# Foam vs. honeycomb for carbon fiber sandwich

### Help Support HomeBuiltAirplanes.com:

#### birdus

##### Well-Known Member
OK, got into Timoshenko (Mechanics of Materials), and nothing on curved beams. Then Shigley, and the standard method of calculating neutral axis shift and stresses. Then Juvinall and nothing on curved beams, but a nice review on constant stress cantilever beams (applicable to Wittman style landing gear legs).

Then Roark's, Chapter 9 is huge and useful. It goes through the whole topic. If the curve radius is 8 or more times the depth of the beam, the errors from using straight beam theory is 5% and less. If the curve radius is smaller than 8 times the depth of the beam, then you need to wrap curved beam theory into your analysis. There are formula for neutral axis shift of wide flange beams and deflections. The expansion of the calculations to composite beams is straightforward. Well, once you get past the composite beam theory, which may warp your brain more than science fiction novels.

Oh, and I have looked and looked for "kick" loads for tapered and/or curved beams. The other engineer in the house did too. The only place our internet searches have turned up anything is - wait for it - a couple threads on Homebuiltairplanes.com. Sure, I found how to calculate recoil forces in guns - simple Newtonian physics. Somebody want to talk about "kick" loads in curved and/or tapered beams, their origin, cite textbooks, learned articles, professors making up expressions that did not get widely used, etc? I am all ears.

Billski
Thanks for that extra info, Bill. Jim Marske gave me some numbers regarding the strength I'll lose with the bends, so I'll probably just start out by building a simple test piece and do some static testing on it. That will also give me some practice with the building technique. I'll use Jim's calculations but add in enough extra Graphlite rods to make up for the loss in the curve. The test spar will probably just be a representative curve of the real one—maybe a 40" radius (or more) extended a couple feet on each side of the bend (maybe an 8-foot piece), which is about 30° where the landing gear mount. Jim told me that a 40" radius will lose about 7.5% in strength, so if I increased the number of Graphlite rods by, say, 10% in count, I'm thinking I'll be okay. The test piece should be enlightening.

Jay

#### wanttobuild

##### Well-Known Member
Why not share the specs and sources with everyone here?

BJC
AMT Composites
Cape Town SA.
My apologies
Ben
H-Minus

BJC

#### harrisonaero

##### Well-Known Member
Check this out:

http://courses.washington.edu/me354a/Curved Beams.pdf

Shigley also has some stuff on curved beams... Roark's has a whole section on the mechanics of curved beams. I still do not know what this "kick" load is. Aero guys rename things that were already identified and analyzed by mechanical engineers before the Wrights ever flew, so sometimes we need translation help. I will look at Shigley and Roark's and see if it will help.

Billski
When in doubt, ask WWBD.... What would Burt/Bruhn do?

Look in Bruhn D3.11 through D3.13 for the description and analysis methods for kick loads.