Longeron and F-770 forward side skin fit

Hi All,

I'm just finalizing things before I drill my longerons to the side skins (yes, some nervous times). The left side was about smack on, and the right side needed tweaking a little, which I did by setting up a temporary clamping mechanism and using a screw jack so that I didn't have to remove it from the fuselage. I've got tolerance town to about 3/64" between the top of the skin and the top of the F-770 just after the bend point, which can be reduced to about 1/32 if a small amount of preload is put on the longeron when I drill and insert the clecoes. Given that we are trying to follow a shallow but sharp bend in the skin profile with a thick piece of angle, I am assuming that a 1/32" tolerance is ok. If not, let me know and I'll pull it out and do some more whacking with the hammer.
In light of the fact that there will be a curve transition, I am assuming that I should hold the longeron flush with the F-770 skin aft of the bend point, and let it bow outwards this 1/32" or so once it is forward of the bend, before pulling it back flush again as soon as possible. Is this train of thought on the right track?
Cheers,
Tom.

EDIT: Don't use this clamp in-situ procedure for adding to the sharp downward bend. Chock the fuselage and remove the stringer (see post #9 of this thread). It is easier.

Longeron

Mine was flush. If you allow it to creep above the skin, watch the edge distance on the hole.
There's also a twist at the forward end. It needs to be right to fit the firewall brackets.
Beat it up some more.

You crush me Larry. I see getting the longeron out when upside down is not going to be easy. I think I'll have someone to track me down to help me flip the canoe so I can access these things. That said I still struggle to see how you can make such a precise bend at an exact point without inducing significant stresses at such a bend point. When I reviewed the plans again, it says:

"Check this bend by using the F-770 forward side skin as a template. The angle should match the upper portion of the skin within about a quarter of an inch."

1/4" seems like a massive tolerance to me, especially when they state that the shallow curve must fit the template within 1/16". With 1/4" gap, I don't see F-721A lining up adequately. 1/32" on the otherhand...
Tom.

tgmillso said:

You crush me Larry. I see getting the longeron out when upside down is not going to be easy. I think I'll have someone to track me down to help me flip the canoe so I can access these things. That said I still struggle to see how you can make such a precise bend at an exact point without inducing significant stresses at such a bend point. When I reviewed the plans again, it says:

"Check this bend by using the F-770 forward side skin as a template. The angle should match the upper portion of the skin within about a quarter of an inch."

1/4" seems like a massive tolerance to me, especially when they state that the shallow curve must fit the template within 1/16". With 1/4" gap, I don't see F-721A lining up adequately. 1/32" on the otherhand...
Tom.

Click to expand...

Could the 1/4 in refer to the location of the bend rather than the edge error?

1/4 inch is for the initial bend/fit. It is assumed that it can be forced into position as long it is fairly close.

I started from the back clamped it flush for 1 to 2 feet and then drilled.
I started getting into some problem at F-705, there were not enough to clearance to the bulkhead so I had to trim the bulkhead top in order to make it flush.

The second area was the bend downward at F-704. One side was perfect, the other was slightly off. I had a supporting sawhorse at F-704, so I took out some clecos, clamped the longeron to the sawhorse with blocks of wood and adjusted the bending (the longeron now being inside of the F-704) until it followed the curve of the sideskin. At the point I also supported the F-704 bulkhead and the F-705 for stability.

Same method was used to fine adjust the twist into the engine weldement.

Alright, I may have one more go at bending it in position, because it's going to be a dog to get out.
T.

I'd check with the mother ship.

Larry's came out great, congrats dude! But it may not need to be so perfect

Mine's probably a 64th off at the worst point. Remember, you'll be mounting the F-721B canopy decks on the other side of the angle, and you'll be filing the outboard edge flush with the side skins. A little gap (given that edge distance is OK) will be easy to fill and make disappear.

Now, if you're making a tipper, even 1 mil error will completely screw up how the whole thing fits, and you'll get scoops, cracks and all manner of horrors (or that what it seems to me while a cajole my tip up frame into proper alignment) ;-)

Method

nilberg said:

1/4 inch is for the initial bend/fit. It is assumed that it can be forced into position as long it is fairly close.

I started from the back clamped it flush for 1 to 2 feet and then drilled.
I started getting into some problem at F-705, there were not enough to clearance to the bulkhead so I had to trim the bulkhead top in order to make it flush.

The second area was the bend downward at F-704. One side was perfect, the other was slightly off. I had a supporting sawhorse at F-704, so I took out some clecos, clamped the longeron to the sawhorse with blocks of wood and adjusted the bending (the longeron now being inside of the F-704) until it followed the curve of the sideskin. At the point I also supported the F-704 bulkhead and the F-705 for stability.

Same method was used to fine adjust the twist into the engine weldement.

Click to expand...

Perfect? No way. I just worked it till it matched both the side skin and curve of the aft deck.
That said, I think the method above is a good way to fine tune it on the ship using lots of wood shims to protect parts, supports and a huge crescent wrench to massage it into submission.

Lifting fuselage for F-718 upper longeron removal

Hi Guys,
So I checked in with Van's regarding how closely the longeron should follow the F-770 forward side skin forward of the sharp downward bend, and Scott Risan said that 1/32" was quite adequate. Anyway, I went ahead and began drilling mine from the tail forward per the instructions, and when I got to the "sharp downward bend", it was now off by 1/16"!!! I was drilling then 100% clecoing, and I suspect that holding it tighter to the skin resulted in the downward bend now being observed as inadequate. Thus, I had to remove and rework. Working alone and with no one to help me pull it all apart and flip the canoe over so that I could extract the longerons, I was at a loss as how to get these things out, and the VAF forums and builders websites I checked were turning up no hints. I figured I couldn't pull them out in situ, as there was so much load on the sides and nothing else supporting them. After blankly gazing at the structure for what seemed like an eternity, I came to the conclusion that if I supported the structure on one side at the F-704 bulkhead and picked up the tail using the tailwheel spring, then I should be able to pull them out one at a time by myself. I cut a piece of 4x2 and wedged it between the seat floor at F-704 and the top of my sawhorse (aka a table with adjustable threaded rods drilled into the legs for leveling purposes) making sure that it fit flush against the seat floor so that there was no risk of bending the flanges on the F-704 or the seatfloor itself. This lifted the forward left side about 2", then I picked up the tail and placed it on my tall sanding table. I then unclecoed the fuselage at the F-706/7/8/11/12 bulkheads up to the upper fuselage J stiffener, and with everything else intact on the other side, the fuselage remained perfectly stiff. The longeron was then removed from the forward end working aft, and in all, it was as easy as pie to get out.
It then got clamped back in the vice and got the beating it deserved, then back in (ok, in and out a couple of times to get it perfect, but that's now an easy process). Now it is within 1/64" at the "sharp downward bend", which although may not be at Larry levels of perfection, it is close enough for me.
I know this procedure may be old hat for many of the old (or more creative) hands, however chocking the fuselage up like this saved me a lot of time and frustration. Hopefully when clowns like me do a google search in the future for "removing F-718 upper longeron removal" this helps them out.

I would check this fit good, with the twist Larry mentions, I would mock up the firewall and engine mount brackets to make sure, because if this is not right, you may have a hard time fitting the top forward skin......down the road.

Tip

Tom
Post that one to the "Tips" forum. That's pretty creative.

Good point Larry. Will do.
Bret, I checked the twist using my Stanley Fatmax digital level (my new favourite toy) and I had to tweak it a few times, but it's now within 0.4 degrees of target which I'm hoping should suffice. The instructions are right that the secondary effects of bending can be frustrating. Every time I would add a fraction of bend, my twist would come undone and vice versa. I will however add this to the instruction in the tips section, as it is a very valid point.
Tom.