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Date: Wed, 28 Feb 2007 09:38:09 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] engine mount 4130 vs 304 SS
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Russell Duffy wrote:

> Greetings,
>  
> As I ponder my choices for mounting the single rotor engine, I noticed 
> something that surprised me.  304 stainless is cheaper than 4130 
> steel.  One thing that's always bothered me about steel tube type 
> construction is the concern for rusting inside the tubes, so stainless 
> is appealing.  There must be a catch here. 
>  
> Is there a good reason I should use 4130 instead of 304?  Either will 
> likely be sized much larger than needed, since I don't have the means 
> to do any proper analysis of the strength. 
>  
> Thanks,
> Rusty (Autoflight drive supposed to be shipping next week)

Something Richard Sohn told me, and I soon after found to be VERY true.  
The HAZ of stainless weldment will rust almost as easily as mild steel 
if not properly treated.  It must either be buffed to the original 
shine, passivated/pickled, or painted.  Since you won't have access to 
the inside of the tube, going stainless may not get you much.

I've often thought of a tube and fabric fuselage made of stainless would 
make a lot of sense.  Most tube airframes are heavily overbuilt just 
from a materials availability point of view.  That is, one size tube is 
just barely under the stress limit, so there is no choice but to choose 
the next larger size which give you an extra 50% margin.  Any increase 
in weight from needing to use larger tubes would be offset by the work 
and weight saved from not needing to paint it.  If there was a way 
around treating the HAZ zone, it could be a winner of an idea.

-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |