Natural fabric materials such as cotton and linen were primarily used for covering the structure of both small and large aircraft back in the day. However the use of fabric covering materials continues to be widely used on many new manufactured aircraft, for restoration of antique and vintage aircraft, and of course used by builders of many experimental and recreational aircraft. Many factors determined the lifespan of fabric covering primarily the environment the aircraft operates in and whether it’s stored inside or outside. On average natural fabrics would last a decade; or more or less.
A benefit if you will, of routinely removing the covering of these older aircraft is that mechanics and owners had the ability to regularly inspect, clean, repair, treat, and paint the generally steel, aluminum, and wood materials used to construct these vintage aircraft.
As a result of longer lifespan synthetic fabrics, one can argue the structure underneath may not be subject to a thorough cleaning and inspection as often as it once had, supporting the need for diligent preparation of the structure being covered. Moisture ingestion in hidden areas, spilled chemicals and oils, dissimilar metals, and other corrosion causing features, may have longer periods of time to attack and do damage. Many areas prone to corrosion are covered and hard or even impossible to access making it difficult to spot corrosion forming.
To talk more about restoring and recovering vintage airplanes, I turned to my good friends at Stanton Sport Aviation where they recently restored a 1947 Piper PA-12 Super Cruiser and a 1946 Piper J-3 Cub. Both aircraft were originally manufactured using natural fabrics for covering.
Under the fabric
The fuselages like similar aircraft of this period are a truss built structure using chromium molybdenum steel 4130 tubing or commonly referred to as “chrome moly”. Originally the steel parts were painted with a common use primer of the day; how many of you recall the thin green appearing zinc chromate primer? Both wood and aluminum were used for fuselage stringers, floor boards, and miscellaneous other features. The wings for both these airplanes were constructed using aluminum spars and wing-ribs.
The first step to any restoration is the careful disassembly, that is if it’s still assembled, and it’s advisable to keep a detailed journal of how the aircraft came apart with lots of diagrams and photographs. The entire airplane and all the attaching parts need a thorough cleaning and detailed inspection.
Typical discrepancies encountered can be cracked, bent, dented, and of course rusted tubing and other steel parts, as well as the occasional cracked weld-joint or fitting. A close inspection of the lower fuselage structure particularly on tail-wheel type aircraft near the tail-post area will generally discover tubing in need of repair or replacement. Other areas prone to rust are those subject to water ingestion such as near the landing gear, windows, doors, and other openings.
Kent Johnson, A&P mechanic and manager of Stanton Sport Aviation, says, ”On the Cruiser we found both the left and right side rear window channels completely rusted through due to moisture ingestion. They likely had been that way for years but until the airplane was disassembled and the fabric removed this went undetected.”
Rust on the outer surface of tubing will be easily seen, but corrosion caused by moisture that has penetrated its way inside the tubing can be difficult or impossible to detect. Rust can be hiding inside these 60 plus year old fuselages and attack the steel structure on the inside of the tubing as well as the outer surfaces. One method used to find corroded and weak tubing is to apply pressure using a sharp instrument such as an awl to corroded or suspected areas of tubing. With most tubes having thin wall thicknesses, a rusted tube will be weakened resulting in the awl denting or penetrating the tubing, identifying an area in need of repair or replacement. Also look for obvious signs such as rust-red discoloration and flaking.
As the company firmly level-loaded increased aircraft production and deliveries, it continued to globally expand its dealer network.
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