Fin Flutter Calculations
- Brahm Soltes
Fin flutter is an important consideration when designing a rocket. It refers to an aerodynamic phenomenon where the forces acting on the fins of your rocket will cause such an extreme moment, that it will permanently deform or snap the wings, leading to a catastrophic failure. Fin flutter is a function of the fin's cross-sectional area, thickness, and Shear Modulus.
Calculating Fin Flutter
There are multiple ways to calculate fin flutter, either by hand, with calculators, or with flow simulations. It is usually best to use two different methods to ensure you are getting comparable results. Beyond that, there are a series of websites that show the formulas needed to calculate flutter, and some Excel calculators that can be used as well. These can be found below in the "Links" section.
Fin Flutter requires you to know 3 things; the dimensions of your fins, the material of your fins, and the maximum airspeed of your rocket. The airspeed requires that you simulate the rocket in OpenRocket or another simulator package. The material of your fins will be needed to determine the Shear Modulus of the material. This is sometimes supplied by the manufacturer, or for homogeneous materials it can be found on Matweb. Be careful with composites like carbon fiber or fiberglass, their Shear Modulus is dependent on the manufacturing method used and the orientation of the grain. Most people use a generalized value for these.
Be careful to keep track of units when calculating fin flutter. Different calculations require different units.
Links
Apogee Newsletter 291 "Fin Flutter"
MatWeb Physical Properties Database
Determining if Your Fin Flutter is Safe
Most fin flutter calculations give you a value of "Flutter Boundary" as a velocity. You must first multiply this value by a factor of safety (Usually 0.6 to 0.8 is used), and then compare it to your maximum airspeed. If your airspeed is greater than the value you found, then the fins will most likely shred during flight. This can be corrected with thicker fins, a stronger material, or shorter, wider fins that generate less of a moment.