Originally Posted by
twistedenginestarter
What confuses me is if the new engines are about the same weight, size and thrust, why is it such a big deal?
As mentioned above, fuel burn is a HUGE deal today. And in fact if they were not pretty much the same size, weight, thrust they wouldn't be a retrofit candidate, so it's not surprising that AB has gone for keeping those parameters fairly stable.
Originally Posted by
twistedenginestarter
....Nowadays surely the flight computers are the most tricky part of flying, so as long as you keep the same flight control behaviour does it really matter if there are slight differences in actual flight responses? Slightly different engines surely can't make much difference compared to the normal variation in loads, weather etc?
Firstly, you're assuming that aircraft are all about flight dynamics and response. if it's a very very simple aircraft sure. But engines do a lot more than just a bit of push. They are THE power source on-board, and every other system on the aircraft is dependent, in some fashion, on the engines' behaviour. So if i change the engine, suddenly my bleed air is a different temperature (because its from a different port) or is limited in mass flow 9because my new engine can only spare so much bleed). Now i have to potentially redesign my ECS and AI systems to adjust - or I have to impose some significant constraints on the engine supplier. Similarly for hydraulic power and electrical power - all the ancillaries are affected.
Secondly, the engine/wing integration is not that simple - there's a LOT of work goes into ensuring an efficient design in terms of minimizing interference between the two, while still getting good stalling behaviour despite having that big thing hung in front of the leading edge. A new engine nacelle could have significant design work requirements to make it work.