I wholeheartedly agree with you.Defining 'short-coupled' as a 3:1 ratio, 1/4 MAC to 1/4 HT with the implication that PIO may result from breaching this figure is not consistent with practice and experience, assuming adequate tail volume. Without checking my own figures relating to the prototype Flitzer Z-1, I can say that the tail arm is a lot less than this ratio and yet the a/c is fully stable in pitch hands-free at Vc and will remains so indefinitely and without an active trimming system (all unnecessary systems eliminated in the quest for utter simplicity) only small adjustments to RPM are needed as fuel is burned off, the tank being slightly ahead of the average CG.
The prototype tail area is smaller than on the plans-built Z-21 type, which was only increased in an attempt to achieve a full g-break with a light weight pilot. The foil used is the USA 35B which has a CP travel, migrating forwards from 45% to 30% approx at Vs and CG does not breach the 30% MAC position. Full (control deflection) pitch-excursions, positive or negative from level flight at cruise result in the a/c returning hands-free to a level attitude with zero pitching oscillation around its original attitude: ie dead-beat damped at Vc.
The natural in-flight stability which provides for relaxed X-country flying transitions to extreme agility with positive pitch input with the mobile CP approaching the CG, creating a highly manoeuvrable machine for a little recreational aerial combat, turn rate being 360 in 7 secs.
So such a hard and fast rule on tail arm is not necessarily correct.
Let's see if anyone else knows of evidence in the literature. You could do your own search too. The place I would start would be in texts on the topic of stability and control, and look for effects of whatever stiffness you have in mind.I wholeheartedly agree with you.
Many things affect to short coupledness.
I bet the stiffness of the main wing can be one aspect.
I use that description too. This issue is prevented way better by simply carrying vertical fuselage walls parallel to the long axis of the airplane through the whole root chord of the wing. Look at any modern jetliner or business jet. It may not look the best, but the air thinks so...Warning: non-engineer words about engineering stuff. (My version of what an actual aero engineer explained to me.)
It causes drag; the aft section of the wing upper surface is pulling air down; if the fuselage pinches inward in the same area, it is trying to pull the air 'in'. Result is a local low pressure area near the intersection, flow separation, and turbulence. Bigger issue the faster you go. The 'fix' (using the term loosely) on some a/c (like the Swift) is a large and increasing fillet at the wing aft end/fuselage intersection.
It's not necessarily a problem with the airplane. At all. PIOs, are Pilot Induced.
it's not flutter. It's not close coupled. It's not a stability thing.
it's the pilot expectations out of sync with reality.
It isn't even required that the plane responds faster than the pilot expects, although that is probably the most common "cause". That Vickers Vimy replica had some PIO issues with some pilots, because it responded so Slowly to input.