the strength of intermediate grade cotton is required. This means the new fabric breaking strength must be at least 65 pounds and the minimum fabric breaking strength at which the aircraft becomes unairworthy is 46 pounds.
Lighter weight fabric may be found to have been certified on gliders or sailplanes and may be used on many uncertificated aircraft or aircraft in the Light Sport Aircraft (LSA) category. For aircraft with wing loading less than 8 pounds per square foot or less, or VNE of 135 mph or less, the fabric is considered unairworthy when the breaking strength has deteriorated to below 35 pounds (new minimum strength of 50 pounds). Figure 3-10 summarizes these parameters.
How Fabric Breaking Strength is Determined
Manufacturer’s instructions should always be consulted first for fabric strength inspection methodology. These instructions are approved data and may not require removal of a test strip to determine airworthiness of the fabric. In some cases, the manufacturer’s information does not include any fabric inspection methods. It may refer the IA to AC 43.13- 1, Chapter 2, Fabric Covering, which contains the approved FAA test strip method for breaking strength.
The test strip method for the breaking strength of aircraft covering fabrics uses standards published by the American Society for Testing and Materials (ASTM) for the testing of various materials. Breaking strength is determined by cutting a 11⁄4 inch by 4–6 inch strip of fabric from the aircraft covering. This sample should be taken from an area that is exposed to the elements—usually an upper surface. It is also wise to take the sample from an area that has a dark colored finish since this has absorbed more of the sun’s UV rays and degraded faster. All coatings are then removed and the edges raveled to leave a 1-inch width. One end of the strip is clamped into a secured clamp and the other end is clamped such that a suitable container may be suspended from it. Weight is added to the container until the fabric breaks. The breaking strength of the fabric is equal to the weight of the lower clamp, the container, and the weight added to it. If the breaking strength is still in question, a sample should be sent
to a qualified testing laboratory and breaking strength tests made in accordance with ASTM publication D5035.
Note that the fabric test strip must have all coatings removed from it for the test. Soaking and cleaning the test strip in methyl ethyl ketone (MEK) usually removes all the coatings.
Properly installed and maintained polyester fabric should give years of service before appreciable fabric strength degradation occurs. Aircraft owners often prefer not to have test strips cut out of the fabric, especially when the aircraft or the fabric covering is relatively new, because removal of a test strip damages the integrity of an airworthy component if the fabric passes. The test strip area then must be repaired, costing time and money. To avoid cutting a strip out of airworthy fabric, the IA makes a decision based on knowledge, experience, and available nondestructive techniques as to whether removal of a test strip is warranted to ensure that the aircraft can be returned to service.
An aircraft made airworthy under an STC is subject to the instructions for continued airworthiness in that STC. Most STCs refer to AC 43.13-1 for inspection methodology. Poly- FiberTM and CeconiteTM re-covering process STCs contain their own instructions and techniques for determining fabric strength and airworthiness. Therefore, an aircraft covered under those STCs may be inspected in accordance with this information. In most cases, the aircraft can be approved for return to service without cutting a strip from the fabric covering.
The procedures in the Poly-FiberTM and CeconiteTM STCs outlined in the following paragraphs are useful when inspecting any fabric covered aircraft as they add to the information gathered by the IA to determine the condition of the fabric. However, following these procedures alone on aircraft not re- covered under these STCs does not make the aircraft airworthy. The IA must add his or her own knowledge, experience, and judgment to make a final determination of the strength of the fabric and whether it is airworthy.
IF YOUR PERFORMANCE IS. . .
Fabric Performance Criteria
FABRIC STRENGTH MUST BE. . .
Loading
VNE Speed
Type
New Breaking Strength
Minimum Breaking Strength
> 9 lb/sq ft
> 160 mph
< 160 mph
≥ Grade A
> 80 lb
> 56
< 9 lb/sq ft
> 65 lb
> 46
< 8 lb/sq ft
< 135 mph
> 50 lb
≥ Intermediate ≥Lightweight
> 35
Poly Fiber system is supposed to last for 30 years parked outside if it's installed correctly. At that rate, the fabric should still be good if it's never seen sunlight. Most likely the poly tac is dried up. I've used some fairly old poly brush and couldn't tell the difference, but supposedly the shelf life is 3 years if I remember right. Jim Chuk
I'm excited! There have been so many helpful responses. I welded the reinforcement plates to the end of my lift struts. I'm waiting for warmer weather to weld the gussets on the rudder peddles and the elevator center area. From what I've learned the pros recommend the temp of 4130 to be above 60 degrees to keep it from shock heating and cooling when it is TIG welded.
I do have a few more questions to throw in.
1. I have two shiny 6 gallon aluminum wing tanks. I guess they are no longer useful?
2. Can I use the big plastic front main tank, or is it best to install a rear header tank and buy the larger 13 gallon wing tanks? I will admit, that would make more space behind the panel.
3. Should I be starting any type of registration or communication withe the FAA?
I am attending and trying to learn at my local EAA Chapter 23. Between that and all of the thoughtful advice and comments here on this forum, I'm getting excited to make some progress.
I have designed an electric elevator and rudder trim system with 24 led bar graph position displays and driven by an arduino. It should be useful for displaying position of the flaperons also. I'm adding a linear actuator to control the flaperons instead of the manual lever. I want to get as much control on the stitck as possible, since I only have my right hand. I'm still unsure where the throttle control would be placed. I'd like to control it using my left hook. A bit unconventional for sure. Thanks for joining me in my thoughts and decision process.
As long as your plane isn't covered yet, I would think about widening the fuselage while you can do it easily. Basicly you just add additional tubing around the perimeter of the doors, and twist the doors to fit the new opening. I made my Avid MK IV 3 1/2" wider by doing this. It's not interfering with the original structure, just adding to it. Another area to consider strengthen is the seat truss. Many of them have ended up bending with landings that didn't seem that hard. I used .040" thick steel in the Avid in the picture. The bungees hook on a bit different on the Kitfox, so you have to leave room for that however. Take care, Jim Chuk
Probably gained same amount at hips, and shoulders. About half way up the door opening is where the new tubing starts to angel back into the original fuselage width. That is pretty close to where your shoulders are. Jim Chuk
Re: Unstarted KF Model1 SN#111- Questions
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