A supersonic speeds trailing edge tabs can lose effectiveness. This
was a huge problem in the early days of supersonic flight, as the
planes would suddenly lose control-ability when the plane went
trans-sonic. It was found a vortex would form from the wings that
would sit on-top of the control surface making it not effective
anymore. They solved the problem by making the entire surface move
instead of just a small one on the rear end. This is why if you look
at any modern fighter jet the whole elevator surface will move.
You should be able to get around that problem and make the tabs work
depending on how you design your fins, but it is a problem to be aware
of.
Also canted fins, airfoils, and spin-tabs are all using basically the
same mechanism.
I cant remember the equation for determining the optimal spin rate for
rockets. I do remember one of the major factors in the equation was
the diameter, pretty sure this is because larger diameter rockets have
more rotational inertia and so they get a larger stabilizing force for
the same rotation rate. Though there are other factors that change the
exact spin speed you should use.
I'll find the paper that described it when I get a chance and forward it.
On Sun, Jul 17, 2016 at 7:59 AM, Zachary Martinez <znm3m8@xxxxxxx> wrote:
Zachary Martinez
Aerospace Engineering | Missouri S&T
On Sun, 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.
The airfoil idea has the same limitations as a spin tab. You can actually
think of the fin-spin tab combination as an airfoil itself. The spin tab-fin
has an optimal thread angle through the air that it will always be trying to
match which corresponds to the angle of attack at zero lift. An airfoil will
also have this optimal thread angle corresponding to the angle of attack at
zero lift. The spin rate of the rocket is always trying to approach the
point where the fins are at zero lift and the only thing slowing them down
is the inertia of the rocket. This will mean that as the rocket gains speed
the spin rate will continue to increase and as it loses speed it will
continue to decrease. The only benefit of using a "classic" airfoil is that
you would get less drag out of it as it would probably be more streamlined.
In all other ways an airfoil and spin tabs are identical.
Since the optimal roll rate for roll
stabilization is mostly determined by the diameter of the rocket,
Can you elaborate on this? I am having trouble visualizing and understanding
why this is the case.
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.