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I'm designing an aerobatic aircraft similar to EXTRA 330SC. I would like to know about stability: lateral and longitudinal, static and dynamic. I read that aerobatic aircraft have neutral static stability. Some books relate Cm to AoA. Others relate Cm to CL. Please detail more about this or provide some bibliography.

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  • $\begingroup$ When I asked Patty Wagstaff how she was able to make her plane do some of the things it did, as I couldn't even imagine how to move my control yoke to make my tail flip over my nose for example, she said the plane was finely balanced to allow it to do that. $\endgroup$ – CrossRoads May 2 '18 at 16:56
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    $\begingroup$ I hope you are designing a model of an aerobatic airplane. If you are building a real one and don't know the basics of its dynamics... I wouldn't one to fly it. $\endgroup$ – Zeus May 3 '18 at 5:17
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    $\begingroup$ This is a pretty broad subject so unfortunately I have to vote to close it (generally more focused questions work well here FYI) I would recommend the FAA’s Pilots Handbook of Aeronautical Knowledge which has a good primer on the concepts of stability in it. $\endgroup$ – Carlo Felicione Jul 22 '19 at 19:08
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If you are serious about designing an airplane that will one day fly and certify, you should, to the very least, read up Raymer, Aircraft Design: A Conceptual Approach. It has a chapter on stability & control, and would be a good introduction to the lingos.

More in depth coverage can be found in Etkins, Dynamics of Flight. It's pretty much the bible for S&C engineers.

Once you feel like you have the theory down, design guidelines for conceptual design (even preliminary design) can be found in Roskam, Airplane Design.

Since design goes hand-in-hand with certification requirements, a knowledge of 14CFR Part 23 and AC 23-8C wouldn't hurt.

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The farther forward the center of mass is from the neutral point, the more stable the airplane is in pitch. When the two coincide, the airplane becomes pitch-unstable. When center of mass is behind, human reflexes are insufficient to maintain controlled flight.

As far as designing goes, center of mass and AoA are different beasts. Center of mass stays roughly the same, while AoA (especially in aerobatics) changes greatly and quickly.

Others here have explained further.

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  • $\begingroup$ Did you mean center of lift, or neutral point? $\endgroup$ – quiet flyer Jul 22 '19 at 18:21
  • $\begingroup$ Doh. NP. Edited. Thanks. $\endgroup$ – Camille Goudeseune Jul 22 '19 at 19:35
  • $\begingroup$ Cm is the moment (coefficient). It cannot be 'behind'. If it 'stays the same', the aircraft stability is neutral by definition. Its derivative by AoA, $Cm_{\alpha}$, stays roughly the same (in the normal operational range of AoA), and if it's negative, the airplane is stable. For aerobatics though, the post-stall behaviour is also very important. $\endgroup$ – Zeus Jul 23 '19 at 1:11
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    $\begingroup$ Or wait. Did you mean centre of mass? Then write it unambiguously, CM or better CG (even though it's less correct formally), or in full. $\endgroup$ – Zeus Jul 23 '19 at 1:16
  • $\begingroup$ Or use math notation: math.meta.stackexchange.com/questions/5020/… As an added bonus, LaTeX uses a very similar system so you gain transferable knowledge in the process. $\endgroup$ – AEhere supports Monica Jul 23 '19 at 6:50

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