Screws and plastics

Tiistai 8.8.2017 - Reino Myllymäki

You can check the success of the restoration work of old aicraft by looking the screws. If you will find some Torx or Phillips screws, something has went wrong.

The Phillips (PH) screw was invented by John P. Thompson who sold his invention to the Phillips Screw Company in 1935. Although 85 % of U.S. screw manufacturers have sold the licence in 1940, the State Aircraft Factory did not use Phillips screws in VL Myrsky.

The Pozidriv (Pz) is derived from the Phillips screw and it is therefore an younger invention. Phillips and Pozidriv screws are almost past now, since the 1967 invented Torx (Tx) is a common solution now.

So, VL Myrsky has no PH, Pz or Tx screws. Only slot head screws and hexagonal head bolts.


The slot head screws of the State Aircraft Factory in connection with a Parkesine abrasion shelter and the alarm buzzer. Photos: Finnish Airforce Museum.

Do not ignore plastics! You can find selluloide "windows" in the aircraft of the 1920s. VL Myrsky has maybe none selluloide parts but Parkesine has been used in VL Myrsky as buttons and abrasion shelters. The Parkesine was invented in 1856 and the selluloide in 1870. Selluloide has been used as film material until asetate substituted it in 1950s.


The propeller controllers button has made from Parkesine and the instruction panel from Paxolin. Photo: Finnish Airforce Museum.

You can find a lot of Bakelite or phenol formaldehyde resin from the VL Myrsky. Some buttons are made from casted Bakelite. Paxolin is a composite material of Bakelite and paper. Paxolin has used for example in the instruction panel of the propeller controller.

The most important use of Bakelite is plywood. Theodor Goldschmidt invented Tego-film in late 1920s. Tego-film is Bakelite extracted in a silk paper. German manufacturers did not succeed in plywood manufacturing by Tego-film. But the Finns succeed accidentially in 1929. Before WWII Finland was the second greatest plywood manufacturer in the World. The number one was Soviet Union.

It was possible to produce weatherproof birch plywood and laminated veneer lumber by Tego-film. Veneer lumber was used in propeller blades, antenna mast, spars and tail strucure. Birch plywood was used in spars, ribs, tail structures and in the cover of the wing and the fuselage. The diagonal plywood was used, too. The grain direction has turned 45 degrees in the diagonal plywood.


The cabin aft glazing and canopy have been made from Plexiglas. Photo: Finnis Aviation Museum.

The third - or is it fourth? - plastics used in VL Myrsky is acrylic (PMMA). It was invented in 1928 and commercialised as Plexiglas in 1933. The canopy and the cabin aft glazing have been made from acryl. It is possible to made curved shapes from acryl. However, it is not very durable material. Nowadays the polycarbonate (PC) is more durable but also more expensive material. The polycarbonate was invented in 1898 but commercialised after WWII.

Avainsanat: aviation history, restoring, old aircraft, VL Myrsky II, MY-14

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