The word dope conjures up different visions in oneís mind. In the world of airplanes this word has been around for years. It was used by early aviators to describe the chemicals that were placed on fabric airplanes. The word dope actually means a liquid used in some craft. It is a term for a working liquid. Dope is still commonly used today to describe a covering process used by many airplane restorers and builders. What about this type of covering process? With several different types of covering processes available does dope still have a place? Lets take a look at this method of fabric covering in detail. We will begin in this issue with a description of dopes, their advantages, types of fabric that are used with this method, etc.
The first chemical developed and manufactured specifically for aircraft fabric was nitrate dope. It is made from nitrocellulose, a product that is highly flammable. Nitrate dope has been successfully used on fabric since early World War I days. It was initially the only chemical applied to the fabrics used such as Grade A cotton. Nitrate dope also continues to shrink fabric as the years go by. The early pioneers learned just how tight to shrink cotton prior to applying the nitrate dope. They allowed for the shrinkage that would occur as the dope aged.
Because of the flammability issue with nitrate dope, another product came on the market during World War II. This product was termed butyrate dope and it essentially replaced the more flammable nitrate on most airplanes. Butyrate dope is a cellulose-based product. Even though it is less flammable than nitrate dope, it will burn. It also retains the characteristic of shrinking with age. This shrinkage must be taken into consideration to avoid damage to underlying structures throughout the years. With this problem in mind, non-tautening dopes were eventually introduced. These dopes have a plasticizer as an additive thus reducing the tendency to shrink. It should be noted that even non-tautening dopes will shrink with age though not to the extent of regular dopes. This shrinkage is due to the eventual evaporation of the plasticizer.
With the advent of butyrate dope the procedure for covering Grade A cotton changed. Butyrate dope was used in place of nitrate to curtail the flammability issue. Several coats of butyrate dope were applied without any plasticizer. After the initial coats were in place, the builder or restorer then added non-tautening butyrate dopes as filler coats. This lessened the tendency to shrink with age.
What about nitrate and butyrate dope today? Can you use it on polyester fabric? The answer is yes; you can use it on polyester fabric. As a matter of fact, a large number of individuals still use this process to cover their airplanes. Ceconite fabric is the most common type of polyester fabric using nitrate and butyrate dopes. This fabric was developed in the late 1950ís and is still in use today. The advantage of Ceconite fabric versus Grade A cotton is two-fold. First of all, certified Grade A cotton is no longer available for purchase in this country. You can manage to obtain cotton but the fabric available does not meet the technical service order allowing it to be used on production aircraft. Secondly, Ceconite fabric can be shrunk with a household iron. Cotton fabric must be shrunk using the right amount of water sprayed onto the fabric. This is not an easy process for the beginner to accomplish. Ceconite fabric is simply shrunk to a predetermined temperature by calibrating an iron. That temperature is normally 240 degrees F. Non-tautening dopes may then be applied to reduce the tendency to shrink.
Nitrate dope must be used on polyester fabric as the initial coat. Butyrate dope will not adhere to polyester fabric, only nitrate. Therefore, the first few coats of chemicals must be nitrate dope. Butyrate dope is then used to build up over the nitrate dopes that are in place. The flame problem still exists but it is less than if only nitrate dope is used.
Advantages of Nitrate and Butyrate
A nitrate and butyrate finish provides one of the deepest, most rich looking finishes available for fabric airplanes. It has a deep, wet look that is almost impossible to obtain using any other type of coating. Dopes do not require the user to wear a fresh air breathing system unlike polyurethane coatings. A simple charcoal-filtered respirator is sufficient to protect your lungs. A dope finish is easy to repair. The color coats are very easy to blend after a repair.
A dope finish will endure for many years. I am personally aware of Ceconite fabric coated with nitrate and butyrate dope that has been on an airplane for 25+ years. It still looks good and meets the test outlined for fabric. An additional advantage is the ability to rejuvenate the finish of butyrate dope. After years of service if the topcoats are beginning to crack and show wear, you can spray on a rejuvenator that will allow you to restore the finish. Applying a rejuvenator will restore the flexibility of the dope thus adding many more years to the life of the finish.
Disadvantages of Nitrate and Butyrate
Covering an airplane with dope is more time consuming than using other systems because it requires more coats of chemicals and more sanding. Shrinking of the fabric is more critical when using dopes. The initial tautening of the fabric must allow for the additional shrinking that will be caused by the dopes as they age. If the fabric is shrunk at a temperature over 240 degrees F the added shrinkage from the dopes may result in a deformed airframe. Other systems use chemicals that do not shrink with age allowing you to apply 350 degrees F to the fabric for total shrinkage.
The problem of flammability remains. Nitrate and butyrate dopes are still flammable. Even though fewer coats of nitrate dope are used, they are still used.
Using Ceconite Fabric and Dopes
Ceconite fabric is a polyester fabric used today with several coating systems. One of these systems is nitrate and butyrate dope. Ceconite does have a covering manual that outlines the steps involved in covering with these chemicals. FAA Advisory Circular 43-13, Aircraft Inspection and Repair, discusses the use of nitrate and butyrate dope on Grade A cotton.
What about covering your own airplane? Can you legally do this if you are not a mechanic? The short answer is yes, you may. If you have an amateur-built airplane you can cover it using any material you desire. In other words, there are no restrictions as to your qualifications or the quality of the chemicals and fabric. Of course, from a safety point of view you should certainly use only covering systems that are approved for production aircraft. It is extremely important that no matter what type of airplane you are covering that you read and follow the directions from the manufacturer.
If you are recovering a production aircraft, the rules are somewhat different. FAR Part 43 discusses the issues of maintenance on production aircraft. FAR 43.3 (d) allows an individual to perform maintenance on a production aircraft without holding a mechanicís certificate if that person works under the supervision of a licensed mechanic. Under this regulation a person may restore an antique or classic airplane without holding an A & P license. What does the term supervision entail? Basically, a licensed mechanic can show you what to do, leave you alone to do it, return periodically to ensure the work is being properly completed, and then sign off the work when complete. The mechanic supervising your work is responsible for signing the logbook. With that in mind, they will certainly want to supervise what you are doing to ensure it is being properly accomplished. They are ultimately responsible for the work that is being performed on the airplane. So, under the supervision of a licensed airframe mechanic you can recover your own production airplane.
Ceconite fabric is the choice when using nitrate and butyrate dopes. It is a polyester fabric that is shrunk using heat. This fabric is available in three different weights. Ceconite light is a 1.8 ounce fabric designed for ultralights, gliders, and light aircraft with a wing loading of less than 9 pounds per square foot. This usually means aircraft with engine horsepower less than 65. Ceconite 102 is the medium weight fabric. It weighs 3.5 ounces per square yard and it is used on most light aircraft. It is a good replacement for Grade A cotton and is very well suited for all fabric covered aircraft that do not operate under extreme conditions. Ceconite 101 is the heavy weight fabric that is used for agricultural aircraft, warbirds, aerobatic aircraft, in short, any airplane that receives rough treatment. This fabric weighs 3.7 ounces per square yard.
Ceconite fabrics are stamped with the following inscription:
FAA PMA APPROVED
This stamp simply identifies the fabric as being certified for production aircraft. It will be found about every foot along the edge of the fabric. Fabric that does not contain this stamp is uncertified and is not legal for use on a production airplane.
FOR USE SEE CECONITE
Ceconite fabric possesses what is known as a Supplemental Type Certificate (STC). That simply means the fabric has been approved for use on production aircraft if certain chemicals are used in conjunction with the fabric. These chemicals are outlined in the Ceconite manual (nitrate and butyrate dope are approved as one type of coating). Most production aircraft will be eligible to use Ceconite fabric under this STC. A listing of all these aircraft is available in the manual. When the A & P mechanic signs off the covering process they will use a number to identify this STC. The Ceconite STC number for all production aircraft is 4503NM.
Steps of Covering With Dopes
The following steps are used in covering a fabric airplane using Ceconite fabric along with nitrate and butyrate dopes.
- Prepare the surface
- Attach the fabric
- Tighten the fabric
- Seal the fabric surface
- Secure the fabric to the wings
- Apply finishing tapes
- Apply inspection rings and drain grommets
- Apply butyrate fill coats
- Protect the fabric from UV rays of the sun
- Apply color butyrate coats