Hi Shawn.
Doh, really wanted and argument with Nick for old times sake but you will have to do instead! Anyway, the L2 was 1980s technology and had an analogue autopilot. I say that knowing full well that it was in fact digital, but it was a digital representation of an analogue control system. It also had a different pitot-static system from the 225.
The 225 is rock steady at 30kts coupled (the ASI reads down to about 20kts). This is in part due to those cunning French using "hybridised" parameters for the control algorithms in a 225 .
In an L2, the autopilot would be looking at IAS only, and responding to any difference between IAS and datum, and responding accordingly. Too much gain and it would overshoot and start to divergently hunt the datum, especially at low speed, so gains had to be kept pretty low hence it was not that keen to get back to its correct speed.
225 uses mostly inertial data from the AHRS, so it knows by integrating the accelerometer data, the speed. The IAS is of course hybridised with this inertial data to remove the long term integration errors, wind changes etc which would otherwise inevitably build up. Because of this hybridised data and precise knowledge of accelerations, it can have much higher gains on the IAS control loop, and they don't have to be a representation of an analogue control loop with all the inherent issues that can create. Discontinuous functions are easy in software!
So the 225 can and routinely does fly at 30kts coupled in IMC, night etc and is rock steady even in turbulence.
When I had a downer on ATC (for reasons I won't bore you with!) I was asked to "slow down to minimum approach speed" by a rather grumpy approach controller. It was fairly windy, and with the minimum IAS coupled to the ILS with IAS mode being 30kts in IMC, the headwind would have resulted in the heli either becoming stationary, or even reversing back up the ILS, something it is quite happy to do. Fortunately sense prevailed (could be something to do with the large fixed-wings behind me) and in the end I only slowed to Vy.
HC
Last edited by HeliComparator; 12th Oct 2012 at 16:07.