GE Aviation’s future fighter engine TECHNOLOGY XA100

Variable cycle engines have a major challenge

For a supercruise engine, you want a high pressure ratio fan, and a large moderate pressure ratio core with temperature and rotor speed margins sufficient to maintain airflow and engine pressure ratio at supercruise inlet conditions (approximately 100F inlet temperature at 40K, 1.5Mn).

For a cruise engine, you want a low pressure ratio fan, and a small, high pressure, hard working core module. You can lower the pressure ratio of the fan by opening up the exhaust nozzle, but this lowers the inlet pressure to the core and reduces the power required from the core module. You can rematch and reduce the airflow to the core for this higher bypass condition, but you end up with a low pressure ratio, lazy core. What you gain in propulsive efficiency with the higher bypass, low pressure ratio fan is offset by the reduced thermodynamic efficiency of the core.

While the YF120 engine demonstrated tons of supercruise thrust during the ATF Dem-Val, it was not as fuel efficient as the YF119 engine at either supercruise or subsonic conditions, indicating that GE had not solved that dilemma.

The challenge for the XA100 / XA101 is to raise (or at least maintain) the pressure ratio of the core module at the same time that you are reducing core airflow. I believe that this is going to require variable geometry in the turbine to reduce the flow area, re-matching the core to a higher pressure ratio, lower flow condition for subsonic cruise. Variable geometry in the hot section is major design challenge, to say the least.

Why variable geometry of fan blades when changing the angle of the fan to the flow should reach a similar result?

Why variable geometry of fan blades when changing the angle of the fan to the flow should reach a similar result?

Variable geometry in jet engines means variable stators (with the exception of the PW V2500 Superfan demonstrator, which did have variable fan blades). Variable inlet guide vanes are common in Fans, and several stages of variable stator vanes are common in high pressure compressors. Moving these vanes more cambered reduces the angle of attack of the fan / compressor blades, reducing airflow at a specific rotor speed and increasing stall margin at that rotor speed.

When you reduce airflow thru the core with a fixed geometry turbine, the pressure ratio of the engine goes down, reducing the thermodynamic efficiency. With a fan that was is fixed in size, the only way to increase the bypass ratio is to make the core airflow smaller. A variable turbine stator can increase the back pressure with that reduced airflow,so you could maintain pressure ratio with less airflow.

Note that the F135-600 engine system in the F-35B is variable bypass engine. Instead of making the core smaller, they make the fan larger by adding the lift fan. The increased total airflow enables the engine to increase low speed lift thrust up to 40k from the normal 28k Mil Power

TOWARDS TOMORROW GE Aviation's US Fighter Engine [6 page PDF attached NOTE DATE 2015]
Jul 2015 Chris Kjelgaard

"In the second of two articles on the engines to power the US fighters of tomorrow, Chris Kjelgaard finds out about GE Aviation’s views and work on adaptive-cycle engines...

...GE Aviation and FLADEs
Neither GE nor Pratt & Whitney will actually build a full-size AETD engine, but under the research programme both contractors must produce designs for one. (In the absence of specific engine dimension requirements from AFRL, both companies sized their AETD designs so they would fit into exactly the same space the F135 engine takes up in the F-35 airframe.)

AFRL closely guards the engine architecture details of the two companies’ AETD designs. But the fact both have adaptive-cycle fans and third airstreams that are complementary to their core and bypass airstreams means they require variablegeometry features. These are needed to produce the third stream of air and direct it to different locations within the engine, as variably required.

McCormick confirmed GE Aviation’s AETD design uses variable-geometry mechanisms, which the company calls ‘FLADE’ features. The term ‘FLADE’ stands for ‘fan-on-blades’, ‘blades-on-blades’ or even ‘fan-blades-on-fan-blades’....

Source: AIR International Magazine July 2015 Vol.89 No.1

Do we have anything more recent on the New Adaptive Cycle Engines from GE and/or P&W??? (XA100 - XA101)