How have engineers managed to increase commercial airliner wing aspect ratios over time?

Question

Over time, the wing aspect ratios of commercial airliners have increased.

For evidence ,see the following data:

• 1980s:

  • Boeing 747-400: 7.91,
  • Boeing 757-200: 8.0
  • Boeing 767-300: 8.0
  • Airbus A310: 8.8

• 1990s:

  • Airbus A330: 10
  • Airbus A340: 9.2
  • Boeing 777: 9.96

• 2000s:

  • Boeing 787: 11

The advantage of a higher aspect ratio wing is that it increases L/D ratios by reducing induced drag, and thus for a given amount of lift, an airliner will incur less drag and burn less fuel. However, this is traded-off against the fact that the wing has to be thicker to counter the increased bending moments from the longer wings. This eats into the drag-reduction fuel savings.

But over time, it appears that engineers have managed to overcome the trade-off, making the wingspan longer without incurring the penalties of increased weight. How have they managed to do this?

Two notes:

1. On the most recent airliners (eg, the 787), there's considerable evidence that the shift to carbon composite wing structures has allowed for this to happen. However, the shift began before composite wing structures were introduced, so there must be other explanations
2. I'm almost sure Finite Element Modeling plays a role, but I'm just not sure how FEM allowed for the increase in AR without incurring the offsetting drag-increasing penalties.

Answer 1

As to what makes higher aspect ratios feasible, no magic here:

1. Aerospace materials have improved over time, in quality and strength.

Carbon gets a lot of hype, part deserved and part not. It's just one of the materials.

Aluminum alloys range in yield strength from 55 MPa for the soft mush laptops and phones are made of to 650 MPa for structural aerospace parts. These are both common alloys currently in use. Steel has a further wider range.

Composites range in strength even more widely, from <100 to 3500 MPa, depending on the fiber, direction, weave, resin, filler, fiber ratio, and the curing method.

It's never just "aluminum" or "titanium" or "composite". Each is a very broad category. Overall, materials, including alloys, have been improving steadily; composites are most prominent at the latest step. Well before carbon, fiber-metal laminates have been cutting weight from the skin.

2. Better design precision.

Today's airplanes are engineered with CAD and FEA - finite element analysis. This allows for modeling the wing's structure in its entirety, down to small parts, and learning the stresses in each specific piece. Then, the pieces that are stressed less can be lightened, and the pieces that are likely points of failure can be reinforced.

The manufacturing methods have also improved, allowing for thinner layers, adhesive bonding, more precise milling and trimming. Overall, older airplanes had to carry a lot of metal that wasn't stressed as highly as it could have been, because it wasn't calculated accurately enough, or wasn't cost-effective to remove.

Today, it's common for airframes to come within a few percent of their calculated design load during destructive testing.

3. Larger airports.

Wingspan isn't just a matter of structural considerations. Larger wingspans have been feasible for a long time. The issue is, the wider it is, the fewer airports can fit the airplane in their gates, and the higher their landing fees. The FAA divides aircraft into design groups by wingspan, and airport design has to fit increasing requirements for each group.

The Boeing 747 and then the Airbus A380 have prompted airports to adapt to larger aircraft. Then, even as they are being phased out, the runways, the taxiways and the gates remain as designed for these "jumbo"/"superjumbo" categories.

This opens up room for slightly smaller aircraft like the A350 or the B777X to make use of these wider facilities. Since these planes are lighter than the 747 or the A380, they don't need as much chord to get the lift they need.

• This is just the technical aspect. Keep in mind that higher aspect ratios were always possible, like used in gliders, but would always contribute more weight to the wing. There's a lot of reasons why higher aspect ratios were desired: increasing fuel prices, newer engines, more airframe stretching anticipated.

Answer 2

Answer: By having more efficient engines at their disposal.

Engineers had to choose lower aspect ratios than what they liked in the past. If you compare the reduction in aspect ratio which coincided with the switch to jets, you will see that larger aspect ratios were possible all along. Only the fuel-hungry early jets demanded a larger wing volume which was achieved by making wing chord larger than ideal. This helped to make flap systems less complex, but overall the aspect ratio was forced down below what would had been possible with more efficient piston engines.

Aspect ratio over year of introduction for different types. Blue dots = Piston engines, red dots = jet engines. More recent models could use more efficient engines and could get back to the higher aspect ratios of the piston era.

The How of a high aspect ratio wing is rather straightforward: Use thicker airfoils at the root and limit the permitted load factor and flight speed in gusty weather.