Obtaining the correct distance between wing spars

After adding F-415a/b ribs (R&L) to the fuselage I measured a slightly longer distance between the main wing sparand the rear wing spar attach point previously built. Inasmuch as this was a clear warning in the builders instructions I set out to discover why and correct it.

Turns out that the intersection of floor rib F-415a, bulkhead F406, and rib 415b was a bit sloppy. This added about 1/2" of distance that needed resolved. So I attempted the clamp the entire assembly and put the squeeze on it.

That did not work because F415a/b on boths sides began to buckle under the load. Without the 3/4 X 3/4 X .063 aluminum angles attached, there was no way to compress the whole After adding F-415a/b ribs (R&L) to the fuselage I measured a slightly longer distance between the main wing sparand the rear wing spar attach point previously built. Inasmuch as this was a clear warning in the builders instructions I set out to discover why and correct it.

Accordingly, I cut and fitted the four pieces of aluminum angle which were then drilled and clecoed onto the webbing and provided sufficient strength to compress the assembly and achive the 25 7/8" distance.

Your distance may be slightly different, but as mentioned in the builders manual - this distance must match the distance on the wing or you will be unable to mount the wings to the fuselage when the time comes. 

Once these strenghtened ribs were clamped to bulkhead F-406, it seemed apparent that ribs F-418 needed to be placed next. They determine the distance between bulkhead's F-407 and F-408.

Although one could as easily just use a simple measurements to place the bulkheads; it seems helpful to use the F-418 ribs (with aluminum angles attached) to confirm this distance.

Joining Bulkhead F-404 to Rib F-415

The rivets which were used to construct Bulkhead F-404 appeared to be interfering with the attaching of Rib F-415A. Accordingly, I drilled out 3 rivets, one on the bottom (top on jig) and two along the bottom (top on jig) of the aft facing web. This allowed the rib to rest flush against the U-shaped piece surrounding the Wing Spar opening.

Once the rib was in place the critical measurement between the forward wing spar and the aft wing spar was within 1/8". With minor shaving of the forward part of F-415 rib, I was able to get the measurement on both side spot on.

In a discussion on Vansairforce.net I was encouraged by an experienced builder to not focus too much on ribs and bottom assembly until all the longerons and bulkheads were in place. While I believe this to be good advice, the criticality of the measure (as noted in the preview plan book) between the forward and aft wing spars leads me to believe that F-415 should be considered as part of that process.

Without having confidence in the measurements along F-450 (between Bulkhead F-401 and Bulkhead F-404), and subsequently F-415A there is no way that I can see to get bulkhead F-407 in the correct placement. Starting from the front  and working toward the back, I am able to verify both the overall distances at each bulkhead and distances between.

Bulkhead F-407 seems critical to the correct attachement of the mid and aft body longerons.

My next challenge is to figure out why I am about 1/2" too long at the 69 1/8" measuring point (DWG-26) at bulkhead F-407. There are only ribs between bulkhead F-406 (aft wing spar) and bulkhead F-407, so the error must be in the aft of those two ribs. It seems like I remember the previous builder, who up to this point has demostrated an uncanny degree of accuracy in the building of bulkheads, etc., mentioned that rib F-415B needed to be reworked just a bit. The angle at the forward end of F-415B seems a bit off and I believe when reworked will reduce the distance to the junction of bulkhead F-407.

Once that position is acheived then the aft and mid-point longerons can be clamped in position. At this point, I just have the longerons clamped in the approximate position ready for a more accurate placement.

After spending a little more time on F-415A & F-415B, I have concluded that they just needed to be tweaked a little bit and clamped. Once I squeezed everything together I found the measurement at bulkhead F-407 to be exactly correct.

However, it is a little difficult to put the squeeze on these pieces made out of sheet aluminum. So I have decided to place the 3/4" X 3/4" X .125 aluminum angles along the four rib pieces that are pictured. That will give me a solid structure to clamp and squeeze together and prevent movement as I begin to drill and clecoe all of these junction points.

One fringe benefit of getting more of the longerons in place is that it actually is beginning to look like a fuselage!

If you have any insights or can see I am about to make a big mistake - please post a comment.

Bulkhead F-402 Dilemma

The previous builder claimed to have built all of the bulkheads, so I wondered why F-402 was still in four pieces. It turns out that F-402 intersects with two ribs (F-413) which attach to the F-401 & F-404 Firewalls.

My biggest problem was that I could not find brackets F-442, which are  attachment points that fit on either side of F-413 ribbing. Eventually the engine mount connects to these F-442 brackets through the firewall.

Furthermore, I could not locate an aluminum angle piece that connects the two ribs via F-442L & R. Are these angles, when not specified by part number always fabricated by the builder?

Eventually I found brackets F-442, but it appears that I have made my first mistake, which corresponded to the first holes I drilled in them and the angle piece between them. I did not allow for the additional angles which run along the top of F-413. This added 1/4" to the called out dimension of 7 1/2"

The plans are very inadequate in constructing this rib assembly. You have to search through three or four drawings to figure it out. I also found two 3/4 X 3/4 X .125 aluminum angles that run the length of both F-413 ribs. Unfortunately one of them is about 7 1/2" too short now as I used part of it to create the angle piece that connects the assembly together.

Although this assembly could have been made much later, I hope it will be useful in verifying the distance between F-401 & F-404. Although I need to complete it before I can place it in the fuselage. Here is a side view of the same assembly.

Probably going to need some help from Van's on this one. I cannot interpret the drawings.

I have the following questions  on this assembly:

1. Are the 3/4 X 3/4 X .125 angles which run along the top of F-413's precut or do I cut them from the stock angle material on hand? How much extra is in a typical kit?
2. Is there any better detail about how the angles attach to F-413 ribs, F-442 brackets, and F-443 brackets? (I am also missing spacers called for in the plans as F-444)
3. Do you attach the vertical angles on the F-401 Firewall first to accomodate this assembly?
4. Am I getting way ahead of myself? Should I be focusing on longerons, bulkheads, etc before tackliing this?

Any help would be appreciated.

Get things straight from the start

In order to get the fuselage started straight and  level you will need to minimally have the following bulkheads in place:

• F-401 The Firewall
• F-404 The Wing Spar
• F-407 The Back Seat
• F-412 The Tail

Starting from the front, be absolutely sure that F-401 is level, at the correct hieght (there was an error on earlier plans of a 5/1/8 vs. 5 7/8) and square or vertical on the jig. Then mark the center of that bulhead on the upper (lower on jig) aluminum angle (need term). This is the alignment point on the front which will be used in conjunction with the alignment hole in F-412.

In my case, I had to center F-404 on the jig, without the string because the crossmember I used was taller than all of the other crossmembers in order to hold both F-404 webs in the correct position. By utilizing the sides of the jig, a carpenter's square, and measuring carefully from F-401, I was able to center F-404. In retrospect, I suppose you could cut out the center section of the F-404 crossmember to allow the alignment string to run the full length of the fuselage.

I next marked a center line on the F-404 crossmember and drilled an alignment hole at a height that kept the string 1/8" - 1/4" above all the other crossmembers. Turns out that this is a much simpler connection point, I am not sure how one would connect to the stainless steel or aluminim angle of F-401

NOTE: Unless you keep the string off  the crossmembers slightly you will not get a true reading along the length of the fusleage.

This photo shows where I fastened a string from the center alignment hole in F412. Then I stretched it to the alignment hole just drilled in F-404 and checked to make sure there was nothing touching the string while measuring the distance between the longerons at each crossmember.

I'm not sure it matters, but I placed the remainging bulkhead into position on the longerons, with the exception of F-410.

Due to the palcement of the holes in  F-410, there is no way to run th alignment string. Well actually there is a way - I discovered it by accident. By putting F-410 in upside down, the string just barely clears both the crossmember and the F-410 bulkhead. Probably best to just leave it out until everything is locked down and centered along the jig.

Using the alignment string as your guide, you can now easily straighten the longerons and attach (clamp per instructions or of your own design) them securely to the crossmembers. In theory, if all of your crossmembers are the same height, your longerons will also be perfectly level at this point. In my case there was a slight variation between F-412 and the remaining crossmembers, so I had to make an adjustment.

Here is what it looked like when I was finished.  The longerons are perfectly level along their entire length until F-404 where they bend down (up jigged). They are also secured to the crossmembers such that there is an equal distance from the alignment string to each respective longeron, thus creating the basis for a straight fuselage.

By definition the bulkheads are mostly vertical because the longerons pass through the notches in each bulkhead. And assuming the crossmembers are place at the correct distance from F-401, the bulkheads are also very close to their final resting places. So far I have not yet drilled one hole, just clamping things together until final measurements are verified.

Let the fuselage begin

After reading the instructions several times, wading through many websites, blogs, and forum posts, I started to build. There is a dearth of specifics on how to do this, so my approach may or may not be best, but things are coming together.

I started with the Firewall bulkhead (401) and attached it as instructed, then attached the last bulkhead (412). Once they were both securely anchored (level, straight, and plum) then I had something to work with.

My good fortune was to have most of the bulkheads already constructed. If that is not your situation then is your next step, fact is you may be better off delaying your jig build until all your bulkheads are complete.

Placing all of the bulkheads in place gives you a real sense of accomplishment, even if they are just hanging there. Your neighbors might even start to recognize it as an aeroplane or something destined for lofty places.  I found a simple and effective way to temporarily place your bulkheads using 1" pieces of aluminum channel from an old shelf rack. If you leave it slighty loose, you can move the bulkhead around and position it correctly.

Bending the longerons seemed a little scary, knowing they are primary in the strength of this project. But doing so was not that difficult. I found that by clamping a block of wood on top of the vice at the exact bend point desired, it had the double effect of more securely holding the longerons and creating a bend that was identical on both sides. I bent both longerons at the same time where possible.

I also found that using the fuselage parts that fit between the bulkheads validated the measurement and placement of my bulkheads. These pieces are clamped in place temporarily.

I have not yet aligned all of the bulkheads. Once I have them all in place I will use string, a laser level, and carpenters lever to ensure they are straight and level. 



Things learned:

1. I am real happy to have a partially completed kit, just this fuselage seems overwhelming.
2. Placing packing tape on the backside edges of your drawings will help keep them from disintegrating. I have turned every one a hundred times its seems.
3. The instructions are not perfect. Bulkhead #407 had no slot for the longeron and no instructions for placing that slot. Don't even think about slotting the longeron!

Building the Jig

Building a jig seems pretty simple for someone with a reasonable set of skills and a few hand tools. And about two days later, I finished it. The challenges were finding long enough and strait enough wood from my neigbors scrap pile (with permission of course), coaxing out the bends and bows, leveling the contraption (on a not so level garage floor), and anchoring it to the floor.

One of the best ideas thus far is the attachment of two identical measuring tapes ($4.88 from Walmart) which run the length of the jig. This provides a solid measuring platform that eliminates a tape measure for placing the critically spaced laterals for holding the bulkheads. Using a carpenters tape is not such a good idea, at least not the ones in my toolbox. The end of my tape measure has about 1/8" of play and depending if you are pushing or pulling this can introduce a 1/4" error. This is way to much for an airplane structure in my opinion.

Be sure to buy a metal ruler. It is much more accurate and you can lay it across the jig for perfect alighnment on both sides of the aisle. I had to get creative to get my jig straight.

Things I learned:

1. The measurements called out for jig building do not align with the measurements on the blue prints for the fuselage. There is 3/4" difference to accommodate the mounting of Bulkhead #1 (F402). You'll need to reposition the tape running along the sides of the jig after the jig is finished, should you decide to use the tapes.
2. Using (2)1X6 X16's instead of (1) 2X6X16 will improve the likelihood of a straight jig or at least the ease of straightening a crooked one. Be sure to put the corssmembers and bracing triangles in place before you disconnect your straightening aperatus - unless you want to start over.
3. You have to start thinking about building this fuselage upside down. It can get a little confusing at first.
4. Liquid nails in reasonable abundance does a good job of securing the jig to your garage floor. Be sure it is straight, level, and well braced before doing so.
5. And just in case your wondering, the instructions for the jig are in the plans for the airplane, so don't try to do this in advance (like I did).

Using a chain, 2X4, string, and small spreader to straighten jig  

The Start of my RV-4 Project

Approaching the final approach of my airline career the prospect of being unable to slip the surly bonds on a regular basis is unnerving. So I found an RV-4 kit in a neighboring state and took the plunge.

Happily, this kit was in the advance stages of construction and the previous homebuilder/owner was employed at an aircraft factory - so he had loads of experience building. Lucky for me as I have no experience.

All evidence suggests a high quality build thus far. This coupled with a fair price (about 1/2 the going value of a new RV-4) and a very good impression of the character of the previous owner all conspired to make the purchase.

This took a suprising amount of space to properly pack.

With borrowed trailer in tow I returned with the goods today - fully loaded. And one speeding ticket through Montpelier, ID for good measure!

The following components were completed by C.N. the previous owner.

• Wings (less the fuel tank sealing)
• Ailerons
• Flaps
• Vertical Stab & Rudder
• Horizontal Stab &Elevator
• Fuselage bulkheads
• 402 - Firewall
• 404 - Spar
• 406 - (partial)
• 407 - Rear Seat
• 408 - 412 Rear Bulkheads
• Bellcranks
• Various sheet metal parts

Things to think about before you make the decisioin to do this.

1. Can you confirm the kit is complete?
2. Are you VERY comfortable about the quaility of the work accomplished?
3. Has there been more than one previous owner?
4. When and where was the project stored during the interim?

RV-4 parts and pieces - hope it's all there!