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One facet of the evolution of commercial transonic transport design since 1970 years is that L/D ratios (alternatively, the M * L/D) have increased only mildly over time. For example, see the following photo. I always found this fact to be incredibly surprising, because over the same time period the capabilities of CFD techniques to optimize airfoils and wings has increased dramatically. I expected L/D ratios to be much higher today than they were in the late 1960s.

The reason I've seen given which reconciles the simultaneous lack of significant advance in L/D ratios (a measure of aerodynamic efficiency) and immense increase in CFD capabilities (which should increase aerodynamic efficiency) is that designers often "trade" improvements in aerodynamic efficiency for reductions in structural weight. Firstly, I'd ask, is this a correct explanation for the only modest improvement in L/D ratios seen over the past 25+ years?

Note: one example that's given of this is that a reduction wave drag achieved by reducing shockwave generation in transonic flow can allow aircraft to fly faster, or fly the same speed at greater wing thickness, and the latter is often chosen to reduce structural weight.

Secondly, this question got me thinking, if the above explanation is correct, are there any references that attempt to model what the "counterfactual" L/D frontier would be for transonic transports if aerodynamic efficiency was not traded for weight improvements? That is, how high could we reasonably push modern L/D ratios for commercial transports if that was our main concern?

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    $\begingroup$ Progress isn't linear, and there's little reason to believe that an advance in a hardware or software design tools would correlate directly to a proportional advance in the end item. Since the beginning of the jet age airliners have been round tubes with swept wings. Improvements on this basic form have been incremental from there. From my perspective it's like asking why architects can't design better houses using CAD than with paper and pencil. Just an observation FWIW... $\endgroup$ Jun 11, 2022 at 17:39

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First of all, I am sceptical regarding your graph. The Fokker F-28 or the Short 360 are way too good while the Boeing 767 looks poorer than what other sources claim. Size alone helps already by making the viscous effects relatively smaller.

Next, despite L/D featuring linearly in the Breguet equation, airliners are not optimized for best L/D but for lowest seat mile cost. This means they fly faster that what achieving a high L/D would require in order to increase their transport efficiency. If we only reduce cruise Mach, the optimization will lead to less sweep and higher aspect ratios like in the Boeing SUGAR studies.

A quick search did not reveal L/D figures for such airliners, but with their braced wings of an aspect ratio of 27 and only 20° sweep I would estimate their L/D to be in the high Twenties if not even low Thirties. Note that some designs use a third engine at the end of the fuselage to ingest its boundary layer in order to reduce pressure drag.

Another avenue is active laminarization by sucking away the boundary layer. Early studies by Erich Pfenniger and others at NASA (PDF!) claimed L/D improvements in excess of 20% and a NASA LaRC study on the basis of test data obtained with a modified Boeing 757 concluded that a 15% reduction in block fuel consumption is possible with laminarization over the forward 50% of the wing of a 300 seat airliner.

L/D gains with different laminarization techniques

L/D gains with different laminarization techniques (source). NLF = Natural Laminar Flow by shaping the pressure distribution, HLFC = Hybrid Laminar Flow Control mixes in some sucking, LFC = Laminar Flow Control by sucking at several chord lines.

This would bring a modern design like the A350 or the Boeing 787, which by themselves already have an L/D of close to 20, to an L/D in the mid-twenties if the active laminarization does not add weight or consume additional energy. Since both assumptions are rather unrealistic, and forty years later we have yet to see any commercial application, I expect that the effort for active laminarization eats up most of its benefits.

To answer your question I need to make assumptions how far you want to move from the current operating point of airliners. If speed should stay the same, we have already reached almost all of the reasonable potential and increases come from lower fuel burn of engines which allow to reduce wing chord a bit.

If, however, flight speed can be reduced by 0.1 Mach, large gains are still possible and I expect L/D values to become 50% higher if economics demands the highest L/D possible.

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