OpenRocket: Large angle of attack encountered

I have been working on a 5" dia, 98 powered, rocket for a while. While running the sim with a CTI N5800 it oscillates between a 55K flight and a 9800 K flight. The msg I get when the stability problem occurs is "large angle of attack encountered". If I add even more nose weight it eliminates this problem. Prior to the addition of more nose weight the rocket sim stability cal is +1.31.

My question is this: If a design has a plus 1.0 cal stability (according to OR), then why would it not be stable every time I run the sim?

Yes, I can change to a VK nosecone, or go with larger fins, etc. and make this design stable (where every sim run is stable), but I'm trying to understand why OR reports a large angle of attack encountered with a supposedly stable rocket.

Thanks for the help!

Moderator, I just realized that I should have posted this in "Rocketry Electronics and Software". If you feel like it, please move my post. Thanks.

what do you get when you plot the stability of the flight? is there 1 cal of stability for the entire flight?

When you get a "large angle of attack" sim click on it and look at the graphs. There should be one for stability vs time. If you look at it you'll find that around the time the rocket goes through mach that the CP moves FORWARD about 1 caliber.

I had similar questions when I started my 54mm minimum diameter project earlier this year. I got a lot of good info from the thread I started:

Cutting stability to the bare minimum. 54mm minimum diameter design

Plot stability vs time, and stability versus velocity. My guess is that the Cp is moving too far forward.

Likely larger fins, or if it is three find try adding a fourth fin.

markkoelsch said:

Plot stability vs time, and stability versus velocity. My guess is that the Cp is moving too far forward.

Likely larger fins, or if it is three find try adding a fourth fin.

Click to expand...

What causes the Cp to move forward,.... passing through Mach?

RKeller said:

what do you get when you plot the stability of the flight? is there 1 cal of stability for the entire flight?

Click to expand...

I just now took a look at stability vs time... and the answer to your question is NO. So, there's my problem!

Yes

Walldiver7 said:

What causes the Cp to move forward,.... passing through Mach?

Click to expand...

BINGO!

Walldiver7 said:

I just now took a look at stability vs time... and the answer to your question is NO. So, there's my problem!

Click to expand...

ChrisAttebery said:

When you get a "large angle of attack" sim click on it and look at the graphs. There should be one for stability vs time. If you look at it you'll find that around the time the rocket goes through mach that the CP moves FORWARD about 1 caliber.

I had similar questions when I started my 54mm minimum diameter project earlier this year. I got a lot of good info from the thread I started:

Cutting stability to the bare minimum. 54mm minimum diameter design

Click to expand...

Thanks for the education!