On Monday, October 22, 2012 6:11:38 AM UTC-7, JohnDeRosa wrote:
I was asked last night "Why don't commercial airliners (747, A380,
etc) have 'super wings' like gliders?" I mumbled something semi-
coherent but didn't really know the correct answer.

So, would high aspect ratio and highly efficient glider-like wings
enhance fuel economy for all airplanes? What are the engineering
tradeoffs for wing design between a hulking airliner and a slim/trim
glider?

If range exclusively was what was being optimized, then the short form answer
to your first question is, "Yes." (Remember the round-the-world-unrefueled
Rutan "Voyager"?) Note *max* range would occur at relatively slow speeds
(equivalent to a glider's max L/D speed), "slow" being defined as relative to
what the wing is otherwise easily capable of aerodynamically.

Your second question is one that college-level, graduate, and post-graduate
courses of study - yea! entire working lives - are directed toward. In other
words, airplane optimization is genuinely complex!!! Even "mere glider
optimization" is seriously complex as can be sensed from the recent Uvalde
Worlds (why those "short-span" Open Class designs?), Greg Cole's "Duckhawk"
(re-reading the recent "Soaring" mag article may be warranted for anyone
pondering design tradeoffs), etc.

Most non-technical people's eyes would instantly glaze over upon seeing some
of the (even relatively basic) graphical presentations commonly used in the
airplane design field depicting results of parametric studies/tradeoffs. (I
think they're pretty cool, but even the simplest require considerable thought
to grasp...and reflect even more considerable computational effort.)

Considering only the wing, and working at the most basic level, a designer can
"play with": span, thickness, chord, thickness/chord ratio, sweep, aspect
ratio, incidence (angle mounted on the fuselage), high-lift devices, etc. And
everything played with influences/interacts-with everything else.

Span may be limited by ground-based infrastructure; this was a major
consideration in the initial design of the 747 "way back when", and the A-380
more recently. Or it could be limited by structural considerations (strength
of materials, since each pound of wing reduces payload).

Thickness - some is required for structural and airfoil shape reasons, but
"too much" limits top speed (thinner generally being gooder for mach
considerations), while "too little" (if that's possible!) will affect fuel
capacity, possibly affect landing gear stowage, and incorporation of high-lift
devices.

Chord - directly affects structural weight and aspect ratio, the latter in
turn aerodynamically affecting climb and cruise efficiencies...

Sweep - necessary at high subsonic speeds to delay compressibility
effects/drag-increase, but increases structural weight for a given span...

Of the things Joe Interested Observer can directly see, sweep is interesting
(to me, anyway!) to consider. Consider Boeings. Though - for any given design
- the cruise mach the airlines tend to use has almost certainly been
influenced over the decades by fuel costs, the early models' sweep angle
tended to reflect their design cruise mach, more sweepback equating to a
higher cruise mach. That distinctly changed with the 757/767/777/787 ships,
the 3-former due (probably) to improved materials (thinner wings possible) and
(perhaps) to improved computational methods of airfoil/flow analysis.

These 4 designs each have high design cruise machs, but less sweepback than
their forebears. Certainly in the 787's case, new materials plays a huge part,
as likely does (further/continually) improving computational fluid dynamics.

Consider also the 737 - its cruise mach has steadily increased throughout its
development, the first generations being distinctly slower than (its
contemporary with considerably more sweepback) the 727, as might be surmised
when considering its distinctly smaller sweepback angle. The latest models are
really different airplanes, despite retaining the same model
number...completely different wings (even before the winglets appeared),
aerodynamically speaking.

Look closely at the 787...a long-range, high mach design. It wouldn't surprise
me, if you ran the numbers (I haven't), if it has the highest aspect ratio of
any "major jet airliner" to-date. Structurally it evidently can (have a high
aspect ratio), and aerodynamically, it's definitely helpful for range.

Next time you get asked the question leading to your post, consider an
accurate answer of, "They DO!" Passenger jet wings simply look different than
glider wings because of all the other factors entering into their optimization
considerations. In airplane design terms, it's difficult to get much more
"mission simple" than a glider.
- - - - - -

This being RAS, take a look back at Dick Schreder's original HP-15...a failed
attempt to utilize extremely high aspect ratio to maximize performance. It
likely ran afoul of structural and aerodynamic considerations, mostly the
latter, I'd guess. The small chord almost certainly meant its airfoil (even if
laminarly executed) was operating outside the theoretical laminar bucket at
slow (thermalling) speeds due to Reynolds number effects, even without
considering profile accuracy. What's the most effective way to hurt average XC
speed?

Bob - is it winter yet? - W.