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Longeron template (F-718 template and F-721 don't match)

G

goldbar

Member
So the answer must already be hidden in here somewhere, but it is not jumping out at me..
So the template for bending the longerons labeled "F-718-l and F-718-r bending template" and the curve of the F-721B-L (and R) do not match ? Huh ? Not the solid line or the dashed line, what am I missing here ? I have the bend nearly complete to the template but in looking ahead at the next step I pulled out the canopy decks (F-712L & R) and lo and behold, they differ by about 1/4" or more (eyeing it).
This has to be some misunderstanding on my part ?
 
Just a question...did you use the original drawing, or did you make a copy of it to cut up? I did the latter, using a large-scale scanner at Kinko's, *but* I held the two up together to make sure the scanner/printer did a perfect 1:1 copy.

If you made a copy and it wasn't exactly 1:1, might that be the cause?
 
Make sure you're reading the drawing carefully. I almost had the same problem on my 9A (longerons are the same) until I realized I was not aligning the canopy decks with the longerons correctly. Once I realized my mistake it turned out the longeron template made a perfect part.
 
From what others have said, the older kits had a "Not to scale drawing". You will need to match the F-721-B not the drawing.
I have a newer kit and mine was scaled perfect. I got so wrapped up in what others had said that I goofed and drilled my F-721-B in assembly in the wrong place. I had to by another longeron.
 
Do not take the plan as guide!

Hello,

As Ken Krueger told me once, do not take the plan as a guide to bent the longeron! The plan is printet on paper and the paper grow or shrinks depending on humidity, temperature and age.

Take the aft canopy deck F-721B as template, those two parts have to fit together. Maybe start with the paper template, but finish it with the aft canopy deck.

You can also check with the aft canopy deck, if the plane is accurate or streched.

Regards, Dominik
 
check the start of bend locations

Just bent mine last weekend, its a perfect match. Double check where you are placing the fwd/beginning of F-721B. It starts at 28-1/4 but the bend doesn't start till 38-7/16.
 
Thank you all for your posts.. It makes perfect sense to match the bend to the F-721, it jast had me concerned that the 721 and template were not the same, I'll finish my bends with the 721.. I appreciate all of your comments and help.
 
Sorry but I'm not convinced that real comprehension has occured yet, Goldbar.
No insult intended. I remember looking at it incorrectly in the beginning too.

The curve shown on the plans only PARTIALLY resides within the 721 deck as Jeremy and another said. See dwg 18

Certainly, you can use 721 as the final arbiter of shape, but make SURE you have it located on the longeron in the correct spot before "correcting" the curve.

My template on dwg 17 and and the 721 placed per dwg 18 matched spot on.
 
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No insult taken rzbill... And yes, you were correct, real comprehension occurred about ten minutes ago, when I realized I had drilled the 721 decks, to the wrong place along the 718 longeron. Somehow, thru this whole process it did not register that the front of the bend, listed as 38 7/16ths, is different than the front placement of the 712 deck, which is listed as 28 1/4. Many kept saying, (my interpretation of what they were saying) match the bend to the 721 deck, but in reality the bend continues rearward and beyond the 721 deck, as the deck is placed at 28 1/4 and the bend starts ten-ish inchs back from that and continues beyond the deck. Anyway, full comprehension has been expensive today. Wonder if I can find this stock locally ?
 
I had kept looking at DWG-17, where the template was located, had I taken a closer look at DWG-18 and the placement of the 721 deck, perhaps some lightbulbs would have gone off... It probably did not help anyone in the know, tat my phrasing of the question was not so clear.
 
Sorry to hear about your misfortune with the $$ longerons. Be careful looking locally. Ensure that whatever you find is both the correct material (6061-T6) and the correct dimensions (including the radius in the middle of the "L"). Many cheaper extrusions such as those found at your local home improvement store do not have the radius, and thus are not structurally sound for use as primary structure in airplanes.

If you were closer to KS I'd gladly offer you my extra piece of longeron stock, which I ordered with my wing kit in case I screwed up a longeron. Luckily I didn't need it but I'm betting at some point in the future someone else will.
 
I spent a couple of days trying to figure the best way to get a new longeron after I goofed mine up. I had almost hit the order button a couple of times but I couldn't stomach the freight. Found a supplier within 3 miles of the house that had it for $16. Look local but yes it must be 6061-T6 3/4"x3/4".
 
Ken, builder support, seemed to think all was not lost, and he had several valid points. Checked w a local metals supplier, and yes, score ! A new start, correct size, material, inside radius, a perfect match, mine for only $28.10.
Wonder how many more of these lessons I'll need to learn before this project flys ?
 
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