Shawn,
What I said was “on the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.” I then cited CFR paragraphs to confirm that a requirement exists to determine airspeed system position error at airspeed down to 20 kts. Those statements were not intended to be contentious, merely statements of fact.
I agree that the accelerated takeoff position error is different from the level flight position error. The takeoff IAS typically lags (as you said), but the CFR requirement is met as long as the indication is repeatable and readable….important for TDP, field length, and H-V envelope avoidance. I would argue that at speeds below 40 kts airspeed system performance is compromised more by rotor downwash than by dynamic to static pressure differential. The best performing helicopter airspeed system I’ve ever seen is that of the S-61N where the pitot-static probes are mounted several feet above the rotor mast.
In any of my work, Vtoss was never predicated on airspeed system performance, but on helicopter performance (OEI ROC WAT Limited performance). Some Category A certified helicopters have selectable Vtoss and others have a fixed Vtoss. For those helicopters having a fixed Vtoss, that value is typically in the 50-60 knot range. Anything lower would cause the WAT curve to be limited by the first takeoff segment Vtoss OEI climb performance requirement rather than the enroute Vy OEI climb performance requirement which produces the best possible Category A WAT limited performance. For those helicopters featuring selectable Vtoss, the minimum value is typically set on the order of 40 kts, because anything lower would cause the operating weight to be so low that there could be very little payload.
I believe it is typically the IFR handling qualities requirement, most likely the lateral-directional stability requirement that sets the minimum IFR airspeed. I think that 50-60 kts is the minimum airspeed at which most helicopters can demonstrate compliance with the IFR handling qualities requirement.
HT