I have two pretty much unrelated topics. I have researched both in the archive and have found in one case the question has been asked before but could not find it answered. In the other case there has been much discussion and answered various ways.
1. What is the purpose of the threaded fork in the aft strut of the cabanes. It is also used in the interplane struts, why. It cannot really be adjusted in or out without twisting the spars, since the other attach points are not "hinges" - Unless you use the universal type joint that has been mentioned, with its concerns of not adequately supporting the spars, since it can hinge. It must really be set to one position, the position in which the cabanes or interplanes will assemble, and then left alone. Furthermore, since it has no opposite left threaded counterpart it is not really a "turnbuckle" so is limited to turning it at least 1/2 turn at a time to get the bolt in again, so its adjustments are somewhat coarse.
2. What plywood (birch or mahog) is used in the centersection floor and the spar doubler plates. It seems both have been the answer depending on the thread you read. My thoughts would be that birch is denser, stronger/stiffer but heavier and its tight grain does not make as good of glue joints. Mahog is all the reverse of that. On the spar doublers strength is good, but so is a good glue joint to the spars, since if the glue fails it hardly matters if the bolts can tear through the plywood - So maybe Mahog to be sure they do not separate from the spars???
On the center section floor it is mostly loaded in compression and maybe some torsional loads. It will be there soaking up compression loads whether it remains glued to the spars and ribs or not, so maybe birch is better. On the other hand, that is a big piece of plywood and the weight of birch over mahog is probably significant if mahog is adequate.
Comments/Opinions? Has anyone seen the answer actually called out in the plans that I'm missing? Is anyone out there flying either of the above configurations and still alive to write about it?
Cabanes and Plywood Types
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johnkerr
- Posts: 78
- Joined: Thu Mar 25, 2010 10:48 am
Re: Cabanes and Plywood Types
Opinion offered. 1. The threaded fork set the incidence of the center section. True the total amount of movement is relatively small it is adequate to the job
I used birch for both the doublers and the center section bottom for the reasons you offer. Purely conjecture but glue starvation from over clamping has been listed as a concern in some of the reading I have done. Remember each of the plies has been glued to it's neighbor so glue must work on birch. As in most probably
Glued wood, the wood will fail before the joint. Prepare some test pieces and get comfortable with your choice
John
I used birch for both the doublers and the center section bottom for the reasons you offer. Purely conjecture but glue starvation from over clamping has been listed as a concern in some of the reading I have done. Remember each of the plies has been glued to it's neighbor so glue must work on birch. As in most probably
Glued wood, the wood will fail before the joint. Prepare some test pieces and get comfortable with your choice
John
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Jackal
- Posts: 62
- Joined: Sat Apr 03, 2010 10:02 am
Re: Cabanes and Plywood Types
John,
Thanks for your meaningful input. Yes I suppose that nothing is really "fixed" and even a "fixed" joint that is not engineered to hinge at all will still hinge some, allowing the function you noted to set the incidence. As I understand it the flat bottom of the wing is to be parallel to the top longerons, which is not exactly zero incidence for the wing chord, and that the same is true for the bottom wing.
Regarding the plywood, yes you are right, the birch is glued on the other side (between the plies) so it must work. I was leaning toward birch ply in both areas for its integrity, which just from handling it seems to far exceed that of mahog.
CORRECTION: While brushing my teeth this morning I thought about my statement (above) that the center section ply will remain in place soaking up compression even if it separates from the spars/ribs. It dawned on me that this is not true since the compression is coming in from the spars as they are lifted. The flying wires angling up in a "V" to the middle of the spars causes the spars to want to move inward (toward their root) thus transferring compression through the spar plate fittings into the center section spars. If the plywood is detached from the center section spars it will not be getting any compression loads until the center section spars fail and allow the root end of the wing spars to come against the plywood. By this time your day is already very very bad and will probably not get any better.
Thanks again John,
-Jackal
Thanks for your meaningful input. Yes I suppose that nothing is really "fixed" and even a "fixed" joint that is not engineered to hinge at all will still hinge some, allowing the function you noted to set the incidence. As I understand it the flat bottom of the wing is to be parallel to the top longerons, which is not exactly zero incidence for the wing chord, and that the same is true for the bottom wing.
Regarding the plywood, yes you are right, the birch is glued on the other side (between the plies) so it must work. I was leaning toward birch ply in both areas for its integrity, which just from handling it seems to far exceed that of mahog.
CORRECTION: While brushing my teeth this morning I thought about my statement (above) that the center section ply will remain in place soaking up compression even if it separates from the spars/ribs. It dawned on me that this is not true since the compression is coming in from the spars as they are lifted. The flying wires angling up in a "V" to the middle of the spars causes the spars to want to move inward (toward their root) thus transferring compression through the spar plate fittings into the center section spars. If the plywood is detached from the center section spars it will not be getting any compression loads until the center section spars fail and allow the root end of the wing spars to come against the plywood. By this time your day is already very very bad and will probably not get any better.
Thanks again John,
-Jackal