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F-728B hole?

s10sakota

Well Known Member
Should there be a third hole in the two F-728 pieces? These pieces get bolted on to the spar carry-through....

Notice on the pic (in red circles) there is a rivet hole in the spar box piece and the angle bracket that I had to fabricate, but there is no matching hole in the F-728 piece.



Since the F-728 is bolted on to the spar carry through (F-704?) it must be made to be able to remove it. If I drill the third hole and put a rivet through the F-728, the spar web, and the angle bracket, then the F-704 won't be able to be removed.

This is kind of difficult to explain typing, but hopefully you understand what I'm asking!

There's never any mention of drilling a third hole in this piece...

 
Mark
Just the two holes. The rivet in the angle is flush behind the support cover.
Look at drawing 11. top right "forward center section,front view" an426ad4-5 all f-704a to f-704m rivets flush head on fwd side. Hope this helps.
 
Uh-oh!!! I screwed up! I already drilled the holes to #30. I thought they got the same AN470 as the other pieces. I look and look at the plans so I don't forget anything or I don't make mistakes like this....and I still make them!

I'm not sure the spar web is thick enough to counter-sink for the larger rivets.

I might wanna e-mail Vans!! :mad:

WAIT A MINUTE! I didn't drill them the wrong size! Ok right now it's just time to step away. I've spent almost all day in the garage and my brain can't even think straight right now.

So...the holes are indeed not overdrilled...they are drilled the correct size. So, does that mean that the spar web (F-704A) gets countersunk for the rivets? Something just isn't adding up to me so I will step away for the night, and get back on it tomorrow with a fresh brain!

Thanks!
 
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Uh-oh!!! I screwed up! I already drilled the holes to #30. I thought they got the same AN470 as the other pieces. I look and look at the plans so I don't forget anything or I don't make mistakes like this....and I still make them!

I'm not sure the spar web is thick enough to counter-sink for the larger rivets.

I might wanna e-mail Vans!! :mad:

You could use a universal head AN470 rivet through the spar amd then notch the flange of the F-782B with generous radii throughout to clear that rivet.
 
I will look at it all again tomorrow. I don't think I drilled them to the wrong size. I believe everything is correct, but my brain has had enough for the night! back at it tomorrow...
 
You're good to go Mark with the #30 holes. Just make sure you use the angle as a backer when you countersink the holes so the centering bit doesn't wander.
To recap, you will have three holes drilled #30 through the web and the angle and 4 holes drilled#40 through the angle for the nutplates. All machine countersunk. So only 3 holes#30 through the web and angle plus the two holes through the web and angle for the two screws. The four rivets for the nutplates are flush on the forward portion of the angle only.
IMAGE_63.jpeg
 
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There should only be the two holes in F728B, you are correct. Those two holes are for screws. The remaining three holes, including the middle one, are #30, as you have them, and attach the angle to the spar web. Only the two screws attach the 728B. I believe you are fine. The rivet callout will tell you whether or not you need to countersink, it looks like you will at least need to countersink for that middle rivet as the 728B sits over it.

Chris
 
They are suppose to be #40

Look at drawing 11 as a AlphaBravoCharlie posted. If you look in zone 3F you can see the rivet call out. Those three rivets are attaching the F-704M web stiffner to the center section. They have to be flush so the Cover Support Rib can sit up against the center section.

You said you drilled them #40. If so, you are good to go. If they are #30 you can use AN426-4 rivets as long as you have the edge clearance on the F-704M stiffiner. If you do not have the edge clearance, fabricate another stiffiner from slightly larger stock. You made them out of 3/4 x 3/4 but you could make them out of 1 x 3/4. It will add nano ounces to the weight of the aircraft.
 
I haven't finished this step yet because I'm still confused. It's just a gut feeling I have that something isn't right, even though the plans are pretty clear here. Normally, my pattern goes like this:

1. ignore gut feeling.
2. drill/cut parts how I think they are suppose to be
3. realize I was wrong
4. order new parts...

So I don't want to countersink the spar web before I'm 100% sure I'm suppose to.

Here's a shot of the plans..



Circled in red it clearly states to use AN426AD4-5 rivets which are flush rivets. It also says that the flush head is to go on the forward side, which is the same side as the bracket.

In the pic below, I can clearly see why the MIDDLE rivet should be a flush rivet as the bracket will need to sit on top of the flush rivet. (rivet circled in red).

What I can't figure out, is why the two rivets circled in green, need to be flush rivets? Why can't I use AN470AD4-5 rivets like the other two middle brackets? As far as I can tell, nothing will go over those two rivets that make them need to be flush rivets.



It just seems to me that the spar web is too thin to countersink for a 426AD4-5 rivet. My thinking is why countersink the spar web on the two rivets (circled in green) if a AN470 rivet would work fine?

Like I said the plans are pretty clear, but it just seems 'weird' to me to use a countersink rivet when I don't have to...

Thanks again for the help and the replies!
 
With the caveat that mine is a 9A rather than 7, in my case, and I believe as well for the 7A, the lower rivet that you refer to is later covered by a piece of of the steel landing gear mount, thus the flush rivet callout. I can't see the top rivet on mine anymore as it's obscured, but it also may be underneath the steel weldment.

I am not sure if anything will be there on a 7, you would need to look ahead in the plans, but I can tell you the flush rivet is required on the nosewheel models.

Chris
 
The beginning of the Vans manual states that for an AN426AD4- rivet, .050 is the minimum thickness of the sheet metal to countersink.

The spar web is .044 making it thinner than the minimum thickness to countersink. This is why I'm leary of countersinking!
 
To make this thread more useful to future searchers, could a moderator please change the title to reference an F-782B, not F-728B? Several of the posts also refer to the part incorrectly.

The 728B is an angle that stiffens the F-728A vertical channel attached to the F-706 bulkhead, the bottom of which helps to anchor the elevator bellcrank. Just yesterday I riveted the F-728A to the bottom of the F-706, and got a bit confused when I saw this thread...
 
From a very quick look at the plans, you may be able to use 470 rivets here if it really concerns you, assuming you are building a tail dragger; a quick look at the plans doesn't indicate anything in the way. The -A model will require flush rivets here. For that reason, and since you wouldn't want to dimple the spar web, countersinking would be the only option, and I am certain quite acceptable in this location. Remember, at the very least the middle hole gets countersunk, so it isn't a structural problem doing so in this location. Maybe you'd rather slightly under-countersink the hole and then shave the rivet head?

Maybe a call to Vans will put your mind at ease?

Chris
 
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