Bjorn’s Corner: New engine development. Part 8. Open Rotor technology

By Bjorn Fehrm

May 17, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

In the last Corner, we looked at why Open-Rotor engines are more efficient. Their propulsive efficiency can be considerably higher than that of a turbofan. We will explore this further this week.

Figure 1. A counterrotating Open Rotor design that SAFRAN ground tested in 2019. Source: SAFRAN.

Open Rotor technology

With an Open Rotor or Fan, we can increase the capture area of air so that an increased massflow can compensate for a drastically reduced Overspeed. We still get the thrust we need in the relationship Thrust = Massflow * Overspeed.

But we need more components than just an Open Fan. With a single fan, you lose shaft-power to air-overspeed efficiency because the air is not pushed straight back by a fan blade. The angle to the freestream means you lose Overspeed in the direction of flight and thus efficiency.

This is true for a fan in a turbofan or a compressor stage in a core as well. If you look into a turbofan from the back of the bypass channel, you see the angeled static vanes that are there to turn the air straight back. The same function has the stator vanes in a core compressor, but here, the turning is larger, so the air is hitting the next compressor stage at an optimal angle for the blades.

The turboprop propeller forsakes this step and consequently loses efficiency. The result is that the propeller downstream air (called the propwash) flows in a swirling pattern (Figure 2), something that all prop pilots know and must compensate for, especially during takeoff, where the pattern can destabilize the aircraft in yaw.

Figure 2. The propwash generated by the C-130 engines. Source: Simulation presented at Reddit.

The efficiency loss can be up to 5%.  That’s why propeller efficiency tops out at about 87%, whereas a fan in a nacelle with its static bypass vanes can have over 90% fan efficiency.

Initially, Open-Rotor designs put a second counterrotating fan stage after the first stage to de-swirl the flow and thus inject more Overspeed in the direction of the engine thrust axis. This forces a complicated dual gearbox between the core and the two counterrotating fans (Figure 1).

The initial GE Open Rotor design, the GE-36, and the following research designs all had this counter-rotating design. Then, in 2014, a GE engineer on the Open Rotor team said why? Why not replace the second rotor with fixed de-swirling stator vanes and get rid of the complication? Analysis and tests showed that the loss in efficiency was negligible, with a considerable gain in complexity and weight. The GE single-stage Open Fan engine was born.

Figure 3. The GE single-fan architecture as shown for the first time at the Farnborough International Air Show in 2018. Source: Leeham Co.

I saw it for the first time on the Open Sky stand at the Farnborough Air show in 2018, where I took the picture in Figure 3. The reduction in complexity was obvious, with the gearbox now at a turboprop level of complexity. The gearbox and fan+stator were also on the cold side of the engine, further reducing complexity.

The initial placement of the Open Rotors at the back of the engine was to get the noisy fan tips (not noise dampened by a nacelle) away from the passenger cabin in the preferred rear engine layout. However, GE continued researching the Open Rotor concept after the 1980s and the GE36. It found the source of the engine’s very high noise.

A lot came from the interaction of the tip vortices of the first fan with the second fan’s blades. Gradually, in research conducted together with NASA, GE could reduce the noise level to the point that today, the Open Fan architecture is at the noise level of the LEAP engine.

Figure 4. The CFM RISE, a further development of the GE single fan concept. Source: CFM.

The advantages of the single Open Fan were so convincing that SAFRAN agreed with GE to launch the CFM RISE (Figure 4) in 2021 as the next-generation engine to the LEAP for the market’s largest engine segment, the 25klbf to 35klbf thrust segment for Single Ailse airliners. We will dive deeper into the CFM RISE in future Corners.

18 Comments on “Bjorn’s Corner: New engine development. Part 8. Open Rotor technology

  1. The obvious question,why don’t turboprops have stator vanes in that case?
    RISE had better live up to it’s promises on noise,,the public has become used to the fantastic achievements of the last 50 years and won’t be be prepared to accept one db more Show mobile phones and LEAP generation engines to people 50 years ago and they would not believe advance in technology to be possible.

    • +1

      I’ll believe it when I see all the claims proved in commercial operation, especially re: noise levels and net efficiency gains. About those uncontained blade failures, I will
      remain silent for now.

      • ‘other’ I was explaining these things last week
        “GE could reduce the noise level to the point that today, the Open Fan architecture is at the noise level of the LEAP engine.”

        The complexity is reduced the noise is just like turbofan – not an ordinary
        turboprop, but still some use their own intuition and or biases….if all this was developed at the pre production phase in a certain ‘other” country they would all singing its praises.

        • Quotes from others asserting this and that prove nothing. Actual *supporting evidence* for this or that claim is always welcome. Until then,
          it’s just idle talk.


    • I think there’s two things: at the time there was no capability nor incentive to fix it.

      Capability: computing power has massively improved and makes this kind of detailed modelling possible

      Incentive: why fix turboprops if jets solve the problem and sell like hot cakes? Now that jets are not the solution anymore the prop noise issue has a business case being solved.

      Not saying the sales pitch will match reality, I can’t know this. But there’s good reason to be found why it may be fixed now and was not then.

      • I would amend that its claimed its fixed but we have not seen (heard) any of it.

        Equally what is the cost? That is a small core that has to do a heck of a lot of work. Pushed to the MAX as it were.

        And as there will be no independent results published, we won’t know for a long time.

        When P&W did the first work on GTF, it never changed the basics. They adjusted around the details but the basic aspect did not change.

        The OR has been a continuous string of changes with lots of claims that were then shot down (I suspect Airbus and Boeing told them to pack it as they were NOT going to build one off air frames that depended on a pie in the sky concept).

        So is this really a final viable if not acceptable offering or does it change once again?

        I remain skeptical.

  2. The advantages of not having a variable pitch counter rotating prop driven by its own turbine stages in its rotating frame are massive. Still there are Volvo Penta DuoProp and Mercury Bravo-III counter rotating boat props. The Volvo Penta is pretty simple with concentric shafts going to each side of a vertical angle drive.

    • I would say the GTF development did change a lot, its started out as the launch engine for the very long range A340 but ended up as the A320 neo engine for high cycle short range duties

      Interesting background on the then early IAE Superfan GTF project and its derivative the Pratt GTF

      Including the real reason RR sold out of IAE
      “But in order to win support from Airbus for consideration of the new GTF engine on the proposed A320neo, Pratt had to go through a number of twists. Airbus wanted the engine to be offered under the IAE brand. After several unsuccessful attempts to win consensus of the IAE partners, Pratt had to fix that issue decisively. So Pratt bought the share of IAE owned by Rolls-Royce, and brought the IAE program fully under the Pratt brand.”

  3. on that impressive graphic of the Herkules you put in, all propellers turn counter clockwise.

    Is that always the case? wouldn’t it make sense if one turns clockwise and one counter clockwise?

      • Quite a lot of advantages to have the separate engines on each wing *counter rotating*. [Its contra-rotating when you have two props on the same engine]
        Its not really that much more complexity as you have a gearbox anyway- cars quite easily fit in a reverse gear

        ‘The arrangement preserves the symmetry of the aircraft when the four engines are operating, and reduces the adverse yaw in case of an engine failure, allowing in turn a reduction in the size of the tail fin by 17 per cent, hence reducing weight and drag. Another consequence has been the possibility to improve by four per cent the lift at low speed and thus to simplify the slats and, as a result, reduce by eight per cent the surface of the horizontal stabilizer. Furthermore, it also reduces the level of vibrations and therefore the noise inside the aircraft.”
        B Abraham, pprune

  4. I’m really enjoyed this series of articles, Bjorn. Thanks for writing these up!

    The GE36 from the 1980s didn’t use a dual gearbox. The front open rotor was connected by a shaft to one series of free power turbines, while the back open rotor was attached to a drum rotating in the opposite direction that contained the other series of free power turbines. Both rotated at the same rpm, and the turbines directly drove the rotors without the use of gearboxes.

    It was the lesser-known PW/Allison 578-DX that used a dual gearbox. I think McDonnell-Douglas preferred the slightly more efficient 578-DX to the GE36 for its planned propfan MD-80 successor, but the much-delayed 578-DX didn’t make it into flight testing until right before MCDD gave up and targeted a turbofan-powered MD-90 instead.

    Also, the non-rotating stator vane concept was not conceived by GE in the 2010s. It was investigated during the 1980s, but GE and Allison chose to use the contra-rotating architecture.

    I hope you can explain how the single-rotor RISE can have a similar rotor diameter to the double-rotor GE36 and 578-DX, yet generates a similar amount of thrust. The deswirling by the stator vane only captures/redeploys a small portion of the total thrust required, so I don’t understand why the 12-13′ diameter RISE wouldn’t have to be closer in size to the much-larger (17.5′ diameter), single-rotor TP400.

    • Thanks for the comments Cascadian.
      Yes, the GE36 didn’t use a gearbox, but there was no shaft connection. The fan blades were attached to a direct extension of the power turbine blades. GE knew how to do it from the CF700 aft fan version of the CJ610/J85 engine.
      The RISE is designed for a cruise speed of M0.78, the GE36 M0.72 max cruise. Therefore, the RISE Specific Thrust/Overspeed is higher to not have too much thrust lapse due to speed. The higher Overspeed enables a lower mass flow, i.e. air capture area.

      • Thanks for the correction. Also, I think I mixed up what the front and back open rotors were attached to — the rotating barrel part would be attached to the rotor that’s closer to the center of the engine, so it would contain the front rotor (not the back rotor, because the rotor blades for the GE36 were in the pusher configuration). Since the GE36 didn’t have a gearbox, that engine was a lot shorter than the 578-DX.

        A higher overspeed for the RISE compared to previous contra-rotating O.R. engines would be the obvious explanation, but I wonder if GE can actually sacrifice FPR that way while still maintaining a large SFC advantage over the current competition.

        BTW — the blades on the GE36 demonstrator engines were designed for M0.72, but the production version would’ve been able to handle much higher speeds. The GE36 would’ve cruised at M0.76 on the propfan derivative of the MD-80, and it would’ve surpassed that on the 7J7.

      • Forgot to ask — how would a single 12-13′ rotor generate the amount of thrust that previously required two rotors? If it’s done by increasing the airflow (and thus the RPM) compared to the levels used by the GE36/578-DX, how are they reducing the noise generated by the increased tip speeds (which were already at about M1.15 for the 1980s propfans)? If the extra thrust is generated without having to increase the RPM levels, what aerodynamic breakthroughs were made to allow the fan blades to create more lift from the same amount of airflow over the blade surfaces?

  5. Talking about Airbus approach, I think they have not been sitting on their hands watching Boeing weakening itself.

    A new article on full thermoplastic NB fuselage prototyping in Europe. Not sure Airbus afterwards is happy about the openness of this article..

    Imagine a wing design optimized for CFM RISE like high BPR engines and contours become clear of some 200-300 seats, a medium capacitry/ range A300/ A310/ A330 replacement.

    On the noise of the CROC’s, I think a large part was because of the wake of the engine struts hitting the fans in the pusher configurations. It made e.g. the Piaggio unacceptable noisy on many airports. The puller configuration of the CFM Rise (& Embraer studies) avoids this source of noise.

    I remember a lot of research was put into reducing the wake noise problem 10-15 years ago, but the problem was never really solved.

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