Effect of non-uniform fin sets on stability?

Effect of non-uniform fin sets on stability?

Post by Will Scarv » Thu, 06 Feb 2003 01:27:22



Hi all,

I'm working on the design of my first truly custom rocket.  It's a
model rocket version of a Killer Whale.  OK, that's a bit odd, I
admit, but my daughter loves Orcas.  Please see the link for a design
diagram:

http://www.angelfire.com/sd/scarvie/rocketry_fleet.html#orca_rocket

As you can see, in the design the forward fins are the same shape, and
are all located the same distance from the nose.  To make the model
more closely resemble a Killer Whale, however, I'd like to move two of
the forward fins a bit forward (to represent the flippers) and one a
bit backward (to represent the dorsal fin).  I can't get Rocksim to do
this for me, and I don't have any practical experience with this kind
of fin arrangement.  I wouldn't need much distance between the
"flipper" fins and the "dorsal" fin to get the visual impression
right.

Anyone know what effect having fins in this arrangement will have on
stability?

As a side note, I'll probably shape the flipper fins more like an
Orca's flippers, but the surface area will be nearly the same as the
dorsal fin, so I doubt this alone would have much impact on stability.

Thanks much,

Will Scarvie

 
 
 

Effect of non-uniform fin sets on stability?

Post by Leonard Fehsken » Thu, 06 Feb 2003 02:53:50




Quote:
> are all located the same distance from the nose.  To make the model
> more closely resemble a Killer Whale, however, I'd like to move two of
> the forward fins a bit forward (to represent the flippers) and one a
> bit backward (to represent the dorsal fin).  I can't get Rocksim to do
> this for me, and I don't have any practical experience with this kind
> of fin arrangement.  I wouldn't need much distance between the
> "flipper" fins and the "dorsal" fin to get the visual impression
> right.

RockSim won't model asymmetric fin setups, but you can get an idea of the
effect by doing two different versions, a side view and a top/bottom view.
As long as the CP/CG relationships on *both* views are OK, you're OK.

len.

 
 
 

Effect of non-uniform fin sets on stability?

Post by Kevin Trojanowsk » Thu, 06 Feb 2003 04:10:05


I don't know the answer to your question, but it's a pretty cool design.
  And great that you're willing to go to the extra design work to
produce a rocket that makes your daughter happy!

-Kevin

 
 
 

Effect of non-uniform fin sets on stability?

Post by Rhhick » Thu, 06 Feb 2003 08:31:43


I had test flown 4 different Shrox prototypes he'd sent me (already built) & 2
of them no symmetry as we customarily think of it. I did cardboard cut-out
CP/CG models from both the top & side view & averaged the 2 CPs get get my CG.
They generated much curiousity on the field beforehand due to their risky,
unconventional look, but flew perfectly (& still do). I'm planning an upscale
of 1 if not both of the more radical ones. --  Richard "NOW, the Team America
team I'm mentoring is starting to get serious" Hickok
 
 
 

Effect of non-uniform fin sets on stability?

Post by Will Scarv » Thu, 06 Feb 2003 08:35:58


Quote:

> I don't know the answer to your question, but it's a pretty cool design.
>   And great that you're willing to go to the extra design work to
> produce a rocket that makes your daughter happy!

> -Kevin

Thanks Kevin.  Sharing this hobby with my daughter is a great pleasure
for me.  It brings me back to days spent building and flying with my
Dad, and she really likes it.

We're going out again this weekend to the local club launch, to send
her new Quest Falcon (suitably decorated with kitty and flower
stickers) up for the first time.  :-)

Thanks again,

Will

 
 
 

Effect of non-uniform fin sets on stability?

Post by Will Scarv » Thu, 06 Feb 2003 08:39:18


Quote:



> > are all located the same distance from the nose.  To make the model
> > more closely resemble a Killer Whale, however, I'd like to move two of
> > the forward fins a bit forward (to represent the flippers) and one a
> > bit backward (to represent the dorsal fin).  I can't get Rocksim to do
> > this for me, and I don't have any practical experience with this kind
> > of fin arrangement.  I wouldn't need much distance between the
> > "flipper" fins and the "dorsal" fin to get the visual impression
> > right.

> RockSim won't model asymmetric fin setups, but you can get an idea of the
> effect by doing two different versions, a side view and a top/bottom view.
> As long as the CP/CG relationships on *both* views are OK, you're OK.

> len.

Hmm.  I'm not following you here, I think.  I _think_ you're saying
that, if I try one version with all three fins up front where the
flippers are, and another with the fin set back where the dorsal fin
will be, and both are stable, I'm OK if I put some of the fins at each
location (two up front and one back, in my case).

But I don't understand how the view (side vs. top/bottom) affects
CG/CP in Rocksim (or in the real world, for that matter)...

I sure appreciate the help though,

Will

 
 
 

Effect of non-uniform fin sets on stability?

Post by Leon Ke » Thu, 06 Feb 2003 08:47:40


I would say that RockSim gives you a good basis for starting
construction.  Once it is built, you can test it by swing-testing it.
Attach a 6' + string securely at the CG (balance point), and swing the
rocket around.  If it constantly points nose-forward, you are OK.
Else, add a bit more nose balast.

Good luck, great design!!!

Leon Kemp
CAR S561 L3, RSO L1

Quote:

> Hi all,

> I'm working on the design of my first truly custom rocket.  It's a
> model rocket version of a Killer Whale.  OK, that's a bit odd, I
> admit, but my daughter loves Orcas.  Please see the link for a design
> diagram:

> http://www.angelfire.com/sd/scarvie/rocketry_fleet.html#orca_rocket

> As you can see, in the design the forward fins are the same shape, and
> are all located the same distance from the nose.  To make the model
> more closely resemble a Killer Whale, however, I'd like to move two of
> the forward fins a bit forward (to represent the flippers) and one a
> bit backward (to represent the dorsal fin).  I can't get Rocksim to do
> this for me, and I don't have any practical experience with this kind
> of fin arrangement.  I wouldn't need much distance between the
> "flipper" fins and the "dorsal" fin to get the visual impression
> right.

> Anyone know what effect having fins in this arrangement will have on
> stability?

> As a side note, I'll probably shape the flipper fins more like an
> Orca's flippers, but the surface area will be nearly the same as the
> dorsal fin, so I doubt this alone would have much impact on stability.

> Thanks much,

> Will Scarvie

 
 
 

Effect of non-uniform fin sets on stability?

Post by Leonard Fehsken » Fri, 07 Feb 2003 00:33:24




Quote:
> Hmm.  I'm not following you here, I think.  I _think_ you're saying
> that, if I try one version with all three fins up front where the
> flippers are, and another with the fin set back where the dorsal fin
> will be, and both are stable, I'm OK if I put some of the fins at each
> location (two up front and one back, in my case).

Not quite.  Look at the model from the top.  Size and position the fins
*for the RockSim* design where you see them from this perspective.  Check
your CP/CG relationship for this "view".

Now repeat the exercise looking at the model from the side.

If the CP/CG relationships for both views are OK, you're OK.

Quote:
> But I don't understand how the view (side vs. top/bottom) affects
> CG/CP in Rocksim (or in the real world, for that matter)...

Different fin areas in different places.  Also true in the real world.

The CP's position depends on the projection presented to the relative
wind.  If the the vehicle is not rotationally symmetric, that projection
will vary with roll angle.

Fins stabilize (i.e., provide restoring forces to) a vehicle to
counteract the effects of perturbing forces (e.g., a wind gust).  If the
vehicle is not symmetric (i.e., it looks different from different
angles), then the response to perturbing forces will depend on where
those forces come from (e.g., the top vs. the sides).  This is why, for
example, downdrafts and updrafts have much more effect on the motion of
an airplane than side gusts.  The wings present much more area to up and
down dafts than they do to side drafts, and the vertical stabilizer is
much smaller than the wings.

The assumption of rotational symmetry is a simplifying assumption.

len.