Hi Pat
The 1/8" gap you are talking about is actually pretty easy to fill to perfection and it doesn't require lots of sanding. In fact, it will hardly require sanding any if you use this technique.
1. Mark the inboard section of the wing at 10% ordinate points, parallel to the centerline. Do the same at the tips. The lines inbetween should be perfectly straight lines (even though there is a change in section shape but it is linear).
2. Take a very rigid batten like a 1" x 1" square aluminum tube or better yet get several (I use this a lot and have a half dozen just for this purpose). Now make sure they are clean and and straight. Wash the milling oil and residue off, dry then several coats apply mold release wax to it.
3. Next mix up about 75% balloons and 25% fumed silica with epoxy to make a somewhat loose batch of putty and I also like to add a little pigment as in about .5% so that I have a reference as to my datum marks when I sand with subsequent filler.
4. Now take your batten and mark the lines with a pencil along the 10% points and spread the putty on the lines with plenty of excess in a ridge.
5. Lay the batten on top of the putty and tape it in place doing a little wiggling to help it settle to a straight line. Do this for your other battens along other 10% points too, then scrape off the excess squeeze with a putty knife when done..
6. When this hardens, remove the battens and lightly sand off the flashing. Repeat the process until you have a grid done that is easy to fill between the gaps, say every 4-6".
7. Now take a 12" drywall knife and glaze between the gaps of your now perfectly fair datum with more putty WITHOUT the pigment. DO NOT try and perfectly fill the gaps, you will drag too much and you will increase our sanding. Instead, just fill what can easily be filled UP TO THE TOPS OF YOUR DATUM, NOT ABOVE IT.
8. When hard, lightly top this with a sanding board but up to this point, you have hardly sanded at all. Then fill one more time or maybe twice of glazing as needed. Sand each layer after it hardens but since all of these are really only to our dataum, very little sanding is needed. The last longboard should reveal perfection.
2 more tips. Change paper often. It stays the same roughness throughout the life of the paper but old paper is like a duff knife, it doe not cut the putty very well making it more work and less fair. I say less fair because sharp paper takes down unfairness quickly without hitting the lows as much since it goes faster with less strokes. Dull paper on the otherhand doesn't cut as quickly and it tends to sand in the lows more which means more work.
And also if you sand the putty pretty much as soon as it is hard but before it is cured, it sands a LOT eaasier than fully cured putty.
Given this technique, you will always have perfectly fair surfaces assuming your battens are good. I developed this awhile back while building boats and have used it for keels, rudders, other foils and to splice on bows to large boats that were getting nose jobs.
This also points to the technique for sanding you asked about. The board should follow the same rules as the battens, i.e. they should be oriented to a projected point off of the wingtip. Anything else will tend to hollow out a low in the middle of the board since it is actually riding on the top of the curved section to some degree.
But I would argue about the 45 degrees. You can and should mix it up a bit with the X pattern being plus minus 15-20 degrees all the way to plus minus 70-75 degrees for a bit of the time with maybe 50% - 75% being the 45 degree pattern. A lot of how I do the pattern depends on if I am trying to concentrate the sanding on something. And the 3M long boards are good BUT there are two kinds. One is flexible and the other straight and rigid. DO NOT use the flexible one for this application! Make a new one if you have to out of clear and dry redwood or fir with handles as stiffeners but the flexible one will ruin your work.
This may sound like a lot of work but it actually isn't. It should be done in a 3-4 nights of part time work leaving lots of time for other stuff while the putty hardens. An added plus is that you will find that there will be no pinholes over the putty which may make this worthwile for that one point.
Good building!
Pat Cohenour <patricktc@cox.net> wrote:
I am in the process of prepping the wings and have some questions. I am doing most of the work with Aeropoxy Lite and the 4? x 30? 3M fairing boards with 80 grit paper. I should add that I?m not looking for perfection, just straight & smooth to the naked eye, without significant negative aerodynamic consequences. Having no prior experience, I guess I?m trying to figure out what is ?good enough? without going all the way to paint. I have worked only on the top of the right wing and should mention that I have by this time essentially covered the entire surface with filler and sanded it down, checking with a guide coat (flat white quick-dry spray
paint); the guide coat showed sanding marks essentially everywhere.
One concern is that top wing skin drops down from ~ 19? out (where the gap is < 0.005?) to the inboard edge about 1/8? (see attached photo. Seems to me that would require an excessive amount of buildup. If this is worked as a smooth transition, will it be noticeable? I assume there would be no significant effect on the flying qualities of the wing. I have the same situation at the wingtip. I have 2 ? 2x2x65? Al tubes that I can use as longboard sanders and am using one as the straight-edge as shown in the photo (BTW, the backlight in the photo is a 2x4? fluorescent shop fixture). I?m sure that
gaps of 0.005? or less are acceptable, but at what point does a depression become noticeable (what depth spread over what length, assuming smooth transition over at least a few inches)? And is a 65? longboard too much? The factory says they use the 3M boards.
Next, I know that the board should be used in an ?X? pattern (45 deg to long axis) but should the board be parallel to the long axis of the wing at all times, or should it be parallel to the LE or TE near the LE/TE, transitioning in a fan pattern to parallel to the spar over the spar (see attached graphic)? Does this make sense to anyone?
And what about either covering the entire surface with filler first and then sanding, or trying to find & fill the low spots, then sanding & repeating?
Pat Cohenour
IV-P in progress
patricktc@cox.net
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