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Maple wood crutch - grain across or vertical?

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Craig Beswick:
According to Windy the growth rings should be horizontal to the engine bolts.

Craig

Chris Brainard:
Like this

Dennis Toth:
Thanks Tim and Chris,

As Chris said growth ring horizontal is what I always did, but I agree with Tim for this situation. Maple seems to be very strong and as long as I fit the blind nuts carefully it should be ok. This is only a Fox 35 and the maple will be supported by a bottom block. My other option is to find a suitable Proto Speed magnesium pan and just use that.

Best,   DennisT

Trostle:

--- Quote from: Tim Wescott on August 15, 2021, 07:52:20 PM ---
Are you talking about the growth rings?  If you're looking at the end of the wood you can see light and dark stripes, often a bit curved.  These are not grain, but rather the tree's growth rings.


--- End quote ---

The "grain" of wood is created by "growth rings"  The type of wood, like "A-grain", or "B-grain", or "C-grain" depends how the wood is cut relative to the "growth rings".

Keith

Dave Hull:
It depends on what parameters are most important to you.

If you want the bearers to be mounted such that the direction of highest stiffness of the wood opposes the main axis of shaking of the engine (roughly parallel to cylinder bore) then you should orient the wood so that the growth rings are perpendicular to the axis of the cylinder bore.

All wood species that I know of have greater stiffness (elastic modulus) in the radial direction than in the tangential direction. In species of interest here (maple, birch), a rough approximation is that the stiffness is 60—100% higher in the radial direction of the tree trunk. (As an aside, for balsa, the ratio is actually 3 times stiffer, so of course we consider that in our wood selection.) The data forms the science behind that selection process.

The grain descriptions that Keith provided are pretty much homebrewed by modelers for modelers. Last time this topic came up, the best I could determine was that JASCO was likely to have begun the labeling of A-Grain, B-Grain, and C-Grain. If someone can find an earlier reference for this nomenclature it would be fun to know. In traditional woodworking nomenclature you have quartersawn (quarter grain) which is our C-Grain, and flatsawn which is our A-Grain. Of course, most pieces are somewhere in between, which fits our B-Grain nomenclature. These make the most sense when you think of a wide, flat board, or in our case a balsa sheet. When we are talking about sticks, the only thing that matters is looking at the end grain and figuring it out. For example, you could take a flatsawn board and then slit it into sticks and depending on which way you turned it, you have radial or tangential grain at your fingertips.

If you are also using motor mounts that are deeper than they are wide to help provide more stiffness, then why give away performance properties where there is zero increase in weight by using wood with non-optimal grain orientation? Of course, with any model airplane work--and most other home projects for that matter--whatever is “good enough” for the builder is, by definition, good enough.

As always, your design choice is a compromise. The compressive strength or resistance to crushing is 5 to 10 times higher in the tangential direction than the radial direction. Nothing is free. The crushing issue can be solved either with metallic pads that are larger than the engine lugs to spread out the compressive forces, or else bed the engine on steel or titanium inserts bonded into the motor mounts. You won’t get the thru-bolt crushing loads that way. This is done on a lot of model racing planes and works very well.

The definitive resource for engineering properties of wood is the Forest Product Laboratories Wood Handbook (FPL-GTR-190).  The source data for this discussion is contained on page 5-2, and 5-9.

Dave

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