I am starting a new thread for this as it is not a DFC90/Cirrus specific question.
having constant IAS mode is a huge benefit
Interesting...
Probably depends on your power reserve.
I do two kinds of flying: low level hacking (UK Class G sort of stuff) and Eurocontrol IFR (abroad, all high altitude).
The former is always done at 23"/2400/11.5GPH so there is nothing to do...
The latter is always done with a wide open throttle, from takeoff until later in the descent. The VS is adjusted post-takeoff for ~120kt (for cooling) and then it drops off, and one cannot maintain any given figure all the way up to even say FL140, never mind the operating ceiling (TB20, FL200). Any IAS value which can be maintained at say FL140 would be far too low for the initial climb.
Can anyone offer a specific detailed scenario where this is useful?
The IAS mode is particularly nice in normally aspirated aircraft. If you only have VS mode, you have to constantly reduce climb rate in order to maintain speed. If you have IAS mode, in theory you can just select one IAS at the start of the climb and then be done with it until leveling off at, say, FL140. In pratice, you still have to adjust it every now and then. Why? Well, in the case of the SR22, one would start the climb at something like 125KIAS, which yields good cooling, good forward speed and a very decent climb rate. However, once past about 7000 feet, it makes sense to reduce to 120 knots and once past FL100, I reduce to 115KIAS in order to maintain a good climb rate (above FL100, one does not need the higher IAS for cooling purposes any more).
Still, it's a nice reduction in workload.
I rarely ever do VS climbs/descents, only IAS. With the KFC225, I can set +500ft VS and it would stall the aircraft if it entered a downdraft. Also the AP has to do a lot more correction to keep a VS than an IAS.
With a turbo, I can maintain the climb rate up to the ceiling but my cylinders wouldn't like it. At the same time the IAS/TAS spread gets bigger so one would adjust the IAS climb figure every now and then. For this, the autopilot has a knob which you turn to adjust the IAS. I usually climb at 115KIAS all the way up, sometimes depending on DA, I have to do a bit more.
I would say that IAS mode is the single biggest improvement over my old autopilot.
If we assume that a periodical adjustment is necessary as one climbs -
What I don't see is why the periodical IAS adjustment should be needed less often (on an autopilot which has IAS hold) than a periodical VS adjustment (on an autopilot which has VS hold).
Given constant thrust, there is a direct inverse relationship between the IAS and the VS.
I use IAS in the climb - and VS in the descent.
I set the IAS to 110 and my NA SR22 does not get hot at that angle, but of course I have not tried at MTOM in very hot weather. I would then, like bosco, use 12o KIAS.
The 110 work very fine from T.O. to FL120 (have not tried higher).
Sometimes I set the VS to 1200 for Take off and then switch to IAS mode and 110 kIAS after leaving the traffic pattern.
I have not tried IAS in descent yet
What I don't see is why the periodical IAS adjustment should be needed less often (on an autopilot which has IAS hold) than a periodical VS adjustment (on an autopilot which has VS hold).
If you assume still air without any vertical movement, you might be correct. However, the reality is very different. VS climbs are very unstable and don't work in convective weather.
Given constant thrust, there is a direct inverse relationship between the IAS and the VS.
In a vacuum? 
Alexis,
of course, all aircraft all different, and yours might be just a tad cooler than mine. But anyway, next summer you will find out that 110KIAS is a bit too low. Never let that CHT go past 380. It's good to keep the engine as cool as possible. And you're not wasting anything by going faster. Yes, your climb will a minute longer, but you will also cover more ground in the meantime.
VS climbs are very unstable and don't work in convective weather.
They are completely stable for me (KFC225) but are obviously power limited.
Say for an initial 120kt, the VS hold might be initially set to 800fpm and will have to be periodically reduced to say 100fpm at say FL180.
If I had IAS hold, I would initially set it to 120kt (which would give me the same 800fpm, obviously!) and after maybe 5000ft would have to start reducing the IAS preset to lower numbers.
Maybe the IAS hold implementation is easier than a VS hold implementation. The latter needs a decent barometer, dedicated to the autopilot.
IAS hold also gives you stall protection, which is a good safety feature, but that's a separate argument.
Yes, I know... I am VERY careful about the 380 F and have never been over that, or maybe only once when it was all still new I saw 390 or so in the engine log.
After reading all those articles by Mike Bush about the IO-550 I am very much aware that this is a very critical.
Two days ago I found a cool power setting: 2520 rpm, 20 MAP, 164 KTAS and only 11.9 GPS LOP (Best Economy). So cool!
They are completely stable for me (KFC225)
Certainly not in convective weather.
