Why didn't it prang?

Why didn't it prang?

Post by Dave Pachec » Tue, 15 Aug 2000 04:00:00



I'll have a launch report of this up soon, but right now I just want
to ask the question.

I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
held my breath and watched it fall horizontally from an estimated 850
feet (probably less because of weather conditions, etc). It landed
horizontally on the sand and loosened two fins. I've repaired it, but
I'm still wondering why it didn't come in nose down? I guess the only
way it would have stayed horizontal is if it was moving forward at a
relatively high speed (IOW it was remaining stable as usual, but just
sideways). But it didn't drift very far to be moving very fast, and
why didn't it arch over at apogee? Can wind cause it?? TIA

--Dave Pacheco
==========================================================
ICQ: 6568274            http://modelrockets.8m.com/
NAR#: 77711             G.Y.R.O. #135

Doesn't "expecting the unexpected" make the unexpected expected?
==========================================================

 
 
 

Why didn't it prang?

Post by Paul Smit » Tue, 15 Aug 2000 04:00:00


Quote:

>I'll have a launch report of this up soon, but right now I just want
>to ask the question.

>I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
>held my breath and watched it fall horizontally from an estimated 850
>feet (probably less because of weather conditions, etc). It landed
>horizontally on the sand and loosened two fins. I've repaired it, but
>I'm still wondering why it didn't come in nose down?

    What religion do you ascribe to?    :)

Paul Smith

 
 
 

Why didn't it prang?

Post by Bob Elli » Tue, 15 Aug 2000 04:00:00


Dave,  

Can't tell you why, but can confirm it.  I got a batch of E18-7's that
appeared to have 10 second delays (timed) instead of 7 seconds.  Two of
the three flites came down flat, not nose first as any other rocket
does.  It looks strange to see an Arreaux falling down slowly from 800+
feet for 3-4 seconds then seeing the ejection charge fire and the chute
deploy, but that's a lot better than seeing it come down balistic for
those 3-4 seconds and having the chute open at about 200 feet. Amazingly
no damage from that, I expected a full length zipper.  
If anyone has the answer to how a nearly overstable (2.3 Caliber on the
E18 reload) rocket can come down flat from apogee instead of balistic, I
want to hear too.

Bob Ellis

Quote:

> I'll have a launch report of this up soon, but right now I just want
> to ask the question.

> I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
> held my breath and watched it fall horizontally from an estimated 850
> feet (probably less because of weather conditions, etc). It landed
> horizontally on the sand and loosened two fins. I've repaired it, but
> I'm still wondering why it didn't come in nose down? I guess the only
> way it would have stayed horizontal is if it was moving forward at a
> relatively high speed (IOW it was remaining stable as usual, but just
> sideways). But it didn't drift very far to be moving very fast, and
> why didn't it arch over at apogee? Can wind cause it?? TIA

> --Dave Pacheco
> ==========================================================
> ICQ: 6568274            http://modelrockets.8m.com/
> NAR#: 77711             G.Y.R.O. #135

> Doesn't "expecting the unexpected" make the unexpected expected?
> ==========================================================

 
 
 

Why didn't it prang?

Post by daz.. » Tue, 15 Aug 2000 04:00:00


Quote:

> If anyone has the answer to how a nearly overstable (2.3 Caliber on
> the E18 reload) rocket can come down flat from apogee instead of
> balistic, I want to hear too.

This would have to suggest that there's a lot of CP forward
movement as the rocket deviates from zero AOA... enough to
give "neutral" stability at 90 degrees AOA so that a rocket
falling sideways will stay that way instead of being swung
nose first by fin action?

-dave w

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

 
 
 

Why didn't it prang?

Post by PeteAlw » Tue, 15 Aug 2000 04:00:00


Quote:

>I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
>held my breath and watched it fall horizontally from an estimated 850
>feet (probably less because of weather conditions, etc). It landed
>horizontally on the sand and loosened two fins. I've repaired it, but
>I'm still wondering why it didn't come in nose down? I guess the only
>way it would have stayed horizontal is if it was moving forward at a
>relatively high speed (IOW it was remaining stable as usual, but just
>sideways). But it didn't drift very far to be moving very fast, and
>why didn't it arch over at apogee? Can wind cause it?? TIA

This is exactly what the Bumbling Brothers' (my brother Bob's and my) NARAM R&D
report was about.

Our goal was to make this a deliberate recovery mode.

The secret is in the angle of attack.

I'll assume you know about Center of Pressure (CP) and Center of Gravity (CG).
CP is the "average" point that air pushes on the model.  CG is the balance
point of the model, and the natural pivot point of the model in free flight.
If the CP of the model is behind the CG, the air pushes on the model (on
average) behind the pivot point.  So if the model points away from straight
forward, the air pushing on the model tends to push the rear of the model back
behind the CG.  The model is stable, and all is happy.

You can find the CG of a model by balancing it on one finger.  The CP is
trickier.  The ancient method was the cardboard cutout method.  Essentially you
cut out a profile of the model from cardboard and balance the cardboard cutout
on a pencil.  Technicaly the balance point is the "center of lateral area
(CLA)."  But this method turns out to be accurate only for a model travelling
directly sideways--a 90-degree angle of attack.  Forunately, a model that tests
stable this way, with the CLA ahead of the CG, is stable--the method is very
conservative.

Then came the great and powerful Jim Barrowman, who showed us the way to
clculate the CP for models that were already moving the way they were
pointing--at low angles of attack.  and lo, Barrowman showed that models that
looked unstable with the cardboard cutout method wer indeed stable.  And fins
became smaller, and models flew straight, and it was good.

But some models that pass the Barrowman test--the CG is in front if the
Barrowman CP (BCP), fail the the cardboard cutout test--the CG is behind the
CLA.  This can be bad if you launch from a short rod in high winds.  An
otherwise stable model goes unstable, and all hell breaks loose.

But our R&D hypothesis is that this condition, a CG ahead of the Barrowman CP,
but behind the Center of Lateral Area, leads to a miracle.  A model that is
stable is stable going forward as long as the angle of attack is low, but one
that can experience a high angle of attack if it is pointing straight up at
apogee.  For then it experiences a high angle of attack for a moment, and loses
its stability.

When the air hits the model sideways, the model wants to point so that its CG
is ahead of the CP.  But now the CP is the Center of lateral area, which is
ahead of the CG.  The model wants to point backwards.  But if the model goes
straight backwards, the angle of attack goes low again, and now the CP is ahead
of the CG.  Argh! the poor model wants to crash, but backwards or forwards?!?!

The poor confused thing compromises somewhere in the middle, typicaly in a
something of a backwards glide.

And so your Arreaux fell sideways, just as a swing-stability tested model
sometimes kind of just wanders in direction as you swing it around your head,
kind of wanting to go backwards, but not having the heart to, but will go
foreward with great certainty if you start it right.

The trick is to make this happen in flight deliberately.  Bob and I use a
little hole punched at the front of the body, so the ejection gasses throw the
model into a tumble.

This got us 4th place in a single flight in D R/G at NARAM and 1st place in
R&D.

Peter Alway

Saturn Press
PO Box 3709
Ann Arbor, MI 48106-3709
http://members.aol.com/satrnpress/saturn.htm
Free scale data at:
http://personal.physics.lsa.umich.edu/alway/space_rocket.htm

 
 
 

Why didn't it prang?

Post by Ted Cochr » Tue, 15 Aug 2000 04:00:00



Quote:



>>If anyone has the answer to how a nearly overstable (2.3 Caliber on the
>>E18 reload) rocket can come down flat from apogee instead of balistic, I
>>want to hear too.

>The Always did an R&D project on this at NARAM. They flew a lot of test
>flights that did this deliberately.

>I'm hoping Peter or Robert can give us a pointer to some text.

Never mind--Peter did it while I was writing this :-)

Nice R&D work, btw!

--tc

My opinions only.

 
 
 

Why didn't it prang?

Post by Ted Cochr » Tue, 15 Aug 2000 04:00:00



Quote:

>If anyone has the answer to how a nearly overstable (2.3 Caliber on the
>E18 reload) rocket can come down flat from apogee instead of balistic, I
>want to hear too.

The Always did an R&D project on this at NARAM. They flew a lot of test
flights that did this deliberately.

I'm hoping Peter or Robert can give us a pointer to some text.

--tc

My opinions only.

 
 
 

Why didn't it prang?

Post by R. J. Talle » Tue, 15 Aug 2000 04:00:00


Two years ago I flew my Mustang on a G80. The ejection charge failed and it
went into a flat spin from more than 1500' up. It landed on the dry lake bed
and loosened two fins. Five minutes of repair and I was airborn again.  I
suspect a neutral CG and small rear fin cord relative to the over all
diameter of the rocket causes the condition we both observed. Sanford, any
words of wisdom from the Guru of AT design?

R J Talley


Quote:


> >I'll have a launch report of this up soon, but right now I just want
> >to ask the question.

> >I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
> >held my breath and watched it fall horizontally from an estimated 850
> >feet (probably less because of weather conditions, etc). It landed
> >horizontally on the sand and loosened two fins. I've repaired it, but
> >I'm still wondering why it didn't come in nose down?

>     What religion do you ascribe to?    :)

> Paul Smith

 
 
 

Why didn't it prang?

Post by Ted Cochr » Tue, 15 Aug 2000 04:00:00



Quote:

>This got us 4th place in a single flight in D R/G at NARAM and 1st place in
>R&D.

I'm also guessing that it spells the end of "conventional" designs in D
and higher (and maybe lower!) RG competitions in breezy conditions, absent
any rule changes.

Folks will start putting together long rockets with four nose-to-tail
strakes/fins, with a span of a BT diameter or two, and enough fin on the
back to keep it stable going up.  Why bother with folding wings and
monocoat and RC gear when you can fly a superroc backwards and be
competitive?

--tc

My opinions only.

 
 
 

Why didn't it prang?

Post by Brett Buc » Tue, 15 Aug 2000 04:00:00


Quote:

> I'll have a launch report of this up soon, but right now I just want
> to ask the question.

> I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I
> held my breath and watched it fall horizontally from an estimated 850
> feet (probably less because of weather conditions, etc). It landed
> horizontally on the sand and loosened two fins. I've repaired it, but
> I'm still wondering why it didn't come in nose down? I guess the only
> way it would have stayed horizontal is if it was moving forward at a
> relatively high speed (IOW it was remaining stable as usual, but just
> sideways). But it didn't drift very far to be moving very fast, and
> why didn't it arch over at apogee? Can wind cause it?? TIA

   Conditional stability.  The inertia is high, the fins stalled before
they could flip it over, model not stable with the fins stalled.

    Brett

 
 
 

Why didn't it prang?

Post by PeteAlw » Tue, 15 Aug 2000 04:00:00


Quote:


>> If anyone has the answer to how a nearly overstable (2.3 Caliber on
>> the E18 reload) rocket can come down flat from apogee instead of
>> balistic, I want to hear too.

>This would have to suggest that there's a lot of CP forward
>movement as the rocket deviates from zero AOA... enough to
>give "neutral" stability at 90 degrees AOA so that a rocket
>falling sideways will stay that way instead of being swung
>nose first by fin action?

Yup.  Another way to look at it is that at a 90-degree angle of attack, there
is is as much "tube action" wanting to swing it tail first as there is "fin
action" wanting to swing it nose first.

Peter Alway

Saturn Press
PO Box 3709
Ann Arbor, MI 48106-3709
http://members.aol.com/satrnpress/saturn.htm
Free scale data at:
http://personal.physics.lsa.umich.edu/alway/space_rocket.htm

 
 
 

Why didn't it prang?

Post by Dave Pachec » Tue, 15 Aug 2000 04:00:00


On Mon, 14 Aug 2000 11:52:08 -0500, "Paul Smith"

Quote:

>    What religion do you ascribe to?    :)

Rocketology. I lit some extra incense yesterday morning.

--Dave Pacheco
==========================================================
ICQ: 6568274            http://modelrockets.8m.com/
NAR#: 77711             G.Y.R.O. #135

Doesn't "expecting the unexpected" make the unexpected expected?
==========================================================

 
 
 

Why didn't it prang?

Post by Dave Pachec » Tue, 15 Aug 2000 04:00:00



Quote:
>But our R&D hypothesis is that this condition, a CG ahead of the Barrowman CP,
>but behind the Center of Lateral Area, leads to a miracle.  A model that is
>stable is stable going forward as long as the angle of attack is low, but one
>that can experience a high angle of attack if it is pointing straight up at
>apogee.  For then it experiences a high angle of attack for a moment, and loses
>its stability.

I was familiar with everything you said before but I'd never thought
of this. It seems to explain it pretty well. Thanks!

--Dave Pacheco
==========================================================
ICQ: 6568274            http://modelrockets.8m.com/
NAR#: 77711             G.Y.R.O. #135

Doesn't "expecting the unexpected" make the unexpected expected?
==========================================================