FlyingCello From United Kingdom, joined Jul 2010, 106 posts, RR: 0 Posted (11 months 3 weeks 2 days 11 hours ago) and read 3693 times:
The 747-8 seems to have the same wing structure as the 744, but lots of mentions have been made of it having been 'relofted'. What exactly was changed? I know that flaps and leading edge devices were modified, but this seems to be something more fundamental...it appears to be a reprofiling of the entire wing...is that correct? Is the 747-8 now supercritcal? Does it mean that the wing cross section is now signficantly different, albeit on the old structure.
In a similar vein, 2013 A380s are going to be delivered with an additional 1.5 degrees twist in the wing...is this the entire wing being given a slightly higher angle of attack? Or is the wing root remaining the same, with this additional twist being introduced outboard? A higher AoA would seem like a massive engineering change...
Any B or A structural guys out there who can shed any light?
zeke From Hong Kong, joined Dec 2006, 7770 posts, RR: 73 Reply 1, posted (11 months 3 weeks 2 days 11 hours ago) and read 3681 times:
747-8 reloft refers to the change in airfoil section to a modern supercritical section, the platform of the clean wing remains essentially the same, the span wise engine location is the same.
A380 the wing twist changes reduces the angle of attack of the wing tip by 1.5 degrees. The wing root remains the same, they have not stated over what span the twist will be incorporated. This is to reduce drag.
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
If you alter the wing incidence angle you don't change the AoA, you change the pitch attitude (the wing AoA remains essentially the same). In this case, as zeke described, they're just altering the twist. The change in attitude will be almost invisible.
FlyingCello From United Kingdom, joined Jul 2010, 106 posts, RR: 0 Reply 5, posted (11 months 3 weeks 2 days 7 hours ago) and read 3517 times:
It makes you wonder how Airbus got to this point. Did all the early analysis give a slightly 'wrong' result? I'm guessing that operational experience has uncovered that the extra twist is beneficial, but I wonder where the models got the original analysis wrong? Or, is the wing loading up in a different way than expected?
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 6, posted (11 months 3 weeks 2 days 7 hours ago) and read 3494 times:
Quoting FlyingCello (Reply 5): It makes you wonder how Airbus got to this point. Did all the early analysis give a slightly 'wrong' result?
Almost certainly. Modern analysis tools are amazing but it's still beyond their capability to do a complete, integrated, aerodynamic + structural simulation at full scale fidelity. They get really close (within a few % or so) but to get real numbers you still need to go out and actually fly it.
Quoting FlyingCello (Reply 5): I'm guessing that operational experience has uncovered that the extra twist is beneficial, but I wonder where the models got the original analysis wrong? Or, is the wing loading up in a different way than expected?
They probably didn't get the pressure distribution exactly right. That's a tough iterative problem; the aerodynamic solvers assume a fixed wing position. The structural solvers assume a fixed loading. But the loading alters the position and vice versa. You need to iterate between the two to get "reality". The most likely scenario, to me, is that Airbus got the actual pressure distribution data during flight test and found out that the actual position of the wing in flight wasn't what they though it was, so they altered the jig position so that the in-flight position matches up with what their analysis says is the lowest drag position.
Aeroweanie From United States of America, joined Dec 2004, 1601 posts, RR: 52 Reply 7, posted (11 months 3 weeks 2 days 5 hours ago) and read 3396 times:
I would speculate that likely scenarios are:
1) Airbus probably desires to increase the maximum take-off gross weight of the A380. In doing so, the root bending moment of the wing will go up. Any easy fix is to increase wing twist to move the wing load back inboard. This was done during the original 747 development.
2) Experience might have shown that the wing torsional flexibility is less than originally calculated during the design phase. This might be tied to the rib cracking issues. To reduce wing bending loads, more twist is an easy fix.
FlyingCello From United Kingdom, joined Jul 2010, 106 posts, RR: 0 Reply 9, posted (11 months 3 weeks 2 days 4 hours ago) and read 3380 times:
I guess we have access to tools now that allow us to analyse performance to a far greater resolution than before...the A380 already seems pretty efficient as it is, so this additional gain is the cherry on the pie. 30 years ago, we may not even have known that it needed fixed! Expensive change all the same.
Stitch From United States of America, joined Jul 2005, 26964 posts, RR: 83 Reply 11, posted (11 months 3 weeks 11 hours ago) and read 2819 times:
Quoting sweair (Reply 10): Too bad they couldn't have done a 777X style re wing on the 748, that could have taken some serious weight out of that frame?
The 747-500X and 747-600X would have had the 777's wing.
With the 747-8, Boeing didn't want to change the structural arrangement of the wing: I am guessing so they could use the existing wing tooling and to be able to grandfather the wing under the existing Type Certificate.