First, love this post.
Second, how about variable pitch fans? Would eliminate the need for reversing buckets. I assume right now fans are pitched for optimum cruise, and therefore variable pitch fans would help most during takeoff and climb and descent. Might they even reduce noise levels during takeoff, if just by allowing a shorter roll and steeper climb???
I'd love to hear more about variable pitch fans.
I forgot variable pitch fans. Improved thrust at takeoff, improved climb and cruise fuel burn, about 2% in climb, maybe 0.6% in cruise (I am less familiar with test data, so please understand that is a SWAG).
The big benefit of variable pitch is reduced wear during climb. It allows efficiency gains and adds about 20% to the cycle life of the engine. Maybe 25%.
The simplified nacelle pays for the variable pitch. Small noise reduction, but since at where the ground absorbs, it doesn't effect the outside the airport noise profiles.
It is good tech.
I'm sure I forgot other stuff.
Much of what you wrote surprises me. If fans are optimized for cruise conditions, why would a variable fan improve cruise? Are normal fixed fans set to a compromise between climb and cruise?
Any why would a variable fan reduce wear during climb? I imagine you'd run the core at some reasonable maximum thrust with either a fixed or variable fan pitch, and take the better climb performance with the variable pitch. The core would see a similar work load in both cases, but less time of climb in the variable case. (I'm not stating a fact, I'm asking if this is true).
I missed your excellent questions.
A fan must have surge margin at takeoff. At cruise that surplus surge margin is wasting fuel. Yes, the fan is optimized as best as possible for cruise, but it will meet the requirements for all opperating conditions, for a fan surge is a bad day.
During climb, the turbine powering the fan is going too slow. The fan is demanding far too much horsepower. By reducing fan pitch, the low spool goes faster during the slow climb phase. Once the plane turns over for cruise it accelerated up to an optimal fan RPM which is really optimal Mach #. The durability during climb is by increasing the RPM/Mach number on the fan, you have more thrust. The aircraft climbs faster. This allows the low compressor (aka booster compressor) to reach a higher pressure. This results in more pressure drop for the combustor and turbine cooling. Every about 50F drop in Nickel temperature doubles service life.
The above is why RR was sold on the triple spool.
The important time in climb is the end of climb scortch. The turn over to cruise is absolutely scortching on the combustor and high turbine (highest metal temperature). If the high turbine isn't the cycle limiting part, as the most expensive part of the engine, send the designers back to engine basic design class.
This is why the LEAP and GE9X have variable turbine cooling. High during takeoff, climb, and a few other conditions. Low turbine cooling for cruise.
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