I would like to add my support for a couple of points here:
The need for HS immersion is probably mostly related to how much high prop thrustline must be countered. With the HS immersed, it's nose-up moment can be tuned to counter the nose-down moment of the high prop thrustline. This would develop a degree of "Power Stability" by maintaining essentially the same RTV relationship to the CG - with all power levels. "Power Stability" is tested by simply seeing how much the TRIMMED airspeed changes with a change in engine power. The proposed ASTM gyroplane standard sets that airspeed requirement at no more than a 10% change with full power or idle power from the median MPRS (Minimum Power Required Speed) power level - reduce or raise power from MPRS to full or idle power changes the trimmed airspeed by no more than 10%. For a turned off engine, this requirement allows 20% trimmed airspeed changes from MPRS power. This is so the pilot would not have to provide so much compensating stick forces when power is changed, so that G-Load stability (CG/RTV relationship) does not change badly with power changes, and so that there is less tendency to bunt or initiate pilot over-reaction upon sudden power changes!
A lot of HS immersion might be necessary to balance a high prop offset, but high immerwsion can lead to other issues. The DYNAMIC stabilizing characteristics of the HS will change according to the acceleration of the air over the HS - engine power. This means that the dynamic damping rate may significantly diminish - change handling and control characteristics, when power is reduced and the HS is not as DYNAMICALLY effective! This can play a part when such a configuration (highly immersed HS) suffers a sudden power loss. Not only may the nose attitude be changing rapidly, but the dynamic characteristics might suddenly be such that the pilot is not familiar or proficient with - over-control and possible PIO initiation of. I noticed this in my High Command (and in another un-named model highly immersed configuration) when descending in rough air at high speed and low power. To the seat of my pants, this became very uncomfortable in the turbulent air. But, upon restoring power back to cruise, it was again very comfortable flying in the turbulent air! Part of this effect in the un-named model was from the low prop thrustline G-Load stability enhancement with power. But, on the High Command, which was more CLT, the affect was probalby from less dynamic stabilizing effect of the less effective HS with power low.
The "T" tail with the thrustline centered HS does very effectively minimize the engine torque issue. I demonstrated this on my High Command by testing the two HS configurations on it. The torque and nose yaw displacement, and need for pilot rudder inputs was greatly diminished with the "T" Tall Tail (a standard Dominator tail). But, I was less able to adjust for better Power Stability with adjustments of the HS mounting incidence. The enhancement of the HS lift with power prevented me from tuning all of the Power instability out of it by simple HS incidence adjustments. I maintain that all gyro configurations are different, and that only final flight testing can determine if you have the right combinations of things. These combinations include, for the HS, how big, how far back, what airfoil shape, angle of incidence mounting, and the immersion factor. Affecting these parameters are the prop thrustline, the center of pressure relative to the CG, and other airframe aerodynamic moments produced by windscreens, enclosures, etc. There are no single answers for any particular configuration. I also feel that even a keel mounted HS can have some amount of immersion effect from the propwash. The Magni has a keel mounted HS with a slightly high prop thrustline. And the trimmed airspeed changes little from low power to full power. There is noticeable torque effect, but those issues are delt with by a laterally offset roll pivot, a very long and forgiving wheelbase, and a very long tail moment arm, and a bit of rudder trim - no noticeable need for excessive rudder action!
Although two different HSs might be a solution or a "fix" to a particular gyro configuration, it should be possible to have the same effect with a single HS, properly placed, sized and angled! Remember, the absolutely centered HS ("T" Tail configuration) may not be the highest immersion possible. The airflow acceleration around the inner portion of the prop is not so great as the outer 1/3 area. The highest immersion HS might be at about the 1/3 distance from the outer diameter of the prop. The placement of the HS in the propwash may affect the cantilever loads on the HS, as well as the turbulence that the HS sees. I have seen arguments that say the swirl from the prowash actually can add to the AOA of the HS surfaces and the drag. The center of the propwash may necessarily amplify some of the forward fuselage disturbances to the airflow - further turbulating the air that the HS sees. So, cleaner airflow on the HS may be best achieved closer to the keel.
I am trying not to promote any particular configuration, there are always tradeoffs in any design and different designers and pilots desire different characteristics. I do feel that the basic stability criteria should and can be achieved in most configurations - but the proof of what works for that configuration is in actual flight testing. But, discussing configuration impacts is fun and can be instructive.
- Greg Gremminger