OK... I did an extensive CFD analysis of this phenomona... I have not run all the flow cases I am interested in, but I will 'splain below....

An aerobatic plane (Say 3D) is set up to be very nearly pitch neutral. Pattern planes are a little more nose heavy as they track and feel a little better overall... (I have flown my Compy SX at a CG that was just a RCH behind neutral actually)

So generally speaking, the CG effect is rather small for pattern/IMAC/3D planes. Trim your plane to the CG that feels/performs the best and don't worry how it affects KE coupling!

I set up my CFD study using a 78 inch span YAK 55. I did the study with the plane in a 35 degree yaw condition with 40 degrees of rudder deflection... similar to about what you would expect flying a KE pass.

I setup the model so that I could adjust the location of the stab on the fuse. I could move the stab to the top of the fuse, run the simulation, move the stab to the bottom, run the simulation, and place the stab in the middle and run the simulation.

I actually fine tuned the center position until I got a net ZERO pitching force.

For each simulation, I am able to determine the pressures and forces on the stab surfaces. Top'n'Bottom, left'n'Right.

With the stab located on top of the fuselage, there was a pronounced Pitch DOWN (towards landing gear).
With the stab located on bottom of the fuelage, there was a pronounced Pitch UP (towards the canopy)
With the stab located in the middle... in just the right spot, there was No Pitch!!

Ever notice on the Giant Scale IMAC/3D aerobatic planes the stab is positioned well below the scale location??? This is done to reduce/null the yaw coupling... it is apparent on pattern planes as well... the stab is located low on the fuselage....


I can't fully explain the exact mechanism that creates this phenomona... I believe it has to do wth the flow that goes around the fuselage at some yaw angle
being captured by the stab... there is a tendency for the flow to wrap around the yawed fuselage and if the stab is high, it captures that flow creating a higher pressure on the bottom surface of the stab (Nose down) and if the stab is low, that flow is captured on the top of the stab (Nose up) and if the stab is located in just the right spot, the flow is evenly split with no net pressure difference.

Interestingly, I did a run with the fuse at zero yaw angle and 40 degrees of rudder and there is a nose up force regardless of the stab location. it is only once the yaw angle is achieved that the other forces dominate the flow.

As a control, I split the model down the middle and ran that case to verify there was no net pitching force... and there wasn't

when I have time I will get better imagery showing the flow and pressures....


There was a post earlier suggesting that the downwash was a dominating factor.....well... it isn't!! lol.. as a pitch neutral plane will fly inverted hands off.... so the wing-tail angles are zero-zero

The yaw coupling is a phenomona that exists at all flight attitudes, upright, inverted, upline, downline .. whatever.... leave the mix on!! (I do)