I square the fin to my table surface first. The fin is your reference point for the horizontal stab and wings. They must be exactly perpendicular to the fin, so I take great care to ensure that the fuse is fixed in a position such that the fin is exactly perpendicular to my flat table surface. At the same time I also ensure that the thrust line is exactly parallel to the flat surface. Fin exactly perpendicular, and thrust line exactly parallel. With the fuse oriented in this way, I can ensure that 1) wings and stab are perpendicular to the fin, and 2) establish the incidence of both. To facilitate squaring the fin, I tack-glue the rudder post into the fin. Then I carefully mark the rudder post with a fine straight line exactly in the middle of the vertical length. This is not as easy as it might seem. The fin may not be perfectly symmetrical (not even the good European kits are perfectly symetrical without defects). In your case you have a curvy fin. You will need to do the best that you can. This fine, straight line on your rudder post is the reference that must be perpendicular to your flat table.

If you do not have a flat table -- really flat, then you will need to consider some other method. I have a granite surface plate that is flat to within 0.0004 of an inch across the entire surface. That is not absolutely necessary, but it makes me feel good. Most things are not flat. You might think they are flat, but they are not. Doors, inexpensive aluminum straight edges, floors -- you name it, they are curvy. Even pool table slates are not very flat. Some are, but most are not. If you use those type of things as references, then you will probably not produce a straight airplane. Most doors for example are often off 1/8 inch or greater across the surface area. The whole door might twist, dip and rise. If you are using a non-flat surface as your reference for wings and stabs, your measurements will be off, and your plane will not be dead nuts straight. You can try to build a flat surface, but it is not very easy, and it may not stay flat for long.

Next I install the stab. I use a surface height gage to inspect the level of the stab in various locations. The bottom at the tips, the trailing and leading edges at the tip and roots. The goal is to get your stab mounted such that it is exactly parallel to the building surface, with the correct incidence on each stab half, and perpendicular to the centerline of the fuse. You can check this last measurement by triangulating the length from the stab tips to a point in the center of the fuse near the nose.

Then I install the wing using the same procedures and measurements. If it is a one-piece wing, do not assume the saddle is exactly perpendicular to the fin. That means you may have to do something to correct the offset. Personally, I prefer plug in wings. This is one of the reasons. I do not reference my wing installation from the tube itself. The tube may fit into the wing slightly crooked. If this is the case, and you install your wing tube perfectly aligned, then your wing will end up not perfectly aligned. Same story for the stab. If you glued the tube sockets in yourself using a very careful procedure on a flat surface, and you know that the tube is dead centered in your wings/stab, OK then you could align your tube with respect to your flat surface. But I like to measure the wings and stab, and not the tubes.

How long does this take? It usually takes me a several days to align and mount the wings and stabs. I do not stop measuring until every possible measurement is within 1/64th of an inch. If you move one thing, something else changes. If you bump something, then you have to recheck all measurements. I don't mix epoxy until I am dead certain it is straight. But once it is done, you should not have to repeat the process until your next airplane.

Everything that I know I learned from somebody else. If there is an abbreviated set of steps that achieves superior accuracy, I would like to learn about it. I continue to learn new techniques by talking to other builders.


Regards,

David