The flutter velocity performed for the “Don’t Debate This” high power rocket was incorrect because the user did not specify the correct supersonic lift slope (CN_alpha) and center of gravity (CG) location for the fins and other errors to be discussed. When proper inputs for airfoil lift slope and CG location are input, the resulting flutter velocity (UF) of the rocket is determined to be Mach 3.29 and the divergence velocity is Mach 4.30. Therefore, the statement on page 3 of "Taming the N5800: Don't Debate This" that "FinSim predicted the potential onset of flutter and divergence well below Mach 3” is totally incorrect. Also, the user input the incorrect value for pressure (P) at the point of maximum Mach number. The user assumed a simulation altitude of 4K feet that is the altitude at the launch point in the Nevada desert. Actually, the correct simulation altitude should be 11K feet or the altitude above the launch point at which maximum rocket velocity occurs during launch.

Table-1 presents a comparison between flutter velocity methods discussed and the Ug method used by FinSim. Notice the other methods that rely on simplified equations over predict flutter velocity by a wide margin compared to FinSim and should only be used as estimates.
 

UF METHOD (Use Table-2)	FLUTTER VELOCITY (UF)	DIFFERENCE
FinSim_Ug	Mach 3.29	NA
NACA TN 4197 equation	Mach 3.89	+ 18.2 %
POF 291 equation	Mach 5.50	+ 67.2 %
Table-1, Flutter velocity computation methods compared to the FinSim Ug method

Use the values displayed in Table-2 to populate the FinSim main screen and fin geometry screen for performing a flutter velocity and divergence velocity analysis. Make sure to select NACA TN 4197 Method under the CN-alpha pull down menu to automatically specify the aerodynamic center (AC). Also, to avoid a common mistake be sure to input the correct values for fin elastic axis (AE) location and fin center of gravity (CG) location. Please refer to the screen shots below for performing a flutter velocity and divergence velocity analysis. Use a similar procedure for other supersonic fin flutter velocity analyses.
 

FINSIM INPUT DESCRIPTIONS	VALUES	NOTES
Semi span (b)	4.0 in	Fin input geometry screen
Root chord (cr)	12.0 in	Fin input geometry screen
Tip chord (ct)	2.25 in	Fin input geometry screen
Sweep length	9.75 in	Fin input geometry screen
Fin thickness (t)	0.195 in	Fin input geometry screen
Maximum body diameter near fins	4.0 in	Fin input geometry screen
Fin Materials		Aluminum, main screen pull down
Shear modulus (G)	3759398 psi	Fin material, aluminum
Elastic modulus (E)	10E6 psi	Fin material, aluminum
Poissons ratio	0.33	Fin material, aluminum
Material density	0.098 lb/in3	Fin material, aluminum
Rocket simulation altitude (P) @ max Mach	11,000 ft	Pressure, main screen pull down
Supersonic Airfoil Lift Slope		CN_alpha, main screen pull down
Aerodynamic center location (A.C.)	0.50	0.5 for supersonic, main screen
Aeroelastic input variables		E.A. and C.G. main screen inputs
Elastic axis location (E.A.)	0.50	Attachment centroid, main screen
Center of gravity location (C.G.)	0.658	Measured or computed, main screen
FinSim Results
Torsion-Flexure flutter velocity (UF)	Mach 3.29	See Figure-5 and Figure-6
U-g Torsion-Flexure divergence velocity (UD)	Mach 4.30	See Figure-5 and Figure-6
Table-2, FinSim input variables for performing supersonic flutter velocity analysis

The following screen shots are intended to help FinSim users predict supersonic flutter velocity and divergence velocity simply and easily. Use the values presented in Table-1 to perform the flutter and divergence velocity analysis.

Figure-2, Determine elastic axis location (xea = Xea/2b) using the Excel spreadsheet