AKAIK the DC-8-70 series was not particularly known for its airfield performance or generous wing yet here it can manage a significant nose-down attitude at very low height. This I take it must mean that it has a very efficient/generous wing at low speeds which is even more surprising since it does not have leading edge slats.
My question is when is an airliner considered structurally 'over-winged'? That is, when do you deem that it could easily manage with a smaller wing to satisfy its nominal mission profile than that installed? The 720, 747SP and A380 come to mind, and -to a lesser extent- the 788/A358.
Can the A380/747SP/788/A358 do a nose-down final approach like the DC-8-70 in the above pic at nominal landing weights? How can you devise a meaningful criteria to decide on 'rightwingèdness'?
Pihero From France, joined Jan 2005, 3588 posts, RR: 72 Reply 3, posted (1 year 7 months 2 weeks 11 hours ago) and read 2686 times:
Quoting faro (Thread starter): it must mean that it has a very efficient/generous wing at low speeds which is even more surprising since it does not have leading edge slats.
No. The reason is in fact the lack of L.E slats. We had a discussion on this subject not very long ago.
ferpe From France, joined Nov 2010, 1916 posts, RR: 56 Reply 4, posted (1 year 7 months 2 weeks 11 hours ago) and read 2681 times:
Quoting faro (Thread starter): AKAIK the DC-8-70 series was not particularly known for its airfield performance or generous wing yet here it can manage a significant nose-down attitude at very low height.
The attitude on approach has very much to do with the flaps and how they are used. Have them extended to a very high angle and 2 things happen:
1. Your pitch attitude goes very much nose-down, good for visibility and avoiding tail strikes at float.
2. If they have a very high angle they generate a lot of drag > you can lower your pitch angle even more and keep power on to speed up any needed go-around power increase. You would have to retract the flaps for this go-around however.
The 2 above have not so much to do with how large the wing is, more what flap type and how the designer use the flap to achieve:
Pihero From France, joined Jan 2005, 3588 posts, RR: 72 Reply 5, posted (1 year 7 months 2 weeks 11 hours ago) and read 2680 times:
Quoting faro (Thread starter): My question is when is an airliner considered structurally 'over-winged'?
When a limiting factor is the minimum control speed, Vmc.
For reasons of economy and operational efficiency, manufacturers do not design over-winged airplanes, but sometimes a requirement - generally from an airline - for special performance will make one so : the best example is the 757 which was required to lift ooff a max payload out of Dever in the summertime, making it the famed hot rod it is anywhere else.
The 388 is another example - or close to - as the wing was designed for the -900 and some built-in growth potential.
Quoting faro (Thread starter): Can the A380/747SP/788/A358 do a nose-down final approach like the DC-8-70 in the above pic at nominal landing weights?
No. The design of the high-lift devices would prevent it.
Quoting Pihero (Reply 3): Quoting faro (Thread starter):
it must mean that it has a very efficient/generous wing at low speeds which is even more surprising since it does not have leading edge slats.
No. The reason is in fact the lack of L.E slats. We had a discussion on this subject not very long ago.
Can you kindly provide the link to that discussion, couldn't locate it on my own...
Quoting Pihero (Reply 5): Quoting faro (Thread starter):
My question is when is an airliner considered structurally 'over-winged'?
When a limiting factor is the minimum control speed, Vmc.
Can you elaborate; I don't quite understand. Vmc limits would stem from empennage size/design no? How would an outsize wing lead to Vmc limits?
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 7, posted (1 year 7 months 1 week 6 days 17 hours ago) and read 2267 times:
Quoting faro (Reply 6): Vmc limits would stem from empennage size/design no? How would an outsize wing lead to Vmc limits?
Vmca only matters in the air if you're capable of flying that slow...suppose Vmca based purely on the empennage size is 90 knots but your minimum stall speed is 100 knots. You can't get to Vmca so that's never a limit on how slow you can fly.
As your wing gets bigger your stall speed drops; at some point it will go below Vmca. At that point, the slowest you can fly is limited by your empennage size rather than your wing size and additional wing area provides you no slower speed benefit.
faro From Egypt, joined Aug 2007, 1456 posts, RR: 0 Reply 8, posted (1 year 7 months 1 week 6 days 14 hours ago) and read 2214 times:
Quoting tdscanuck (Reply 7): Vmca only matters in the air if you're capable of flying that slow...suppose Vmca based purely on the empennage size is 90 knots but your minimum stall speed is 100 knots. You can't get to Vmca so that's never a limit on how slow you can fly.
As your wing gets bigger your stall speed drops; at some point it will go below Vmca. At that point, the slowest you can fly is limited by your empennage size rather than your wing size and additional wing area provides you no slower speed benefit.
Ok, got it; thanx. That is from a control perspective.
How about structure weight though? Is there some agreed-upon criteria about when you are hauling around what would be deemed to be too much wing structure for the (nominal mission)? Do structures people use some rule of thumb figure in terms of %/ratios of OEW, MZFW, etc beyond which wing weight is deemed non-optimal for the mission? It's interesting to see how A & B interpret this differently in launching short baseline models vs long baseline models with the A330/787/A350.
ferpe From France, joined Nov 2010, 1916 posts, RR: 56 Reply 9, posted (1 year 7 months 1 week 5 days 13 hours ago) and read 1941 times:
Quoting faro (Reply 8): Do structures people use some rule of thumb figure in terms of %/ratios of OEW, MZFW, etc beyond which wing weight is deemed non-optimal for the mission? It's interesting to see how A & B interpret this differently in launching short baseline models vs long baseline models with the A330/787/A350.
If one want to get some feeling for the weights involved in a modern aircraft this is the best reference I have found:
Go down on this PianoX model of the 787 and you find the structural weights for the different pieces. Now this model is based on a version of 787 from 2005 with 216t MTOW but you also get the relations, e.g. wing 12,61% of MTOW. The wing at the time was slightly different (58,7m span instead of 60m) but the information is the best we got. Relations don't change that much when you grow the frame to regain payload-range so one can use these relations.
The wingloading for the final 788 at 228t ended up at 700 kg/m2, higher then A350 (600) but still not where the 77W is at 800 kg/m2.
Now B could use this wing also for the 789 (780 kg/m2) and finally for the 7810X. That this final frame is a shorter range model is typical, the shorter the range design point, the lighter and therefore smaller the wing. Why, well to my reasoning the climb segments gets more pronounced in the overall flight profile for the shorter range frame (lower weight is good) and the cruise segments for the longer range one (lower drag due to lift is good at the higher FLs desired).
A does the opposite, they produce an optimal wing for the mid model (333 or 359) at around 600 kg/m2 for the initial weight variants and then shorten the fuselage to get a longer range frame (drag due to skin friction is lowered, ref 332 or 358) with a lower wing loading still (until they up the MTOW to the same as the mid model to get even more range).
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