- Category: Quickie Plans
- Published: Sunday, 01 June 2008 01:00
- Written by Jun Su
- Hits: 1624
Finishing the composite airplane is quite a bit more important than simply obtaining an attractive paint job. The finish on a composite aircraft serves to protect the structure from weathering and deterioration from ultra violet radiation (sunlight). The finishing materials also give the airplane its final aerodynamic shape. Using the proper materials and techniques, the finishing process is fast, pleasing (both esthetically and aerodynamically), and provides for long maintenance-free service. Use of substandard materials can limit the life of the finish, result in an overweight airplane, or even limit the service life of the airframe. Sanding is done constantly during the finishing process and extreme caution must be exercised to avoid damaging the structure. A poorly executed finishing job can destroy the structural integrity of the airframe. Even the finished color of the composite aircraft can effect its structure. The finishing process is as important to the structure of the composite airplane as basic materials and techniques used in fabrication are. Proper techniques must be adhered to for safety as well as to obtain an attractive airplane.
The Quickie is very sensitive to weight growth. You may easily add 20 pounds during the finishing process if you try to finish the entire aircraft to sailplane standards (smooth, wave-free surfaces). For that reason, we think that only light pilots(under 160 lb. should consider finishing their entire Quickie to those standards.
There is one part of the aircraft that must be finished to a smooth and wave-free surface - the canard. We have found that unless the canard is smooth and wave-free, serious degradation of performance and flying qualities results. This section will tell you how to obtain a smooth and wave- free finish on the canard.
The rest of the airplane, in order to keep it as light as possible, should be sanded with very little filling, then primed, and then painted. This will allow some of the fiberglass weave to remain showing, but your Quickie will still look good.
Remember, build it light and finish it light; every pound of weight that you save during the construction and finishing will make the aircraft much more fun to fly in the coming years.
FINISH COLORS AND HEAT
The materials used in amateur-built composite airframes are predominately epoxy resin systems with fiberglass reinforcement over a variety of plastic foam cores. The epoxies and the foams are all sensitive to high temperatures. Some epoxies, cured at elevated temperatures, retain their physical strength to temperatures not found outside an oven. Others, including most room temperature curing epoxies such as the RAES and RAEF systems, soften and loose their rigidity at only moderate temperatures. The common plastic foams are also heat sensitive and tend to soften and (some) swell with moderately elevated temperature. Elevated temperatures could potentially cause a softening of the fiberglass load bearing material, a swelling of the foam core, and general distortion of the airframe. To achieve elevated temperatures you would have to bake your airplane or find some other means of heating it. The sun is a potential source for this heat. In still air, on a hot sunny day it is possible to obtain surface temperatures that approach 250° F. The color of the surface determines how much solar heat it will absorb. White surfaces absorb very little (10%) of the sun’s heat while a black surface (95% absorbsion) will heat up tremendously. The accompanying graph shows the relationship between color and surface temperature. White has been choosen as the standard color for fiberglass sailplanes to preclude any possibility of excess temperature due to solar heating. The same criteria apply to the Quickie, and white is recommended. Trim colors in non-critical areas are acceptable as desired. Such as the fuselage, vertical surfaces, and the underside of wings and canard. Dark trim colors are definately not recommended on the upper surface of the wings and canard! If you would like further information on the subject read the September 1975 issues of “Soaring” magazine.
TOOLS AND MATERIALS
The tools and materials used in finishing the composite airplane are simple and straight foreward. A low density microsphere/epoxy mixture (dry micro) is used for coarse filling requirements. Automotive type polyester body fillers (Bondo) are very heavy and not recommended for aircraft finishing work. Medium to light surface filling (less than .030 in) is done with a light weight polyester spray (or brush) filler/primer called Feather-Fill. Feather-Fill is noteworthy for its ability to fill medium thicknesses in a single spray or brush coat and for its easy sanding to a smooth surface. Dupont 70S dark gray laquer primer/surfacer provides an effective ultra violet radiation barrier with its 15% carbon-black content as well as an excellent finish sanding surface in preparation for the finish paint. The actual finish paint type is largely a matter of the builder’s personal preference. Automotive finishes in laquer, enamel, acrylic laquers, acrylic enamels, and the polyurethanes are all acceptable. We find the acrylic laquer is easy to work with, easily patched and readily polished to a high gloss.
The enamels and acrylic enamels are low cost and easy to apply, however, they are not readily repairable if chipped. The polyurethane finishes offer the best gloss for the longest life, but they are high cost and virtually impossible to repair. There is a polyester paint, known as Prestek, commonly used in sailplane circles to achieve a glass-smooth finish, but
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