Covering the Tail Section
By: Bud Connolly
Many kit buyers start with just the tail section since that is the least expensive way to begin a kit. I offer the following tips based on my own experience. For many of us, this will be our first experience with fabric covering, and we may be a bit apprehensive.
Starting on the tail section is a great way to learn how to cover with fabric. The parts are all relatively small and easy to manage on a common table top. If you make a mistake and ruin one piece of covering, it's easy and inexpensive to strip it off and do it over. Most builders won't make a major mistake though, and after finishing the tail section pieces you will feel very confident about tackling the larger wings and fuselage without any worry. It's a real confidence builder to do the tail sections, and you'll be proud of your work."
Topics:1. Finishing tape requirements along edges and over seams.
1. Finishing tape requirements along edges and over seams.
When it came time to put the fabric covering on the tail of my plane, I had several questions so I called Dave Goulet at the Challenger factory. Specifically I wanted to know if finishing tape was required along the edges of the tubes, especially over the sewn seams.
Dave said that this is not required. It's a personal choice. The finishing tapes will provide a bit more abrasion protection on the tube edges, but they are not needed for strength or safety.
Although the FAA does require such tapes on STC aircraft, there is no requirement for that on experimental aircraft. Actually, the FAA also requires at least medium weight fabric on STC aircraft, whereas we can use the lightweight fabric on our experimentals.
Dave added that if I don't use the finishing tape, then I should be sure to keep the sewn seam exactly along the centerline of the tube if at all possible. Also, make sure I have put a good coat of Poly Tak cement on the tube before putting the socks on, and then after shrinking, to make sure I have a good Poly Tak application along the outside tubes where the fabric is. That's what I did, and I prefer the smooth look of the fabric without the finishing tapes.
2. Installing the socks and shrinking the fabric.
Putting the fabric socks on the tail and shrinking them tight is the first "magic" experience the new builder is likely to have. It's at this point that tubes and fabric come together to become, and look like, a part of an airplane.
Before putting the socks on, lightly sand the perimeter of the tubing where the fabric will be glued, then wipe the tubes down with MEK to clean them as much as possible. Cover all the gusset edges and rivet heads with fabric tape. Brush a good coat of thinned Poly Tak on those tubes, then when dry (15 minutes), brush on a second coat and allow that coat to fully dry as well.
The socks can now be pulled over the tail section. Poly Tak the fabric down to the tubes at the open end after trimming off excess fabric. But, do not put any Poly Tak on the rest of the perimeter tubes at this time. You will want to be able to shift the fabric around the perimeter tubes a little as you shrink it in order to keep the sewn seams aligned along the tube centerlines as closely as possible.
Shrinking the fabric is done with the iron in three steps. The first step is to shrink it at a temperature of 250 degrees (Fahrenheit). That will begin the shrinking process but will not bring the fabric up tight against the tubes.
Next, increase the temperature of the iron 300 degrees and repeat the process. This will give a definite tightness to the fabric. During this shrinking process, try to apply heat to each side of the tail surface equally in order to keep the sewn seams lined up on the tube centerline. Since the perimeter tubes are not actually glued down yet, you will be able to move the fabric a little on the tubes and adjust the appearance of the perimeter.
Final shrinking is done at 350 degrees for a full tightness. Just repeat the procedure done at the lower temperatures, applying heat to each side of the tail surfaces.
It's important not to apply heat in excess of about 400 degrees because the dacron fabric will go from tight to suddenly not so tight as the temperature hits around 450 degrees. Once the fabric gets that hot, it will relax a bit and cannot be retightened again. That's a big no-no and will spoil your day. The only fix for fabric that has been heated beyond its maximum point is to throw it away and start over with new.
After you are satisfied with the alignment of the sewn seams and all wrinkles are ironed out at 350 degrees, you can then glue down the perimeters. Brush thinned Poly Tak along the entire perimeter where the fabric touches the metal tubes. Try not to get too sloppy with the Poly Tak, but if some gets on the fabric where there is no metal under it, that's not a real problem. Later you will be painting the fabric and the Poly Tak spots can covered up with the base layers of the paint.
3. A modification that gives tube junctions a smoother appearance.
I used one of those cutoff type composition blades on my table saw to round the ends of the tubes. They are available for table saws to cut tile or other very hard material. The blades are cheap, costing less than $10.
I pressed the exposed tube end of the tail section against the side of the blade and slowly ground the tube down to match the curve of the tube that was joined to it. I took the grind right down to where the tubes seemed to transition almost perfectly at the point where they meet at 90 degrees.
4. Three methods used to calibrate the temperature of an iron.
It's important to shrink the dacron fabric at specific temperatures. Most important of all is to not exceed the maximum temperature limit of the fabric itself or you will cause the fabric to permanently relax and lose tension. This will happen around 450 degrees or so.
I have seen three different ways to calibrate an iron in order to establish the settings for 250, 300, and 350 degrees. Here is a brief description of each one.
Use a glass mercury thermometer.
The first way is to use a mercury thermometer that will accurately read up to at least 350 degrees. These are available from the fabric supply companies that sell the dacron fabric we use, like Poly Fiber. They recommend using a silicone heat sink compound that comes in a tube and which can be obtained from an electronics parts store. You embed the thermometer bulb in a dab of the compound in order to get a good thermal contact with the sole plate of the iron. This is an inexpensive way to check iron temperature settings.
Use a metal coil thermometer.
The next way is to go to a good model airplane hobby shop and buy one of the little metal coil thermometers that are used for calibrating the irons used to shrink the covering material used on model wings. These can be set on the sole plate of the iron and allowed to come up to temperature. They indicate the temperature with a little pointer and scale. This is also an inexpensive method, costing about $15, more or less.
Use an infrared digital thermometer.
Another way to calibrate an iron is to use an infrared, digital readout, handheld thermometer such as a Raytek sensor. These cost about $90 and are available from Poly Fiber. While they are rather expensive, they have some definite advantages.
No contact has to be made with the iron. A red spot formed by a laser is pointed at the iron, and the device immediately tells you the temperature of the iron at that exact spot.
You will find that irons are not the same temperature over the entire sole plate. There are both hot spots and cooler spots, with the hottest spot usually right around the upper center of the iron in the vicinity of the steam hole vents.
I opted for the Raytek sensor after talking with Poly Fiber. Irons take a number of minutes to come up to temperature and stabilize. During the initial heat buildup they will overshoot the desired temperature by 75 degrees or more, then, over a period of minutes, will stabilize down to about plus or minus 25 degrees.
Also, if the iron is accidentally dropped, the calibration should be rechecked. The calibration can also be affected by using a different length or type of extension cord. The Raytek can immediately and easily spot any such calibration changes, and the calibration can be checked prior to each use.
You may not want to spend that extra money on a Raytek sensor, but I assure you that if you had all three calibration methods sitting in front of you (for free), each time you start to iron some fabric, you would happily select the Raytek over any other method. Your choice.
Most builders do not use a Raytek but rather select one of the first two choices to calibrate their irons. That's fine as long as you make sure you are not getting any overtemperature condition during shrinking of the lightweight fabric we use to cover the airplane.
These calibration devices can be seen at the Poly Fiber web page, and either Jim or Dondi Miller can answer any questions you might have about fabric shrinking, painting, or whatever else you want to know about fabric.
More about the Raytek:
The Raytek digital thermometer is a neat tool. Looks like a little raygun type thing (Startrek) that you aim with a laser spot at the place you want to read the temperature. It's supposed to be accurate to within 5 degrees for the higher temperatures used to iron the fabric. It would probably be fine too for lower temperatures once you've tested it to find what the error might be, if any.
A Raytek is so handy to use that you can do periodic checks of your iron while you're working on the fabric. I have found that the inexpensive internal thermostats used on $25 irons will drift away from the original setting due to internal heat buildup. This effect becomes more pronounced the longer the iron is on.
The real reason I got one is that Poly Fiber convinced me that dacron is great stuff but really does need to be shrunk at specific temperatures in order to get the proper tension. And especially, I wanted to avoid any accidental overtemperature.
As another application, a Raytek would be perfect for spot checking something like a bearing temperature or even a CHT temperature versus what the panel instrument says. They will work at distances up to a couple of feet.
When you press the trigger, a laser spot appears. Put the spot on what you want and an LCD window shows the temperature of that spot. Mine is set for Fahrenheit, but I think there's a switch if you want it to display in Celsius.
Click here: Raytek - MiniTemp Handheld Infrared Thermometer - MT2, MT4
"A search of the Internet using the word Raytek will show various online stores that sell the Raytek MT4 for about $75 or less."
By: Bud Connolly