Here are pictures of a couple of the many dozens of test panels I've made of fiberglass/foam sandwich construction. The S-glass/epoxy composite face sheets are about 0.005" thick, with a 0.25" thick sheet of HighLoad 60 Styrofoam in between. I've been trying to get the best combination of surface preparation for the foam and exterior painted surface finish for the Y-tail and wing tip airfoils, and some of the results have been promising. The picture of the panel on the right shows a mirror-like glossy finish obtained only with run-of-mill aerosol-can white spray lacquer from the hardware store. This was done by applying release agent to a Mylar caul sheet, followed by spray painting it and letting it dry for a few days... then I hand-laminated the sandwich panel with a wet-layup technique, applying the painted side of the caul sheet against the epoxy. After vacuum-bag curing, I peeled off the release-coated caul sheet, leaving a very glossy exterior finish with the paint layer adhering very well to the S-glass/epoxy composite skin. A similar panel that was painted after vacuum-bag curing has much rougher and uglier finish, as seen in the comparison photo on the left (pardon my feeble photography skills). About the only way I could figure out how to capture the finish of these panels was to snap a picture of the reflection from the 4-light bulb fixture in my office.
Composite molders do a similar thing by "gel-coating" the mold before laying up, but by using a glossy caul sheet on the mold side, I can have a not-so-smooth mold surface, and still get a glossy exterior finish... and not just for looks either... you get improved laminar air flow. All in all, it seems clear to me that painting the outside from the inside is the way to go!!! This allows an unskilled painter like me to hide roughness and surface imperfections underneath the paint layer, not on top.
These featherweight panels are amazingly strong and stiff, as the flexural tests are proving now !!... quantitative results will be published here as soon as we have declared victory on achieving our "exit criterion" in the panel design's delivered strength & stiffness. As engineers should know, but sometimes forget, a project's progress can come to a grinding halt in the pursuit of the "optimum".. for example: a directive from your boss and/or customer saying something must be "as light as possible" or some such vague nonsense.. so how do you know when you're done making it lighter?... Remember: nothing is perfect, therefore you can ALWAYS make ANYTHING better... be it stronger, lighter, faster, shinier, more efficient, etc. To exit the endless merry-go-round of incrementally improving your design, you need a finish line.. an exit strategy... a success criterion clearly defined as meeting a pre-established requirement... then you MOVE ON! As one pithy Russian general said it best: 'Better' is the enemy of 'good enough'
As a related side note from my personal experience circa 1999: At Beal Aerospace -- which is now nothing more than a case study on how NOT to run a private space launch company -- we used to joke that the technical requirements were far less important than our wealthy owner's ever-shifting "desirements." He wants the rocket to be bigger one day, simpler the next.. then he would get on a make-it-cheap-like-a-hillbilly-would kick, followed by orders to make it prettier, quicker, tested more, tested less, able to handle even bigger payloads, etc. We were getting good at redesigning & rescheduling, but not building & flying.. Well, the joke was on us.. after burning through untold millions of his money, he laid us all off, shut the place down, and our giant rocketship never flew... lesson learned.
No comments:
Post a Comment