Bjorn’s Corner: New engine development. Part 7. Open Rotor Propulsive Efficiency

By Bjorn Fehrm

May 10, 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 started a discussion around Open Rotor engines after looking at geared versus direct-drive Turbofans. We now look deeper at the Open Rotor Propulsive Efficiency.

Figure 1. The CFM RISE Open Fan engine. Source: CFM.

Open Fan technology

The major advantage of an unshrouded propeller or fan is that the swept area, and thus the mass flow, can be increased without the surrounding nacelle becoming large and thus heavy and draggy.

A nacelle with a thrust reverser typically weighs about one-third of the engine it covers. The larger the turbofan fan, the larger the mass of the nacelle with thrust reverser and the larger the drag of the outside of the nacelle (the drag on the inside past the fan is included in the engine data).

To increase the massflow radically and, by it, lower the Overspeed for increased Propulsive Efficiency, the non-shrouded Open Fan offers advantages. We can measure this advantage in ByPass Ratio, BPR, so it can be compared with the BPR values for Turbofans.

Figure 2 shows the bypass ratios of some well-known engines, and turboprops. We have also included the typical BPR of an Open Fan like CFM RISE.

The diagram, which is from a SAFRAN publication about Open Rotors, shows the coupling between the ByPass Ratio (BPR, X-axis), the Propulsive Efficiency (left Y-axis), and the Fan Pressure Ratio (FPR, right Y-axis).

Figure 2. The diagram couples Bypass ratios, Propulsive Efficiency, and Fan Pressure ratios for typical aircraft engines. Source: SAFRAN.

Propulsive Efficiency

From previous Corners, we know that the parameter that decides the Propulsive Efficiency is the Overspeed (Specific Thrust in engine speak), i.e., the acceleration the fan gives the air when passing the engine. This acceleration is coupled to the Fan Pressure ratio (FPR), as this is a pressure parameter that tells us the acceleration induced on the air when it passes the fan.

So, the important parameter when it comes to Propulsive Efficiency is the Fan Pressure Ratio (FPR). The lower the FPR, the less acceleration of the air, the lower the Overspeed, and the higher the Propulsive Efficiency.

To keep the thrust constant, we need to increase the engine mass flow, which we achieve by increasing the fan swept area, which is helped by an increased ByPass Ratio, BPR.

So the Fan pressure ratio is connected to the Propulsive Efficiency and the Bypass ratio primarily to the massflow of the engine. It’s, therefore, misinformation when people talk about the ByPass Ratio, BPR, as a direct measure of Propulsive Efficiency. It’s an indirect parameter that is used to get to the engine thrust level by setting the massflow of the engine when the Overspeed is set.

So, the FPR regulates the engine’s overspeed. It does this directly in the Bypass channel. What about the Core? We will discuss the Core in detail later in the series.

Right now, we can say the FPR into the core will form the pressure gain of the first compressor stage. The engine designer regulates the core’s exhaust speed, so it is 25% higher than the Bypass air. This optimizes the engine’s overall efficiency (why is a rather convoluted thermodynamic story).

We see that the most produced turbofan, the CFM56, has a Bypass ratio of around 5 during cruise and a pressure ratio of around 1.65 (read right up from BPR 5 in the diagram). The high FPR induces a higher Overspeed to the CFM56 bypass air than the latest engines.

For the LEAP-1A30, we shall read along the BPR 10 line, which gives us a Fan Pressure ratio of 1.4. For the PW1130G, we find an FPR of around 1.37.

A design like the CFM RISE will have a BPR of over 50, which gives a Propulsive Efficiency of around 95% and a cruise FPR of around 1.07.

The 95% could be compared with the 83% cruise Propulsive Efficiency of the PW1130 GTF turbofan from Part 4. So, a design like the CFM RISE gains 12% in efficiency only through the step from a BPR 12 turbofan to a BPR 50+ Open Fan?

Not quite. There is a step before we come to the Propulsive Efficiency of an Open Fan. It’s the efficiency we have in transferring shaft power from the core into the fan-induced Overspeed of the air.

It’s called the fan efficiency factor. We will discuss the ins and outs of fan efficiency in the next Corner.

65 Comments on “Bjorn’s Corner: New engine development. Part 7. Open Rotor Propulsive Efficiency

  1. Just how risky is this? The last generation of engines was difficult enough and with the size of the 737/A320 market you really have to commit to volume and reliability
    Can Boeing hang it on to a plane that their future hangs on?

    • Grubbie:

      My take is to split thyis into two areas.

      One is the tech part and a real world install (the mounting on the A380 will give some idea but CFM is only going to release the up side and not any downside)

      The tech part includes the mounting of it vs a jet engine and what aircraft mfg would risk RISE The claim is you can mount RISE on a Jet but it would have to be high wing. And the diameter of the Prop comes into play – picky details as to mounting on a low wing aircraft which is what Airbus is working in and on.

      And can they make it go as fast as a Jet can? I doubt it but stay tuned.

      The other is the public and how they view Props as archaic and even dangerous. Jet engine are nicely covered up and hidden on a real aircraft (grin)

        • Sounds like marketing / PR sweet talk…a bit like what we hear on a daily basis from eVTOL CEOs.

        • Fully agreed.

          We will get some idea when the test article goes on the A380 and hopefully reports from people around the field they test at.

          If not then its the delays that occur. The more and longer the delay the worse the results were.

  2. Is bypass ratio constant or different for various thrust setting and atmosphere conditions?

    From my experience bypass ration was geometric difference of core inlet section rest going through bypass channel.
    Thank you

    • The higher power the engine is developing, the lower the bypass ratio. It ‘s not by a lot; typically, the variant is within 10%.

      • This is correct and why still today P&W uses EPR: Engine Pressure Ratio for takeoff power indication.

        P&W still uses EPR (a “core” EPR – i.e. LP turbine exit to fan face) as the primary thrust parameter in all of their large commercial gas turbine engines, and I would assume all of their military derivatives as well. The PW4000-112″ engines on the 777 are definitely EPR.

        Rolls Royce, on the RB211-524 and -535, used an “integrated EPR” that is a combination of the core PR and the fan PR. I recall that the Trent 800 went to a “core” EPR like P&W.

        GE has used N1 for the primary thrust parameter for all of their commercial engines (not sure about earlier military variants). For commercial transports, the first use of N1 as a thrust setting parameter was on the 747 and the DC-10.

        However, EPR does have its issues especially with ice buildup, but that’s mostly been rectified with reliable EAI.

  3. The major risk is the fan disk stress and the variable pitch blades life limit. De-ice will complicate the design.

    • Currently a TP has to have the whole propeller taken down and serviced at much sooner intervals than the drive portion.

      Turbo Props are not new and that is what this really is, a very high tech pushed to the MAX (pun) TP.

      • The RISE power way exceeds any turboprop. More in the class of naval gas turbines. The LM series or RR naval gas turbines run without a fan stage. So it will an intense work to get it reliable and cost effective

        • the russians might disagree with their NK-12, 15000 SHP in the most powerful version (roughly 34,000 lb of thrust)

          Airbus might also with their TP400-D6 turboprop with 11,00shp and ~25,000 lb of thrust.

          • Yes, and it makes more noise than a freight train.

          • It won’t but that does not mean its solved either.

            Pure noise is not always the issue, its now it manifests itself.

            I had a case where we had 3 Generators in the same room and standing in places in that room even with ear muffs and ear plugs it got through somehow.

            My hearing was not great and I did not notice but one guy who ran the tests for me wanted out of them as he could tell it was doing his in (much younger guy).

            I went in and listened carefully and he was right.

  4. Seemingly, Boeing will not make the same mistake which it committed in the mid-1980s with the 7J7 by having chosen the GE36 propfan as the sole engine for it while Airbus picked the conventional CFM56-5A1 for the A320’s market launch after carefully examining and discarding the propfan over reliability, cabin noise and technology maturation risks. Had Boeing offered the 7J7 with both propfan and the CFM56-5A1 (or a specific variant of it) as engine options for the customers to choose from, Aviation history could have been very different today…

    For a deeper dive on why Airbus stuck to the CFM56-5A1 over Propfan when the entire Aviation world was almost smitten by the promise of Propfans and why the decision turned out to be monumental, would recommend:- Airbus vs. Boeing: Strategy Wars, Tactical Dogfights, High-G Maneuvers and the Photo Finishes

    • The issue with the OR was the rear mount requirement and not being able to mount a jet engine and rear mount costs structure weight, ergo, OR has to offer a lot more to offset that and you are stuck with OR as jet mount won’t work.

      Now CFM would ask mfgs to risk an aircraft design on RISE (my view is its morphed into a Turbo Prop). P&W who is the leader in TPs did not go down that path.

      And the public does not like Props unless forced to, and that is true in Prop happy Alaska (we have highest number of aircraft per capita than any part of the US let alone the world).

      But the public in Alaska kept dinging AK Airlines about the lousy Dash 8 service between the two biggest cities (400 some miles) and AK gave up and put an E175 on the route (prior to the Dash 8 it was all 737 service.

      While we are prop happy that is still a small subset of flyers vs the public and the public up here despite the prop exposure and it being a major and sometimes the only way to get to Villages in the bush, do not like them.

      I used to fly SeaTac to Bellingham and that was Dash 8, no issue for me. But that is no E175s and AK is getting rid of the Dash 8 fleet as fast as they can.

      Those flights were always full and a number per day vs the 2 a day E175 service but AK has decided props don’t cut it.

      • Climbing higher in a jet in a mountainous area for weather probably a factor ?

        • No, the route has not major mountain ranges to get over (Fairbanks to Anchorage or the reverse)

          The public kept pinging on AK until they switched to the E175.

          They ran the route fine and to keep in mind, Alaska is full of mountain ranges and there are routes through them or around them and that goes back to the piston jobs still doing work all over the state.

      • Just 2 remarks. The GE36 was not thru development and was way off from 20 000 cycles on-wing. The Q400 is not that reliable, Bombardier did not invest in systems upgrades to make it as reliable as the CRJ900 or the ATR72-600. ATR did the investment to the ATR42-600 and ATR72-600 hence their market dominance.

  5. Will the brace wing test plane using the MD-90 fuselage use this engine?

    • No, P&W has been selected to put a GTF on it.

      RISE does not have a running test article let alone a in service engine (test Article being worked on to test on the A380 test bed)

      Boeing needs to see how it works with a real world engine not the pie in the sky RISE.

      It should be noted that a real engine in the RISE design is 2035 or so. My view is its simply a delaying tactic so CFM can make a GTF out of the RISE program to compete with RR and PW.

        • Duke:

          As I see it, what RR and PW say is not relevant, they both put their money into the GTF and that speaks volumes

          PW risked a huge loss for that vision. The Boards supported it, amazing compared to Boeing sucking money out of the company.

          What CFM would be doing if not for the free money that had to go into a Prop job, yea, I think we know how that goes. Saved by the EU, they get a new core and gearbox experience.

          • Also, the article says: The advantage of the thin truss brace wing is lost in the wake of this type of engine… fascinating projects.

          • Sam:

            What portion says that? I seem to be missing something.

            RISE has touted it can be mounted on a wing forwards (or the rear also forwards)

          • Got it. Was not sure I could link into it, sigh.

            From that description of all the stuff they plan RISE can do, Swiss Army Knife of engines.

            Or the combo running back that slices and dices, bulls ahead and can catch the ball.

    • Looking forward to the debut of both wing struts and propellers on a NASA test
      aircraft. What’s next, a tailwheel?

  6. This weeks good news Boeing will probably exempted to sell 767 for another 5 years after ‘ 27. Because they didn’t develop an alternative (e.g. re-engine). Now it gets rewarded.

    Didnt deem a good idea first because it could hurt 787, then it could hurt the NMA, then all the other problems

    Don’t worry, they’ll find some heroic story around it soldiering on, name it innovative approach, supporters will inhale.

    • Jon has great reporting, my understanding is if the orders are in before 2028, those can be produced and into service anywhere. It will be interesting to see how that plays out.

      The 767 production is so limited that any emissions affect is like .00001%.

      ICAO etc all blow their horns loudly, but the holes left to drive aircraft through are as big as Maryland.

      So them thar big boys in ICAO see Airbus does not compete and lets take a whack at Boeing. Right idea for the wrong reasons (Boeing needs to be whacked legitimately or not until it reforms)

      Same as EICAS, I have yet to see a study that says its proven better than current MAX alert systems. Its whiz bang computer because that is how computers work. No one has ever researched it and, oh, having lights is a good idea.

      And if Freighter ops want 767s, just like the MD-11 and 757, they will go out and find them and its still the same impact (or lack).

      But no, lets make a huge deal about the Emissions are falling rather than pay attention to those really important issues out there.

    • As noted in the article – Domestic Use Only for those produced after 2028.

      Wendover has an excellent video on Fedex/UPS and their fleets. In it, he talks about fleet usage.

      It does not pay to spend $75 million in Capex on a new aircraft, when it flies some 4-5 hours a day. You can do the same thing flying an A300 that costs you peanuts up front, with increased fuel and maintenance expense.

      Orders in the future, don’t count. Under ICAO rules, the aircraft must be produced before the deadline. So if it isn’t made by 2028, it has to stay in the US. If Canada signs on, those planes won’t be able to fly to Anchorage.

      • Frank P:

        If you would note, I said already booked orders. I don’t have the reference but I believe that is correct.

        I had to laugh, clearly you don’t understand Alaska and Canada. You just have to fly off the Canadian and you can get to AK just fine.

        That said I don’t know transiting airspace applies vs landing in a country that thinks ICAO is reality.

        FedEx has bought new 767s and 777s. In fact they are all new. UPS has bought and is buying new build 767s and bought new build 757s.

        FedEx also bought new MD-11F.

        So the match shifts and it depends on what type of Freighter ops you are running, clearly package freighters are different than the bulk haul fleets and even those have bought new aircraft.

        • “You just have to fly off the Canadian and you can get to AK just fine.”

          Gosh, I can see that working really well for flights from the east coast to Alaska…

  7. Another excellent article Bjorn. While I agree it is probable we will see some efficieny gains with the Prop Fan, UDF, Open Rotor (insert new marketing name here), bottom line is that is still a 120 year old propeller. Governors, deice, FOD, torsional vibration, back-driving issues, ice-shedding damage, blade loss, and of course, the deal breakers, noise and vibration. No, I think ducted fans are here to stay.

    • Its clear you dont understand what a prop fan does.

      Its not a propeller, no more than the front fan on a geared turbo fan is. As well you show outdated ideas on what modern turbo props achieve, maybe that comes from not recently flying in say a 70 seater

      • Any device that pushes air backwards is a propeller, in my humble opinion.
        Call it a Prop Fan, call it a UDF, call it an Open Rotor………it pushes air backwards.

          • Not sure a dictionary is the way to solve this. A physics text might be more appropriate.

          • @ John
            The physics definition concurs with the dictionary definition:

            “A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working fluid such as water or air.”


          • The fan in turbo fan is also a series of blades accelerating air backwards
            Modern turbo fans have them at the front but some early versions had them at the rear of the engine.
            A Unducted fan is just a development of the enclosed turbo fan
            Submarines now have pump jets with multibladed ‘fans’ but arent propellers either

          • When you enclose it, that is an all new animal.

            A TP has a gear box that reduces RPM, because, ahaha, props don’t like to turn fast.

            Actually they are all lifting surfaces, but there is a reason they call it an Open Rotor.

            Its a plane, not its a duck, no, its Super Dog!

      • Thinking you haven’t recently flown a kite, or gone waterskiing.

    • We can consider all sorts of sources — maybe even Feynman or Dirac did a dissertation somewhere on the subject.

      My point is that, for those who wish to chide and lecture others, it might be prudent to first do a little due diligence👌

  8. Abalone,
    Thanks! Seems many are throwing some shade on Bernoulli these days. Considering symetrical wing aircraft are quite common, I think it warrants a look.

    • Yea that was my take when I learned to fly.

      Ok, we can fly upside down why?

      I believe the theory is that air is the same as water, aka fluidics and boats can/do plane.

      Seems Bounouli aside you need a bit of AOA to clunk along.

  9. Long-haul carrier Emirates sees $4.7 billion profit in 2023 as airline takes flight after pandemic

    DUBAI, United Arab Emirates (AP) — The long-haul carrier Emirates announced Monday it saw record profits of $4.7 billion in 2023 as the airline fully took flight after the turbulent years of the coronavirus pandemic disrupted its operations.

    Emirates, owned by Dubai’s government, announced revenues of $33 billion, compared to $29.3 billion the year before. Profit the year prior had been $2.9 billion.

    In November, Emirates announced a $52 billion deal to purchase 90 Boeing 777 aircraft, 55 of them 777-9 variants and 35 of them 777-8s. Emirates will also add an additional five 787 Dreamliners to its previous order of 30 aircraft.


    Seems some carriers can do just fine without an open rotor design (not that it would work for a widebody. Or would it? ).

    Emirates is also spending $2 billion on retrofitting it’s older interiors.

    I wonder how much airlines are willing to spend to get new aircraft tech into service and risking planes not being able to work.

    • With an eye on the ongoing PW GTF fiasco, and previous such episodes with other engines (e.g. RR Trent), one can imagine that airlines will be *very* wary of any new engine tech…

      • A bit of reality is advised.

        The issues with the GTF have been resolved. Agreed the issues are still ricocheting in the engines as they have to get them back through the shops to do the upgrades.

        New deliveries (E2/A220/A320/21) should be doing well.

        But that is the reality of any new engines, they have teething issues. Its been so long since a high cycle engine was done that its been forgotten.

        • So has the RR Trent 1000/TEN series, but you keeping harping on RR’s engine problems when they have been fixed and RR like it’s competitors, GE and PW, they are all working to improve the time on wing (durability) and efficiency of their current offerings?

          • Bran.

            How many times have we heard RR and PW say the problems are “fixed”? Complexity is the mother of failure.

          • Branaboy:

            You miss the fact that the 1000/TEN never achieved the performance of the GenX.

            Its not that RR has not solved it, its that they failed to solve it in the 1000, transferred that to the TEN, then had to back solve it for the no longer produced 1000. Two engines and you wind up with under 30% of the market is bad news.

            I just read a report of a 787 with RR engines (1000) that has been grounded for 7 months with engine issue. Why that is not corrected I do not know but its still having knock on affects.

            On the other hand the GTF is exceeding its target SFC (and a bit better than LEAP) which RR never did.

            My wife just flew a Delta A220-300, loved it (now that is a one off data bit so take it for what its worth but Delta has done good with it)


    Breeze Airways Indicates Interest In A Stretched Airbus A220 Variant

    Breeze Airways is eyeing a stretched version of the A220-300 for future fleet expansion, considering the potential for an exciting new airplane model.

    Breeze Airways, the airline making headwinds as a new American ultra-low-cost carrier (ULCC), has reminded Airbus that they are interested in a stretched version of the popular A220-300. The airline already has 23 of the A220-300 in service, with almost 70 on order. This ULCC is already retiring its 16 Embraer E190 and E195 and is looking to Airbus for inspiration.

    • They want it bigger and with longer range. That is not so easy to solve without redesigning most of the aircraft. A simple stretch with the same wing and fuel load with just beefed up landing gears, brakes and APU update is doable with a limited investment and a MTOW increase of 12-15%.

      • @Claes

        That’s the thing, isn’t it? What do airlines want? Are they willing to pay for a long range, stretched variant or is the meat of the market in a couple of plugs and a loss of range?

        In the A320Neo specs in wiki, they have this:

        6,500 km (3,500 nmi; 4,000 mi) with 165 passengers, with 150: 6,850 km (3,700 nmi; 4,300 mi)

        As I see it, if carriers want to keep it at 3 stews (150 pax) and it needs to have the same range as the A320Neo (for a like for like replacement) or to get to 4,000 nmi – then it’ll have to beef up.

        If airlines figure they can make do with a 3,000 nmi-ish range and save some capex on the variant, they’ll ask for a less expensive stretch.

        • Yes, there is a difference between what you want and what you are willing to pay a good extra for. Boeing has often done a stretch and kept wings and MTOW the same to limit cost. Airbus could do the same on the A220-500 but ideally they would need another engine with 12-15% less SFC and 15% more thrust with the same mass. That is not so easy to find as the new 25k engines like LEAP-1A/C at 3000kg is a bit too heavy. The RR Pearl 10X is maybe sufficient but is side mounted and not really designed for short jumps. RR might not be ready to pay Airbus for the certification cost for a revised engine for the “Fokker 100 of the 30’s”

          • It would be interesting if Airbus is wiling to fork up what is needed for PW to come out with an upgraded GTF that changes the parts they were conservative on (for good reason).

            It would loose the PW-1000 commonality but it would be a true A320 replacement with another jump in fuel efficiency.

  11. The fan in turbo fan is also a series of blades accelerating air backwards
    Modern turbo fans have them at the front but some early versions had them at the rear of the engine.
    A Unducted fan is just a development of the enclosed turbo fan
    Submarines now have pump jets with multibladed ‘fans’ but arent propellers either

    • Is it me or is that a repeat?

      Regardless, jets always had fans, its where the air goes and how thrust is achieved that makes them different.

      When you have all the elements of a Turbo Prop, its a TP

      Airbus decided to rename the things you put on the ends of your wings but its still a Winglet.

  12. Uh-oh

    (Reuters) -The U.S. Justice Department said on Tuesday that Boeing Co had breached its obligations in a 2021 agreement that shielded the planemaker from criminal prosecution over fatal 737 MAX crashes in 2018 and 2019 that killed 346 people.

    The Justice Department said in a court filing in Texas that the U.S. planemaker had failed to “design, implement, and enforce a compliance and ethics program to prevent and detect violations of the U.S. fraud laws throughout its operations.”

    • Frank P:

      And that has what to do with the RISE design?

      2nd time here you jumped in with a total off subject comment.

      Save it for on subject (my opinion of course)

      • And what is the RISE-related content in your dissertation above on 767 freighters and EICAS…?

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