ACK! countersinking rudder trailing edge??

prkaye · #1 10-21-2006, 03:47 PM

ACK! I think I have ruined my rudder trainling edge strip (that thin wedge). The plans say to machine countersink it. So I did one side, no problem. Then I did the other side... the part is so thin (especially near the front of the wedge) that when I countersink both sides, the holes end up enlarged (and ugly) !!

Question 1: I guess I'm countersinking too deeply??

Question 2: when I get the new wedge I'll have to match drill it to the trailing edge of my rudder skins. Problem is that the skins are already dimpled... can I just match drill through the dimpled holes??

Question 3: I ruined a rib as well.. same question, can I match drill a replacement rib through already dimpled skins? I REALLY don't want to have to get new skins and re-do everything.

Question 4: if you dimple a part the wong way (hypothetically speaking, of course lol), is it OK to just re-dimple it back the other way, or will this fatigue the material around the dimple too much?

lorne green · #2 10-21-2006, 04:36 PM

Phil:
You may not have c/sinked too deeply. I just finished the t/e wedge on my 7a. When you c/sink the one side it looks fine but c/sinking the flip side you enlarge the hole to what appears to be too large a hole.........remember the two dimpled skins fill the counter sunk holes. The rivet shank will fill the space as it expands against the skins and then to the wedge. (Unless you've really gone to town on your depth!) If you c/sink too shallow your rivet heads will stand proud of both skin faces. The epoxy/proseal method of bonding the t/e before riveting will ceretainly help bond the three pieces together.

algrajek · #3 10-21-2006, 04:54 PM

I agree with Lorne. Unless you have gone WAAY too deep, you will be fine. And, yes you can match drill through the dimples. But carefully, and you can re-dimple the other way, but that depends on how many holes. A few here and there should be OK.
Al

prkaye · #4 10-21-2006, 05:11 PM

so the way to check the depth of my countersinks is to put a rivet in there, right? see if the rivet head sits flush...

This proseal stuff... can it be purchased at a hardware store, or is it also aircraft specific?

captainron · #5 10-21-2006, 05:32 PM

The way to check the depth of machine countersinks is to make a gauge out of the dimpled skin of the same thickness that will fit into the countersunk hole. Get a piece of scrap skin the right thickness, drill and dimple it, and test fit it into the countersunk hole. You should make the countersinks shallow, then increase the depth until your "test gauge" just fits and the two parts touch together with no gap.
You can use JB Weld here if you're not ready yet for Proseal.

prkaye · #6 10-21-2006, 05:42 PM

Thanks guys! Another mistake I made was using my air drill for countersinking... going slowly with a cordless drill sounds like a much better approach.

I am a bit concerned about match-drilling through already-dimpled skins. When I get my new ribs, would I be better not to match-drill these, but just to drill out the 3/32 holds directly in the ribs without clecoeing to the skin? Just rely on the accuracy of the positioning of the pre-punched holes to get an adequate fit with the skins?

captainron · #7 10-21-2006, 05:52 PM

Just finish drill to #40 or #41, dimple and cleco together. Don't match drill.

Stephen Lindberg · #8 10-21-2006, 10:04 PM

Quote:

Originally Posted by captainron

The way to check the depth of machine countersinks is to make a gauge out of the dimpled skin of the same thickness that will fit into the countersunk hole. Get a piece of scrap skin the right thickness, drill and dimple it, and test fit it into the countersunk hole. You should make the countersinks shallow, then increase the depth until your "test gauge" just fits and the two parts touch together with no gap.
You can use JB Weld here if you're not ready yet for Proseal.

I recommend against machine countersinking to such a depth that a dimpled test gauge will fit flush in the countersunk hole. This is too deep and will result in a very large diameter hole. Countersink only to the depth of a AN426 (flush) rivet or just a few thousanths more. Consider this: riveting together two dimpled skins is essentially the same situation as I have just described. The lower skin dimple is no larger than a flush head rivet and it receives the dimple of the upper skin.

captainron · #9 10-22-2006, 09:54 AM

Lindy, I went out to the shop and made some test samples, since I couldn't remember exactly the method I used on my rudder AEX wedges. The best results I got was using the method you suggested; I dimpled some .025 test pieces and countersunk a piece of .125 aluminum angle to a depth where the rivet head is .004 below the surface. They riveted together nicely. The rivet heads are about .034 thick, plus an extra .004 puts the depth of the countersink at about .038". I don't have the thickness measurement for the AEX where the drilled holes are, but it looks to be about .080", which when countersunk from both sides, would leave a tiny remnant of the original hole size. Good call!