When an engine is made smaller they can often do the opposite remove the first low pressure compressor stage to reduce the core size. You can see this on the E-175 and MRJ geared turbofans.
RRUltrafan wrote:What determines the capability of the amount an engine can grow and be developed without changing the design?
Matt6461 wrote:[sits down, waits for Lightsaber's reply. Has a thought based on engine trends but it's really more a question to Lightsaber - am I thinking in the right direction?]
- (1)that max core efficiency is dictated by max temperature
- (2) given that an engine family can have a broad range of thrust levels that broadly correlates with increases to OPR
- (3)given that the highest-rated members of engine families, all else being equal, tend to have slightly lower SFC (TXWB-97 is an exception, but not all else is equal there)
...given these trends I notice, it appears that the highest growth models within an engine family pay an SFC price for their higher T/W. OPR doesn't add fuel efficiency past some point unless max temp rises with it, it just gets more work from the engine (while spinning the fan faster - lower propulsive efficiency).
If I'm right about that, then there's some crossover point where the benefits of uprating:
(1) lower engine weight for given thrust
(2) lower engine nacelle drag for given thrust
(3) lower engine acquisition cost for cheaper-to-develop derivative versus lower-SFC but more expensive optimized engine
(4) shorter MLG and/or ability to fit underwing
...where these benefits of up-rating are outweighed by:
(1) lower SFC
(2) higher maintenance cost and on-wing time for highly-stressed up-rated model.
The range of up-rating within a family seems to settle around ~33% difference between most- and least-powerful family members (CFM LEAP, for example, spans 24k thrust to 32k).
That might be totally wrong - engines are mostly a black box to me at this point. Hopefully Lightsaber will correct where necessary or just tell everyone to skip this reply.
Engines are always designed for 15% of growth.
Lightsaber wrote:You did an excellent job.
Lightsaber wrote:See... We have other engine experts.
Lightsaber wrote:Limits come in multiple areas, but can be bypassed by minor aerodynamic changes such as enlarging the low compressor to give more air to burn at the same temperature with minor changes. This does reduce bypass ratio and hurts TSFC.
Lightsaber wrote:But then technology marches on. CMC PIPs will increase the LEAP and PW1100G allowed thrust. However, this will probably be for hot/high thrust.
As PIPs roll in, the higher thrust engines benefit more. Thus shrinks stop selling as the cost per flight difference shrinks.
But then technology marches on. CMC PIPs will increase the LEAP and PW1100G allowed thrust. However, this will probably be for hot/high thrust.
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