QuickTalk 6 - FINAL FINISHING SUGGESTIONS
- Category: Q-Talk Articles
- Published: Sunday, 31 October 1982 06:11
- Written by Jim Masal
- Hits: 1413
After building a composite airplane and seeing the finished product in its raw form, the builder is usually torn between two overwhelming desires. The first grips the internal pilot who yearns to throw the paint on and fly off into the wild blue yonder. The second is the craftsman who envies the perfect surface contours that only composites can provide.
The builder usually ends up somewhere between these extremes with a finish that looks fine from a distance, but wouldn't stand up to Oshkosh judging standards. The problem doesn't rest with the builder's desire to have a perfect finish, but simply his patience and techniques. Lately, it has become rather obvious that the final finish is the standard by which the composite airplane is judged by the public. A less-than-adequate surface usually nets a turned-up nose, although each fitting and layup may have been made with exacting detail.
Many flight line arguments have resulted from the so-called "best" finishing techniques. Although oral discussions on the subject are common, written documentation is at a minimum. One exception to this is Bob Walter's extensive article on "Finishing Composite Aircraft" which appeared in the February 1982 issue of SPORT AVIATION. Mr. Walters is best known as the builder of the Dragonfly prototype, but his hints are relevant to all composite craft. This feature is on QBA's list of "must" reading material and should be reviewed by all builders.
It would probably be safe to say that QAC's plans do not cover the subject adequately, especially for the first time builder. Although what needs to be done is stated, there is not enough detail to explain how it is to be accomplished. This shortcoming is probably being noted at QAC. A circular on final surface finishing was available from QAC at Oshkosh this year. Chris Young (#469) sent us his copy and we have printed it in full for your benefit. Although written for a Q2 audience, the ideas pertain to the Quickie as well:
The Q2's high efficiency is due, in part, to its advanced computer-designed airfoils. Because the canard is asked to work hard, its airfoil shape must be fairly true to the intended planform. The hotwiring technique coupled to the finishing techniques outlined in Chapter #3, pages 15-17 will give you a fairly accurate airfoil and meet the performance needs of the aircraft.
We have found, however, that by employing sailplane finishing techniques to the airfoil finishing, particularly the canard, we have been able to lower the stall speed 3-4 mph and raise the top speed by 2-3 mph. It also has the effect of reducing the stick forces in rain and bug build-up.
The end result will give you an airfoil with a surface of .001" to .002" for a 2" square. Also, because your finish sanding will be from the leading edge to the trailing edge, you will have many tiny sanding scratches or troughs in the direction of the air flow, to help attach the air flow over the canard.
Because the final steps involve the sanding of the hard final finish paint, we recommend that the airfoil be painted with a polyurethane or catalyzed acrylic enamel paint.
Sanding Spline: 9" x 9" board made from plans on page 3-15.
Hand Sanding Spline: Made from extra 3/8" bulkhead foam. Cut a piece 11" x 4" and sand a small radius on the edges of the foam on one side.
Window Squeegee: The type used to clean windows. It is used to wipe away wet sanding debris from the airfoil.
Sanding Dye: A mixture of 90% Acetone and 10% lacquer. You can use any visible color such as red or black. This is wiped on the airfoils to show high or low spots and sanding scratches. Care must be taken to check the effect of the dye on the surface paint. Since it is applied on the final surface finish, you may prefer to use a polyurethane paint such as Imron, Ditzler Deltron or some of the catalyzed enamels such as DuPont Centauri or Prestech systems. The dye may soften lacquer or lift some enamels. You can use an old plastic shampoo bottle to keep the dye in.
Band-Aids: To wrap around your fingers to protect them when you wet sand.
Glazing Compound: Such as Meguiar's Auto Seal #H-7 or TR-3 Resin Glaze as a final treatment after sanding the airfoil.
Sandpaper: Use open coat 80 to 180 grit on the Featherfill, 180 to 600 grit wet and dry for finish sanding. Amounts will vary but ten sheets of 80, 120, 180 and fifteen sheets of 180, 220, 320, 400, 600 grit wet and dry should do the job.
A straight, smooth canard or wing will begin on the jib bench. If you spend a little time on your cores before you apply your fiberglass, you will save a lot of hard work and extra weight on the aircraft when you get to the finishing stage. After you have glassed your wing, we assume you will either have been through Chapter 3, or will soon do so. The fill and coarse spline sanding is explained in the education section Pages 3-15, 3-16, and 3-17. Stop sanding when you feel you have attained the recommended .005" per sq. 2" over the canard and wing. The critical area of the canard is the first two-thirds of the cord, leading edge back along the entire span. A benefit can be had by insuring that the leading edges stay in contour and the first one-third of core on the bottom is spline sanded. Leading edge contour can be checked by making female templates from hot wire templates in the area of the leading edges of the canard and wings. As you sand, do not sand into any glass. If you sand to glass, stop and apply another coat of Featherfill. When you have met the .005" per sq. 2", move on to the next phase - the performance finishing of your flight surfaces.
(* Featherfill is a two-part brushable or sprayable polyester filler. It is recommended because of its price and easy application and sanding. Other fillers of this type are available, such as Streling, Prestech, Bondo Dynafill, etc. Their use is your decision, however, we will use Featherfill as the example of a polyester primer surfacer in this article.)
You may wish to check surface wave by the rule and feeler gauge method at this time. Mentally divide the wing into 6" grids and check at every intersection. Also, by holding your hand flat on the surface, palm down and lightly moving it leading edge to aft, you can detect bumps and dips fairly easy, after a little practice. If you plan to go ahead with the sanding operation, you will most likely need to spray another coat of Featherfill on the canard. Assuming that an adequate job of foam core preparation and glass layup was done, this should be your fourth coat of Featherfill. Make this coat wet, but keep it a light coat. You may wish to add a little lacquer thinner to let it flow out evenly. You will now use 120 grit paper and sand in a different direction, leading edge to trailing edge - in a straight line. Hold the board in front of you on the leading edge applying about 5 lbs. down pressure and push the board straight back to the back edge of the wing and then pull it forward to the starting position, repeat this twice in the same spot. DO NOT move the spline board sideways. You will notice the sandpaper scratch marks in the Featherfill that go straight back to the aft edge of the canard. From here on out, try to imagine these marks as record grooves and the spline board as the needle, never allow your sanding strokes to go in any direction but straight back and forth.
Now move the spline board over about 3" and repeat the two back and forth strokes. Do not push the spline over to the next position with the sandpaper against the canard surface. Lift the spline off the canard at the end of the stroke and place on the new position. Remember the record and the needle. Use this technique for the remainder of the sanding operation. As we go on, sanding scratches will get smaller, but still remain slightly at the end of all our work. They will be one of the factors for the high efficiency that your canard will have as the result of all this. Repeat this process up and down the length of the canard until you are satisfied with the finish. The final sanding for the .001" to .002" will be done on the actual finish paint, preferably some type of epoxy or polyurethane paint, so you don't need perfection at this point. We are establishing a sanding pattern and knocking down the last of the major bumps in the finish.
You will notice also that you can clearly see the low spots in the Featherfill, even as little as .001" by the shading difference of the sanded Featherfill to the unsanded Featherfill. Try to sand until all of these disappear. At this point you are well beyond the minimal surface criteria as outlined in the education section. Now you may lightly spray your UV primer on the airfoil. At this time you need to make another sanding tool like the small blocks the sailplane people use to sand with. Take a piece of 3/8" thick light foam left over from the bulkhead material and cut it into a rectangle 11" x 4". Sand a small radius on the edges of one side. You can wrap the foam block with wet and dry sandpaper. If, for some reason you need to sand the primer (dust, bugs, orange peel, etc.), use the sanding block and sand with 320 to 400 grit paper and use water. Use a window squeegee to wipe the water off the airfoil as you sand. Remember to sand in a straight line, leading edge to trailing edge and squeegee the water off in the same manner.
If you are happy with the work you've done so far, proceed to the final painting of the canard and airframe. One thing to consider at this point is the fact that you will be removing some paint from the canard surface, so it is suggested to use two coats of paint over the canard, (also the wing if you wish to continue the refinement). One coat equals one full cross coat, lengthwise and spanwise. Let the paint dry and cure completely. Most polyurethanes are cured enough in 5 days to sand. Now you will use 400 grit sandpaper and finish with 600 sandpaper using the 11" x 4" white foam sanding block. Also you will make use of the acetone and lacquer paint to make an indicator dye. This dye is wiped on the canard surface with a small, soft cloth, staining the entire top surface. This mixture has the advantage of drying almost instantly and will show small dips and imperfections in the paint. This dye also will show the sandpaper marks from the sanding operations.
After the dye is placed on the surface, start sanding lightly with 400 grit paper, straight back to the trailing edge moving out to the tip and back until the dye is sanded away. Be careful because in your zeal to sand out the dips and imperfections, you may sand too far into your finish paint, which of course, will require another application of paint.
Wipe the sanding residue off with the squeegee and stain the surface again with the dye. Move up to the 600 grit paper and sand in the recommended manner until the 400 grit sanding marks are gone. Again, hopefully, you have not sanded too much of your finish paint away. One way of gauging your progress is to place your hand palm down on the wet surface and lightly move it from the leading edge to the trailing edge. You should be able to feel the larger dips and bumps with your entire hand, and the smaller irregularities with the tips of your fingers. If you are lucky enough to be working in an area where there are fluorescent lights hung, you can look at their reflections on the surface after you wipe away the excess water with the squeegee. The reflections of the lights will appear straight, and be without bends or joggles. You can even hold a fluorescent light up over the surface to help you see your progress. If you should happen to sand through the finish paint, you must apply more paint and continue to sand. The leading edge should be sanded very lightly and care must be taken not to change the leading edge, so be careful!
By now you have arrived at the desired finish, but you will need to apply a surface sealer to seal the surface and shine the surface (use Auto Seal Glaze or TR-3 resin glaze). Using the 3/8" white foam blcok, wrap a terry cloth around it and using the same aft straight strokes, apply the glaze. You will see the sanding marks but after this step the canard will look like the sharp edge of a razor blade, a satin sheen with a look of functional purpose.
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