Hi folks,

I´m about to bend the H-tail feather spars to my plans built 750. The manual gives me a dl for the stabilzer front spar blank of 141mm while the flanges are 20mm and web 107mm. As I suspected the dimmentions didn´t add up when I ran a test piece. I ended up 3 mm short on the web after compensating 1mm for each of the flanges. The next time I tried, I had the web + flanges -2mm compensation for the bend radii (107+(2x20)-2=145 dl) now I am 1mm on the plus side on the web. All I need to do now is reduce another mm and I´m good to go. But I can´t help but wonder if I'm missing something or if the developed length is wrong? Any thoughts? I´m no engineer so if I had to guess I´d go with the first option.

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Comment by Timothy Aanerud on December 12, 2012 at 10:43pm

It takes time to get to know your sheet metal brake.  There are formulas for determining developed length.  I googled "developed length" and found some formulas.   They do work, but I've found it is simpler to just cut a few test strips of metal and bend them and then make adjustments until you get the desired size.  Flaperon spars have given me a fare amount of trouble.  It took 10 tries before I had 4 spars that were within spec. :-(

Comment by Ralph E. Wishart on August 7, 2012 at 3:18pm

Thanks Clint,
I made the spar after the web height. It worked allright.

Comment by Clint L Gosch on August 5, 2012 at 2:58pm

I struggled with that as well on that assembly and others. I came to the conclusion that it really matters how you measure the web, I interpreted the plans that the 107mm is the overall distance from top of flange to bottom of flange, in this case you have the bottom flange at an angle, this complicates things.  As I recall, my spar web was about 2mm short.  I finally decided to build the form blocks for 75T1-2 and 75T1-3 to see how everything fit together, the ribs fit fine.  I do want my project to be built as "perfect as possible", but I would like to fly my project before I have grandchildren, with that in mind I'll have to settle for "built as safe as possible".  I do not think 2 or 3mm will affect safety as long as the other parts fit well.  So far I have found that scratch building, even with very good plans, is about 50% tool and parts fabrication and 50% fitting and fretting.  If you are in anyway uncomfortable I would recommend calling Caleb or Roger, as you probably know they are great resources.  Anybody with more/different experience, please chime in!!

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