What is the lightest way to cover my airplane? This is the question asked by builders of ultralight or very light aircraft. Letís take a look at how to cover one of these aircraft the lightest possible way while providing maximum protection for the fabric and providing a safe operation for the aircraft.
COVERING AN ULTRALIGHT
One of the main considerations in covering an ultralight or very light aircraft is weight. The builder does not want to increase the empty weight of the aircraft appreciably with the fabric and coatings. Obviously, this can present a problem. Along with the weight restriction, the builder also needs to protect the fabric from the UV rays of the sun and still have the airplane look nice. With all of this in mind, a compromise method of fabric covering is often required for lightweight aircraft.
The choice of fabric is important for an ultralight. You will want the lightest weight fabric possible that still meets all of the standards needed for larger airplanes. Do not short change yourself by using an unknown fabric. You will not need a certified fabric for an ultralight but you will want to ensure that the fabric you are using has been tested and is regularly used on larger airplanes. Ceconite has a lightweight fabric that weighs only 1.7 ounces per square yard. This fabric is designated as uncertified but is acceptable (and widely used) for aircraft. It meets the specifications of the technical service order required for fabric that is used on aircraft. The uncertified lightweight fabric is 66 inches in width. Poly-Fiber has a similar type of fabric that weighs the same.
How much fabric is typically required and how much weight will be placed on the structure with the covering system? A good base of comparison is a J-3 Cub. This airplane has an area of about 730 square feet. When the airplane was originally manufactured, Grade A fabric and dopes were used to cover the airplane. When completed, the average weight of dope and fabric added to the aircraft was about 75 pounds. This is a lot of weight to add to the aircraft structure.
Fortunately, there are now alternative methods of covering that will reduce this total weight substantially. The average ultralight will require approximately 500 square feet of fabric to cover the surfaces. At our weight of 1.7 ounces per square yard that means the total weight of the fabric will be about 6 pounds.
Now, the trick is to not load it down with coatings. Many ultralight builders do not understand that polyester fabric and coatings, if properly applied, will provide a long lasting finish that will usually have a total weight of less than 23-25 pounds. For this weight you have a fabric system that will withstand being outside in the weather for long periods of time and will also provide a safe operation. Your aircraft will have the same service life as a certified airplane covered with Ceconite fabric and dope products or with the Poly-Fiber covering system.
Many ultralighters leave off the silver coatings (aluminum pigment) to save weight. Chemical UV blockers are often added to the topcoat paint in an effort to provide UV protection. No substitute will ever be as effective as coating fabric with aluminum pigment dopes. The bottom line with an ultralight or very light airplane is this. You should place the same type of fabric covering on one of these airplanes as you would on a larger experimental or production type airplane. This will assure that the fabric covering will last for years. If you want the fabric on your airplane to last a long time and provide safe operation, then use one of the following methods of covering.
CECONITE FABRIC AND DOPES
Use Ceconite uncertified light fabric. Use lightweight finishing tapes. Attach the fabric to the frame using Super Seam fabric cement. Shrink the fabric according to the directions provided in the Ceconite manual. Attach the fabric to the wings and control surfaces using some sort of mechanical means such as rib lacing or screws. Do not rely on fabric cement alone to hold the fabric in place on lifting surfaces such as wings.
Obtain nitrate thinner and non- tautening nitrate dope or Rand-O-Proof. Mix the dope (or Rand-O-Proof) and thinner in equal amounts and spray two full cross-coats onto the surfaces. Let them dry for 2-4 hours.
Using a Randolph product called Rand-O-Fill spray on one full cross coat. Let this dry for a few hours. If any dirt or dust is evident at this stage you may lightly scuff sand the surfaces using #400 sandpaper.
Select a butyrate color topcoat. Mix the topcoat with equal parts of butyrate thinner. Spray on one full cross-coat of color.
This method will provide you with a long lasting finish with a minimum amount of weight should you choose to cover your ultralight using nitrate and butyrate dope.
POLY-FIBER COVERING METHOD
Poly-Fiber offers two options for covering an ultralight. Both of these methods use the lightweight fabric. The first option uses a topcoat (final color) that has a UV blocker added to protect against the UV rays of the sun. This is a reduced coat system with no aluminum pigment and will not provide the protection that a full coat system will provide. It will, however, save about 12 pounds of weight. The total weight of fabric and coatings will be about 10-11 pounds. Understand that you are saving weight but losing full protection from the sun.
The best method is to use the regular Poly-Fiber system that is placed on all aircraft. This option will ensure complete protection against the UV rays of the sun. The most important factor in determining the life span of a fabric is UV protection. The full Poly-Fiber system with the silver Poly-Spray applied will give the maximum protection for any type of aircraft.
Use Poly-Fiber light fabric. Use lightweight finishing tapes. Attach the fabric to the frame using Poly-Tak fabric cement. Shrink the fabric according to the directions provided in the Poly-Fiber manual. This manual has an excellent section devoted to covering ultralight/very light airplanes. Attach the fabric to the wings and control surfaces using some sort of mechanical means such as rib lacing or screws. Do not rely on fabric cement alone to hold the fabric in place on lifting surfaces such as wings.
Brush or spray on one coat of Poly-Brush that has been thinned 1 part of thinner to 3 parts of Poly-Brush. This coat will provide the base for all subsequent coats.
Apply one to two coats of Poly-Spray thinned one part thinner to 4 parts of Poly-Spray. Apply these coats to the upper surfaces of the aircraft only. This will provide UV protection for the top surfaces that are exposed to the direct sun. Let the Poly-Spray dry overnight and then lightly sand the surfaces using 320 grit sandpaper.
Spray on two coats of Poly-Tone color thinned 1 part thinner to 3 parts of Poly-Tone. If you decide to not use the Poly-Spray discussed in step three, then add Poly-Fiberís UV blocker to the Poly-Tone prior to spraying. This will at least provide minimum protection from the sun. Eliminating the Poly-Spray will save approximately 10 pounds of weight but it will be at the expense of not having full protection from the sun.
Do not cover an ultralight using either of these systems and then place an automotive topcoat over the fabric. Automotive paints are high build and usually heavy. They can actually double the weight of a covering system. It is not unusual for 15 pounds or more to be added to the aircraft using an automotive paint.
So, the bottom line is thisóuse a system that is designed for ultralight aircraft and finish the job using the topcoat paint that is part of the system. Use lightweight fabric and a coating system that will provide the protection needed for the fabric to last for years. Covering an ultralight using sail cloth and other common materials will usually not be durable for long periods of time. An ultralight flies in the same airspace as regular airplanes so donít skimp on materials when it comes time to cover it. Do it right.