My fins are typically so-called "hexagonal" shaped: symmetrical on each side
and with a 7.5 degree taper on each side of the leading and trailing edges, the
center being flat and with a rounded leading and trailing edge. They are also
tapered from the root--typically by a factor of four (a 0.75" root tapers to
0.188" at the opposite side)--halving the weight as compared to a fixed
thickness. This means that they have no right angles and are accordingly
difficult to fixture for machining; this is where a very high end shop can
price well below a conventional machine shop...and below the cost of my doing
it myself. It sometimes turns out that the very high end is the optimal
solution....
Making these fins asymmetric between the two sides--to create differing
lift--seems to me a hard problem: the taper of the leading and trailing edges
is set by hypersonic aerodynamic experience; given a fixed fin shape, this
makes changing the shape between sides difficult. In principal, I could use two
fins that are slightly differently sized compared to the other two--and I think
that an elegant solution--but reducing the production run increases the cost....
Happy to consider alternative ideas about how to induce asymmetric lift w/o
increasing cost above the cost of some sort of tab.
Bill
Sent from my Commodore 64
On Jul 17, 2016, at 11:44 AM, Wyatt Rehder <wyatt.rehder@xxxxxxxxx> wrote:
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.