Questions re Polikarpov-fighters.

[Kari Lumppio]

Originally Posted by Arsenal VG-33

...there is no interest in the “modern” or “obsolete” definition, but only on the strenth vs weight ratio. The highly stesses stainless steel 30Kh GSA used on the I-18 airframe (120-140 kg/mm²) was very effective on that way. “Modernity” is a kind of industrial problem, and technically not always the best. In 1939, light alloys were giving 40 kg/mm² resistance, at best.

Hello!

I am sitting on the fence on the topic of this thread. But about the materials. With the values you give steel with strength 120 kg/mm2 has less than 4% better strength/weight ratio than "light alloys".

But can you make whole airframe out of steel? No! Usually steel is used only to make the load carrying members like fuselage tubular frame which has to be covered to get the aerodynamic form. Many times it was aluminum "roof" and fabric covering below that. Steel fuselage frames most often were statically determined meaning there was no alternative load paths if the frame tubes were damaged. I do not know if this was generally taken in account in design.
BTW the delta wood was similar to steel in the use - it was used only where high strength was needed (spars, stringers). Not for whole airframe.

For light alloys the design is usually totally different. The whole "cover" was load carrying element which was supported by frames and stringers against buckling/collapsing (some reversible buckling of panels is usually allowed for). Conservatively designed stiffened structure usually had redundancy and provided multiple load paths even when damaged ("fail safe"). Curtiss design is a good example.

In the real light structures stability is usually of more concern than material strength. Even in tubular frames. If so, then the much higher density of steel becomes literally a burden. There is no use for the extra strength of steel over light alloys then.

Plus. Was the "30Kh GSA" steel really stainless? Was it weldable, and how easily?

As far as I know steel frames usually were of chromolybdene (spelling) steel like AISI 4130 (good weldability). It rusts easily, the high-strength version (4130N) rusts like hell. I have right now one test series in environmental chamber downstairs, a work project. It was covered with rust spots within hour when in 60 degrees Celsius/95% relative humidity. Aluminum 7075 - both clad and bare - have got only some oxidation spots after couple of weeks. Another thing to ponder in design.

Regards,
Kari