captplaystation, variable geometry - both the fan and the hot section - has been a wet dream of aircraft turbine engine designers for decades (variable geometry compressors have been SOP for a long time). The problem has been cost, weight, and especially reliability have not been there to support it. The biggest problem is reliability - if the variable geometry on that automotive turbocharger fails, the piston engine will probably still run well enough to drive it to the dealer. If that happens on an aircraft turbine, your best case is probably in inflight shutdown - and it can easily lead to an engine overspeed, and it's not like you just pull over to the side of the road when it breaks
. On a car, one failure per 100,000
miles might be acceptable, on a commercial airplane one failure per 100,000
hours would be totally unacceptable. Hence aircraft turbine designers MUST be more conservative.
Gysbreght, it's both. As the fans have gotten bigger relative to the core, the accel characteristics from min idle have gotten much slower, and those big fans start out slower to boot. For example the CF6 typical ground idle was ~60% N2 and ~28% N1. The GEnx ground idle is more like 70% N2 and N1 is ~21% N1. When you advance the throttle on the GEnx, N2 goes up fairly quickly, but it takes a
long time to get to 40% N1. From 40% to 100% the CF6 and GEnx are pretty similar.