Any tip on for bending the longerons, is the flat part of the angle, the top,the other side the outside
AND, It's not nearly as difficult as it sounds. Just take your time. When I did my first RV-6 back in 1989, I was surprised at how easy it was.If I understand your question, yes the "flat" part will be the top, the curved part will be the side. You didn't state which RV you're working on, so this is only general information.
Go slowly and don't try to do too much with each hit. Following what others had done, I used a shot-filled dead-blow hammer, but there's probably any number of ways to do it. There was a learning curve to it, but I had another person help to pull the bend into the angle from behind me while I was coaxing it (beating) with the hammer. A vice securely mounted to a workbench in front of me was used to hold the angle in place, and aluminum angle pieces were used in the vice to prevent marring from the steel jaws. And whatever you do, don't forget that one side is a mirror of the other. Ideally, when you're done, you should be able to lay the two top surfaces back to back and there should be no gap between the two (where appropriate) and the curves should mirror each other. I think Van's said the tolerance should be within 1/16" of the template and each other.
I’ve said it before and I’ll say it again… If you follow the instructions exactly, it really isn’t very difficult.
Cheers, DaveH