So the rocket weighs 70-80lbs on the launch pad and you have 250lbs of
thrust? Do you have a thrust curve for that engine/motor?
Monroe
-------- Original Message --------
Subject: [AR] Fin stability and flutter margin
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
Date: Sat, December 29, 2018 7:44 pm
To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
I've sized the fins for my 250 lbf biprop based on common guidelines I
found but I'd appreciate a sanity check. The rocket will be 14 feet
long, 6 inches in diameter and will weigh about 51 lb empty with a
propellant load between 11 - 16 lb (10-15 second burn). I was planning
on using four aluminum trapezoidal fins with a 10 inch root chord, 4
inch tip chord, 6 inch height, and 0.125 inches thick. From the flutter
calculations in Apogee Newsletter 411, this should provide a 50% flutter
margin at burnout. Is that reasonable?
The fin mounts allow position adjustment of a few inches in 1 inch
increments but I was considering configuring them for somewhere between
2 and 3 calibers of stability. Is that a reasonable balance between
stability and tendency to weathercock? I plan to fly out of FAR on the
Baxter rail but I'm not sure what the typical winds are there.
When placing the fins to achieve a target Cp, is there a practical
difference between a taller fin closer to the CG vs. a shorter fin
farther back? I would think the aerodynamic moment would be the same
either way for a given set of conditions . Shorter fins farther back
would be preferable from a weight and drag standpoint.
I'm using OpenRocket for simulation and the results seem reasonable over
a wind range of 0-15 mph.
Thanks,
-Bob