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Author Topic: Maple wood crutch - grain across or vertical?  (Read 2154 times)

Offline Dennis Toth

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Maple wood crutch - grain across or vertical?
« on: August 15, 2021, 04:02:01 PM »
Going to start a project that will require a full length maple crutch. I have some long motor mount wood that will fit the pattern but the way it was cut would have the grain running vertical through the piece. This is a FOX 35 ship and the maple on its side is 1/2" thick. I don't think it will crack but wanted to see what others think before hacking up really nice maple.

Best,    DennisT

Offline Tim Wescott

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Re: Maple wood crutch - grain across or vertical?
« Reply #1 on: August 15, 2021, 06:35:20 PM »
I can not envision what you're trying to do here -- take a pic of that part of the plan, or post a link?

If it's an old design they may have used maple in a way that required cross-grain strength because they didn't have anything else -- I could be utterly wrong (because I can't envision what you're doing) but could you use aircraft plywood?  Or build a box that looks like the plans piece, but has a lot of hollow inside for lightness sake?
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Offline Brett Buck

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Re: Maple wood crutch - grain across or vertical?
« Reply #2 on: August 15, 2021, 06:46:42 PM »
Going to start a project that will require a full length maple crutch. I have some long motor mount wood that will fit the pattern but the way it was cut would have the grain running vertical through the piece. This is a FOX 35 ship and the maple on its side is 1/2" thick. I don't think it will crack but wanted to see what others think before hacking up really nice maple

    As a general rule, the grain has to run the long way. But the way you describe it I am not sure we are talking about the same thing. Typically a crutch is made by 1/2x3/8 cross-section, length long enough to go from the nose through the second former (and maybe further), with the grain running the long way. That is typically separated by 1/2" thick medium balsa with the grain running from one mount to the other from right behind the engine to the second former. Epoxy it all together on the bench to make sure it is absolutely planar, then sand it flat.

     Brett




Offline Dennis Toth

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Re: Maple wood crutch - grain across or vertical?
« Reply #3 on: August 15, 2021, 07:26:53 PM »
OK, what I am building is a simple rat racer type ship. The fuse is from two pieces of full length motor mount stock, this forms the crutch, then a top and bottom block is added for some shape. The crutch pieces I have the grain running down the length but if you look at it from the end the grain is vertical for the width (1/2" x 3/4") I need in order to get the shape (similar to a speed pan shape). My question is really about the wood splitting, particularly when installing blind nuts?

Best,    DennisT

Offline Tim Wescott

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Re: Maple wood crutch - grain across or vertical?
« Reply #4 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.

See the picture.

For some kinds of woodworking you do care about the direction of the growth rings with respect to the wood (i.e., the difference between A grain and C grain balsa is how the wood is cut with respect to the growth rings).  But I don't think you care here.

As long as you're not jamming anything into the holes that's just too big to fit, you're not going to split the wood.  While maple may be a bit stronger in one cross-grain direction than the other, I have never heard of or experienced any splitting (of maple) along a growth ring boundary.  If you have to pound in the T nuts -- the holes are too small.  Otherwise -- don't worry.
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Offline Craig Beswick

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Re: Maple wood crutch - grain across or vertical?
« Reply #5 on: August 15, 2021, 08:07:34 PM »
According to Windy the growth rings should be horizontal to the engine bolts.

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Offline Chris Brainard

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Re: Maple wood crutch - grain across or vertical?
« Reply #6 on: August 15, 2021, 11:15:28 PM »
Like this

Offline Dennis Toth

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Re: Maple wood crutch - grain across or vertical?
« Reply #7 on: August 16, 2021, 06:37:37 AM »
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

Offline Trostle

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Re: Maple wood crutch - grain across or vertical?
« Reply #8 on: August 16, 2021, 10:53:31 AM »

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.


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

Offline Dave Hull

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Re: Maple wood crutch - grain across or vertical?
« Reply #9 on: August 17, 2021, 01:51:49 AM »
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

Offline Brett Buck

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Re: Maple wood crutch - grain across or vertical?
« Reply #10 on: August 18, 2021, 09:12:21 PM »
OK, what I am building is a simple rat racer type ship. The fuse is from two pieces of full length motor mount stock, this forms the crutch, then a top and bottom block is added for some shape. The crutch pieces I have the grain running down the length but if you look at it from the end the grain is vertical for the width (1/2" x 3/4") I need in order to get the shape (similar to a speed pan shape). My question is really about the wood splitting, particularly when installing blind nuts?

    OK, I knew we were not communicating. I would probably rather have them vertical than horizontal but to be honest with you, I never even looked at which way the growth rings go at any point in the last 50+ years. As long as you drill proper clearances for the bore of the blind nut you shouldn't split it.

      Brett

Offline Serge_Krauss

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Re: Maple wood crutch - grain across or vertical?
« Reply #11 on: August 20, 2021, 12:57:28 AM »
I guess I too am having trouble understanding, because the question seems simple. I think the grain direction is most relevant to the direction of compression loads from engine mounting screws. 'seems to me that it should be lengthwise  and in the direction of the mounting screws. For a profile that would be perpendicular to the fuselage sides to counter compression loads from the engine mounting. I have never experienced splitting from blind nuts (I recess the plywood doublers to accept the blind nuts so that they are flush with the surface of the plywood. For a full-fuselage stunter, I would still orient the grain
 to resist compression loads from mounting screws.

SK


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