Leeham News and Analysis
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Leeham News and Analysis
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Bjorn’s Corner: New engine development. Part 28. Wrapup.
By Bjorn Fehrm
October 11, 2024, ©. Leeham News: We have done an article series on why engine development takes longer than airframe development. Part of the reason is that advancements in engine technology can deliver considerably higher fuel consumption reductions than airframe advancements.
The change of engines for the A320 series and 737 MAX delivered a 15% improvement in engine efficiency. In contrast, the airframe improvement was less than half, mainly by stacking cabin seats closer together.
Figure 1. The 4:1 gear ratio Utrafan demonstrator in the Rolls-Royce test cell. Source: Rolls-Royce.
What improvements come from engines versus airframes for the next generation?
Engines
The last generation step in engine fuel consumption reduction was 15%. It’s important to understand how this was achieved. The CFM56/IAE V2500 generation had a ByPass Ratio (BPR) of around five, depending on engine thrust rating and flight phase.
The Pratt & Whitney GTF and CFM LEAP more than doubled BPR to above 10 and up to 12, depending on the thrust version and flight phase. The core efficiency improvements were more modest, even though this improvement has delivered the problems, not the larger direct drive or geared fans.
For the next generation, we can’t double the bypass ratio once more. This would mean BPRs of 20 to 24. Such engines would be too large and heavy. About 15 is planned, but it requires a change in gearbox principle from the GTF gearbox with a ratio of ~3:1, where the ring gear is connected to the fan, to a gearbox where the planetary ring is connected to the fan.
The gear ratio is then 4:1, which is needed for making efficient BPR 15 engines, Figure 1. However, a 15:1 BPR with a modest increase in the core temperature and pressures does not deliver a 15% reduction in fuel burn over the GTF and LEAP, more like 10%.
The alternative route is to skip the outer containment of a turbofan and go open rotor. Now, the BPR jumps to over 50, but we have new losses as the tip losses that are cut by the nacelle are coming back. All in all, together with a modest improvement in the core, the CFM RISE shall deliver a 20% improvement.
Airframe
The last round of airframe improvements of around 5% came mainly from cabin densification, which increased high-density seating from around 170 to about 180 for both the A320 and the 737-8. Airbus also wanted to count the change from a wing fence to a winglet, but this change was available on the A320ceo as well.
For the next generation, the improvement area will mainly be in induced drag. A folding wingtip can give up to another four to six percent in drag improvement, whereas a more extreme truss-braced wing can give seven to nine percent.
Composites shall give lower airframe masses but given the trend to larger wingspans, the strength of the composites will be used to enable the induced drag improvement without increased airframe mass.
Conclusion
We see that engine improvement delivers double the airframe improvements or more. This is why engine OEMs pursue the advancement in technologies we have discussed and why it’s so hard to achieve a product that delivers such improvement together with the reliability and durability that the previous generation has achieved.
The way to reduce the trouble level for new engine technology is to introduce several years of maturity testing into the engine programs, something only Rolls-Royce and Airbus have done to date.
For (unshrouded) fan blades the same AR related rules should apply as to wings ( my assumption ).
What kind of tip refining is available in this domain?
( In recent years winglets on wind turbine blades appeared.)
Thanks a lot for this interesting and detailed series of articles.
It almost seems like the airframe developers can’t do much other than wait for new engines to mature.
If it is already known that truss-braced wings can give seven to nine percent increase in efficiency, then why is only Boeing working on this? One would expect that with its vast resources, Airbus could leapfrog Boeing. Does this mean that Airbus doesn’t believe in it or would it infringe NASA patents?
Interesting question, Airbus have come far on the wing of tomorrow project but it does not appear to be truss-braced.
What makes you think that NASA has any commercially interesting patents on TBW?
The basic concept has been around for decades (Hurel-Dubois), and has more recently been worked on by the Europeans and Chinese. Any concepts previousy published/disclosed can no longer be patented.
Also, note that BA recently pulled engineers from the TBW project and assigned them to MAX-7/10 certification…says a lot.
In the past, various commenters here have opined on the nuisance aspect of the truss for ground handling crews, with obstruction/collision risk an important issue.
“BA pulled …”
This may be less indicative than one would expect:
“too small bedspread effect” however you pull it something will stay exposed i.e. limited human resources.
“obstructions”
I see more issues in managing stress distribution.
wing root to truss joint to truss root to wing root is a closed stiff triangle.
It’s Transonic Truss Braced Wing (TTBW) that may have patents I’d have thought.
Hurel-Dubois et al are definitely sub-sonic truss braced wings.
It seems to me that the airframers are a lot less pressured to obtain efficiency improvements than the engine manufacturers (I used to be an engineer for RR). There’s a lot that can be done to make airframes lighter and stiffer that the airframers turn their nose up at, where an engine manufacturer would grab it with both hands. Weight reduction in all the a/c (and engine) accessories seems to me the lowest hanging fruit.
Just because there are (or might be) patents doesn’t imply that those patents are commercially interesting.
If a patent can be easily circumvented via a non-patented route, then it’s essentially worthless.
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Effectively, engines just hang there, doing their thing.
In contrast, airframes have to be continually loaded and unloaded, they interact with air bridges and ground vehicles and other aircraft, and they have to be pleasant for those inside them. All of that creates barriers between the concepts of “possible”, “feasible”, “practical” and (commercially) “desirable”.
The blended wing is a perfect example: great as a paper concept from an aeronautical engineer, but a headache for airlines, airports and regulators.
I think its clear the Airbus does not feel the need to go to a new tech, aka TTBW.
So far the TTBW is theory and like RISE, you have to consider the acceptance by the public. Frankly there is no way to prove it until you do it (or offer it) and see what the Airline do.
In the meantime Airbus merely has to offer up a new wing at least for the A321 and they get a jump. Not as much as TTBW but its a lot quicker and loaded and ready to go.
What Airbus does in regards to the A220/A320 conundrum, no predictions. The A320 can be replaced by an A220-500, but it sure is not common to the rest of the Airbus lineup.
Boeing needed the C Series and Airbus did not. They got themselves boxed in there and there is no clear course. If Airbus customers continue to want a viable A320 they would have to put a wing on it and it pretty much has to be a dedicated wing. A220 Customers wanting the -500 would mean a dual path until Airbus replaced the A320 series.
Still its not the worst problem to have (not like having a strike going on!). Airbus is now in the position they can force things if they want to. Or they can just keep a loss going on the A220 and cough up for the A320.
Boeing is clearly going to be a distant number 2 in Single Aisle at best and the 777X is once again delayed. People will be looking at the A350-1000 when it was a toss up.
The 787 could be a bright spot but then it does not have an F version so Boeing has to spend money to do it or give up the new mid sized freighter market.
It seems a low cost GenX engine and a -400 Fuselage would be very appealing. Boeing ain’t going there either.
If I had a way forward it would be for Boeing to declare bankruptcy, reorganize themsleves. Not sure what the legal aspects on the Defense contracts would be, usually all obligations are negated. So, KC-46A etc at a selling point you get a return on it. But not being a financial guy………
As long as Boeing keeps making its defense products even in reorganization, the US DOD does not care. Yea they would not be happy with price increase if that is what happened but if they can put up with all the F-35 garbage, KC-46A, T-7A, MQ-25 are small potatoes.
GE has now had 5 years longer than expected to work on the GE9x. They would have been extremely foolish not to use this time to do lots of testing as well as work on the known issues. The thought occurs to me that most engines probably enter service with known problems because the manufacturers have just run out of time.
The A320NEO and 737MAX entered service on time because they were essentially just an engine swap (Although the MAX airframe turned out to have been rushed with fatal consequences)
GE has other engines they are testing and unless its required by Boeing they won’t do any more testing than regs require.
And no I do not think they enter service with known issues. They know there will be issues but not what they are.
GenX shaft coating as a part of serialized production being one of the more dramatic ones.
No one should be allowed to make a change like that and not have it tested.
Thanks for the article series Björn, it’s been very educational.
Barney Berlinger
Our gear technology firm has already designed/analyzed an increase in the gear ratio for the PW1000G engine by 39% to ~4.18 to 1 IN THE SAME ENCLOSED GEARING PACKAGE AS THE CURRENT MODEL. Our AI modeling indicates an engine weight reduction of 5%+ at the new higher engine speed and current power with the fan running at the current design speed. Current design engine speed 12,000 RPM and new speed would be 16,667 RPM. Can this advancement lead to much better fuel consumption?
hp rpm going up is not the intended path.
target (afaics) is longer blades with lower rpm ( tipspeed!”)
higher bpr for similar hpt speeds.
We can easily accommodate that change in configuration, but we are concerned about the higher drag of the larger fan/nacelle size. How would you reconcile these tradeoffs? Would fuel efficiency be increased with the engine running at the designed power/speed–24,000Kw and 12,000 RPM–with the larger fan? Thx. BB
Sometimes the engine position need optimization on wing to get the desired interference drag.
I doubt the TBW will happen. Too many issues and not enough benefit above a new higher aspect wing with folding tips. Airbus’s solution is a “gull” wing which provides enough room for the RISE engine or a 15/1 BPR geared turbofan mounted at the inflection point without crazy long landing gear.
I have read statements about he RISE being ready for EIS in 2035. My guess is that this is optimistic. I’m 74 now but would love to fly on a RISE equipped aircraft to see the thing changing pitch (blades and stators).
How do you know all the issues and what the benefits are for a TTBW?
It hasnt flown before ( at trans sonic speeds not propeller speeds) and the known unknowns is why they have a full scale demonstrator in the works to find out !
I agree with Duke. We simply do not know on the TTBW as far as its tech working and how it all works out with integration.
We also do not know public acceptance. Its not what they are used to.
Ergo, Airlines might not even order it because they don’t know how it goes.
RISE is also speculative. But its also a one off install. You won’t be able to put a jet engine on the same wing. So the same issue, tech has to be proven, airlines have to be convinced and once again they have goo reason to be leery of public acceptance of a prop job.
I tend to think the TTBW has less acceptance issues than a prop job and by quite a bit, but any reluctance and that is an opinion as there simply are no facts (there are facts about how the public feels about a prop job)
I truly admire your passion for aviation and new technologies. To be honest, for the average passenger, seeing an open-rotor engine on the wing of a new Airbus or Boeing might be surprising—almost like a throwback to the 1950s. They might wonder, “Why are they using propellers again, even if they have variable pitch?”
This was huge fun and very informative. Many thanks!
Let me also add my thanks to Bjorn for writing a very educational series of articles.
As of May of this year, GE still lists “Develop Advanced Thermal Management System and waste heat recovery system” as one of its objectives for the CFM RISE. I’m bummed that Bjorn wasn’t able to learn more about how CFM was going to achieve this goal and what efficiency numbers CFM was expecting from its implementation.