Fuselaage welding

I am building up a PA14 fuselage (2+2) from plans. I have noticed that the upper longerons are slightly bowed inward after the clusters have been finish welded. Should these be straightened or left as is? What is the best way to straighten them?

That is something not abnormal. You can cut wooden sticks to temporary bow out longerons to avoid this. Just about a 1/2" bow out will do it,when welding is complete longeron will come back straight. To straighten existing bow out gentle use of a rubber mallet will do it,(eyeball it with some type of straight edge) but gentle is the operative word here.

Don't hit it in one place with the mallet. Put a 2x4 along the tube (so you have a long surface to surface area) and hit it out. You may put a kink in it by hitting it in one place, and cause a stress riser.

I finally decided to straighten my longerons after welding. I took some aluminum and hardwood blocks split them lengthwise,clamped them together, and then drilled 3/4" holes lengthwise. I then place one half on each side of the longeron tube and clamped a long stiff square tube to the outside. I repeated the clamping on the other end of the square tube beyond the area I wanted to bend. I then used another block half on the inside of the longeron where I wanted it to be pulled from. I then used a big C clamp to pull the block and longeron out towards the external square tube. It took quite a bit of force and some overcorrection but the end result looked good. You need to leave some room between the external square tube and the longeron to allow for the overcorrection springback situation. Try pulling a little at a time in different locations to get it just the way you want it. I found some near the ends and some in the middle seemed to work best. The drilled blocks helped to prevent collapse of the longeron tubes at the pressure points.