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  #1  
Old 11-10-2019, 09:20 PM
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N546RV N546RV is offline
 
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Location: Houston, TX
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Default Horizontal stab attach - sanity check

OK folks...so I'm at the point of fitting the horizontal stab and getting the attach holes drilled. I did the holes between the rear spar and the vertical attach bars tonight - those are pretty much a piece of cake after checking level and alignment about 400 times. The holes in the forward splice angle, though, I'm spending a whole lot of time reasoning things out before I drill, since it seems like this is an area where people frequently encounter problems.

The main thing I'm running into is that, as far as I can tell, it is 100% impossible to drill these holes and maintain "proper" edge distance (that is, 2x from the center of the hole). This seems to be a fairly well-known issue, and digging through past discussions indicates that a) the mothership knows this and reassured builders frequently and b) since the bolts are loaded axially (vs in shear), edge distance is less critical.

In an effort to be super thorough with positioning these holes - as opposed to simply taking the measurements from the plans as gospel truth - I did a whole lot of drawing on the aft deck with a Sharpie. With a little work in Photoshop for some colored overlays, we can end up with a nice little visual representation of all the pertinent pieces (I've omitted the spacers since they don't seem likely to have edge distance issues):



The real fun here comes in dealing with the overlap between the longerons and the angle riveted to the bulkhead. The most critical point in terms of edge distance seems to revolve around the lateral position of the outboard bolt holes. The important edges here are the inboard edges of the longerons, and the outboard ends of the bulkhead angle. It stands to reason, to me, that the best way to position this hole laterally is right in the center of the overlap of these two pieces, since this maximizes the edge distance on both pieces.

Hopefully this close-up annotated view makes this a bit more clear (note that I've removed the depiction of the stab attach angle since it's not critical for this particular case):



Note that the total overlap of the longeron and bulkhead angle is ~5/8", which in turn implies that the best edge distance I can get on both pieces is 5/16" if I really nail that centerline.

So the first question is: does this make some semblance of sense, and does my approach here seem reasonable?

Then we get into the question of how to actually go about drilling these holes. There's not a lot of detail in the construction manual here, so I suppose the naive way to do the drilling is to get the stab clamped into place, mark out the hole locations on the stab attach angle, and drill straight down through all the pieces. The main thing I worry about there is accurately getting the hole position marked on the angle, since most of the pertinent measurements need to be made relative to the longerons or other things that are beneath the stab. I suppose I could drill up from underneath, but I don't feel like that really solves any problems.

One thought that occurred to me was to drill #40 pilot holes just through the aft deck and attached pieces, then clamp the stab and spacers in place and drill that hole up from beneath, up to #30. Then I have to worry about being sure I drill square while probably working in an awkward position, likely with an angle drill.

So maybe the real question is: how did everybody go about marking and drilling these holes? After typing all this, I feel like it may be a rhetorical question to ask if I'm overthinking this. Which is why I'm typing all this tonight and then letting all this stew overnight before I go actually put holes in anything.

After all, I don't want my plane to end up in Vic Syracuse's hall-of-shame thread...
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  #2  
Old 11-10-2019, 11:24 PM
rapid_ascent rapid_ascent is online now
 
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Default

Well I didnt do it correctly but I have some thoughts. I tried it today.

I used the center of the rivet heads that go through the longerons and drew a line on the aft deck. This line can be continued up onto the flange of the F-714. The other dimension is 1/2 from the edge of the flange which can be measured from the 1 to 1 detail on the drawing.

Now you need an angle drill and it needs to have a long bit. Mine was too short resulting in the head hitting them bar on the F-714. I started with a #40 bit so I had some hope of making a correction. Problem #2: Drilling perfectly vertical. Even though I thought it looked good mine was not. A drill block of some sort might help.

I only tried drilling the 2 holes that go through the longerons so far. On one side my F-710 appears to be too short so not enough edge distance. On both holes I seem to be too close to the flange of the F-710.

Id suggest you check the fit of your F-710 before drilling anything.
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  #3  
Old 11-11-2019, 07:06 AM
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N546RV N546RV is offline
 
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Quote:
Originally Posted by rapid_ascent View Post
Id suggest you check the fit of your F-710 before drilling anything.
I did take a look at that last night. The part is F-810 in my case, but I presume it fits up the same way, with the top/outside of the angle against the bottom/inside of the longerons. I was particularly concerned about how the ends of F-810 fit against the inside of the longerons, and I was happy to see that they're nice and snug. Looks like when I did that bit of fitting God knows how long ago, I made things nice and precise.

Knowing that those pieces fit up together tightly is a large part of the reason for my idea of using the midpoint of the overlap area as the lateral position of the hole; if there was a gap, then I'd need to shift the hole inboard a bit to compensate.

Using a drill block is a good idea; I'd thought about that but wondered about possible issues with the tight quarters, but much like you, I'm concerned with my ability to eyeball a nice straight alignment.
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  #4  
Old 11-11-2019, 09:35 AM
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wirejock wirejock is offline
 
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Default Drilling HS

Youre both right on. I made sure the 710 brace was nested in the longerons. Sounds like you're good there. I also used the rivets, after verifying the measurement, as one of the lines. After hundreds of measurements and markings and a call to Vans, the holes were drilled top down with a sharp #40. Then a mirror was used to look from underneath and verify the holes were where the belong. Next, drill in increments up to target dimension.
Came out perfect.
I called Vans because the edge distance math didn't work out. Just as you calculated.
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