Design/modify your fins so they generate lift. Airfoil to me always
implied airplane wing, but you are correct. If you generate lift in
the same direction on all the fins it should induce a roll without
having to cant the fins. You might be able to do this by gluing a
foam/wood/composite attachment to one side of each fin. It probably
wouldn't be the simplest method since you have to do a bit of design
calculations, but it should work. Since the optimal roll rate for roll
stabilization is mostly determined by the diameter of the rocket, you
shouldn't need adjust the lift force from launch to launch unless you
are drastically changing the speed profile, mass or the shape of the
rocket.
On Sun, Jul 17, 2016 at 7:08 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Every fin is an airfoil, by definition. You're going to have to be more
specific.
On Sunday, July 17, 2016, Wyatt Rehder <wyatt.rehder@xxxxxxxxx> wrote:
You could make your fins airfoils, glue something to the side of the
fin to have them make lift. Wouldn't need to cant the fins in that
case. Or just make your fins airfoils from the start.
On Sun, Jul 17, 2016 at 3:55 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
If you assume a spherical cow then there is no fin setup that can give
a constant roll rate other than 0 for any accelerating rocket. An
ideal fin tab or canted fin will spin the rocket through the air at a
constant "thread pitch", nominally with 0 spin rate at launch and
apogee and maximum spin at maximum velocity, though in reality inertia
would slow the spin up and spin down.
On Sun, Jul 17, 2016 at 1:29 PM, William Claybaugh
<wclaybaugh2@xxxxxxxxx> wrote:
Yep, supersonic. Still interested in a non-canted fin solution theory,
if
any one knows of one.
Bill
On Saturday, July 16, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
I imagine your rockets go supersonic, so you'd be better off with a
canted fin (or a trim fin at the end of a fin) than a trailing edge
fin tab due to the change in behavior of a trailing edge control
surface in the transonic and supersonic regimes.
On Sat, Jul 16, 2016 at 11:37 PM, William Claybaugh
<wclaybaugh2@xxxxxxxxx> wrote:
I'd be interested in a theory to describe this problem. My own
interest
is
in using fin tabs to assure spin at a specific rate (9-10 hertz),
which
appears--to me--to be cheaper and more reliable than trying to
carefully
align the fins to just the correct offset. Aligning them dead
straight
and
using a fin tab appears to be much simpler....
Does anyone know of such an analysis?
Bill
On Saturday, July 16, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Gary Cooper posted this to the FAR facebook page, though he's
flying
in northern California:
https://youtu.be/I-FhXvLtk-U
A homebuilt motor that appears quite reliable, and trim tabs on the
fins to get rid of the tiny amount of roll the rocket was doing.