Originally Posted by
TangoAlphad
I'm going to go for a horrible generalisation here...
So could we say that a type which may be considered under powered or at least, not the pokiest machine, be flat rated to allow you to achieve the most out of it within the reasonable temp range and types with a more comfortable excess (i.e modern transport jets etc) de-rate?
Not really.
Like everything engineering related, this is about compromises, and not paying for benefits that you don't really need.
Flat rating boils down to this:
You engine is limited by (a) thermodynamics (Basically, when are you going to melt the turbine) and (b) mechanics (when is something going to break).
Thermodynamically you can keep on getting more thrust if the OAT is lower. But in order to utilise it you have to beef up everything structurally. This means you are going to be carrying extra mass. Always. Including when it is hot outside.
So, with flat rating you might loose the opportunity for extra thrust, but it is on cold days when everything else perf wise is working in your favour anyway.
Powerplants aren't my area of expertise but pretty much everything modern is flat rated (I'm sure someone will correct me). There is simply no engineering reason not to (unless it is some upgraded legacy design).
Simply say "we need this much thrust at this temperature. Do the mechanics for that much thrust and flat rate below that temp" Then if your jet can take off from main base at Max Mass at that temp, it'll be able to do it at every lower temp as well.
Whereas your generalisation is more about de-rating rather than flat rating. If your jet is 'underpowered' shall we then you won't get to do a reduced thrust take off as much or as often.
I thrust everyone else can correct me if this is all a load of bollocks.