The keys to good broadhead flight include not only tuning your bow, but tuning your arrows. Spine should be adequately stiff. Borderline spine is not so good with broadheads. Spine testing your arrows is very beneficial if you can do it. Making sure you have adequate F.O.C. will help. I like to keep it over 10%. Putting a helical fletch on your arrows will help a lot also.

Combine all this with a well tuned bow and you'll have great flying broadheads.