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When looking at different frames, there are lots of descriptions for different ways the arms are set up. Are there objective descriptions of how these arrangement change the flight characteristics rather than just "I like the way this one feels"?

For example, looking at the iFlight frames, the XL5 and SL5 are almost identical, except one is a "True-X" arrangement while the other is a "Squished-X" arrangement.

A few such terms that'd I would really like to see some solid description of the flight characteristics:

  • True-X
  • Squished-X
  • Stretch-X
  • Deadcat
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As a designer and manufacturer of frames here's my experience (6+ years, 10,000 flights, FPV only, 107 pilot w/HAM)

  • True-X: If you build well and balance your load these will fly in a symmetrical fashion on roll/pitch. If you set your rotation to 900 degree/sec (my normal) then roll and pitch (freestyle) will be similar in duration, stick movement, and hand-eye. Generally the best to learn on as a point of reference.
  • Squished-X: (lateral x) I fly a few of these, they have a "float" and just like the stretch-x they are slightly different - but not much. I use this layout for our high-resolution filming work, great for sliding pans and holding a line.
  • Stretch-X: I don't fly these too much anymore. I found no advantage for my flight style (all freestyle FPV, no racing).
  • Deadcat: Flies like a lateral-x in my opinion.

Other factors beyond frame shape influence performance. ESC on arms versus AIO is huge as you move weight around and change the inertial mass (resistance to acceleration). The same is true of distribution of arm mass. Stack hight is also important.

If you are a new pilot lean towards pure-x.

In general the frame contributes around 10% of the final flight feel. Electronics, tuning, weight, weight distribution, drive train (motor+prop), battery size/type, battery mounting, and your skill in construction are all as important as the frame.

I can change KV by 10% and swap props and get 20x different flight than changing out arm ratio.

Focusing on pure-x first will attenuate your hand-eye to notice the difference later.

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Unfortunately it really seems to be a personal preference thing.

I tried logging the same move on both pitch and roll on a stretched x frame to determine exactly how much difference there was and I couldn't find any difference in the results.

Stretched x are generally preferred for racing, but Thomas Bitmatta decided on squished x for his JS-1 frame.

For definitions:

  • True X - Symmetrical in both X and Y dimensions
  • Squished X - Wider than it is long
  • Stretch X - Longer than it is wide
  • Deadcat - Wider on the front than the rear
  • Hybrid X - Wider on the back than the front (See XBEE MCK frame).

The primary advantage of squished X and Deadcat is that you can keep the props out of the frame of the camera if you are running low camera angle.

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