Actually "flicking" a knife open isn't technically a result of centrifugal force or gravity. It is a result of inertia. In order for something to be a gravity knife, the blade must deploy simply according to the position it is in in an unlocked position. That is, the blade slides out when held downward and the lock is released. To open a knife with centrifugal force, you would have to hold the knife and spin around until it flies open. Anybody's knives pass that test?
Don't take a lawyer into court, take a physicist! Actually a high school physics teacher would do.
NC doesn't have a blade length restriction, though people often think we do because folks import that whole idea from other places. I'm not sure if any of our cities have restrictions, but I've never heard of any. You can carry most knives as long as they are NOT concealed, unless some one wants to label it a "dirk, dagger or bowie knife". The funny thing is, even if you have a concealed carry permit for a gun you still can't carry any knife other than a "normal pocketknife" concealed. !!!??? The definitions of the things in quotes above are all contingent on case law, and I still think they have to prove intent.