Leeham News and Analysis
There's more to real news than a news release.
Leeham News and Analysis
- Boeing’s new fighter award moves away from money-losing fixed price contracts March 24, 2025
- Airbus still struggles to deliver planes on time March 24, 2025
- Bjorn’s Corner: Air Transport’s route to 2050. Part 14. March 21, 2025
- How good is the COMAC C919? March 20, 2025
- Boeing CFO Brian West: OK on tariffs for now, backlogs and inventories help March 20, 2025
Bjorn’s Corner: New engine development. Part 27. Long developments
October 4, 2024, ©. Leeham News: We do an article series about engine development and why it has longer timelines than airframe development. It also carries larger risks of product maturity problems when it enters service than the airframe of an airliner.
We looked at engine technologies and why several are challenging to bring to a mature technical state. Then, we compared previous engine developments with the present generation. Memory is short. What we could see is that previous generations of engines carried larger reliability and durability problems than the present generation engines.
Now, we discuss the development timelines for key engine versus airframe developments.
Figure 2. The GE-36 Open Fan of the first generation, from 1988. Source: Wikipedia.
Airframe versus Engine development
Airframe
A friend of mine is an experienced program manager developing a new generation tube and wing airliner. He is managing a program that employs new technology in the propulsion area.
While the propulsion technology poses technological and other challenges, the airframe part does not. “We have come to a point in airframe development and certification that, if it’s a conventional tube and wing airframe, we have very few unknown unknowns left, if any. We know how to do this type of development, certification and production”.
This assumes the airframe follows the classical design principle of a tube and low-wing airframe with wing-hung engines and a classical horizontal and vertical tail empennage. There are very few in-market variations of this theme once we come to an airframe size where the engines hung from the wing allow an acceptable tall landing gear.
The advantage of this design compared to previous versions with engines at the rear, high wings, or T-tails are significant. These variants are structurally less efficient without rendering any aerodynamic advantages. The concentration of the engine, landing gear, and wing forces in a confined area at the inner part of the wings is structurally efficient. The aerodynamic loss that the wing-mounted engine with pylons generates is small compared to the advantages of the layout.
The other area of development over the last decades has been the material system for structures. Composites have increased their part of the primary structure. However, it has happened very gradually, and the experience of material change has been collected over the last 50 years (the first large composite structure was the vertical tail of the Airbus A300).
The fly-by-wire development followed a similar path, with the first application in Concorde entering into service 48 years ago. The development has been gradual, with analog systems for the pitch channel at first, gradually applied to the roll channel, and finally, recently, the yaw channel.
Once digital technology and its software were sufficiently understood and safe, analog technology was gradually replaced by digital computed flight laws.
Engines
Engine technology has several technology areas that have gradually developed in ways similar to airframe technologies. Examples are turbine blade and nozzle development with ever-better casting methods (directional crystal, single crystal), cooling routings, and coatings.
However, there have been developments that have been larger technological leaps. Examples are fan gearboxes, where the main problem was not the gearbox itself (planetary gearboxes in aviation are over 100 years old) but the technology to predict problems months rather than days in advance.
It took Pratt & Whitney 30+ years to perfect and prove this technology. The application of a geared reduction between fast-running compressors and turbines and a slower-running fan is now accepted and established technology.
Another technology is Open-Rotor or Fan technology. GE flew the first generation 1986 on a Boeing 727 (Figure 2).
Figure 2. GE-36 on 727, flying in 1986. Source: Wikipedia.
The sound level and vibration were very high, and as the fuel price went down from the 1973 crisis, the technology was not developed to a state where it could be used on aircraft.
GE never stopped developing the Open Fan. In several programs with NASA, it kept chipping away at the main problem, the sound level (which is pressure waves and, therefore, a primary source of surface pressures and, thus, vibrations).
Around 15 years ago, it understood the source of the sound in detail and what to do about it. The next problem was the engine’s gearbox complication. The initial Open Rotors had been using concentric drive shafts from double planetary gearboxes to achieve a de-swirled thrust flow.
The suggestion to simplify the setup to a fixed de-swirler and to turn the engine around came about a decade ago. The detailed understanding of the fan flows, and the ensuing simulations showed that fixing one of the rotors could be done without any engine efficiency loss.
The CFM RISE engine, which builds on the GE Open Fan work, will be mature enough for large-scale entry into service by 2035. At that time, the total development time for the technology will be 55 years, a considerable investment in time and money.
Conclusion
The largest reduction in fuel burn is from a new generation of engines. While an engine can gain up to 20%, it’s generally understood that we talk about less than 10% from airframe advancements.
Fuel reduction advancements are the most effective way to reduce environmental emissions from Air Transport. They’re instant with the entry into service of a new generation and require no new legislation or subsidies from the authorities.
It’s the only environmental improvement technology that has been effective so far, and given the progress and pace of alternative propulsion concepts and the production and adoption of SAF, it will remain the most effective way this side of 2050.
You can mess up a traditional wing and tube aircraft as well pretty easy. Just push technology to the edge and you get into problems like hydrogen embrittlement on very high strength steel, so complex avionic and control system that to simulate every combination failure mode and correct actions require massive computer power and skills. Then simple failures like recently on a 737MAX component that got the bearing installed the wrong way can trigger a failure route as the component freezes up.
@Claes:
I am scratching my head on the Steel Emgrittlement. In regards to airframe there is little steel invovled (gear?). I don’t get how you can mess it up as its a well understood aspect regardless.
The only surprise I know of was the GP7000 that blew out over Greenland . But that was an engine aspect so its not airframe.
Agreed that control systems are uber complex, but that too is well understood and no surprises with lot of examples to go from (F-16 anyone?). So yes a lot of testing but if you make changes you have a solid known base.
And a failure to install a bearing right has nothign to do with Tube and Wing or any other airframe type, that is basic mechanics. Impacts can be severe in an Aircraft crash but mechanics messing things up is as old as mechanics.
HSS steels in mainly landing gears are made stonger and stonger with a wear protection of hard chrome. As you get higher strength you get more sensitivity to hydrogen embrittlement and sometimes in combination with corrosion, for each generation you want to use higher strength steels to save mass. When you grind the chrome to dimension you risk overheating in the steel below the chrome and start the hydrogen embrittlement. Just see the number of gear collapses at airliners.
“Just see the number of gear collapses at airliners.”
I’ve only ever seen gear collapse due to locking linkage failures.
Even crashes seem to rip articulation elements and not the oleo leg ( though it may get bent )
@Claes:
I have seen the gear punch up through the wing, linkages not lock and a rash of those on the Q400 (Dash 8) were bad maint.
So while in theory its an aspect to consider as far as maint and refurbish goes, its nothing to do with Tube and Wing.
There has been a number of cracked wheel axles and cylinders due to a combination of corrosion and hydrogen embrittlement. Airbus enforces the use of Barkhausen noise NDT to discover overheating under the hard chrome after grinding and I think industry will follow on new models with even stronger HSS steels used.
Nice walk down memory lane with the UDF. If only the 7J7 was actually made.
Forget the Green aspect, are SAF safe to use?
As far as SAF goes? Probably. But then we had the fuel cooler (heater?) freeze up on the 777 into Heathrow.
Or the ice up of the GenX engines that may be a climate change aspect.
They have tested SAF and so far no reported issues. But you never know when something new crops up, or a different base that acts differently.
I think apart from engine development, we might see priority changes in speed versus economics/ restriction.
Aircraft fully optimized for 20% slower airspeeds up to 1500km. More allowance on noise..
Lower utilization, longer flight lenghts, lower fuel burn/ polution?
Modify old starting points, assumptions.
Speed vs economics is indeed an interesting one.
The ATR and Dash 8 are in stark contrast. The ATR can’t go as fast, has sold more and currently the sole surviving TP in its class.
The Dash 8 could operate at lower speeds and be almost as economical to run as an ATR. An ATR could not go as fast as a Dash 8 and make up time or in the case of getting more ops in a day, do that.
But then its a shift to jets in the US and the ATR is not exactly a sales sensation (I believe about 60 a year).
Alaska Airlines (aka Horizon and that weird relationship) elected to drop the Dash 8 and go with the E1-175. Western US is a natural for a Dash 8 profile. Cities tend to be a bit further apart (speed) and it has no route issues in terrain profiles for one engine out.
Ethiopia and India/Pakistan/Burma would be the other natural arena for the Dash 8.
There is a quality difference between ATR -600 models and Q400. That is the main reason for ATR dominance. The CRJ’s are pretty good but their upgrade programs was never started on the Q400’s.
Care to explain more about quality of the ATR vs the Q400 ? I’m interested on what you base that comment on.
Seems it was based on a series of landing gear collapses. Goodrich was the maker
“More allowance on noise..”
Can’t see any reason why. Some should suffer more so others can fly? That won’t work. No aircraft, no noise and no climate impact. A louder engine wouldn’t be acceptable. Therefor even a far more fuel efficient engines can’t be louder.
@keesje
I can see some trade offs on speed so long as it is at lower volume airports. Ultimately, fewer flights equals fewer revenue opportunities.
As far as noise goes those restrictions are not coming down anytime soon. I rank that up there with assuming scope clause is going away
There is a trade off as with increased speed as you can fly higher into thinner air. Still eventually you need to land, so trip length is a factor on max cruise altitude
When I read the paragraph “airframe versus engine development”, I think of the old team of engineers in charge of pylon design in conjunction with aerodynamicists and “nacellists”. Today the trend seems to me to be for both the aircraft manufacturer and the engine manufacturer to set up teams to optimize their mutual INTEGRATION. In my opinion, the aircraft manufacturer will retain a form of primacy in the sense that the 3D definition of the wing is itself linked to the wing-fuselage junction and the positioning of the landing gear, two subjects that he must master. We were told that aircraft design was done around the landing gear bay .
I can’t imagine GE’s or PW’s design offices not having catalogued the various wing options (TBW, gull wing, flying wing, etc.) before embarking on their own choice of engine manufacturer, which themselves are very heavy in terms of costs and time, generally further upstream than those of the aircraft manufacturer. This reciprocal duplication of upstream studies seems inevitable to me, and should be a fascinating thing to experience from the inside. Happy young engineers!!
Would love to see more detail on “concentric drive shafts from double planetary gearboxes”
Sounds devilish complicated
Yea. I worked with what the Mfg (Sumitomo) called a Cycloidal Drive. Truly strange monkey motion of flat roller bearings. It confused people so much they came out with a Hand model to show the concept. I still have mine.
Worth a looking at if no other reason than to twist your brain a bit.
https://www.youtube.com/watch?v=1ijFove42Kw
Also a lesson in over hype. Its a nice compact system, but Sumitomo claimed it had serious shock capability (rock crushes and the like) and was bullet proof.
I found a group putting out serious amounts of metal, after some sleuthing it was found that they had done a slight frame undersized for the application. Small conveyors and I would not have thought it would be an issue but fail they were. They replaced them with look alike units. Something in the area of 5 hp motors and only rated for 4. Mild application as conveyors have a take-up roller that bounces up and down so the start is cushioned.
They failed too often regardless. So great ideas and drive theory have to be proven out.
Quite a lot of rather intriguing rpm reduction solutions around.
Another sample is the “single tooth pinion gear”
see https://www.jstage.jst.go.jp/article/jsaeronbun/53/4/53_20224469/_pdf
but here talk is about dual _planetary gears_ with coaxial power offtake.
Obviously more parts than the single planetary reduction used in GTF and future RISE.
apropos & @Bjoern: why is it “GE RISE” when CFM does the RISE and GE used to be tasked with the core engine while Safran did the LP ( fan and such ) modules?
Very interesting – y0u have to admire the minds that came up with that.
Not many a/c applications at a reduction of 11:1. Perhaps a motor in the landing gear strut driving the MLG wheels at 1/11th of the RPM. That would save fuel during taxi.
Or a flap/slat drive, between motor and torque shaft. Is that mechanism easy to lock, or does it backdrive?
On a speed/ utilization balance, maybe authorities should set targets on efficiency and make taxes, landing rights connected to those.
Like in F1, set e.g. restricted cilinder volume, hybrid requirements & let engineers get the maximum out of it to win the competition.
Didn’t work for the 2027 engine polution restrictions, a government gave 5 year emission waivers, protecting the local manufacturer.
They did . ICAO efficiency numbers ( and adopted by FAA) set around 2017 to come into effect around 2027
The 767 waiver will only effect a small number of planes for US domestic use only
Its directed at the CO2 output per passenger, which effectively is a fuel efficiency number . Planes are already highly efficient
For ships propellers they don’t use de-swirlers as such.
But a device that looks similar but free turning called a Grim wheel – after it’s inventor.
“SIEM explains that the Grim vane wheel is located immediately behind the propeller. The vane wheel blades have turbine profiles in this inner section whereas they have propeller profiles at the outer blade section. In the turbine section, kinetic energy is taken from the propeller slipstream, which is directly transformed again within the propeller section into an additional thrust. The main function of the vane wheel is to extract energy from the propeller slipstream in the turbine portion and convert this energy into additional thrust in the propeller portion.”
The QE2 liner had them after it’s engine conversion but they got damaged very quickly. Seem to be making a come back
https://maritime-executive.com/article/nyk-to-research-energy-saving-devices-for-use-on-50-bulkers
A very interesting article in AW yesterday
https://aviationweek.com/aerospace/emerging-technologies/airbus-details-next-gen-engine-study-options