Building An Airplane From Plans
Dec 31, 2010
I took some time today to pick up some carriage bolts, nuts and washers and put our rolling 4’x12′ table on level feet so it made a better platform for assembly of the horizontal stablizer. I used scrap pieces of 2×4 and 2×6 about a foot long, drilled a 7/16″ hole in the end and screwed a 3″ long 1/2″ carriage bolt into the bottom with a washer and a stop nut.
As mentioned, assembly of the rudder was moving along fine until it became apparent that the ruddder was not “square” with the world. My (Dan’s) thought is that this issue is somewhat easy to avoid but just as easy to fall into if you are not careful. The bottom line is that the rudder structure, like the wings, elevators, ailerons and flaps are thin ridgid structures that are expected to be “straight” when assembled. The geometry of the shape suggests that drilling the holes for assembly needs to be done carefully so as not to create a potato chip (warped wing) affect with the final product. A first order analysis of a 3 dimensional box shows how sensative the final shape is to the initial dimensions. If the one leg of a 2-1/2″ x 37″ rectangle is extended by 1/16″ the resulting shape (a rhombus) when coupled to a 3 dimensional assembly will be forced to satisfy the simultaneous relationship of each perspective side. If you attempt to adjust one perspective into a particular orthoganal alignment it will affect one or both of the other associated perspective views. Since the sheet dimensions are fixed (it is unlikely you will distort the sheet in its “x and y” dimensions) the sheet must accommodate the change by shifting in the “Z” dimension. How far will the part move in the Z dimension? A first order guess would be if the long side of a 2″ x 32″ rectangle is adjusted by 1/16″ the far end will raise by the ratio of 32/2 or 16 times the 1/16″ which is a full 1″.
All of this is somewhat accademic but points out what I mentioned to Tim in an E-mail that, while it is important to measure, mark and cut parts as close as possible, it is the assembly process and “rigging” of each part during assembly that defines the “trueness” of the plane. In other words, you can produce precision ribs and spars for the wings, for instance but when the skin is attached to the wing it is very easy to imagine (for me anyway) how the wing could be “warped” and just as easy to imagine how a slightly less than perfect wing structure could be properly rigged when the skin is match drilled to the ribs to obtain a straight wing.
This relatively small adjustment in the match drilling, shown above, was responsible for a rudder that was close to an inch out of “plane” (no pun intended) when sitting on a flat table surface. The cause was the slightly out of square original sheet that was bent into the basic shape and then held in edge alignment through the trimming and assembly process.
The correction was not large and the slight offset in the otherwise very perfect rudder was absorbed when up-drilling to the #30 bit size. A close call and a very valuable lesson.
Had some time on Christmas Eve to trim the elevators and from the lessons on the rudder assembly made sure everything was square before cutting out both elevators.
December 24th. 2010:
The components for the horizontal stabilizer have been completed over the last few months and it is now time to match drill and assemble these.
Shown above are the horizontal stabilizer ribs and spars before match drilling. On the opposite side of the table are the elevator skins, ribs and root ribs with the elevator horn ready for assembly.
This step in building is one of those points where you can see things moving fast and of course, it can be the point where mistakes can be costly. I try to keep in mind that mistakes will be made and they can always be corrected, it’s just a matter of being as careful and deliberate as you can so as not to rush into anything and reduce all the previous work into a collection of aluminum scrap. By the same token, it is important not to be paralyzed by fear of making a mistake. The only way to avoid making any mistakes is to do nothing.
December 24th, 2010 (2 hours)
Trimming the elevators, like the rudder that they share the same control surface basic shape with (Z03-01) needs to be done with some care but after you do the first one the process should go pretty fast. Tim layed our elevators out while I was matchdrilling the rudder. As I realized the rudder was skewed after assembly all construction was halted and we found ourselves struggling to understand what went wrong. As described in the “Rudder Fix” page, the correction was straight forward as was the plan to avoid this problem is the future.
The day after I corrected the rudder I re-marked the elevators after checking them for square and cut them to size. The procedure is pretty easy.
1) Insure your table surface is flat (not necessarilly level but being level in two dimensions is an easy way to insure it is flat). and press your elvator flat to the table.
2) Trim the square end of the elevator so it is actually square. This will be the datum (reference) for the angle cut and needs to form a right angle with the bent edges of the elevator.
2) Clamp the elevator with the proper 2-1/4″ height at the widest point (see SNX Z03-01). We used cleco clamps to hold the elevators in this position and found that marking the overlapped surface when the shape is in the correct spot helps, since re-measuring them every time you take the clamp off is not an efficient way to proceed.
3) Mark the angle at the root end of the elevator. You can lament over this step (like Tim did) for along time but my plan is to simple cut the part slightly long and then trim with a vixen file to the final size to correct for any errors in marking. Since Tim had the parts already marked I simply had to confirm his marks and make any adjustments.
4) Cut the bent edges of the sheet metal with a band saw with the cleco clamps still holding it. We had a piece of wood that we slipped inside the elevator that was cut to an approximation of the final shape but I found that it was useful for the next step (file with vixen) it was not that critical during band saw cutting.
5) Finish cutting the large areas of the elevator skin with large snips and file completed cut to where they were marked with a vixen file.
One other thing to note, small variations in the root rib assembly (one of our root ribs that was clecoed to the elevator horn showed a slight curve when clecoed to the horn) will tend to create an imperfect hinge line along the back of the elevator assembly. Match drilling the root ribs assembly to the elevator skins might be best delayed until the horizontal stabilizer is ready to be mated with the elevator. This way the hinges can get properly aligned and any small adjustment in the attachment of the root ribs to the skins can be done without producing any latent (embedded) strain in the elevator / horizontal stabilizer asssembly.
Match drilled rudder to ribs and hinge….found it was slightly skewed from original cutout and bend and never corrected. OK, maybe more than slightly skewed. About 1 inch potato chip shape. No way to fix this easily without making it look like it was in an accident. Can salvage horn without ribs and hinge. Can use the skin for other sheet metal parts (ribs, etc.). If this is like the elevator spar I can make all the parts over again in a fraction of the time it took to do the first time. Maybe one day, 8 hours from beginning to end if all goes well
Trimmed final rudder skin to length and shape. Cut and drilled hinges for rudder.
(side track to build enclosure for noisy compressor)
Bought sacrificial ruler to drill hinges. Studied hinges for rudder to make sure cuts lengths and locations were understood. Trimmed sample rudder to size and marked final rudder skin.
Made T14-03 (optional ribs for fiberglass tips). Assembled rudder horn, assembled elevator root horn. Made elevator trim piece for and attached to elevator horn.