Bjorn’s Corner: New engine development. Part 10. Airframe integration

By Bjorn Fehrm

May 24, 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 the nacelles used for a turbofan engine and for an open-rotor engine. Now, we go one step further and look at the integration of modern engines on an airliner.

Figure 1. Boeing 737NG (left) and MAX (right) nacelles compared. Source: Leeham Graphic from Boeing 737 images.

Turbofan airframe integration

As the fan size increases of the turbofan and open-rotor engines the airframe integration becomes more challenging. Well-known is the effect of the CFM LEAP-1B engine integration on the low-slung Boeing 737 MAX and the problems it created, Figure 1.

As the nacelle diameter increases, the engine/nacelle combination moves forward and up to get adequate ground clearance while still keeping the engine exhaust stream below the wing.

The effect is that the nacelle, as a rotation body that creates suction pressure on its top, will cause the aircraft to have a nose-up pitching moment at high Angles of Attack (AoA). For the 737 MAX, which has a classical mechanical flight control system, it resulted in the flawed MCAS stability augmentation add-on.

The pitch-up effect is not limited to the 737 MAX. Other aircraft manufacturers share the same problem with large engine nacelles, as modern engines’ fan sizes increase to increase the ByPass Ratio (BPR). However, a modern Fly-By-Wire control system can handle an uneven pitch moment curve better than a mechanical control system.

We got the present installation problems when BPRs went from 4 to 5 in the CFM56 / IAE V2500 generation to 9 to 12 in the Pratt & Whitney GTF / CFM LEAP generation.

As the next-generation turbofans increase the BPR further to gain efficiency (through larger air massflow at lower Overspeeds), the installation effects of larger nacelles will increase the airframe integration problems.

Open Rotor airframe integration

Much has been written about Open-Rotor airframe integration and how problematic it should be. Let’s use the information we have on the CFM RISE to dissect the problem.

CFM has published the graphic in Figure 2. We use this as a reference for our discussion.

Figure 2. CFM RISE airframe integration examples. Source: CFM.

Mass

An Open Fan engine is not a light engine. As we understand it, a 30klbf CFM RISE would weigh around the same as the equivalent thrust CFM LEAP when including a nacelle and thrust reverser (the RISE reverses the blades for thrust reverse).

The heavy engine needs an efficient airframe integration. The most mass-efficient installation of the three in Figure 2 is the front one, as the forces of the engine, landing gear, and wing are concentrated around the wing root of the aircraft. The engine weight also acts as a counterweight to the air’s wing bending.

In the other architectures, the forces are spread, and each needs a beefy structure to channel the forces into the rest of the airframe.

Placement

The CFM RISE fan has a diameter of 13ft or 4m, the diameter of the ATR72-600 propeller. To house such a propeller/fan on a low-wing design, you raise the nacelle on the wing. This means the de-swirled fan stream will impinge on the wing surfaces and cause scrubbing drag. CFM says this effect is included in its 20% efficiency improvement over the LEAP.

Blade Out

The next objection to the Open Fan is the blade-out scenario. The turboprop has this scenario. It has achieved certification and proven safe over the last 50 years. The CFM RISE fan blade is not larger or heavier than an ATR72-600 propeller blade and spins in the same RPM range.

Therefore, an open fan engine that follows the safety rules from the Turboprop segment should not be more demanding to certify than an equivalent blade energy Turboprop engine.

Summary

The Turbofan has well-known airframe integration problems that increase the larger the BPR of the engine.

The CFM RISE has the same integration problematic as the propeller of an ATR72. Using the same certification criteria should, therefore, be possible.

44 Comments on “Bjorn’s Corner: New engine development. Part 10. Airframe integration

  1. I’m still trying to figger out the precise differences between an ‘Unducted Fan’, an ‘Open Rotor’, and the simply and truthfully named ‘Propeller’. So confusing!
    A falsifiable prediction: more new, obfuscatory names will be forthcoming, and not just in this domain. 😉

    “Beware of occupations requiring new clothes.” -H.D. Thoreau.

    • In principle the separate turboprop reduction/accessory gearbox is the main difference as it is integral in the UDF. The blade designs is a bit different as the UDF can keep a high propulsive efficiency at higher speeds.

      • I’m interested to see how the UDF / Open Rotor / whatever it’s called this week plays out in practice and in service. We’ll see how it goes.

        verily.

        • Yes, my biggest worry is flutter at max speed at different altitudes even acoustically trigged, anti-ice system durability and wear/fatigue in blade attachments where it rotates to desired pitch angle at this power levels. Historically it has been a problem to integrate Engine and prop systems into the same FADEC (TP400-D6).

          • “Historically it has been a problem to integrate Engine and prop systems into the same FADEC (TP400-D6).”

            could you elaborate?

  2. Wings and propellers are more closely related than you think too.
    I always liked the little Piaggo Avanti as it turned all the nomenclature on its head.
    Fuselage provides lift , propellers push not pull, the tailplane pushes up not down and so on.

    • As far as I know the Avanti has normal positive stability with the tailplane pushing down to offset the center of gravity ahead of center of lift. Foreplanes do produce lift

      • Its not a canard, inspite of looking like one but a “three lifting surfaces” . The description of the aerodynamics is too complicated for me to describe correctly
        It also has the same cabin diameter as larger jets -Citation XLS and Falcon 50 and with the wing box behind the cabin. The shape apparently provides more laminar flow than similar size cabins.
        The high cruise speed with wing mounted pushing propellers could have relevance for a modern ducted fan type

    • Wings and propellers are more closely related than you think too.”

      Yes.
      conformal transformation of a linear 3d arrangement onto a cylinder ( or a polar space )
      regular prop blades ~~ un-swept wing
      scimitar shapes ~~ are the transformtion of a swept wing layout.

  3. Was surprised to learn that the RPM range is the same as turboprops, thought it would be higher. With the blade size being simlar as well I take it the RISE should be an evolution of the turboprop?

    • That is my opinion.

      Generally if its shrouded its a jet be it low or high bypass.

      It the prop is out in the open its a Turbo prop.

      You can break them all down further (low bypass/high bypass aka Turbo Fan)

      OR was at one time a direct drive counter rotating beast. Call it kind of half way in between a jet and a turbo prop.

      The RISE is far closer to a Turbo Prop (with GE Characteristics)

      • Sails are also lifting surface so there is a plethora of lifting surfaces that need sub indention to know what you are dealing with.

        Mid size warships no longer have sails (big news) but they do have Jet Engines for main propulsion (though various mixes of diesels thrown in which have turbo charges which …… etc)

  4. The big advance in airframe technology – we’re told – is for a truss braced wing. Surely the airframe form on the left would be best suited to that, enabling a much higher aspect ratio and reduced weight at the wing root since the truss should take most of the bending moment?

    That would also allow for the nacelle to be lowered and the scrubbing drag from/on the wing to be lowered. Of course, the scrubbing drag from the truss would have to be added.

  5. A CFM RISE weighs 30klbf? That’s 30,000 lbf. I don’t think so. You need to check your math.

    • You measure mass in lb of kg, thrust in lbf or N. So thrust is probably around 30k lbf and mass of powerplant slightly less than 4500kg

  6. Embraer’s next-gen turboprop (delayed until more efficient engines are available) looks like the image on the right.

  7. Are pusher open rotors feasible? Could the jet exhaust exit through the core of the rotor with the inner part of each blade acting like a turbine?

    • The MD-80 and 727 Open Rotor demo airplanes were pushers.

    • You loose your anti swirly thingies unless its forward facing , so its as depicted in Fig 2.

      It would work, but you don’t get all the Zap 9 and Z7 that goes with it (or the theory of Zap9 and Z7)

      As Duke pointed out, its been done on the Avanti. But tail mounting comes with a penalty and LCA do not use that anymore. Not a show stopper for a business aircraft but then they almost all mount gets in the tail (Honda aside).

      • There is nothing around that says you can’t do the un-swirl up front as a “pre-swirl”. i.e. guide vanes first introducing swirl which is neutralized by the counter swirl introduced in the rotating part.

        • While I doubt it, that would be one for Bjorn to answer definitively.

          I do know all current conceptual forms have the TP or the RISE pointing forward, ie the never to be Embraer TP but pointing forward.

  8. Based on the parallel you draw with the ATR 72, the question arises: why doesn’t ATR choose a “mini RISE” for its future ATR evo? Wouldn’t the weight of this mini RISE, compared with a slightly hybrid turboprop, be one of the reasons not to go for it? and/or a possible aerodynamic interaction with the ATR high horizontal tail

    • Theres no advantage for existing high efficiency turbo props to have a new engine based on ducted fan tech.
      The size difference rules out any ‘scaled down’ RISE type as the takeoff power is way way less than for a 6 abreast 150 seater.
      Its difficult enough for engine makers to
      even build a new TP type engine as the market is so small and the savings in fuel in mostly short haul routes dont add up.

  9. Based on the parallel you draw with the ATR 72, the question arises: why doesn’t ATR choose a “mini RISE” for its future ATR evo (with the right pylon) ? Wouldn’t the higher weight of this mini RISE, compared with a slightly hybrid turboprop, be one of the reasons not to go for it? and/or a possible aerodynamic with the ATR high horizontal tail ?

    • Not worth it on at least one if not two fronts.

      First is the low production numbers. I believe ATR is making 60-70 ATR a year. That is vs a possible 1300 MAX/A320 type a year.

      I would also suspect the uber high cost for exotic materials and the engineering involved puts it vastly above a standard TP.

      All those other concepts like Hydrogen, semi electric are going no where (the costs are so high you would drive or take a train).

  10. OK Bjorn,you are going to have to explain exactly what the difference between a turboprop and an open rotor is.
    I don’t know and reading these comments,it seems that no one else is completely clear either

    • A Turbo prop is a turbo jet engine with a shaft drive to gearbox and propellers separate

      The open rotor is a turbofan with the gearbox integral and the unducted front fan blades looking like propellers. They look like propellers too when inside the fan casing

    • Well, propeller is an aerodynamic device pushing mass driven by a power source that is not involved in creating thrust.

      a jet engine is an intrinsic powersource/mass accelerator.
      diverting power to a bypass stream increases propulsive efficiency.
      ( which makes sense in relation/proportion to core efficiency. )

      IMU the (prop)Fan tag stays as long as the core exhaust is still providing part of the thrust.

      • GTF has a gearbox and turboprops usually throw any useful byproduct out of the back

        • Airbus decided to call winglets by an S name. That did not change the fact they are winglets.

          So, an Indy care is far more like a road car than it is an airplane (though it has some airplane like characteristics but so do cars these days)

          The OR was rear facing and counter rotating.

          RISE has a lot in common with a TP than it does open rotor.

          Variable Pitch Prop: Check
          Gearbox: Check
          Jet Drive Section: Check”

          A very high tech Turbo Prop.

          And you have to wonder on the blade weight and energy they have if the shed.

          You can add in an angle wing to complicate things.

    • I know nothing but here’s my take.
      A propeller is like a small wing slicing through the air generating lift in a forward direction.
      A rotor blade grabs a chunk of air and pushes it backwards, generating thrust by leaning on the air’s inertia. The deswirling blades extract more thrust as the angled air bounces off them.

  11. “Picture NG vs MAX engine front.”

    I’d supplement that with a side view comparison.
    ( IMU moving the larger frontal area engines up and out it at the problem core. )

  12. It also should be noted that the GE Catalyst engine is severely behind schedule and has dramatically delayed its only application in the Denali TP

    Again this is more about modern compliance, but the question is, an all new type TP aka RISE and what issues come up not even known to exist that are not solved as its not even in a test article form yet?

    The engine to run on the A380 is a prototype at best and that is not going to be the production form.

  13. ‘As the nacelle diameter increases, the engine/nacelle combination moves forward and up to get adequate ground clearance while still keeping the engine exhaust stream below the wing.

    The effect is that the nacelle, as a rotation body that creates suction pressure on its top, will cause the aircraft to have a nose-up pitching moment at high Angles of Attack (AoA). For the 737 MAX, which has a classical mechanical flight control system, it resulted in the flawed MCAS stability augmentation add-on.’

    A nose up pitching moment at high angles of attack.

    Doesn’t sound like MCAS was put in there to make the 737Max ‘feel’ like the 737NG, as Boeing said it was.

    • Frank P:

      All jets do that. That is what the auto trim function does in the MAX and the Airbus aircraft all use computers to smooth it out and keep it in bounds per the specifications.

      You can have some but there is a limit and the FAA at the time deemed it over the limit. I don’t have the knowledge to convey it other than in general terms, but it was on the edge and Boeing had a point it was not over.

      You are aware of Speed Trim? If not, it is in the MAX and every other jet including Airbus. Its a computer function that adjust speed of elevator movement as speed goes up.

      Much like a car with variable ratio power steering.

      Airbus uses what is called bump steering. Move the Hand Controller and it acts UP at a given rate, stops when you let go. Unlike a steering wheel that you do not return to center position to stop the thing form keeping on changing.

      I drove Rock Trucks that had bump steering. Easy enough to get used to, but it is different.

      Don’t get me wrong, once Boeing went down the route to deal with the FAA issue, they turned it into a monster. Low speed you need far more elevator movement to affect, high speed a lot less so.

      So, while not an anti stall item, it only was supposed to react if a stall was imminent. The MAX was not trying to do a loop, but it did have a bit more up movement than an NG.

      The Problem with AOA (my opinion) is its a false requirement. Yea its great for fighter pilots who live on the edge of stall if not over. But a passenger jet stays well away from a stall and stall is stick shaker (warning) or stick shover (very MCAS like). Both are used. .

      And into that area there are dragons. You can stall an aircraft at any speed (yep, 600 mph and you can stall it). Only one AOA.

      So now you have a problem, no speed trim as this is a stall approach smoothing system that is not speed related but AOA aka Stall related.

      So, worst case ops is low air speed as low air flow and you need more a lot of movement. But, using just AOA, you don’t know if its a low speed or high speed event.

      I will leave it to Bjorn to assess if you can do a speed trim function or not, but as MCAS morphed, it got very aggressive movement wise and no tone down for high speed (which you have to get into the program as well).

      Again personally I don’t think AOA belongs on a commercial aircraft and for a long time it was not. It in fact came in when airline pilots became predominantly out of the military pipelines and they wanted their comfort even though they don’t fly AOA (its only a couple of deg accurate so its not precision).

      One way to think about MCAS was it being a slippery slope as base solely on AOA, there was no speed component and clearly Boeing trying to force it into the system cherry picked AOA failure data because they did not want to deal with it.

      Ok FAA, fine, if you want it here it is and quit annoying us.

      Clearly that was a lethal decision for those two MAX flights.

      With my background I was stunned that it was not cross referenced to at least both AOA, in the controls world that is both stupid and fatal. I assembled who programs for machinery and the selection matrix was complicated by what kind of failure mode was best.

      Boeing threw MCAS in as a sop to the FAA. Clearly the people invovled knew that management wanted it done and gone and caved to the pressure (and any managers bread and butter was and is with Boeing cave to pressure as the default, its how you get ahead in that we care less world)

      The A320 has loading issues, you have to load it correctly or you get an out of CG condition. Aircraft are a bunch of balancers of how to make it work. MD-11 had so called relaxed stability and a lot of crashes.

      Airbus (and any FBW system) has a huge matrix of computer faults to ensure that they don’t wind up flying on manual alone.

      We have seen some 757 emergencies when the hydraulic systems failed and the backups also failed.

      FBW is uncontrollable without the computers but you fault a mfg for using a computer program to deal with an area of flight?

      You do realize how contradictory that is?

      Airbus has worked its way through issues over time as they occurred. AF447 is one that stands out because it suddenly dumped an aircraft onto pilots not prepared to deal with it.

      And yes you can program around Pitot failures with an alert when you are no where close to terrain and in level flight. That is an arbitrary decision as to when to shuck over to alternative control laws.

      You can program in the right reaction the pilots are supposed to do (85% power and 5 deg nose up).

      They made flying an FBW easy and then did not train crews on the real emergencies and it was fatal for AF447 as well as others.

      And in no way am I defending Boeing. But you should understand that its not a simple black and white thing its portrayed, its in depth technical and its seriously complicated.

      • FBW from Airbus has happily flown the plane, as requested by the pilots, into the runway approaches, the forest , the sea. You name it.

        The pilots have also stalled – at high altitude and low altitude- before crashing, even though that ‘cant happen’ under normal law. There lies the conundrum or in plain language, you cant idiot proof something

        • Duke:

          I don’t think pilots are idiots but it is an area that a high degree of skill and judgment is required and you can’t stop all acts.

          But it is true, if there is a way to find a way around it, people will do that.

          I saw a well protected roll up door almost get taken out. Snow berm in front of it, so the tug with 3 or 4 dollys rammed its way through it, then he hit the brakes and as the door is going up his hood is going under it. It just cleared the top of the cab when he got that far.

          The guy knew the door and had it all figured out, assuming the braking was as good as he needed it to be!

        • ‘as requested by the pilots,’

          Key difference.

          Pilots didn’t request the aircraft to fly itself into the ground on those two Max flights, did they?

      • ‘So, while not an anti stall item, it only was supposed to react if a stall was imminent. ‘

        Stall is imminent.

        System reacts to not have the aircraft stall.

        Not an anti-stall system.

        If you say so.

        ————————

        ‘They made flying an FBW easy and then did not train crews on the real emergencies and it was fatal for AF447 as well as others.’

        If by ‘real emergencies’ you mean they didn’t train the first officer to not panic and continually pull back on the stick, even when ordered not to…then yes.

        they didn’t train crews for real emergencies.

  14. Well done as always Bjorn……

    Of note is the ASA529 crash. The Brazillia lost a hollow Prop Blade due to internal corrsion and cracking. The aircraft went down off airport under control but unable to continue flight. I didnt know if this entered into your articles research.

    • I know of one blade off on a C-130 that sliced through the fuselage and hit critial structure and it crashed.

      Those blades are huge and heavy. As we saw with the space shuttle, a piece of foam can take you out with the right velocity.

      The AHJ would have to accept bullet proof on the props and that has yet to occur.

      The Qantas A380 met all the engine blow up specs, but they had never designed for what happened. The book said it would not do that.

      The good news was it was over built and survived but it sure was not planned.

      I know of a number of aircraft that were lost due to runaway props.

      DC-6 as I recall and once they suspected it, orders were to ditch rather than hope it survived. We had one do so when I was a kid a mile out from the island we lived on.

      The is a raw story about a P-3 that went down in the Aleutians due to runway prop. I think they lost another one in the Philippines to that as well.

Leave a Reply

Your email address will not be published. Required fields are marked *