Bjorn Fehrm's Fundamentals of Airliners, Part 6, has a good interview illustrating my rationale for the mixed-engine quad proposal:
https://leehamnews.com/2015/01/19/funda ... he-engine/
let’s start with the question we put to CFM through their director of Strategic Communication, Jamie Jewell.
In Power Points around the LEAP, it is described as having lower overall pressure ratio than GEnx-1. It would mean lower static takeoff ratio than 47 (which is the ICAO Annex 16 ratio you give for the highest revving GEnx-1). Is this correct?
A: Narrowbody aircraft have different design objectives than widebody aircraft because of high cyclic operation, which can be up to 6-8 flights per day. The expectation for LEAP is 20,000 cycles on wing, first run, in order to achieve low maintenance cost. The message of the chart is that the cycle design and temperatures for LEAP have been selected for the right balance of performance and operating cost. High cycle temperatures resulting from high operating pressure ratio could drive down time on wing and increase maintenance cost.
As stated by CFM, it designed the LEAP to have lower OPR (thus lower thermal efficiency) than the GEnx-1. Just as I have been saying in this thread, the NB engine intentionally trades some fuel efficiency for better mx costs in a high-cycle environment. Incidentally, lower operating temperatures also mean less advanced materials in the core and therefore lower initial production/purchase cost.
The fuel efficiency sacrificed is on the order of only a few percent, which is why the tradeoff is worth it despite the fact that fuel is a much bigger portion of operating cost than engine mx.
Now consider the application of this dynamic to a mixed-engine quad:
- Let's suppose the Big:Small ratio of engine thrust is 2:1 (e.g. a 60k big engine and a 30k NB engine).
- Assume the SFC delta between the two engines is 3%.
- Next let's assume that, in cruise, we run the big engine at 90% of max thrust and the small engine at 30% of max thrust (which should still be close to the "cruise bucket" in which SFC remains relatively constant with RPM as a percentage of max RPM).
- Under those parameters, the smaller engine is contributing only 1/7 of the engine thrust.
- Given that the smaller engine has 3% higher SFC, the cruise SFC delta due to the smaller engine is only 0.4%.
That's a very small delta that might be counterbalanced already by the following fact:
-versus a uniform quad, the "big" engine is 33% larger than the uniform engines. If we're talking ~180k total thrust, a 60k engine's economies of scale vs. a 45k engines (lower trip clearance losses etc.) may already cancel out the cruise SFC delta attributable to the smaller engine.
But we haven't yet discussed the main benefits: Tailfin size reduction
For a quad, the tailfin is typically sized according to the OEI yaw moment contributed by the outboard engine. Under my proposal, the size of the outer engines shrinks by, say 1/3, which should enable something like a 1/3-smaller tailfin (assuming other critical conditions don't intervene).
On the A380 - and probably any VLA - the tailfin is disproportionately large: ~7% of wetted area (Swet). http://www.dept.aoe.vt.edu/~mason/Mason ... Berger.pdf
So a ~30% reduction of tailfin area saves >2% of Swet and on the order of 1% of OEW. Less engine wetted area and weight per lb-T for the smaller engines
The next generation of aircraft are going to pay a significant - though still justified - price in engine drag for the next-gen engines like Ultrafan. A 70k Ultrafan, for example, will use a 140in fan. https://aviationweek.com/air-transport/ ... 47-testbed
Compared to Trent 1000's 118in fan, that implies >50% greater Swet and drag for equal thrust.
In that scenario, engines would account for ~20% of total airplane parasitic drag (Dp).
While the drag penalty is worth it for engines that provide all of your cruising power, my proposal would allow offloading excess engine power (i.e. power not needed to maintain cruise) to the smaller engines, whose less-efficient power would come with a much lower Dp penalty.
If 33% of engine power comes with 33% less drag, that's ~11% less engine Dp or a >2% delta to total Dp. Of course using an older-tech engine would mean a higher SFC delta but we can divide its impact by 7 as outlined above. The drag-SFC tradeoff (inclusive of tailfin effect) would be neutral even if SFC delta increased to 15%, leaving a much bigger delta to engine mx and acquisition cost. Where that "tradeoff between tradeoffs" regarding fuel vs. mx and acquisition would be optimal is a matter of detailed design.
Plus the smaller engines will weigh less, per lb-T, than uniform quad engines or the larger engines. Why? Because they're thrust-bumped for higher ratio of thrust to dry weight (T/W) ratio than would be an optimal cruising engine.
The sweet spot for modern engines is ~5 T/W. Within engine families there's variation, but the larger thrust-bumped variants always face penalties in SFC and mx cost for higher T/W (unless additional tech intervenes, as RR claims it got the TXWB-97 to SFC parity with the weaker versions via additional tech).
Because the smaller engines will be at least 6 T/W instead of 5, you'll save a couple thousand pounds of OEW inclusive of pylons (engine dry weight is ~60% of propulsion system weight).
If a ~2028 VLA used PIP'd LEAP/PW1100 small engines, I'd expect the deltas versus Ultrafan, per lb-T, to be as follows:
- -30% acquisition cost
- -30% cost of life-limited parts (LLP)
- -35% Swet/Dp
- +10% cruise SFC
Combining these parameters into a DOC accounting, we have the following building blocks:
- >2% delta to Dp from a smaller tailfin = >1% delta to L/D
- -1% delta to OEW from smaller tailfin
- -2.5% delta to Dp from the NB engines = ~2.5% delta to L/D
- -0.5% OEW from the NB engines
- +10% SFC for the smaller engines, contibuting +1.5% SFC at plane-level
Combining these factors in a terribly rough way (post is already long enough), we get ~4% lower trip fuel burn.
- If 1/3 of engine cost is 30% lower, we get -11% delta to engine acquisition cost.
- If engines are 1/3 of plane acquisition cost, we get ~3.5% lower plane acquisition cost
- Similar to acquisition cost, a -11% delta to LLP cost
- If engine mx is 2/3 LLP cost, that's -7% engine mx delta
- some positive delta to engine personnel/administration cost for mixed types, can't say what. It would have to be a 14% delta to negate the LLP delta under these parameters.
- OTOH, the NB engines used are ubiquitous so their personnel cost might be lower than for Ultrafan-gen widebody engines
Now to sum up the DOC costs, let's say fuel, acquisition, and engine mx are 40, 25, and 10% of DOC respectively. Given -4%, -3.5%, and -7% delta to the respective DOC components, my proposal would give approximately -3% DOC delta.
That's before incorporating the added complexity of mixed-engine maintenance, but for that factor to negate 3% DOC would require a doubling of total engine labor-related costs and would require the NB-engine-specific costs to be about six times
as high as normal. I can't even see those costs being twice as high as normal, so I'm pretty convinced that the proposal would pencil out if my aero-structural and acquisition cost estimates are in the ballpark.
A concluding note about why this proposal might be something for a late-2020's quad but not an earlier one:
The proposal works largely because of the very high price - in acquisition, maintenance (LLP's), and drag - of Ultrafan-gen engines. Absent the 30% deltas between Ultrafan-tech and older tech for these factors, the proposal would rely only on the tailfin delta and probably wouldn't be worth the OEM's design effort even if it conferred a <1% DOC advantage. Future planes will be facing increasingly large non-fuel tradeoffs for Ultrafan-level fuel efficiency; a proposal such as mine is the only way to avoid these tradeoffs as it's the only way partially to avoid using Ultrafan-level engine technology: A twin or uniform quad can't afford to sacrifice 10% engine SFC to avoid Ultrafan drag and other costs; only a plane that cruises primarily with Ultrafan and takes off with more help from older/cheaper engines can partially ameliorate the coming fuel/other cost dynamics.