Tailcone attach question, Page 32-4, step 8

[Aiki_Aviator]

Hi All, I am wondering if someone could clarify an instruction in the plans relating to the QB fuse and its attachment to the tailcone.

Step 8 states:
"Remove the F-1074 Tailcone Forward Topskin. {OK, got that} Machine countersink the holes in the F-1070-L and -R mid-side skins and the F-1046-L and -R Mid Fuse longerons as instructed in Figure 3.

Now I understand the countersinking the Shim, and even the 1 hole under F-1074 on the F1070-L/R skins ready for dimpled top skin (F-1074), however, how can I get to the longerons under F1070-L/R to both countersink and then dimple??

Is that a mistake including Longerons in the countersink list, (i.e. is it countersinking the skin all the way along to the existing QB rivets??)

Am I missing something simple??

Items on the attached pictures:
A - Rivets already existing in the QB Fuse into the longerons (already countersunk.
B - Holes, I believe, needing to be countersunk in the longerons and dimpled in the skin F-1070-L/R.

C - 3 holes needing countersinking ready for skin dimples
D - Holes, I believe needing to be countersunk (longeron) and dimpled (skin)

If this is correct, how do I get at the Longerons to countersink? Do I need to drill out a whole lot of rivets??

Any help greatly appreciated.

 

[Phil]

I'm going back a couple of years but I think the plans are right. You countersink through the skins and into the longeron. IIRC the idea was to allow the skin to slip and slide into position so everything would line up and not have tension/oil canning.

If you countersink the longerons and dimple the skin the skin will naturally rest in the csnk's. With this method you put it all together and get the skins to fall in a natural position; then you csnk through them.

It's been about 2 years since I did it, but I'm pretty sure that's how it all played out.

 

[Aiki_Aviator]

I'm going back a couple of years but I think the plans are right. You countersink through the skins and into the longeron. IIRC the idea was to allow the skin to slip and slide into position so everything would line up and not have tension/oil canning.

If you countersink the longerons and dimple the skin the skin will naturally rest in the csnk's. With this method you put it all together and get the skins to fall in a natural position; then you csnk through them.

It's been about 2 years since I did it, but I'm pretty sure that's how it all played out.

Hi,

When you say "You countersink through the skins and into the longeron." Do you mean countersink the skins so they accept a rivet, and if they slightly touch the longerons, so be it??

 

[Phil]

Exactly. Hopefully someone who has done this recently can chime in, but I'm pretty sure that's how I remember doing it.

Fiberglass makes you forget all about the earlier parts of the build.

 

[flion]

I'm working off the scanned plans here, not my actual ones in the hangar, but the illustration on that page (pictured below) clearly shows what rivets get what treatment.

 

[Aiki_Aviator]

I'm working off the scanned plans here, not my actual ones in the hangar, but the illustration on that page (pictured below) clearly shows what rivets get what treatment.

Hi Patric,

Yep, I have that diagram, however the confusion arose when noting Step 8 on that diagram (i.e. top right), where it refers to countersinking the Longeron. (i.e the three holes). Unfortunately on the aircraft the "Longerons" are not exposed at this step in the process. Therefore making the diagram suspect.

Then given that written step 8 detail ALSO refers to countersinking the Longeron where none of the Longeron was exposed, I guess I was making sure I did not damage the entire skin with an assumption.

Could you imagine needing to get new side skins at this point and reattaching because you countersunk rather than dimpled, based on the assumption that they just mixed up the instructions and mistakenly inserted "Longeron" when they did not mean to???

Lets face it, the longerons all the way to here have been countersunk and the skins dimpled. Why stop right here??

Sorry, I saw that as confusing and so did 4 other people helping me the other day.

Not so clear" I believe.

Any other comments from people on this issue?

 

[civengpe]

Hi Patric,

Yep, I have that diagram, however the confusion arose when noting Step 8 on that diagram (i.e. top right), where it refers to countersinking the Longeron. (i.e the three holes). Unfortunately on the aircraft the "Longerons" are not exposed at this step in the process. Therefore making the diagram suspect.

Then given that written step 8 detail ALSO refers to countersinking the Longeron where none of the Longeron was exposed, I guess I was making sure I did not damage the entire skin with an assumption.

Could you imagine needing to get new side skins at this point and reattaching because you countersunk rather than dimpled, based on the assumption that they just mixed up the instructions and mistakenly inserted "Longeron" when they did not mean to???

Lets face it, the longerons all the way to here have been countersunk and the skins dimpled. Why stop right here??

Sorry, I saw that as confusing and so did 4 other people helping me the other day.

Not so clear" I believe.

Any other comments from people on this issue?

I am glad you asked this question. I am about to turn the page in the plans to this section and was unclear about this as well.

 

[paul330]

IIRC, these holes get match-drilled after the skins are fitted because of the extra metalwork associated with the door frame. Because of that, you can't get at the underlying longerons to c/s them to accept a skin dimple. Therefore, you c/s the skin instead.

Again, IIRC, the minimum skin thickness for a 3/32" c/s is 32 thou so you are on the limit. Section 5 of the build manual shows an example of countersinking a skin sandwich where you slightly go through to the second layer. You just have enough skin thickness, so use care not to over countersink. But if you remove a little material from the longeron, that's OK and the hole in the longeron keeps the bit centred so you don't get a mis-shapen hole in the skin.

 

[Phil]

Yeah. That's it exactly.

 

[Aiki_Aviator]

To ALL,

Thanks very much. Yet again, it has been invaluble.

It is a great group of people able to share information and experiences in a positive and supportive way. I love the community feeling to this forum and wish it were more popular in all other walks of life.

Best wishes to all of you.

 

[flion]

the confusion arose when noting Step 8 on that diagram (i.e. top right), where it refers to countersinking the Longeron. (i.e the three holes). Unfortunately on the aircraft the "Longerons" are not exposed at this step in the process.

I guess I can understand the confusion, though it made perfect sense to me as I constructed the mid-fuselage (slow build). Because the longerons are not pre-punched and yet they have to mate closely at the tail cone joint, you can't simply match-drill them to the skins ahead of time past a certain point (those last 20 rivets or so) without giving up all your adjustment. Yet, once the skins are on, you can't countersink behind them, so you have to countersink the skins once the match-drilling is done. Those last three rivets are an exception, you can still dimple the top skin (F-1074) but you have to countersink what's under it. Maybe one day Vans will pre-drill and pre-bend those longerons and then we can countersink and dimple in a more consistent fashion. In any case, I have no problem with you asking the question and sorry if it sounded like I thought it was a dumb one; I was just pointing out where the instructions spelled it out.

 

[Aiki_Aviator]

I guess I can understand the confusion, though it made perfect sense to me as I constructed the mid-fuselage (slow build). Because the longerons are not pre-punched and yet they have to mate closely at the tail cone joint, you can't simply match-drill them to the skins ahead of time past a certain point (those last 20 rivets or so) without giving up all your adjustment. Yet, once the skins are on, you can't countersink behind them, so you have to countersink the skins once the match-drilling is done. Those last three rivets are an exception, you can still dimple the top skin (F-1074) but you have to countersink what's under it. Maybe one day Vans will pre-drill and pre-bend those longerons and then we can countersink and dimple in a more consistent fashion. In any case, I have no problem with you asking the question and sorry if it sounded like I thought it was a dumb one; I was just pointing out where the instructions spelled it out.

No problems. All good.