Over-stable - what effect?

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. >>

> This is a very interesting notion, particularly interesting because I believe
> it has in effect been achieved.  You'll notice some high-power types that,
> before they deploy their chutes, will decend quite a long way in a horizontal
> attitude (usually also rotating around the long axis for some reason or other).
> . . .

Yes, that's exactly what I'm talking about, and it sure *seems* like
we're seeing
that effect, but I can't say for sure in a quantitative way.  For
example, I've
seen rockets like the Estes Longshot (small fins, long body) turn *away*
from
the wind, time and again. I have a modified two stage Maniac that does
the same
thing. I have an idea that the typical calculation of CP is based on
some
assumptions, like airflow along the axis and that the formula won't work
if you
turn the rocket sideways. Then the real CP may be in some other
location. And
if you could anticipate the shift, you could use it to your advantage.
But I'm really just guessing.

Steve

 
 
 

Over-stable - what effect?

Post by Alan Jon » Fri, 12 Mar 1999 04:00:00


On Tue, 09 Mar 1999 11:22:16 -0700, David Urbanek

Quote:

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

Interesting concept.  How does your model rocket know your desired
path?  There are many definitions of stability and I think you could
give a better one here.

<snip>

Quote:
>The further apart the CG and CP are, the slower the oscillations of your
>rocket.

Dang!  You had a real good explanation going until you got this one
backwards.

 Even more to the point, if the CG and CP are far enough apart,

Quote:
>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.

I've been in this hobby a long time and I have yet to see an
overdamped rocket.  Can you cite a well known example or present a
realistic design with overdamped dynamics?  In particular a lagre
static margin as you describe seems to run counter to what you would
need to approach an overdamped design.

There is little to fear from having too much static margin.  You can
expect more weather***ing and flight path dispersions and more drag
due to excesively large fins.  There is absolutely no reason that ALL
rockets should designed like high performance vehicles.  Indeed if a
practical model with overdamping could be designed and built it might
prove to be an interesting oddrock.

Quote:
>--
> David Urbanek  (NAR 73974)

Alan  Jones

 
 
 

Over-stable - what effect?

Post by RobEdmon » Fri, 12 Mar 1999 04:00:00


<< 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. >>

This is a very interesting notion, particularly interesting because I believe
it has in effect been achieved.  You'll notice some high-power types that,
before they deploy their chutes, will decend quite a long way in a horizontal
attitude (usually also rotating around the long axis for some reason or other).
 This indicates the condition you describe, a CP in 90 degree angle of attack
flight (when forces on fins and tube are primarily "drag" rather than "lift"
forces, at which the fins aren't any more "powerful" than an equivalent side
area of tube) that is basically where the CG is.   It would be intresting for
someone to identify one of these types and do a study of its liftoff behavior.
Keep in mind though, that even with this setup, the weathercock won't be
completely eliminated,  because the off-the-rod velocity of the model prevents
the angle of attack from ever being truly 90 degrees.  Eventually the model
will reach a speed at which the residual crosswind component is small enough
that the fins will start acting more like fins, and the model will turn toward
that crosswind.  Although, I guess you can imagine a situation in which the
model has picked up enough downwind drift prior to that point that it ends up
being simply turned back to about vertical.  This might be about what happens
on those straight up windy day flights.
RE

 
 
 

Over-stable - what effect?

Post by RobEdmon » Fri, 12 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. >>

No, it doesn't.  Stability only reflects the model's tendancy to return to its
orientaion with respect to the oncoming airstream.  It has no tendancy to
follow any particular path. For example, once a model weathercocks, it will
never "unweathercock", there is no tendancy for the model to return to a
vertical path.  It simply stabilizes with the nose aimed along the new velocity
vector.  Now, the path and the orientation do contribute to one another,
primarily because the thrust vector is fixed to the orientation.  Thus if the
rocket is somehow turned in a given direction, it will acquire a component of
velocity in that direction, and then in turn the orientation will shift to make
it point more in that direction.    But this is basically all they do, there is
no tendancy to return to a path.   In lifting aircraft, which do have a phugoid
mode of dynamic stability, this is another story.
RE

 
 
 

Over-stable - what effect?

Post by RobEdmon » Fri, 12 Mar 1999 04:00:00


Good explanation, but two problems:

<< Gravity dampens the
oscillations until you are once more at rest. >>

Gravity can't dampen oscillations.  It is the source of the oscilations in the
first place.   Only friction in the swing mechanism or with the air contributes
damping force.  Gravity is a conservative force, you can always re-extract
gravitational potential energy you have added to a system.

<< You want the rocket to oscillate until it finds up again. >>

As I was describing before, the model has no knowledge of "up".  It does not
care what direction it's traveling, only that the nose points into that
direction.  The fact that you may sometimes observe the oscillating model
returning to "up" is a product of the fact that it wasn't pointing away from
it's velocity axis long enough to acquire a significant sideways component of
motion.
RE

 
 
 

Over-stable - what effect?

Post by sallen » Tue, 16 Mar 1999 04:00:00


Quote:

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

For a rocket that is 'overstable' the most undesirable effect is that
the rocket is probably to heavy.  If you think about it, to improve the
stability, you either add nose weight (move cg forward) or add fin
surface area (move cp aft).  In both cases, there is additional weight
or structure that makes your rocket not fly to the optimum altitude.
This may not be an issue if difference is only a few hundred feet
(depends on motor, etc.)  Remember this, the heavier the rocket, the
better the recovery system will have to be.

sallen1

 
 
 

Over-stable - what effect?

Post by Paul Dimin » Tue, 16 Mar 1999 04:00:00


Quote:


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

In addition, the rocket will tend to weather*** (turn into the wind) more
than usual.

Paul.