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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
what is it called when the cross section of a beam is optimised like an H or T beam, is it area inertial moment? Im an EE student and my ME's are in class this week so im kinda on my own here...
im trying to build light weight but stiff stuctures using ribs, beams, or spars or what have you. Im mostly interested in stiffness, strength, and low cost (minimum material).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
We called it second moment of area. However, when designing a structure you also have to consider resistance to buckling. For instance take a T section and an I-beam of the same second moment. The T section will buckle easier under some conditions.
Box sections and tubes are good all-round building blocks.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok i am thinking of using fiberglass layers on styrofoam or balsa, becuase im using way to much carbon fiber and fiber glass.
in my previous builds, i have way more strength present, then is actually being used, and its still not stiff enough. and CF is expensive, so i must optimise the cross section for my purpose.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
Yes, a honeycomb, balsa or foam core will increase the second moment greatly by moving the strong material away from the neutral axis.
ok thats what i thought, but i didtn know it was called a neutral axis, thats where the strong material contributes least to strength and stifffness, yet proportionally to mass.
The balsa also helps resist buckling of the thin (single layer CF/FG) walls out of parallelism/perpendicularity while under load, even though the balsa i use has no real strength.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
pics
CF on 0.5 inch styrofoam, M10A grenade (inert) and scissors for scale...
2 CF layers on the bottom, 2 on top, 4 layers total. 0.5 inch styrofoam visible. Still too flimsy, i now realise i used to much of my hi-$ CF, and still dont have the stiffness i need. Dam it.
single layer CF wrapped on both sides of 0.25 inch thick balsa, very light, very stiff.
A close up while wet. 0.012" thick times two layers. 3k tow, 2x2 twill, 644 Ksi AS4.
abcdef Ducted lift fan for tilt rotor... 11 inch dia. prop, 12 inch dia. duct...
Closeup, bass wood on the servo holder, FG and foam for the duct.
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Questions about I and T beams, material stiffness and strength.
