last replacement rivet is driven and the tail cone assembled.  Next, we move on to the center section.

Step 8.5, Bending the longerons

A longeron is structural member of the airframe that runs in the longitudinal direction, i.e., from propeller to tail. It's principal function is to transfer loads from the skin to the frame or vice versa.  For the RV-7, the longerons come as 3/4" aluminum angle about 15' long.  They must be bent to match the curve of the fuselage around its widest point at the cockpit.  Behind the cockpit, and ahead of it as well, the longerons are straight, but are bent into a tapering angles that follow the double cone shape of the fuselage.  Time to get to it and bend some longerons.  First thing is to mark the points along their length where the curve starts and ends.

While the pieces are still clamped for marking, I cut the aft ends according to the plan.

A hole drilled into the vertex makes a nice radius.  Inside corners like this should always be radiused like this to discourage cracks from forming there.  It works like this:  If you're stress, you like to hang out at the corner.  But if there is a nice gentle curve to the corner, you're not sure where to stand.  Consequently, you and your buddies (more stress) can't get a crack started there because you're too spread out.

To bend the angle, clamp the angle into a vise and find a BFM.  It takes a considerable force to bend the angle.  This is not a job for the timid.  It surprises me how frequently the project that otherwise employs digital calipers and measurements of angles to a tenth of a degree should also require the use of a big mallet.

I gave it three good whacks and then moved the angle in the vise forward about an inch.  I remember thinking that this is not so difficult.  I should be done in an hour!   Ha Ha Ha.  The instructions say to repeat as necessary and so I did.  Back and forth along the section to be curved until the curve matched the template printed on the plan.  It was at this point I realized that although my curve matched the plan, the longeron was also bending in the vertical dimension.  Well I just won't stand for any unauthorized bending so I flip the longeron 90 degrees in the vise and pound the bow out of it.  Now its flat in that plane as it's supposed to be, but making it straight there undid some of the curve.  And so it goes, back and forth for two or three hours.  Just when you think its about done, you notice that it is also twisted.  %&$#!  A large Crescent wrench is good for undoing the twist, but it also affects the curve at the same time.  More pounding.

Eventually the angle tires of all of this wrestling and will submit.  

Eventually it gets done and its time to bend the forward section of the longeron.  This is the part that is in front of the cockpit and tapers downward as the fuselage gets smaller toward the engine.  So I level the longeron on the bench and get the digital protractor ready.

At the point 28.25" inches from the front, the longeron makes a sharp 5.6 degree bend downward.  More pounding, but this time it goes quickly.

With the angle bend downward all that is left is to add a 17 degree twist that will allow the longeron to follow the curved front section.  The big Crescent wrench makes another appearance.  To get the proper angle, I had to rotate the angle more than 90 degrees before it would snap back to 17.  It's a bit scary since I've got so much time invested in the part.

Almost there.

Now to bend one more just like it.  Only reversed, of course.

And there they are.  I'll just add that they really don't look like much laying there on the floor, but 

it's great relief to be past this step all the same.  

Step 8.4, Bulkheads galore

This is bulkhead F706 and the Border Collie at 1 o'clock is Roxy, my ever faithful tech inspector.  The keen observer may detect a slightly incredulous look on her face as she looks on.  It seems that she is concerned with F706 being too flimsy to be considered airworthy.  My canine companion would be correct were we not building in the semi-monocoque fashion.  As you will recall from our previous discussion in step 8.1, when using monocoque design the strength is mostly in the skin not the frame.

On this bulkhead the aluminum is very thin and will be an interesting test of the theory.  The edges are dressed and the mating pieces are match drilled.  Then the corners are fluted in an attempt to get the hoop to lay flat.  Only partially successful, I would say.  The material is so light weight that too aggressive manipulations will only distort the metal.  So I go as far as I dare and will let the skin's pre punched holes line everything up.

Up next is the elevator bellcrank assembly, sans bellcrank which will be assembled later.  This assembly mounts the bellcrank centered in the fuselage tube at the bottom of bulkhead F706 pictured above.

The next two bulkheads are assembled without much fanfare.  The corners require a signifcant amount of fluting to get them to lay flat, but at least these bulkheads are made from somewhat thicker material.

After the fluting, match drilling and priming they're pretty much done.

The next bulkheads are the F711 and F712 which both require some pieces to be fabricated from aluminum stock.  On the F711, 1/8" bar stock is trimmed down as pictured below before riveting.

At right, the F712 is riveted together.  The tie-down for the tail is trimmed to fit next.










Carol took care of this part for me.

And while she was at it, Carol also assembled the elevator bell crank.  Shown below with the hardware already pulled and ready to install.

The last bulkhead I did was the F710 which only requires a piece of angle stiffener to be attached at the top.

And here they are all lined up in order.

The next section will be bending of the F718 longerons.  I've heard that this step can be a maddening experience as the aluminum angle becomes quite obstinate and unpredictable when asked to bend.  No worries here as I've been taking some profanity lessons and feel ready to meet this challenge head on.