Written by Bjorn Fehrm
May 17, 2024, ©. Leeham News: We are publishing a series of articles related to engine development. The objective is to understand why engine development schedules were longer than airframe development, and the risk of product maturity issues was greater.
To understand why engine development has become such a difficult task, you need to understand the fundamentals of engines and the technology used to underpin them.
In the final corner, we looked at why open rotor engines are more efficient. The propulsion efficiency is significantly higher than that of a turbofan. This week we'll take a closer look at this.
Figure 1. Counter-rotating open rotor design ground tested by SAFRAN in 2019. Source: SAFRAN.
open rotor technology
Using an open rotor or fan can increase the air capture area and thus compensate for the greatly reduced overspeed due to increased mass flow.We are still getting the momentum we need in our relationship Thrust = Mass Flow * Overspeed.
However, it requires more components than just an open fan.If you have one fan, you lose. Efficiency from shaft power to air overspeed This is because the air is not pushed straight back by the fan blades. The angle to the free stream means you lose overspeed in the direction of flight and therefore efficiency.
This also applies to the fan and core compressor stages of turbofans. If you look at the turbofan from behind the bypass channel, you'll see stator vanes that are angled to straighten the air. The core compressor stator vanes have the same function, but here the rotation is greater so the air hits the next compressor stage at the perfect angle for the blades.
Turboprop propellers skip this step, making them less efficient. As a result, the air downstream of the propeller (called propeller wash) flows in a swirling pattern (Figure 2). This is something that every propeller pilot knows and must compensate for, especially during takeoff, as its pattern can destabilize the aircraft's yaw.
Figure 2. Propeller wash produced by a C-130 engine. Source: Simulation presented on Reddit.
Efficiency losses can be up to 5%. Therefore, propeller efficiency peaks at around 87%, while fans in the nacelle with static bypass vanes can achieve fan efficiencies of over 90%.
Originally, open-rotor designs placed a second counter-rotating fan stage after the first stage to eliminate flow swirl, injecting more overspeed along the engine's thrust axis. This requires a complex dual gearbox between the core and her two counter-rotating fans (Figure 1).
The early GE open rotor design, the GE-36, and subsequent research designs all employed this counter-rotating design. And he said in 2014 that his GE engineers on the Open Rotor team said: Why not? Why not replace the second rotor with a fixed anti-swirl stator vane to eliminate complexity? Analysis and testing showed that the loss in efficiency was negligible, but the complexity and weight increased significantly. The GE Single Stage Open Fan Engine was born.
Figure 3. GE's single-fan architecture first unveiled at the 2018 Farnborough International Air Show. Source: Leeham Co.
I first saw this at the Open Sky Stand at Farnborough Airshow in 2018. So I took the photo shown in Figure 3. Gearbox complexity was now at turboprop levels, and the reduction in complexity was obvious. The gearbox and fan + stator were also placed on the cold side of the engine, further reducing complexity.
The original placement of open rotors at the rear of the engine was to keep the noisy fan tips (noise dampened by the nacelles) away from the passenger compartment in the preferred rear engine layout. However, GE continued to work on his open rotor concept and his GE36 well into the 1980s and beyond. The cause of the very high engine noise has been discovered.
A lot happened from the interaction between the tip vortex of the first fan and the blades of the second fan. Research conducted in conjunction with NASA has allowed GE to gradually reduce noise levels, and now open fan architectures have reached the noise levels of his LEAP engines.
Figure 4. CFM RISE is a further development of the GE single fan concept. Source: CFM.
The benefits of a single open fan were so compelling that SAFRAN announced CFM RISE (Figure 4) in 2021 as LEAP's next generation engine for the single thrust 25klbf to 35klbf segment, the largest engine segment on the market. We have reached an agreement with GE to release it. Airse airliner. We'll discuss CFM RISE in more detail in a future segment.
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