DE Spar drilling question

[docmatt]

I'm getting ready to drill my spars for fittings.  Referring to pages 41-42C,  it looks like one hole for each fitting does NOT get a phenolic insert.  That also jibes with the "make 30 inserts" on page 42C.  Why would this be?  I am guessing it is to avoid drilling a large hole on the spar doublers.   I can't decide if it is better to have a 1/4" unreinforced hole  or a 3/4" reinforced hole.  Between the multiple belts and suspenders in the spars at those places, I'm not sure it matters much.   What did you guys do?
Also, the picture on page 41 is grossly out of scale, so don't head scratch over that page too much..

[rockiedog2]

Hi Doc
I havent built a DE but looking at plans p 42(in my very early proof plans) it looks like if a bushing was installed in the hole in question the edge distance to the spar butt would likely be very close.  Unless somebody came up with a good reason not to I would just put a 1/4 bolt in there.
I fall back on the Pitts and Acrosport root fittings....they don't have any bushings anywhere. They're also just a simple 1"  strap fitting w/o the huge offsets for adding more bolts/bushings....seems like I remember just 3 quarter bolts thru the spar in each fitting. I haven't heard of any of those pulling out even dogged on like they are
I didn't say this but I wonder if the lawyers got involved in the DE root fittings. But we can't be too safe...if building one I'd follow the plans best I could. I know Leonard would be pleased with that. Right Boss?
Where is The Boss anyway? I haven't seen him around here yet

Spencer

[docmatt]

Thanks Joe,
That makes sense at the wing root, but what about the strut fitting?  Keep it hush, but after talking to Murray, I decided to make the strut fitting a simple 1'' strap.  I still can't decide if I should add an insert to the first hole.  Seems like with all the doublers and plywood there, it will act like a solid block of wood and hold the insert just fine.  I sorta doubt if the wrong decision will make me fall out of the air though.

[Tom H]

DocMatt,
We built Stubby's wings per latest plans.  In my plans book, I see that I found updated drawings for pages 40, 41, 41A, 42, 42A, 42B and 42C, which all show being revised 4/17/2010.  Could be others; these are the ones I found with a quick look.  Probably can be found in the old yahoo site files section.

Regarding bushings, on p 41, for example, the revised dwg shows two bushings being eliminated, and the three remaining are all far from the spar edge.  The original shows a bushing nearer the edge.

We discussed the reasons for the bushing during our build.  I believe the main reasons for the bushings are, one, to act as crush limiters if the bolts are drawn down too much, and two, to spread the load to a greater area in the spar wood.  The 1/4" bolts apply a concentrated load to the harder, stronger bushing material, and the larger diameter of the bushing spreads this load to a larger area of the spar wood. 

Hope this helps.
Tom H

[docmatt]

Thanks Tom,
I spent some more time laying things out today.  I feel confident putting bushings on all of the bolts.  The way that I made the block at the wing root allows plenty of material for a 3/4" hole.  Now with my family back from the Holiday (I had to work), I'll get my son to help me hold the spars on the drill press and get 'em done. 

matt

[Tom H]

DocMatt,
A couple posts up you indicated that you will use a simple 1" strap at the strut fitting.  I would be cautious about doing it that way.  The original plans show, on each side of the spar, a strap welded to 1/8" plates, and this whole assembly bolted to the spar (with same arrangement on opposite side of the spar).  The revised drawings show each of these two part pieces replaced with a one piece part, cut from a sheet of steel.  And, as I said, a couple of bushings are eliminated, but the same number of bolts.

The forces on the fitting at the wing/strut junction are large.  I recommend following the plans on this one.

I had a message commenting on tightening the bolts holding these fittings to the spar.  The bolts are loaded primarily in shear.  I don't know if there is a recommended torque for the bolts.  We tightened the bolts snugly, did not use a torque wrench.  We used elastic stop nuts on these bolts, and, since the bolt heads are more or less forever hidden, safety wired the heads so the nuts could be checked for tightness in the future.

Tom H

[rockiedog2]

I'd go by the plans too...

[Dan_]

The reasons for the bushings as Tom stated, are to stop the crushing of the wood and to keep the bolts from elongating the hole by spreading the force over a much larger area. (the area around the outside circumference of the bushing.)  Some make the bushings slightly smaller than the thickness of the spar so that the bolt squeezes the wood the proper amount without crushing.  A big bonus is if your bushing material is strong enough, you can then put the specified bolt torque on the bolt without crushing (damaging) the spar wood.  

Using steel bushings, I see no reason you couldn't use castellated nuts and cotter pins...  Comments..?  Some use epoxy to seal the wood before placing the bushing.  You would then have to ream the hole after the epoxy cured.

Pic 1 elongated bolt hole... (bottom hole)


pic 2. steel looking bushings...


Attached is AC 43.13-1B 9/8/98  on AN Bolt and nut torque values

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