3/4” x .028 cm tube

[okawbow]

For the engineer types out there...would a LEXL fuselage madewith 3/4” x .028 tube in place of the 5/8” x .035tube be as strong/ stiff? Would it be weldable?

[okdonn]

I'm not an engineer, but I found this when considering similar substitutions.

https://www.roguefab.com/tube-calculator/

Dia  wall     safety factor   Z moment of inertia
5/8 0.035   0.9203           0.009002524
3/4 0.028   1.122             0.010976292

[okawbow]

So, if I’m read it correctly, the larger tube is stronger. It is the same weight per foot, or just slightly lighter, so should be ok.

[okdonn]

I didn't look up the weight per foot, but yes, it looks to be stronger. However, last time I looked, all the 0.028 tubing was considerably more expensive, if that's a consideration.

[okawbow]

I didn't look up the weight per foot, but yes, it looks to be stronger. However, last time I looked, all the 0.028 tubing was considerably more expensive, if that's a consideration.

I actually can get both sizes of the .028 4130 tube for $4.00 a foot less than the .035 sizes. They have 7000ft of the 3/4 and 5000 ft of the 5/8. I will save over $600.00 on tube. And it’s only 35 miles away. I plan on picking up enough for the build Monday.

[okdonn]

Go for it!

[Vince Carucci]

I calculated the .75" x 0.028 to be about 1.2 times stronger than the .625" x 0.035.
This should work out well for a truss style frame. 
You should consider that the thinner walls may dent easier. 
And thinner walls might be a bit more finicky when welding.  

By the way, buy replacing the main tube of the landing gear with 7/8" x 0.035, you effectively double the strength of the tube with a 1.5 oz. penalty per linear foot.

[okawbow]

I accept that the thinner tube maybe more finicky to weld, but I have more time than money. I’m sure if I go slow and use the right tip and flame, it will work ok.

[Dan_]

Make sure you test for correct torch angle...

Try an angle and apply heat.  Withdraw the torch and both tubes should cool at the same rate.  You will be watching the color change.  When you have the right torch angle for the cluster the color of the tubes will change the same.

Of course you only know if you have the wrong angle with mig is when you blow a hole...

How to determine oxyacete tip size...

[scottiniowa]

I actually can get both sizes of the .028 4130 tube for $4.00 a foot less than the .035 sizes. They have 7000ft of the 3/4 and 5000 ft of the 5/8. I will save over $600.00 on tube. And it’s only 35 miles away. I plan on picking up enough for the build Monday.

Unless things have changed tremendously, I would encourage to again check the prices... It never use to be this way...and almost always .750 x .035 being the cheapest of all tubing... A volume thing.    If what you say is true, great..

Best of success.

[okawbow]

You’re scaring me now! LOL. I did call them to confirm the price. I guess it’s an overstock thing. Sure hope it’s not a mistake.

[okdonn]

OK - 0.028 is more expensive than 0.035 at Spruce - where are you getting this price, I might need to stock up!

[okawbow]

I’ll report back tomorrow if the tube looks ok and the price is not a mistake.

Chuck

[okdonn]

I just remembered that several (6-8) years ago there was a discussion about Chinese vs German and American 4130, the general consensus being to avoid the Chinese if at all possible, due to inconsistent quality, impurities, hard spots that cracked, etc. I don't know if that's still the case or not - just reporting what I read on the internet :-)
"If it's too good to be true ---"

[okawbow]

I’ll check it over carefully. Hopefully it will be ok. Won’t cost too much to find out, and always good for other projects.