Minimum liftoff peed so that fins provide stability

Minimum liftoff peed so that fins provide stability

Post by DAL » Tue, 02 Jan 2001 13:57:02



I was wondering what the minimum and recommended speed (in ft/sec) a
rocket should be travelling at when it clears the launch rod so that
the fins are providing stability.

I'm building a modified Mean Machine and I want to determine whether
the Estes D12 will still provide for an acceptable liftoff and flight.
Rocksim 5.0 has a default setting of 44 ft/sec, but I don't know if
this is acceptable.

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Minimum liftoff peed so that fins provide stability

Post by see sig 4 ad » Tue, 02 Jan 2001 16:47:42


Stine=30mph/44fps, Milligan=35mph/51.3fps

Quote:

> I was wondering what the minimum and recommended speed (in ft/sec) a
> rocket should be travelling at when it clears the launch rod so that
> the fins are providing stability.

> I'm building a modified Mean Machine and I want to determine whether
> the Estes D12 will still provide for an acceptable liftoff and flight.
> Rocksim 5.0 has a default setting of 44 ft/sec, but I don't know if
> this is acceptable.

> Sent via Deja.com
> http://www.deja.com/

----------------
Andrew MacMillen
NAR 77472 L1
andrewm_at_hawkfeather_dot_com
!spambot bait address below!

----------------

 
 
 

Minimum liftoff peed so that fins provide stability

Post by James Boswort » Tue, 02 Jan 2001 21:37:54


Quote:

>I was wondering

Now THERE is a subject typo that should have started the year off with
a few smiles!  ;-)
 
 
 

Minimum liftoff peed so that fins provide stability

Post by Chuck Pierc » Tue, 02 Jan 2001 23:37:41


On 01 Jan 2001 12:37:54 GMT, James Bosworth

Quote:


>>I was wondering

>Now THERE is a subject typo that should have started the year off with
>a few smiles!  ;-)

I noticed it, too, had a chuckle, then let it slide.  I don't know if
it qualifies as "Freudian" but it's a good one, all the same.

Chuck

 
 
 

Minimum liftoff peed so that fins provide stability

Post by DAL » Wed, 03 Jan 2001 00:42:49


It could be particularly since I was annoyed that I had to send the

down for 5 days now.  I'm pretty p---ed off.



Quote:
> On 01 Jan 2001 12:37:54 GMT, James Bosworth


> >>I was wondering

> >Now THERE is a subject typo that should have started the year off
with
> >a few smiles!  ;-)

> I noticed it, too, had a chuckle, then let it slide.  I don't know if
> it qualifies as "Freudian" but it's a good one, all the same.

> Chuck

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Minimum liftoff peed so that fins provide stability

Post by Brett Buc » Wed, 03 Jan 2001 04:30:29



Quote:

> Stine=30mph/44fps, Milligan=35mph/51.3fps

 Assuming the model is statically stable:
  buck: Vr=Vw/tan(10 deg)

   Vr= rocket velocity
   Vw = wind velocity in "rod frame"
   10 degrees = minimum angle of attack to avoid stalling fin airfoil.

  Note that this has only to do with the stability, and nothing to do
with the likely flight path angle.

  Brett

 
 
 

Minimum liftoff peed so that fins provide stability

Post by DAL » Wed, 03 Jan 2001 05:11:37


Using your equation and assuming a 2mph wind, then:

Vr=2/0.1763 = 11.34mph

Why is this so much less than the previous estimates?


Quote:


> > Stine=30mph/44fps, Milligan=35mph/51.3fps

>  Assuming the model is statically stable:
>   buck: Vr=Vw/tan(10 deg)

>    Vr= rocket velocity
>    Vw = wind velocity in "rod frame"
>    10 degrees = minimum angle of attack to avoid stalling fin
airfoil.

>   Note that this has only to do with the stability, and nothing to do
> with the likely flight path angle.

>   Brett

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Minimum liftoff peed so that fins provide stability

Post by David Weinshenke » Wed, 03 Jan 2001 06:37:04


Quote:

> Using your equation and assuming a 2mph wind, then:
> Vr=2/0.1763 = 11.34mph
> Why is this so much less than the previous estimates?

Because it's usual to figure on a launch speed high
enough to work with higher wind speed than 2mph.

-dave w

 
 
 

Minimum liftoff peed so that fins provide stability

Post by Brett Buc » Wed, 03 Jan 2001 08:15:26


Quote:

> Using your equation and assuming a 2mph wind, then:

> Vr=2/0.1763 = 11.34mph

> Why is this so much less than the previous estimates?

   Because planning on only 2 mph wind is not generally a good ideas for
other reasons. The best example would be avoiding large tip-off angles
and subsequent low trajectory, which while stable, isn't necessarily a
good idea. But the question was stability, and the way that stability is
significantly affected by subsonic airspeed is when the assumptions made
for stability are violated. One of the primary assumptions in most
stability analysis (all except cardboard cutout, which is in error for
other reasons) is that the fins continue flying and do not stall. A
conservative stall angle for most airfoils is about 10 degrees.

   To me the real issue is on the other end of the scale. For instance,
a 20 fps wind gets you 113 fps for a launch rod speed. This is
super-conservative, because although the fins may be stalled, the lift
doesn't actually go to 0, and the model is still driven in the right
direction to reduce the angle of attack. This violates the stability
analysis assumptions, so how conservative it is depends on factors
outside a simple stability analysis. For instance, how fast it recovers
depends greatly on the restoring forcem, the inertia, the shape of the
lift curve post-stall, and on and on. It probably defies closed-form
analysis. Egg lofters, superrocs, and other models with typically small
fins (despite huge stability margins), and high inertia are the models
of most concern.

    It's my contention that short periods of instability right at the
end of the launch rod are more common than we think, because the effects
are masked by other things, and the model accelerates through it before
significant deviations occur.

   The fact of the matter is that even the 2 mph is conservative when
talking about stability. An old issue of Model Rocketeer (late 60's)
described an R&D report wherein an analysis and experiments were done
with no launch rod at all. I repeated some of those myself, and was
surprised at how many times I got away with no ill stability effects. Of
course, it was those times it *did* go ape-doody from either stability
or extreme tip-of that make it a bad idea.

    Brett

 
 
 

Minimum liftoff peed so that fins provide stability

Post by Leonard Fehske » Thu, 04 Jan 2001 03:25:33



Quote:
> Assuming the model is statically stable:
>  buck: Vr=Vw/tan(10 deg)

SO in a dead calm, the rocket is stable at 0 fps?

len.

 
 
 

Minimum liftoff peed so that fins provide stability

Post by Brett Buc » Thu, 04 Jan 2001 05:08:47


Quote:


> > Assuming the model is statically stable:
> >  buck: Vr=Vw/tan(10 deg)

> SO in a dead calm, the rocket is stable at 0 fps?

> len.

   Yes.

  Brett

 
 
 

Minimum liftoff peed so that fins provide stability

Post by Leonard Fehske » Thu, 04 Jan 2001 06:33:51



Quote:



>> > Assuming the model is statically stable:
>> >  buck: Vr=Vw/tan(10 deg)

>> SO in a dead calm, the rocket is stable at 0 fps?

>> len.

>   Yes.

>  Brett

Only if you disregard motor thrust asymmetries, fin and motor
axis misalignments, mass asymmetries, fin airfoil differences,
...

Wind is not the only source of potential forces and
concomitant changes in pitch or yaw.

len.

 
 
 

Minimum liftoff peed so that fins provide stability

Post by Matt Stu » Thu, 04 Jan 2001 06:36:05


Quote:
> > > Assuming the model is statically stable:
> > >  buck: Vr=Vw/tan(10 deg)

> > SO in a dead calm, the rocket is stable at 0 fps?

> > len.

>    Yes.

Here's my idea of minimum rod speed... it's more of a thought
experiment than a proof.

We know that the CP moves forward as the AOA increases.

We know the rocket goes unstable when the CP moves
forward of the CG.

So, for a given rocket, if you can figure out what the AOA is
that places the CP and CG at the same point (neutral), then
you should be able to figure out the minimum rod speed to
achieve that AOA when added to the wind vector.

You'll still get a value of 0 fps in dead calm, but that's no different
than any other method.  I'd always assume a 5mph wind for
safety.  The nice (or not-so-nice) thing about this method is that
it is a unique value for any given rocket since it involves the
actual starting CG and CP points (and possibly the moments
of intertia).

I don't know the AOA-to-CP relationship, but I'm guessing
Mr. Fossey does.... it should be an easy thing to add to RockSim,
if it's a valid technique.

Any thoughts?

Matt Stum
NAR#73280 L2