Thorp Air Command - T18.net

Hi all I'm chasing advice re the longerons.
The project I am working on came with the 580 -3 ( the long 3/4*3/4" ones ) and and -5 the 1"*1" longerons. they were partially bent and pre drilled with 3/32 pilot holes.
That would have been be good except unfortunately the holes in the longerons were not perfect and are not aligning up with my side skin templates. I'd prefer to go with the rivet pattern from my side skin as the skins will be machine cut with the hole pattern made per plans to high accuracy. The problem is that about 25% of the rivets would end up going into a holes that are not perfectly aligned from the longer on to the skin and that probably isn't going to be good enough.
I think that I have a few choices.

1. Live with it and accept that some holes will not perfectly line up, just drill them out to use larger 5/32" rivets and that should hopefully fix most of the variation but will leave me with about 5-10% of rivet holes that won't meet spec
2. Shift the longeron forward or backwards by half the rivet spacing and drill all new holes, that would mean that the majority of rivets would be going into good holes and meet spec however it would leave the longerons with double the number of holes in them half of which would be unused although they would only be 3/32" pilot holes. This would make the longerons slightly lighter which is good but but what is the effect on its strength? Doing it this way would leave me with only a couple of rivets going into problem holes where there may be 2 holes right next to each other.
3. I could replace the longeron or splice in a new section where the holes are worst aligned. I can only get up to 12FT lengths of the 3/4" angle in Australia. Completely replacing the main longerons with a single length would be an expensive option. So splicing in a shorter length might have to do but where would I place the join and how would I make the join.

Are there any suggestions or other options out there?
Thanks
Stuart

If the holes are off by half a hole, you can clean up the #40 hole to a #30 and continue. Or as you suggest drill out to a #5 rivet and to make it look like a 4 rivet was used, buy a MS rivet as it will have a 5 shank with a 4 head and nobody will know the difference. The guys around the states call them "cheaters"
I would not be afraid to use option two but would not use the option three splice although I saw a plane it had been done on.

You said 'slightly bent'. Can you straighten and switch sides. Put the holes up? Then rebend.

The root of the problem maybe that the longerons were "pre-drilled." I learned quiclky with my project that even though the parts were "pre-marked" and should have lined up using the machined holed tooling method when drilled, this was not the case. About 2% of the holes did not match up.

As Bill suggested I would go with the "cheater rivets" and make life a whole lot easier.

RB

Most of us do not prebend the 3/4" angles. They are riveted on flat and bent around as you cleco up the fuselage. This tigtens up the skins and makes for a great line of rivets. If the joggle at the rear stab fittings has not been done, I would consider unbending them and swapping sides. The #40 holes in the flanges should not be a worry.

_________________
Bob Highley
N711SH
SN 835
KLAL

Thanks guys, good ideas
I have heard of the cheater rivets before but haven't really considered them it might be worth getting some of those, I may well need them for other areas!!
In this case though a reasonable number of the #40 rivet holes are almost 100% offset so rather than using cheaters going straight for the -5 rivets along the whole length of the longerons might be the better option from both the cosmetic and structural points of view? At least on the 1*1 longer one where I am not worried about edge distances.

Having said that the idea of straightening them out swapping sides and re bending hadn't occurred to me, unbending doesn't usually go well, but if it can be done then I like the idea. None of the joggles have been put in yet and I have a bit of extra length to play with.
When relaxed the existing bend in the W42 longerons is only about 3.5" in the centre from straight. Just with it's own weight the bend straightens out to about half of that so a bit of gentle pressure might just bring them back to straight. Assuming I can straighten them out without introducing too many other problems then how concerned should I be about the effect on the strength or causing early fatigue in the angle?

From my reading of the old newsletters the I thought that putting the longerons on flat and then bending had the potential to pull the skin too tight and I thought I had read one report where it actually flattened the skin between frames a little. I planned to putting some bend on the longerons then rivet them to the skin before then attaching the skin to the frames.

Rich I think you are right, a number of other parts I have in the project are pre marked but when I check dimensions against plans I see a number of the centre marks are slightly off. If you were to drill them them without lining them up first then you are going to end up with some mis alignment. In my case it is compounded when you have one part cut from a CNC machine and you try and match it up to what is obviously a manual marked and drilled part.
Stuart

Most of my parts came from Ken Knowles or Phil Tucker and the were "center punched ." As a result during the "early years" of building I found out drilling both the skin and rib holes W/O fitting the parts in place left me with several "misplaced" holes ! I placed all my hope in "matched hole tooling !" After the first few FUBARS, I drilled the skins that were pre punched and marked the prepunched hole locations on the ribs with a red sharpie marker. Starting at the nose of the rib I found the first holes lines lined up. Drilled and clecoed those holes and then pulled the skin over to the next set of "red marks." Some were slight off, but after drilling with #40 and clecoed, then drilling with #30 all was well with the world ! Live and learn !

RB