1) Operating ceiling (if you can do 20k then only a turbo engine will improve on that, but at substantial costs and bringing significant oxygen flow rate issues)
2) Climb rate (if you have 250HP then that will fix the climb rate for a given weight, more or less)
3) De-ice / anti-ice ?
But, if everybody looked at it that way, they would all be flying 30 year old planes, because there is not one single thing in e.g. an SR22 that gives you additional mission capability
1) Ceiling = 28K (turbo'd)
2) 310HP
3) Yup, booted up etc
And to answer your final comment - your absolutely right, hence this thread's entire raison d'etre !
Mind you, can't say I'd ever fancy something where the wings move much faster than the pilot ...
FF
