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.  

Step 8.3, Bulkhead F705

This posting mainly concerns the assembly of the F-705 bulkhead which is located just behind the seats.  I find bulkhead to be a somewhat curious term. Nautical of origin, no doubt, but what is it?  It turns out that a bulkhead is simply a vertical divider.  Usually a bulkhead is also a structural component that adds rigidity to the hull. So in our case, the F-705 is a major structural component that stiffens the cockpit and provides a mounting location for the wing's rear spar and the seat belts.

I begin the F-705 by match drilling the lower horizontal to the central bar.  The central bar attaches the wing spars on the ends and the lap belts.

Next some doublers are  cut from bar stock and match drilled to the central bar.  More on these later.

But first, we take some time out for an extended period of plan gazing.

It's a busy sheet and it takes a while to figure out what needs doing.  Eventually, we decide to press on with the lower seat belt attachments.

And then as usual, we prepare the matched drilled parts for priming.  My philosophy on priming has been to only prime the part's mating faces -- that is, the parts that come in direct contact with each other.  There seems to be many deeply held beliefs regarding primer among the online community ranging from "it's completely unnecessary" to "your plane with fall out of the sky within seconds if you don't use just the right brand of primer."  I find it difficult to get too excited about the subject.

I begin assembly with the central bar doublers that reinforce the rear spar carry through.

The plans call for a 4 degree bend on F705K and so I finally got to use my Harbor Freight brake.  The brake was not very expensive so I wasn't expecting much.  It worked perfectly although the C clamps are kind of a pain to use.

This is the top piece that closes the loop on the F705 bulkhead.  

Oops, I see it's time for a screw up:  It seems that I have added two rivets on each side that were supposed to be added later. No worries here.  I've become expert at removing rivets.

Carol adds the seatbelt tiedowns and presto!  We have a (nearly) finished piece.

Step 8.2, Main spar carry through

The spar carry through is a really just a big splice that sandwiches the two main spars from the wing and carries the lifting load from the wings to the fuselage.

The two big pieces above form the bread of the sandwich while the meat is, of course, the wing spars.  It is interesting to note that the wing spars to not overlap or meet in the middle as might be supposed.  In fact, the spars only extend about 5 or 6 inches into the carry through.  Just beyond the big holes on the ends.  It is sobering to think that 10 feet of wing is attached by only a few inches of the fuselage spar carry through.

The sides of the spar carry through are riveted to the fore and aft sides.

The side pieces will carry the load vertically to spread it out through the fuselage's side skins.  The control column mounts to the spar carry through via these to milled aluminum attachments:

Van allows some of this aluminum to be trimmed back for weight savings.  Here is Carol laying out the trim lines.

After a pass through the bandsaw and a little filing we get these two very pretty mounts that are now 50 grams lighter.  I'm not sure that I would go through the effort again for a mere 1 3/4 ounces weight savings.

Next there are web stiffener blocks to drill and attach.  These blocks discourage the spar web from bending under load. 

And then there are nut plates to attach on the spar flanges.

The next step is to assemble the control columns to each other and to the mounts.  Van plays a little trick on the unsuspecting builder at this point when the manual says it is possible to remove the columns from the mounts as a unit.  Well that is a time saving tip if I've ever heard one, so I plunge on ahead with the column assembly only to find out that it is impossible to remove the columns from the mounts without first removing the aileron pivot bolts.  The feed through bar (grey bar below) is the only thing that can remain connected.  Anyway, once the columns are assembled and adjusted for parallel, they are disassembled and set aside.

The next step is to fabricate some wood spacers that stand in for the wing spars so that the carry through can be bolted together.  The thing about these spacers is that they must be exactly 1.438" thick to mimic the thickness of the spars.  No problem, I'll just laminate some scrap 3/4" MDF and trim it down on the table saw.  Naturally, after the sawing it was about 1/32 too thin.  Suddenly I'm reminded of the old saying, "I cut it three times and it's still too short!"  So I laminated some very thin cardboard in between the halves and I was just bit too thick. Crap! This sounds like a job for  Captain Belt Sander. Problem solved. And we didn't even have a chance to thank him.  

That's about it for the spar carry through.

Ta Da!