All I'll say is that your F bird is far too short. Not that I want the competition on the E, F, and G records, but I've never seen a bird that was stable on an F10 that was shorter than 10 or 12 inches, even with nose weight. Are you using Rocksim to test out the design before you build them?

Warren

All I'll say is that your F bird is far too short.
Warren

As long as the CG is ahead of the CP, "short" is not an issue. Think of it this way -- if you epoxied 3 fins and a nosecone onto that motor (and did nothing else), it would fly just fine. In fact, it would fly fine w/o a nosecone. And it would be a LOT shorter than what you had.

IMHO, your rocket was not too short. I believe your fins were too small.When you add length, your fins must continue to keep the CG ahead of the CG. When you add things like chutes and electronics and shock cords, remember that things may shift.

This is what everyone has troubles with. Trying to make a rocket with the smallest fins possible for less drag is tough. Many high altitude flights do summersalts and cart wheels. As your rocket gets longer as John said you need your fin span to be bigger. I would test your rocket before hand if its a altitude attempt on a smaller motor to see how it goes. And right beofre you launch check your CG CP one more time just incase. Shorter rockets are more effected by Cg Cp then others in my opinion.

Are you using Rocksim to test out the design before you build them?

Like I said in my first post, I used Rocksim to verify the stability using both the Rocksim algorithm and the Barrowman equations. The Rocksim stability margin was 1.45 and the Barrowman stability margin was 1.05.

The CG, by the way, is a little bit behind the front edge of the motor.

I would test your rocket before hand if its a altitude attempt on a smaller motor to see how it goes. And right beofre you launch check your CG CP one more time just incase.

I did both. I flew it on a C11-3 with a motor adapter and it went just fine. I did it to make sure that the reinforced streamer I was using wouldn't shred in the 100+mph deployment that Rocksim predicted for both the C11-3 and the F10-8. It flew just fine with the C11-3. The CG of the final configuration was right on what was modeled in Rocksim

The more I look at this, and thinking back to the behavior, the more inclined I am to believe that the way I packed the electronics and/or streamer there must have been a little lateral CG offset. The data I took in the unstable flight (below) tends to support this. T

This graph shows the first half second of the flight. You can see the rocket rattling around in the launch tower until it clears the tower at about 0.275 seconds on the plot, which also corresponds to the altitude of 3.5 feet. From there (and in fact from the moment it took off) the Parrot measured a small lateral acceleration that built up as the angle of attack increased. By about 0.5 seconds on the plot (0.35 seconds after liftoff) the rocket is going sideways with the wind causing about 12 Gs of lateral acceleration.

What makes me think that there is a lateral offset to the CG is the fact that the Parrot measured about 1G of lateral acceleration before the first impact with the launch rails, and this is in the same direction that the wind was making the rocket pitch over. Also, the rocket didn't straighten out after it burned off some propellant like the one in September did; it was doing cartwheels all the way until it was flopping on the ground. For my next try, I'm going to work out a different method for mounting the electronics and verify that I can roll the rocket on its side without wobbling.

Adrian, I think you are "barking up the right tree". I had cartwheels with an H268, but a wee bit 'o noseweight cured things (sidebar - WM - I need you to post that flight on the records page, will send you the info). I also had a J570 do some skywriting, but I added a bit of length and some lead.

We are all pushing the envelope. Shorter airframes, tinier fins, bigger motors, etc. At some point, if your little toe crosses the line, things get weird.

Not to belabor a point, but this is indeed why simming and flying are two very different things... I've had G, H, and J rockets misbehave. As such, when I flew my M, it had a substantial amount of lead up top. That was too expensive to do aerobatics with 😳 It cost me some altitude, but it beat the heck out of having it get loopy.