Richard:
Thanks for the offer, you are consistently generous with your knowledge.
I don't use fin cans--I attach the fins via a countersunk screw from the inside
of the motor / airframe tube--which generates my search for an analytical
understanding.
I happen to have handy an assembly technique that allows very (laser) accurate
alignment w/ the central axis of the tube but is not suitable for laying on
milli-degree offsets. Thus my interest in an "add-on" solution.
I'm always interested in a good shop; I generally use a guy in Colorado that
Kevin Sages used for his projects and who builds a lot of the parts for THADD,
among many other very top end machining projects. For standard stuff I find
eMachineShop.com suitable.
Bill
Sent from my Commodore 64
On Jul 17, 2016, at 12:56 PM, Richard Hagensick <r.hagensick@xxxxxxxxx> wrote:
Don't know if attachments work. If not we used "Handbook Development for
Astrobee F AIAA-25865-264. Our BALLS rocket this year required a cant of
.383 degrees. You really can't get that kind of accuracy unless you CNC the
fin can. In fact, it is quite difficult to get 0 degree cant by any manual
method. Bill, if you want to try CNC I have a contact for a CNC guy in WY
that does a great job. He did both my 4" and 6" fin cans for me. I can sent
you a pic of the 6" if you want to see what we are using.
Richard Hagensick
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Wyatt Rehder
Sent: Sunday, July 17, 2016 11:34 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: HPR trim tabs
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.
<Handbook Development for Astrobee F AIAA-25865-264.pdf>