Questions re Polikarpov-fighters.

[Kari Lumppio]

Hello!

This thread is on the borderline to bother with an answer.

About the steel 30ХГСА. I was not familiar with the Soviet/Russian material codes but my own searches and the given links helped.

To be stainless steel the material should contain at minimum 11% chromium, usual amount is around 18%. Steel 30 HGSA data is given here:
http://www.metaltrade.ru/steelinfo/30xgsa.htm
The steel contains only 0,8-1,1% chromium. No way it is a stainless steel. It is a structural steel and is it possible that some of the letters "GSA" mean drawn, seamless tube? Because that it is, the 30 HGSA steel.

My suspicion was awakened because stainless usually is not that strong. Welding makes it weaker and prone to corrosion if it is not post-treated by removing the low-chrome layer either chemically, mechanically or both.


Wood is light and IMO good choice for structures with low loading. Case in point is MiG-3 rear fuselage which was work of art. Veneer layers bonded over mold; light, strong, aerodynamically and estethically beautiful piece. Actually all material choices with MiG-3 make engineering sense if you ask me.

For high load structures like wing spars wood is not the best material. Steel strip spar caps do not help if they cannot be bonded reliably to the wooden structure, screws are not enough to transfer the load. As Soviets found out too? Another problem is joining the wooden parts to the rest of airframe. Many a Il-2 did throw off their wing(s), rear fuselage or tail when they were hit in some hard spot. In one case Il-2 lost it's tail after getting AAA hit on wing gun barrel. The big wooden wing spar box in Jak-7 (Jak-1 too?) ate much of the wing inner volume which could have been used to store fuel. Short endurance was the achilles heel for many Soviet fighter types. Bingo fuel for La-5 - even FN - meant that Brewsters could turn the tables in battle.

One problem with wood and also composites like carbon fiber is that they have no plastic deformation like metals have. Shot through aluminum skin makes hole, but the shock damages wood (typically veneer layers, plywoood) in larger diameter area around the hole. This is even more pronounced with carbon fiber laminates, BTW. Most dangerous are outwardly invisible damages in laminates (delaminations).

BTW About the AISI 4130N test pieces and corrosion. We are testing something else where the corrosion of outer surfaces does not factor in. It was problem (nuisance) known beforehand.

Cheers,
Kari