Get that seat back height correct!

For many of you, this is your first project. This is mine as well. Mistakes happen. Here's another example of one I made and corrected.

I assembled the section between the firewall to F6 (seatback) and then started on assembling the fuselage. I didn't pay close attention to the height of the seat back. I had assembled it by placing it on the plans. But the builder needs to keep in mind that there are many areas of adjustment before proceeding. My seat back height was too high, but I didn't realize it. I proceeded to set up my 2x4 on my work bench by locating the F10 bulkhead position and then leveling the top of F10 to the top of F6. Thinking I had things where they ought to be, I notched F10 for the 3/4" stringer angle. I did the port side 1st. But when I got to the starboard side I could see that I was going to cut into the large hole that F10 has for which the elevator control passes through. Since I bought my bulkheads from Peter Beck, I called him to discuss this hole location. He immediately recognized that I had an issue with my set up. After some discussion and measuring, we determined that I had not set my F6 top to 6" above the forward longerons.

So what I had to do is lower the seatback top and reposition my 2x4 so I could relevel my F10 top with the lowered height of F6 top. Now everything was where it needed to be. In the photos below you can see I simply repaired the 1st notch I had cut on the port side of F10 and continued with my building.

Click on photos to enlarge

Tool for bending forward upper longeron.

I need to take a couple of steps back to show you how I bent the upper longeron to match the angle of the rear stringer that runs from behind F6, all the way back to F12. You can't really bend the upper longeron until it is mounted in place because you don't know the angle to which to bend it. Here is the tool I made to bend mine.

Click on photos to enlarge

I used a scrap 2x4 and cut a slot at one end of it just wide and deep enough to fit the upper longeron into. My 2x4 just happened to be 27" long, but it gave me enough leverage to bend the forward longeron inward.

It is important to have bulkhead F10 in place and notched for the 3/4"x1/16" angles, making sure they are level with the top of the forward longeron. Then temporarily mount the 3/4"x1/16" stringer and bend the forward longeron inward until the angle matches the stringer. A second pair of hands is nice to hold the stringer while you bend the longeron, but not required.

Mounted F12 to Stringers

Click on photos to enlarge

I continued work on attaching the stringers today. This is tedious work because it requires you to assemble and then unassemble the peices several times before you have what you want. I used a scrap peice of the 1/16" angle as a shelf to attach the stringers to. I attached this to the front side of F10 as shown in the photo above. The angle makes it easy to level the stringers at F10 side to side. The stringers will be held in place with one 1/8" rivet attached vertically through this shelf. Due to lack of space to drill the vertical rivet hole, I figured out the appropriate location of the vertical rivet hole and dismounted the angle and drilled it 1st. Then I remounted it and put the stringer back in place and used a Sharpie to transfer where the hole was to the stringer. Then unassembled everything and carefully drilled the hole in the stringer. Then I went to the other side and did the same thing. Very tedious!

Next I attached F12 to the stringers. I was having trouble figuring out the horizontal distance F12 is from F10. The plans have the distance from F10 as it is laid out on your 2x4, but I had to cut the end of the board off because it interfered with F12. So my measurement was gone also. I finally found it on Sheet #3 (Rudder & Fin Assembly) where it indicates "about" 17 3/4" between the front and rear fin spars. Well, these spars mount to bulkheads F10 and F12 respectfully. Not very clear if you ask me. I trimmed my stringers so that I achieved this distance. In lue of a countersunk rivet to mount F12 to the stringer, I will leave the cleco in place until it is time to put the side skin on where I will use a "hole finder" and use this hole to attach the skin as well. I have to remember to use a long rivet here.

It might be time to replace some clecos for rivets and then some top skin.

Stringer splice to go beyond 8 foot stock length.

Click on images to enlarge.

I didn't get much done this Labor Day weekend. We had a family wedding to attend. I wanted to get something done so I worked on lengthening the stringer past the standard shipping length of 8 feet. I cut off 36" and added 41" which puts the splice between F9 & F 10. I'll let you know what my final length is when I install F12. I'm leaving the cleco's in place until I know for sure I have enough length to work with.


I used a 5" piece of 3/4" x 1/16" aluminum angle (same material as the stringer) for the splice. You have to round off the outside corner edge to get it to fit inside the 6061-T6 aluminum angle due to the inside radius. I used an aggressive file to knock off the corner edge and then used sand paper to give a nice radius until it matched perfectly with the inside radius. I will use 1/8" rivets to secure from the top only, so as to save the side rail for attaching the skin to.

While talking to Dave Thatcher on the phone on "what I do next", he reminded me that there are two anchor nuts that hold the front fin spar to F10 which get mounted later on to the front side of F10. He said not to rivet the side skin on until I have mounted these nut plates. I leave little notes like the one above to help remind me of these things!

 Click on photos to enlarge.

Because C3 thru C6 protruded out the bottom slightly, I had to modify my bottom skin flange just outside of ribs C3 & C6 as you see in the photo above. Basically I cut a small wedge out of the flange so I could get the skin to angle upward towards ribs C1 & C2 on one side and ribs C7 & C8 on the other side.

 Then I attached a piece of bent aluminum to extend the C2 & C7 rib flange down to the skin as shown in this photo.

Other ways to fix the problem were discussed, like tearing out the seat pan & ribs C3 thru C6 and redoing them, but I opted for this fix. I discussed with Peter Beck why ribs C3 thru C6 might have been longer in the rear and he was going to review his forms. There are some trimming involved with these ribs anyway and, had I followed the plans and laid the wings directly on the floor, or placed a flat piece of plywood between the 2x4's I used to raise my structure off the floor, I would have just had to trim the ribs C3 thru C6 from the top side to make the seat pan fit correctly. Had I known what was happening, I would have chosen to do this.

All in all, if I had not mentioned this to you, you would never have known about this mistake as it is on the bottom of the airplane where no one will see it.

Seat Pan Mistake & Correction

Another mistake I made that I would like to communicate had to do with the center section. The plans say to lay the center section spar on a flat level surface and attach the wings and create the rear center section spar. Then you are to attach the ribs C1 thru C8. Well, my garage floor in flat and level so I used it. However, what I did next sort of got me in trouble. I was having trouble working with a wrench when everything was laying flat on the floor, so I placed two 2x4's under the wings to raise everything up off the floor so the wrench would work better. When I went to install my ribs, I made them fit the top of the front & rear spars. Well, ribs C3 thru C6 were slightly taller in the rear than ribs C1 & C2 and C7 & C8. This made ribs C3 thru C6 protrude out the bottom by about 3/16". I didn't discover this until I flip the center section over to put on the bottom skin.

See next post for how I corrected for this mistake.

Here is the reworked wing tip.

Click on photo to enlarge

As you can see from this photo, my first attempt at the leading edge of my left wing tip did not have a satisfactory result. What I decide to do is to start on my right wing and try to figure out how to do it correctly. Then I came back and redid this wing tip. The trick is to roll this with a different radius at each end.

Trying to get the wing tip skin to mate well with the wing skin was a bugger as well. I had a neighbor help me with this process. He held things in place while I drilled the holes. This was truely a trial an error process for me. I hope filler will hide my poor workmanship. Most who have seen these corners say I'm being too hard on myself. You be the judge.

Next came the center wing tip rib. I had to reshape it to get it to fit properly. I was concerned that the other wing tip was going to be different from this wing tip, but as it turned out, I had to make these changes on both wing tip structures. So, whether they are different from the plans, the important thing is, THEY ARE A LIKE!

Wing tips were a problem

Click on photo to enlarge

 I have now uploaded my CX4 build photos for everyone to review and since I'm starting this blog after I have started, I will try to pick a couple of issues that gave me trouble. One of these was the wing tip structure. I had several issues that challenged my building abilities.

First was fitting the rear wing tip spar. I should point out that I bought Peter Beck's spars/center section and his bulkhead and rib kits. I personally thing Peter has a good product and if this is how you want to get a head start on your project, I would recommend his kits. As you can see in this photo, the rear spar had to be positioned in a different position than called for in the plans. No big deal really, just do it. I have found this similar issues throughout the build process. I guess it's just part of "scratch building" from plans. Dave Thatcher has been a great help with dealing with these types of things.

First Post

Why blog? Well, I have been the editor of the Quickie Builders Association newsletter, Q-Talk, for nearly 5 years now and the newsletter seems to be as relevant today as a typewriter in the age of computers and the Internet. After 20 plus years and 5 editors, the Q-Talk newsletter will come to an end this December. I believe the Internet has replaced it. Yahoo Groups allows you to discuss any subject or ask any question to other like minded people and get a response almost instantly. The newsletter used to be the means by which this type of information got communicated. By the time I publish the newsletter now, a topic is old. The newsletter used to be the archive of all the information that was known about our little airplanes. It was always strongly suggested that you buy all the back copies to inform yourself about what had come & gone with this particular airplane design, before you started your project. Now you are asked to search the Yahoo Groups email archive to get yourself up to speed. Searching the Yahoo Groups archives is way easier than trying to remember what issue a brake mount modification was in.

Also, instead of writing an article about an issue you have overcome with your project and then sending it to the newsletter editor, guys started their own blogs to record their journey with their airplane. And this is the reason I started this blog. Not only for me to record my journey, but to document my build process for those future builders.