Help: Overstability?

Help: Overstability?

Post by Rick Polzell » Tue, 01 Aug 2000 04:00:00



I know that moving the CG forward is good.  Up to two body widths.  What
happens if you add too much nose weight moving the CG into "over stabile"?
What happens to the rocket in flight?  I am asking this because I just
finished my V2 and slightly increased the nose weight (1/4 - 1/2 oz) over
the stock clay provided.  But I am a little worried that it may be too much.
Does anyone know the CP of the V2?  I could then check to be sure.
Thanks in advance for the help.

--
Rick

 
 
 

Help: Overstability?

Post by dondod.. » Tue, 01 Aug 2000 04:00:00


Generally, being overstable is not a real problem, except maybe in very
windy conditions with an underpowered motor, where the rocket may have
more of a tendency to weathercock severely.

I suggest that you try your design with rocksim.  Enter actual values
for weight and center of mass.  Then try launching it with different
wind speeds.  You should be able to determine what wind speed is safe to
launch in.  Using an engine with higher initial thrust will also help to
reduce weathercocking.

Don



Quote:
> I know that moving the CG forward is good.  Up to two body widths.
> What happens if you add too much nose weight moving the CG into
> "over stabile"?
> What happens to the rocket in flight?  I am asking this because I
> just finished my V2 and slightly increased the nose weight
> (1/4 - 1/2 oz) over the stock clay provided.  But I am a little
> worried that it may be too much.

Sent via Deja.com http://www.deja.com/
Before you buy.

 
 
 

Help: Overstability?

Post by Larry W. Hardi » Tue, 01 Aug 2000 04:00:00


Excessive static stability results in two things:
1) a greater tendency to weathercock (point into the wind)
2) decreased dynamic stability (tendency to wobble)
The wobble you can (but usually don't) get from too much static stability
is a fairly high frequency oscillation.  A marginally stable rocket which is
short and fat can exhibit a low frequency wobble.  The weathercocking
is the thing you probably have to worry about and can be avoided by
either refusing to fly in high winds or by using a high thrust engine to get
the rocket to come off the rod at a high speed.

Larry

Quote:

> I know that moving the CG forward is good.  Up to two body widths.  What
> happens if you add too much nose weight moving the CG into "over stabile"?
> What happens to the rocket in flight?  I am asking this because I just
> finished my V2 and slightly increased the nose weight (1/4 - 1/2 oz) over
> the stock clay provided.  But I am a little worried that it may be too much.
> Does anyone know the CP of the V2?  I could then check to be sure.
> Thanks in advance for the help.

> --
> Rick

 
 
 

Help: Overstability?

Post by Barry Lync » Fri, 04 Aug 2000 04:00:00


The F72-15 by rocketvision will achieve the thrust you need to overcome the
wind- Mine left the pad like a firecracker!  Unfortunatly I did use a chute
and the high winds took it to another county.
Was a pretty thing, too!

--
Barry E ***
NAR#76085 SR L2 TRA# 7961 L2
http://www.FoundCollection.com/~be***/


Quote:
> Excessive static stability results in two things:
> 1) a greater tendency to weathercock (point into the wind)
> 2) decreased dynamic stability (tendency to wobble)
> The wobble you can (but usually don't) get from too much static stability
> is a fairly high frequency oscillation.  A marginally stable rocket which
is
> short and fat can exhibit a low frequency wobble.  The weathercocking
> is the thing you probably have to worry about and can be avoided by
> either refusing to fly in high winds or by using a high thrust engine to
get
> the rocket to come off the rod at a high speed.

> Larry


> > I know that moving the CG forward is good.  Up to two body widths.  What
> > happens if you add too much nose weight moving the CG into "over
stabile"?
> > What happens to the rocket in flight?  I am asking this because I just
> > finished my V2 and slightly increased the nose weight (1/4 - 1/2 oz)
over
> > the stock clay provided.  But I am a little worried that it may be too
much.
> > Does anyone know the CP of the V2?  I could then check to be sure.
> > Thanks in advance for the help.

> > --
> > Rick

 
 
 

Help: Overstability?

Post by Greg Cisk » Fri, 04 Aug 2000 04:00:00




Quote:
> Excessive static stability results in two things:
> 1) a greater tendency to weathercock (point into the wind)

Correct.

Quote:
> 2) decreased dynamic stability (tendency to wobble)

To ass a little to what Larry said:

I think this may be because a rocket that is too stable is
also too heavy. As such it will not generate enough speed
to allow the fins to do their job, i.e. stablizing. And this is really
what you have to worry about by adding too much nose weight.

You still want it to generate enough speed. You must ensure
that the weight of an over stable rocket does not interfere
with the rating of the engine you are using (if you know what
I mean). If you are too heavy and too slow, the rocket will
not be stable at all once it leaves the launch rod.

So you need balance :-)

--


 
 
 

Help: Overstability?

Post by Hilty Information Syste » Sat, 05 Aug 2000 04:00:00




Quote:


>> Excessive static stability results in two things:
>> 1) a greater tendency to weathercock (point into the wind)

>Correct.

>> 2) decreased dynamic stability (tendency to wobble)

>To ass a little to what Larry said:

ROFL!

<snip>

Sorry Greg...

tah

Tod A. Hilty  NAR #72099
Hilty Information Systems

Member MTMA, NAR Section #606  

Mantua Township Missile Agency
http://web.raex.com/~markndeb/rockets/mtma/

"I'm going to put the wheels of the bus back on... just in case"
   - BlankReg, Max Headroom: 20 Minutes Into the Future

"I speak for myself _and_ my corporation!  Deal with it!"
   - blankreg

   - remove nospam.ever, and replace with apk for reply

 
 
 

Help: Overstability?

Post by bob fortun » Sat, 05 Aug 2000 04:00:00


Quote:

> >> 2) decreased dynamic stability (tendency to wobble)

> >To ass a little to what Larry said:

> ROFL!

Sounds perfectly fine to me.  It's a well known fact that something too
assed has a tendency to wobble.  Rosanne Barr immediately comes to mind.

Bob

 
 
 

Help: Overstability?

Post by Peter Cl » Sun, 06 Aug 2000 04:00:00


Quote:


>> Excessive static stability results in two things:
>> 1) a greater tendency to weathercock (point into the wind)
>> 2) decreased dynamic stability (tendency to wobble)

Stine's Handbook covers this in some detail (chapter 9). There seems to be two
dynamic stability issues with overstability.

The first is coning that occurs when the CG is way forward and the source of
thrust -- which is to say, the motor -- is a long way behind it. This would be
a result of achieving stability with an absurd amount of nose weight.  That
would mostly be an issue with scale models.

The second is "overdamping" when the fins are so large that the rocket's
direction of travel is changed by their lift before they rotate it to straight
ahead.  Such a rocket flies nose-first, but not always in a straight line; it
makes the flight look more like one with neutral stability.

In practice, neither of these things happens often, and probably won't happen
if a reasonable combination of fins and weight distribution is used. Most
rockets fly just fine when overstable, IMHO, assuming they get off the pad and
up to speed before the wind turns them over.

Peter W. Clay NAR 18619 SR L1
"What is a tree, but a nut that wouldn't listen to the pessimists?"

 
 
 

Help: Overstability?

Post by L.C. » Mon, 07 Aug 2000 04:00:00


I think it happens more than we know. A rocket takes a right angle in the sky.
The next flight, no such behavior is exhibited. It's called a fluke, but it's
really an
over-stability problem, IMHO.

-Larry C.

Quote:

> [good stuff deleted]
> In practice, neither of these things happens often, and probably won't happen
> if a reasonable combination of fins and weight distribution is used. Most
> rockets fly just fine when overstable, IMHO, assuming they get off the pad and
> up to speed before the wind turns them over.