faro From Egypt, joined Aug 2007, 1489 posts, RR: 0 Posted (7 months 1 week 2 days 22 hours ago) and read 8302 times:
IIRC, the A380 has today the lowest CASM of any operational airliner. Its wingspan is also constrained by regulatory gate width requirements so that it has a relatively stubby wing.
How is such a low CASM achieved despite an aspect ratio of around 7.5. Comparatively, most newer airliners have AR ratios around 9 or higher. An AR of 7.5 is comparable to the L1011-500 and VC-10, some rather outdated aircraft.
What is it about the A380's wing that counters its relatively low AR? In terms of lift to drag ratio, is it more or less efficient than the 77W's wing?
dw747400 From United States of America, joined Aug 2001, 1252 posts, RR: 1 Reply 2, posted (7 months 1 week 1 day 17 hours ago) and read 7945 times:
AR is certainly important, but it is not the only measure of efficiency. I'd bet that despite the similar aspect ratio the A380 wing utilizes many aerodynamic advances that make it far more efficient than any other jetliner wing of similar aspect ratio. That said, a higher aspect ratio would certainly have had aerodynamic benefits (though the structure of the shorter wing is lighter).
Combined with other new materials and systems--plus, as above, the economies such a large aircraft--and the A380 is incredibly efficient. Certainly not having the constraint on span would have offered other opportunities for further improvements, but like all designs it was a trade off, and apparently still good enough.
ferpe From France, joined Nov 2010, 2665 posts, RR: 58 Reply 3, posted (7 months 1 week 1 day 12 hours ago) and read 7838 times:
As I have this all in a large excel model, the easiest is to copy the relevant parts including some other notable frames and then look at the different values. I have not studied this before and not looked beyond a quick glimpse so everyone comment. I will leave my first thoughts below the table:
1. This part of my airframe model shows wing data but also drag and lift values on an overall frame level.
2. What one can see is that only the latest frames of the longhaulers have better aspect ratio then the A380. If we would look at only a wing level we would see a L/D (the true figure of merit as it shows how much drag is created for the lift you need to fly level) that was clearly worse for the A380 then e.g. 788, 359 and -9X.
3. What you see in the table is the drag created on an airframe level when lifting the aircrafts weight. Here the very efficient packaging on the A380 benefits the OVERALL L/D as the dominant drag components, parasitic drag (where skin friction drag is dominating) is relatively lower for the A380, it has a low wetted surface per transported pax due to it's whale like packaging. So the frame L/D when cruising at Cl 0.5 (I have put all at Cl 0.5 at FL370 by adjusting the weights so that we can compare better, except for 320 and 738 as they seems to be designed for a higher Cl cruise) is only 1 unit lower then the soaring eagle -9X .
So the A380 is doing pretty well on an overall L/D despite its stubby wing, part of that is low wingloading and a good transonic design = low compressibility drag and then the low overall parasitic drag. These two compensate a somewhat higher induced drag level (Dp is all drag independent of lift and Di is all drag dependent on lift, i.e. also compressibility drag) .
Edit: Just a caution, don't look at the drag per pax etc figures (oops, should have taken them out). FL370 is the average weight FL for the A380, some other frames like the 748i has this FL as the last just before it is time to descent. No wonder they have a lower overall drag level per pax as the frame is at 69% of MTOW vs 78% for the A380. I can't show different frames on different flight levels easily so that is why I adjusted the weight to get them all at the same loading of the wings at FL370. The picture only tells a good story around L/D at that FL. The drag coefficients and the basic wing data (effective span including wingtip devices, wingloading, spanloading etc) is OK to compare as well.
ferpe From France, joined Nov 2010, 2665 posts, RR: 58 Reply 4, posted (7 months 1 week 1 day ago) and read 7669 times:
As I have the wetted areas for the frames (you need them to calculate the dominant drag factor, skin friction drag which is the major part of the parasitic drag (or drag independant of lift)) I revised the table to include the frames total wetted area and then I divided it with the cabin area that the airplane transports. I did not use Pax count as these are not calculated using the same standards (ref the A380 vs 748i debate re seating).
While I guessed on the low wetted area per pax in the previous post it was quite interesting to see the result, it is highlighted (click on the tables to get them in sharp resolution):
Here we see that while the A380 might not be top at induced drag reduction is in a class of it's own when it comes to avoiding the most prevalant drag, skin friction. Normally induced drag (the compression drag is tiny in the Cdi so it is essentially only induced drag) is from 45% down to 1/3 of the drag as the frame gets lighter, we can see that at average cruise weight it is 50-50 for the A380. It compensates by being efficient in other corners.
One can also see why the 748i can't compete, neither with the tube+wings (787 to -9X) nor with the double decker A380. it is simply to much wetted area for what it does, it is an inefficient single level frame with the hump making it double decker on to short a distance, lots of wetted area not contributing to the cabin floor area. As a part double decker it should have a lower factor then the -9X, it does not. And while the wing is a shade better on aspect ratio and span loading then the A380 it is to little to help. Both the A380 and 748i lacks span compared to the new DA crop, A380 because of the 80m dilemma, 748i because it is a base design which is 40 years old.
r2rho From Germany, joined Feb 2007, 2441 posts, RR: 1 Reply 5, posted (7 months 1 week 20 hours ago) and read 7578 times:
Very interesting analysis ferpe.
Another thing to note is that the A380 wing was designed to eventually carry the weight of the -900, which is another constraining factor leading to more "stubbiness" (an A388-only wing could have surely been shorter at the root). That despite that the A388 has turned out so efficient shows that they must have really optimized the aerodynamics to extract the best out of the given constraints. Even then, it looks like Airbus have found they can optimize further, as is shown by the new wing twist applied to the BA frames and onwards.
prebennorholm From Denmark, joined Mar 2000, 6128 posts, RR: 55 Reply 6, posted (7 months 1 week 8 hours ago) and read 7384 times:
Thank you very much, ferpe, for an extremely interesting analysis.
Just one extra comment: The low wetted area of the A380 comes at a price. It has a relatively small cargo volume.
Cargo volume offers potential airline revenue just like cabin floor area does. The large Boeing planes (744, 748i, 773ER) offer a substantially larger cargo volume to cabin floor ratio.
The A380 was made as a pax plane - pax and their baggage. That's perfect when that is your customer base. If, however, your customer base is pax + baggage + substantial high volume cargo on sectors considerably shorter than max range, then the A380 is soon maxed out and loses points to the competition.
Always keep your number of landings equal to your number of take-offs, Preben Norholm
ferpe From France, joined Nov 2010, 2665 posts, RR: 58 Reply 7, posted (7 months 6 days 14 hours ago) and read 7255 times:
Quoting prebennorholm (Reply 6): Just one extra comment: The low wetted area of the A380 comes at a price. It has a relatively small cargo volume.
You are right, the double decker layout gives only half the length to the underfloor area. But that is not all, the A380 engineers seems to have chosen a MLG design that worsened this problem even further, the design impinged on the cargo area at great length:
The MLG is staggered behind each other and protrudes therefore into the cargo area over a long distance (on an already short under the floor area) and made it 1 abreast LD3, 748i and normal tube designs avoided that by focusing the MLG directly behind the center wingbox, for the 748i even when it had a 4 strut MLG like the A380 to spread the pavement loading:
Here not to scale screen-shots from the ACAPs with an LD3 summary for each frame type that I added:
The grey area shows the stowage space for the MLG in the underbelly.
Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 13, posted (7 months 3 days 16 hours ago) and read 6746 times:
Quoting Stitch (Reply 11): What is the current gear good for? I heard 640t (with the brakes good for 625t with brakes on all 20 main wheels).
If I recall correctly, the main limitation was not primarily the landing gear itself but pavement loading.
Which was also why the A388F was supposed to get that additional center bogie, just like the A389 later on.
Quoting ferpe (Reply 7): the A380 engineers seems to have chosen a MLG design that worsened this problem even further, the design impinged on the cargo area at great length:
What would have been a realistic alternative?
When looking at the (excellent) comparison of the floor plans I believe one thing to remember is that in the 747 the lower deck occupies almost half of the main cross section versus less than a third in the A380. Plus the six-wheel A380 body bogies simply couldn't be stowed near-upright like the shorter 747 ones.
Keeping the center area flexible for use either for additional cargo space in the lighter version(s) or for an additional wheel well in the heavier ones looks like a pretty decent concept given the needs and circumstances.
An A389 would probably take away some space for the additional MLG leg there, but if the total length is sufficient, it should still provide somewhat decent cargo space, even if the increased passenger load with their luggage will keep eating into it. But how could a full double decker ever be a first-rate cargo hauler anyway?
Effectively you're paying for the somwhat better cargo ratio in the 747 with the empty crown space above the cabin you can't use.
The wing was already designed for the larger version from the start, as was the MLG layout. Effectively the A388 is already a somewhat compromised shrink of the initial, full design. They already had their hands full with pushing the 80m box to its limits airport-wise with the initial version (let's ignore the originally proposed even shorter double-shrunk -700 for now), but the design clearly points to an A389 being the full implementation.
That the A388 today is already quite efficient even while having to haul around much of the structure of a future A389 looks like a pretty aggressive but forward-looking design strategy to me.
ferpe From France, joined Nov 2010, 2665 posts, RR: 58 Reply 19, posted (7 months 2 days 9 hours ago) and read 6439 times:
Quoting astuteman (Reply 12): My guess would be the launch weight of the 388 plus 30t - i.e. 599 tonnes
OK, so here the table with a projected A380 in 3 versions, std, -900 with the -800 wing and finally -900 with sharklets:
The total wetted area increases (fuselage length in red increases) therefore the parasitic drag increases and the L/D decreases somewhat. The sharklet equipped A380-900X claw back half of that by reducing drag due to lift, see the effective wingspan and aspect ratio in red. That a full doubledecker is an effective transport form for pax is seen when we divide the total wetted area with the cabin area, we are now at a sensational 4.9 m2 wetted area per m2 of cabin space (click on the table to see better).
It still does not make sense to look at total drag per m2 cabin area as I have the different birds at very different weight in order to get them to Cl 0.5 at FL370. We could look at drag independent of lift (and therefore weight) divided by cabin area, you could do that exercise. It will track our wetted area per m2 cabin closely as skin friction drag is the dominant part of the non lift (=parasitic) drag.
One interesting drag factor which decreases is the fuselage pressure drag factor or the form factor drag. This is modeled as an increase of the skin friction drag due to airstream overspeed to get around the fuselage (there is no such thing as drag due to frontal area, it is called pressure drag and is taken as the increased speed needed to traverse that whale body with all it's twist and turns. We can see that this drag likes the longer and slimmer body (better finesse ratio which is fuselage dia to length)).
If you look at the range you see I took an existing -900 column I had which had TXWB engines (TSFC around 0.52 instead of 0.56 ), that is why these ladies fly some 200nm longer then the -800 despite being heavier and having higher drag .
WingedMigrator From United States of America, joined Oct 2005, 2066 posts, RR: 56 Reply 20, posted (7 months 2 days 3 hours ago) and read 6360 times:
Quoting Klaus (Reply 13): If I recall correctly, the main limitation was not primarily the landing gear itself but pavement loading.
Interestingly, the A388 has a lot of room to grow in pavement loading. 569 tons on 20 wheels is only 28.5 tons per wheel, well shy of the 32 routinely seen on other aircraft. 32 x 20 = 640 tons before you need a fifth leg... My numbers may not be all that accurate, but for sure it has plenty of room to grow.
Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 22, posted (7 months 1 day 17 hours ago) and read 6249 times:
Quoting WingedMigrator (Reply 20): Interestingly, the A388 has a lot of room to grow in pavement loading. 569 tons on 20 wheels is only 28.5 tons per wheel, well shy of the 32 routinely seen on other aircraft. 32 x 20 = 640 tons before you need a fifth leg... My numbers may not be all that accurate, but for sure it has plenty of room to grow.
As far as I'm aware it's not just the weight per wheel but also the area the wheels are distributed across which is constrained.
astuteman From United Kingdom, joined Jan 2005, 9599 posts, RR: 97 Reply 24, posted (6 months 3 weeks 6 days 14 hours ago) and read 5835 times:
Quoting ferpe (Reply 19): OK, so here the table with a projected A380 in 3 versions, std, -900 with the -800 wing and finally -900 with sharklets:
Many thanks my friend. I and one or two others used to model this sort of stuff, but I've stopped now - yours are so thorough.
I've always been of the view that when (and if) an A380-900 (as originally described by Airbus) was launched, it would end up outranging the A380-800 a bit, whilst offering 14%-15% more capacity.
I don't see much of a movement in trip cost either, especially if one assumes TXWB's
25 ferpe: Well modeling the way you did it (and I as well before taking it to a full drag model) makes a log of sense, it can bring a pretty close understandin
26 Matt6461: Ferpe - thanks for posting this. Can you share your excel sheet? Also, are you using a program? How are you coming to your numbers?
27 ferpe: Hi, I have been advised to not share the model I have made but it is not very difficult to do one for yourselves. I pretty much gave a cook-book desc
28 Pihero: Hi, Guys ! There's a very good illustration of the A380 wing efficiency, through a picture I discovered on Flicker : A380 humid air takeoff Here we se