I'm suffering a rather unpleasant assembly experience with my RCRCM Strega. This kit is a glass lay up from late 2010. The issue I'd like to discuss; is that the installation of the servo mounting frame is distorting the top wing skin. I'd like to ask what process do you guys use to install these servo trays in molded wings?
Prior to these two Strega kits all my molded ships have had structural foam between the inner and outer layup fabric of the wing skins. RCRCM is using what I think of as a throw back material for this, balsa. I think this is the root of the issue I'm having.
Balsa has most of its strength with the grain (span-wise). There really is little strength across the grain (chord-wise). I'm also shocked that the balsa wing skins are made up of sheets that are butt joined. I don't see any scarf joints in the wing skins.
I fear that there is some force (hinge or glue) that is compressing the balsa sheets against the grain, causing the wavy distortion in the top wing skins.
I think RCRCM knew this might be an issue as they put what I think of as a stiffening carbon patch on the inside of the top skin to resist this force. (Mine has a patch that missed half the servo opening)
My process is something like:
Scuff the inside of the top wing to allow the glue more purchasing power. I also scuff the glueing surface of the servo frame, to make sure it is flat and the allow a better surface for the glue
I then slip the frame into the wing and mount the waxed servo to the mount with all the applicable screws. This way the frame is stressed as it would be by the servo when glued into the wing. I then align the servo and tray in the servo pocket with the help of the push rod to ensure it is aligned with the control horn. With the wing upside down and on a soft towel I push the servo against the top wing skin. And with a capillary applicator apply thin CA glue the accessible parts of the servo frame. After this cures I remove the servo from the servo frame and again apply thin CA glue to make sure I have a solid glue joint with the frame and the wing skin. After the CA cures I reinstall the servo. Flip the wing over and I'm greeted with a very friendly wing that is waving to me with its surface.
My questions are:
Is the CA shrinking when it cures.
Am I distorting the skin while holding the servo tray in place? And the CA is holding this distortion after it cures.
Should I be using epoxy and some holding fixture?
Another variable is that one of my wings is showing this distortion and the other shows little or no distortion. As these wings are built from a natural material, there may be a lot of variability in the density of the balsa. I suspect the friendly wing has “very weak punk wood" in the area of the servo mounts. This wing also has a warped aileron that I'll need to try to straighten out. I suspect there isn’t any post cure heat treatment for the epoxy before the wings are pulled from the plugs.
I was shocked that the wing skins of the RCRCM Strega are balsa with all the problems wood inherent in wood. Structural foam is rather uniform in its properties and between manufacturing batches. I'm sorry to say that I'm growing fonder of my old eastern European molded gliders.
All the best,
Konrad