## Need assistance calculating CP

### Need assistance calculating CP

Hi all,

I have made a payload for a small video camera which includes a hooded
mirror.  Trying to imput the hood as *one* fin is impossible in VCP and
spreadsheet to manually enter my measurements, but can't figure out how to
add more than one set of fins.

Can I get some help?  I'd like to get this thing in the air, but prefer not
to prang.

Do I need to build a hood on the opposite side of the airframe?  I'd prefer
not to.

TIA

Ned Nassif

### Need assistance calculating CP

Quote:
> Hi all,

> I have made a payload for a small video camera which includes a hooded
> mirror.  Trying to imput the hood as *one* fin is impossible in VCP and
> spreadsheet to manually enter my measurements, but can't figure out how to
> add more than one set of fins.

First enter it as three fins.  It is thicker than any fin.  Secondly
when you build the rocket put a dummy hood on the other side so it flies
straight.

Quote:

> Can I get some help?  I'd like to get this thing in the air, but prefer not
> to prang.

> Do I need to build a hood on the opposite side of the airframe?  I'd prefer
> not to.

> TIA

> Ned Nassif

--
Jerry Irvine, Box 1242, Claremont, California 91711 USA

Produce then publish.

### Need assistance calculating CP

<<Do I need to build a hood on the opposite side of the airframe?  I'd prefer
not to.>>

You shouldn't need it on the actual rocket, but maybe you could do it in the
sim as a "work around" if it won't let you put in just one.

### Need assistance calculating CP

Hey Ned,

What kinda worked for me in the past was plugging in 6 fins (or shrouds
or whatever) recording the results, then changing it to 3 fins and
looking at the difference.  Then I did 4 fins and changed it to 3 fins
and looked at the figures.  In hindsight a ratio like 6 (fins) to 4 is
the same ratio as 5 is to 3 might be more logical.  From there you might
be able to extrapolate what 3 is to 1 might be.  Was that confusing or what?

Bob

Quote:

> Hi all,

> I have made a payload for a small video camera which includes a hooded
> mirror.  Trying to imput the hood as *one* fin is impossible in VCP and
> spreadsheet to manually enter my measurements, but can't figure out how to
> add more than one set of fins.

> Can I get some help?  I'd like to get this thing in the air, but prefer not
> to prang.

> Do I need to build a hood on the opposite side of the airframe?  I'd prefer
> not to.

> TIA

> Ned Nassif

### Need assistance calculating CP

Quote:
> Do I need to build a hood on the opposite side of the airframe?  I'd
prefer
> not to.

Think about it.  Won't the rocket tend to "pull" towards the direction the
hood is facing?

### Need assistance calculating CP

Thanks Bob!

Also thanks Ray and Jerome.  I've seen matching hoods and I've seen em on
just one side.  Jerome, it would seem obvious that that would happen.

Ned

Quote:
> Hey Ned,

> What kinda worked for me in the past was plugging in 6 fins (or shrouds
> or whatever) recording the results, then changing it to 3 fins and
> looking at the difference.  Then I did 4 fins and changed it to 3 fins
> and looked at the figures.  In hindsight a ratio like 6 (fins) to 4 is
> the same ratio as 5 is to 3 might be more logical.  From there you might
> be able to extrapolate what 3 is to 1 might be.  Was that confusing or
what?

> Bob

> > Hi all,

> > I have made a payload for a small video camera which includes a hooded
> > mirror.  Trying to imput the hood as *one* fin is impossible in VCP and
> > Rocksim (maybe I'm going about it the wrong way).  I have also
> > spreadsheet to manually enter my measurements, but can't figure out how
to
> > add more than one set of fins.

> > Can I get some help?  I'd like to get this thing in the air, but prefer
not
> > to prang.

> > Do I need to build a hood on the opposite side of the airframe?  I'd
prefer
> > not to.

> > TIA

> > Ned Nassif

### Need assistance calculating CP

<< Also thanks Ray and Jerome.  I've seen matching hoods and I've seen em on
just one side. >>

Same here. I should add that for high thrust motors, matching hoods might
provide a bit more stability, and also reduces sideways torque on the airframe
by equalizing the drag. I've always flown mine with just the mirror shroud and
no matching shape on the back. It works very well for most
motors/configurations, but with a small rocket and a big, fast motor it tends
to corkscrew a bit. I've seen another guy in our club who uses a dummy shroud
on the back, and flies it on a I435 and it goes arrow straight every time.

### Need assistance calculating CP

Well,

Why not eliminate the fin altogether? That is, keep the mirror internal,
with a clear piece of skin for it to look through? This would simplify your
aerodynamics, and should still allow for a broad field of vision, unless you
are seeking to look straight back down the rocket? (In which case you could
increase the OD of the stage holding the mirror, and again use a clear
substance for skin in the selected area, allowing the mirror to look
straight back down the exterior of the tube, and still keep a relatively
clean
aerodynamic shape?)

A newbie to the group and to rockets, but what do you think? As a point of
curiosity, can you let me know what solution you do come up with?

Jack

Quote:
> Hi all,

> I have made a payload for a small video camera which includes a hooded
> mirror.  Trying to imput the hood as *one* fin is impossible in VCP and
a
> spreadsheet to manually enter my measurements, but can't figure out how to
> add more than one set of fins.

> Can I get some help?  I'd like to get this thing in the air, but prefer
not
> to prang.

> Do I need to build a hood on the opposite side of the airframe?  I'd
prefer
> not to.

> TIA

> Ned Nassif

### Need assistance calculating CP

Quote:
> Well,

> Why not eliminate the fin altogether? That is, keep the mirror internal,
> with a clear piece of skin for it to look through? This would simplify
your
> aerodynamics, and should still allow for a broad field of vision, unless
you
> are seeking to look straight back down the rocket? (In which case you
could
> increase the OD of the stage holding the mirror, and again use a clear
> substance for skin in the selected area, allowing the mirror to look
> straight back down the exterior of the tube, and still keep a relatively
> clean
> aerodynamic shape?)

Hey Martin,

Good thinking.  I wasn't clear when I posted my question... I've got the
camera stripped down to fit in a BT-60 (1.637") airframe, and built the
thing before doing my r&d.  Since my post, I have started a new booster with
much larger fins, as the one I was going to use was something I already had
lying around.

Quote:
> A newbie to the group and to rockets, but what do you think? As a point of
> curiosity, can you let me know what solution you do come up with?

> Jack

Welcome Jack.  Stick around!  I'll report back with my results.

Ned Nassif
--