Over-stable - what effect?

Over-stable - what effect?

Post by Chris Olsso » Tue, 09 Mar 1999 04:00:00



Being a neophyte, I have an elementary question.

What is the undesirable effect of a rocket being over-stable?

I understand why an unstable CP-CG relationship is undesirable - if the C of P is for'ard of the
C of G then the tail wil try to overtake the nose and the whole thing will tumble arse over tit.

I also understand why a stable rocket is desirable and that a healthy distance between the C of P
and the C of G is typically something like one to two and a half times the diameter of the
rocket.

What I do not understand is the concept of over-stability.  What is wrong with being too stable?

I'm talking here about hobby rockets which are designed to go straight up.  Of course, highly
manoeuvrable rockets such as air-to-air missiles need only marginal stability because stablity is
the inverse of manoeuvrability.  In an air combat situation a guided missile needs to be very
agile, but that is an irrelevant matter to model rocketry.

For the purpose of getting a hobby (or amateur) rocket to fly reasonably vertically, what is the
problem with being "too" stable?

Cheers,       Chris Olsson

 
 
 

Over-stable - what effect?

Post by Alex Merica » Tue, 09 Mar 1999 04:00:00


An overstable rocket will weather-cock easily, even in light wind.
The LOC Onyx and Estes Fat Boy are too prime examples of overstable rockets.
Even in a light breaze they will head up-wind.

Beyond that I don't know of any problems with safety.

Quote:

> Being a neophyte, I have an elementary question.

> What is the undesirable effect of a rocket being over-stable?

--
Alex Mericas
NAR 62956 Level 1 Insured
President, Austin Area Rocketry Group

 
 
 

Over-stable - what effect?

Post by MODROCK » Tue, 09 Mar 1999 04:00:00


Quote:
>Being a neophyte, I have an elementary question.
>What is the undesirable effect of a rocket being over-stable?

From my little ( and non technical experience) I have noticed that over-stable
designs  typically fly fine. Again, this is just my experience.
I have built several large model rockets that have had over-stable designs and
they flew straighter than anything else I have. My over-stable rockets tend to
lift off slow ( Big Bertha like ) and majestic which I prefer anyway.

Have fun and build 'em stable.
Joe A.
IAR #202
321Liftoff!

 
 
 

Over-stable - what effect?

Post by GCGassaw » Tue, 09 Mar 1999 04:00:00


Quote:
>>>An overstable rocket will weather-cock easily, even in light wind.

The LOC Onyx and Estes Fat Boy are too prime examples of overstable rockets.
Even in a light breaze they will head up-wind.<<<

The Fat Boy is a poster child for extremely short rockets with small moments of
inertia. It's the shortness that's mostly causing that effect. See also Estes'
R2D2 kit. (OK, R2 is a better poster child...)

I know that over-stable model are suppsosed to weathewrcock too easily but I do
have to wonder if that's really so true or so significant. Because if you add
noseweight to a bird, which moves the CG more forward, you are also making its
moment of inertia greater because you've added more mass to an area far from
the CG. Which means it is less likely to be perturbed to begin with.

At any rate I'd rather have a model that's over-stable than risk being slightly
under-stable

- George Gassaway

 
 
 

Over-stable - what effect?

Post by Dan Loegerin » Tue, 09 Mar 1999 04:00:00


Quote:

> For the purpose of getting a hobby (or amateur) rocket to fly reasonably vertically, what is the
> problem with being "too" stable?

OK everybody, I'm going to take a stab at this one - correct me if I am
wrong...

The problem with being overly stable is when the rocket is at relatively
low airspeed (liftoff) it will be much more prone to "weather-cocking"
if you are launching in any wind.  In other words, the rocket will do
its best impression of a cruise missile as it leaves the pad and turns
into the wind.
The reason this happens is that the CG is the point that the rocket will
tend to rotate and the CP is is the point where the rocket is "pushed"
by the force of the air.  Picture it this way, you have two components
to the airstream that is acting on the rocket, one is vertical and is
caused by the rockets thrust as it accelerates upward.  The second is
horizontal and is caused by the wind at the ground.
This will be somewhat simplified, but bear with me here.  If you launch
two rockets in a 5mph wind with all things being equal except the
location of the CP, the resulting forces will be different.  A typical
rocket with a ratio of 2 calibers will have, for argument sake, a force
of 10 pushing on the CP trying to rotate the rocket.  Now if you launch
the second rocket with a ratio of 4 calibers (twice the first one) the
force will go up to 20 (also twice the first one!)  

There is one simple solution when flying overly stable rockets - dont
fly in any wind! :)

I hope this helped answer your question.

Dan Loegering

 
 
 

Over-stable - what effect?

Post by John Hatt » Tue, 09 Mar 1999 04:00:00


Quote:

>The problem with being overly stable is when the rocket is at relatively
>low airspeed (liftoff) it will be much more prone to "weather-cocking"
>if you are launching in any wind.  In other words, the rocket will do
>its best impression of a cruise missile as it leaves the pad and turns
>into the wind.

A good example of this is the Estes Phoenix. When flying on a D, it
tends to "acquire a target" pretty early on if there's any wind.

Quote:
>There is one simple solution when flying overly stable rockets - dont
>fly in any wind! :)

Yep.
---
John Hattan              Grand High UberPope - First Church of Shatnerology

 
 
 

Over-stable - what effect?

Post by Ryburn Ros » Tue, 09 Mar 1999 04:00:00


Quote:
> What I do not understand is the concept of over-stability.  What is wrong with being too stable?

> I'm talking here about hobby rockets which are designed to go straight up.  Of course, highly
> manoeuvrable rockets such as air-to-air missiles need only marginal stability because stablity is
> the inverse of manoeuvrability.  In an air combat situation a guided missile needs to be very
> agile, but that is an irrelevant matter to model rocketry.

> For the purpose of getting a hobby (or amateur) rocket to fly reasonably vertically, what is the
> problem with being "too" stable?

Well, personally I think a lot of the issue is overexagerated. I would
guess that it would be the slow liftoff speed more than anything else,
which could be present if you made a rocket overstable with a lot of
weight far forward. I launched my V-2 on an F20-7 in about 10-15 mph
winds and even though it balances at the bottom of the nosecone (the
front one), which is 12" out of 20" it still boosted straight up. After
it ejected the parachute tho, it drifted like crazy. My understable
rockets like the Sidewinder tend to weathercock more, especially if the
CP is somewhere near the center. I would guess that quite a few of the
rockets that weathercock come off the rod fairly slowly and are more
susceptible to the winds effects since it is higher, relatively, than it
would be if the rocket was screaming off the pad. If a rocket leaves the
rod at 30 mph with 15 mph winds it will probably be much more likely to
weathercock than a rocket that is moving at 60 mph off the rod with 15
mph winds.

Hope this helps,

--
Ryburn Ross
http://www.geocities.com/capecanaveral/hall/3641/wframes.html

 
 
 

Over-stable - what effect?

Post by Steven Roger » Tue, 09 Mar 1999 04:00:00


Quote:

> Being a neophyte, I have an elementary question.

> What is the undesirable effect of a rocket being over-stable? . . .

> . .  if you add
> noseweight to a bird, which moves the CG more forward, you are also making its
> moment of inertia greater because you've added more mass to an area far from
> the CG. Which means it is less likely to be perturbed to begin with. . . .

That makes sense to me, but I also wonder if the location of the "real"
CP is not also a factor. I haven't tried to work this out
quantitatively,
so check my rationale . . . Isn't the CP somewhat of a dynamic quantity
that depends on the angle of incidence?  I'm wondering if the CP for the
wind blowing on the side of the rocket might be significantly different
from
the effective CP for the rocket when its moving fast with the pointy end
foreward.

I've noticed that some rockets tend to turn into the wind, some actually
will
turn away from the wind, and others will be blown downrange while
remaining
vertical - yet all these rockets are "stable".  My theory is that the CP
for
the sideways profile can be far enough away from the end-on CP that you
won't
always get the same kind of weathercocking performance for a given
caliper
separation - in addition to other effects like the moment of angular
inertia.

If this is correct, the trick would be to design a rocket that has a
side-on
CP located at the CG, but an end-on CP located behind the CG. The CP
seems most
suceptible to dynamic effects on rockets with unusual shapes - like 5-8
small fins
rather than 3-4 big ones, tubes for fins, a ring fin, etc.  I think it
may be
more tricky than just looking at noseweight to classify a rocket as
"overstable".

Does anyone have quantitative info to confirm or deny this notion?

Steve

 
 
 

Over-stable - what effect?

Post by US Rocket » Tue, 09 Mar 1999 04:00:00


Quote:

> Because if you add
> noseweight to a bird, which moves the CG more forward, you are also making its
> moment of inertia greater because you've added more mass to an area far from
> the CG. Which means it is less likely to be perturbed to begin with.

Yes, you've increased the polar moment of inertia by adding weight to the front,
but it is still not a great distance from the CG. Add some weight to both ends of a
Mean Machine, and you get a high polar moment. Also remember that the Reynolds
numbers on model rockets are high and the mass is relatively low, so the air has a
greater effect on them. Polar moment is visible in some mid-power, and can cause
some strange recovery events in HPR.

-- Jimbo Franz --

www.USRockets.com

Perspective is everything

 
 
 

Over-stable - what effect?

Post by David Urbane » Wed, 10 Mar 1999 04:00:00


Satiability is the ability of the rocket to return to the desired path
after a momentary force deflects it from this path.

If your motor was perfectly aligned with the CG of the rocket and there
was no wind etc., then a cylinder would fly straight.  Unfortunately the
motor thrust will not be perfectly aligned, the air friction is a
chaotic system and there are numerous other forces that will disturb the
rocket.

The CG is the mathematical abstraction which sums the net gravitational
forces.  Happily, this is easy to figure.  CP is the mathematical
abstraction which sums the net aerodynamic forces and is a SWAG
(Scientific Wild Ass Guess) at best.

Disturb the rocket in flight and it will try to rotate.  For this
example lets say the disturbance will create a clockwise rotation.

CP in front of CG (nose of rocket is front).  The rocket's aerodynamics
will try to make the rocket spin in the clockwise direction (magnifying
the disturbance).

If CP is right on the CG.  The rocket's aerodynamics will do nothing to
change the rotation.

CP is behind the CG.  Aerodynamic forces will create a force that will
try to rotate the rocket in the counter-clockwise direction (correcting
the disturbance).

In the latter case the size of the corrective force is proportional to
the distance between the CG and CP, so it would look like a large
difference is best.  This is true up to a point.

The actual behavior is called dampened harmonic motion.  If you sit on a
swing and let some one give you a push, you'll experience dampened
harmonic motion (if you just sit there).  Gravity dampens the
oscillations until you are once more at rest.  The nose of of your
rocket will oscillate the same way until it, too, is pointed upward.

The further apart the CG and CP are, the slower the oscillations of your
rocket.  Even more to the point, if the CG and CP are far enough apart,
then the rocket won't even oscillate.  It will just choose a new path,
more or less upward if the disturbance is small relative to the other
forces acting on the rocket.  That's what you're trying to avoid.  You
want the rocket to oscillate until it finds up again.  If it's over
stable, then it will just choose a new path and fly that direction until
another force disturbs it.  You have to get the CG/CP pretty far appart
to achieve over dampened behavior like this, but it's possible and you
don't want to do it.  Staying between 1 body diameter and 2.5 body
diameters is a good rule of thumb, but it still won't assure stability.
--
==========================================================================
"By the time we've made it, we've had it." -- Malcolm Forbes
     __

     /_/

http://public.surfree.com/urbanek

 
 
 

Over-stable - what effect?

Post by Alan W. Holm » Wed, 10 Mar 1999 04:00:00



Quote:

>A good example of this is the Estes Phoenix. When flying on a D, it
>tends to "acquire a target" pretty early on if there's any wind.

I disagree with your example.  The Estes Phoenix is *far* from
overstable.  Actually, it is only marginally stable.  Why do you think
that they give you clay to put in the nosecone?  (Hint: they don't
give you the clay so as to make the model overstable)

Alan Holmes

 
 
 

Over-stable - what effect?

Post by Tim Van Millig » Thu, 11 Mar 1999 04:00:00


Quote:

> If this is correct, the trick would be to design a rocket that has a
> side-on
> CP located at the CG, but an end-on CP located behind the CG. The CP
> seems most
> suceptible to dynamic effects on rockets with unusual shapes - like 5-8
> small fins
> rather than 3-4 big ones, tubes for fins, a ring fin, etc.  I think it
> may be
> more tricky than just looking at noseweight to classify a rocket as
> "overstable".

> Does anyone have quantitative info to confirm or deny this notion?

> Steve

Steve, is there any way you can post a picture of what you are trying to
describe on a web page?

Tim

 
 
 

Over-stable - what effect?

Post by Tim Van Millig » Thu, 11 Mar 1999 04:00:00


Quote:

> What is the undesirable effect of a rocket being over-stable?

> What I do not understand is the concept of over-stability.  What is wrong
> with being too stable?

You can check this out in computer simulations using the RockSim 4.0
software. You can design the rocket that is overstable and vary the wind
conditions to see what happens.

the Free demo version on the Apogee Components web site can also be used
to perform these simulations.

Tim Van Milligan Apogee Components, Inc. 1431 Territory Trail Colorado
Springs, CO 80919-3323 USA

Tel: 719-548-5075 web site: http://www.ApogeeRockets.com reply email:

 
 
 

Over-stable - what effect?

Post by Steven Roger » Thu, 11 Mar 1999 04:00:00


Quote:


> > If this is correct, the trick would be to design a rocket that has a side-on
> > CP located at the CG, but an end-on CP located behind the CG. The CP seems most
> > suceptible to dynamic effects on rockets with unusual shapes - like 5-8 small fins
> > rather than 3-4 big ones, tubes for fins, a ring fin, etc.  I think it may be
> > more tricky than just looking at noseweight to classify a rocket as "overstable".

> > Does anyone have quantitative info to confirm or deny this notion?

> > Steve

> Steve, is there any way you can post a picture of what you are trying to
> describe on a web page?

Did Rob's post make it clear? - he's on the same track. The basic idea
is that the
CP as typically calculated is an approximation based on certain
assumptions, like
airflow near the axis. Could it be that the real CP is significantly
different when
those assumptions don't hold?

Think of something like the "Totally Tubular" compared to the Alpha. TT
has Lots of
fin area for wind blowing near the axis, but not much cross-sectional
fin area.  The
Alpha is more closely approximated by the balancing cardboard cut-out.
Given the
same caliper of stability and same side-wind, they won't weathercock the
same amount
because the sideways CP of the TT is not the same as the sideways CP of
the Alpha, though
they might have approximately the same CP for airflow near the axis.

Make sense, or have I been shut up too long with spent motor casings?

Steve