There are a LOT of variables to that question - first two would be the size and shape of the fins. The larger the semi-span, the more prone to fracture due to flexing. Conversely, the shorter the semi-span, the more likely they will survive. Generally full tip to tip fiberglass or carbon is considered normal for birds that approach mach.

Warren

Thanks Warren. That is the type of info I was looking for. I am thinking keeping the speed to .85 mach maximum and then building another rocket fully glassed for speed.

Actually, finding the speed of cardboard or plywood or fiberglass or carbon fiber is a very worthy endeavor and I'd do that if I were you. But from a safe distance, of course. Let 'em fly!

Safe distance?...you mean I can't use a lighter and light the fuse?

Fin flutter is a real interesting problem. If you'd like, come by my EZ-Up (probably next to the admin EZ-Up on the flight line) and I'll show you some fins meant to handle Mach 2 or better.

Warren

Ok, anyone have a estimate for the safe speed of 1/4" wood fins? A few of the engines I am looking at for OF will push it just about mach. The only reinforcement on them is f/g on the motor mount and on the external fillet. I am looking to find what the largest motor I can safely fly for altitude. This will be for my extended Magnum.

Just try it, (what ever motor) and let us know how it worked!
Maybe we will know what not to do next time. 😉