Stability Question

Trying to learn all I can here.... so even though I trust my latest Custom Kit (Serin) to fly well .... I did a cut-out method CP determination and compared it to the CG (fully loaded with chute, motor, etc.). I was aware that this was a super conservative method (gives CP when angle of attack is 90 degrees) but was a bit surprised to get a static margin of negative one! No matter how many times I repeated the process, the CG fell nearly a full inch aft of the CP.

I'm going to fly this cute little bird (4-fin, 14"x1" skill level 1) regardless, on an A8-3, this weekend... but I just wanted to know if I can expect these strange static margins to show up whenever I use the cut-out method. Saving my pennies for Rocksim... may download demo and check it out soon... .

Thanks.

How did you make your cut-out?

Hello again, Dave:

Cut from framing matte board, a profile of exact measurements of the tube diameter x length, 2 fins cut to scale, my best nosecone side view, etc. with measurements within a mm or two. One-side lateral view. Sorry, no pics available. Kinda did this by Harry Stine's instructions... balanced it on a sharp ruler's edge.

I think the small fins are contibuters to my results... clipped deltas, inverted (leading edge at 90 degress to BT), and only about 10 sq cm each. The CP fell 20 cm from the nose, 14 cm from the tail. Far closer to the middle of the craft than I expected. The weight of the A8-3 I put in drove the CG a full 3 cm behind it, even with a 12in plastic chute installed close to the plastic nose. A slightly larger fin, swept back to a point behind the engine hook, would have improved things a lot...

As per RockSim the cardboard cutout CP for the Custom Serin is 9.3 inches from the tip of the nose cone; worst case CG is 9.95 inches from the nose tip with and A8-3 motor installed, that's a static margin of -0.66. The Barrowman CP is 12.17 inches from the tip of the nose cone. The RockSim Cp is 12.38 inches from the tip of the nose cone giving a static margin of 2.49 with an A8-3 motor installed. The Serin should fly just fine on the A8-3. The static margin with a C6-7 is still 1.91. The cardboard cutout method is very conservative, that's why I choose not to use it. These results were from the Serin file archived ar EMRR:

https://www.rocketreviews.com/rocksim/custom_serin.rkt

Try out VCP freeware for Barrowman CP estimation visit:

https://v-serv.com/vcp/

Gary A. Crowell Sr. the creator of this program frequents these forums.

Bruce S. Levison, NAR #69055

Bruce:

Thanks for the Rocksim run .... I knew there was something odd about the cutout method here... but I will take issue with the computers estimate of the cutout CP. I got the cardboard here to prove it (will bring with me to Chagrin ). Anyway ... I downloaded that freeware and learned real fast. With my measurements entered, it drew a perfect rep of my Serin and gave the same CP you got, giving me stability at around 2. I'm going to be using that program a lot.


See ya on the 19th.... Thanks again

Jim,

For advanced instructions on how to use and abuse VCP turn on your popup blocker and Vist Mark Fisher's web sit at:

https://www.mrhq.org/software/vcptips/vcptips.shtml

This is where I got the idea and experience to tweak RockSim files with a word processor.

For flight simulation freeware down load wRASP at:

https://www.wrasp.com/

Mark Fisher also has a tips page for the wRASP program at:

https://www.mrhq.org/software/rasptips/rasptips.shtml

Bruce S. Levison, NAR #69055

Bruce -

1) Thanks for the links and tips.... looks like I have a lot to keep me occupied on those sites (I understand very little of the math at this time, especially the mass calculations).

2) Got my altimeter from PerfectFlite about 72 hours after the order... wow. Can't wait to start working with it.... will probably put it in my Quest Gamma Ray (remember the one you helped me find last fall?) for testing purposes.

3) Got out to fly that Custom Serin this morning... in Sagamore Hills Park. No wind at all, (5) excellent flights with increasingly heavy clay payloads. A8-3's can be a lot of fun on light rockets in small parks! I proved a point you made last week .... over-stability is no problem at all when the winds are kind!