Glider Wings on a 747?

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?

Sign me "I ain't no AeroE".

Thanks, John

Most "normal" people would probably be surprised that modern airliners have pretty good glide performance. I have a table in some textbook that quotes values of around 17:1 to 19:1 for various models that were current in the 1980s when I was in school. This is about what the Schweizer 2-22 I learned in could do. These glide ratios are typically at about 200kts or a little more, so they sure do penetrate! And in fact, they do tend to have long, high-aspect ratio wings with winglets (more and more of them). But, given the huge range of speeds they need to fly, the requirement to store lots of fuel, handle very heavy wingloading, etc, there are a range of compromises required. Sweep angle for high mach numbers, accomodating tons of lift augmenting devices (slats, fowler flaps), fuel tanks, etc. are all things that glider manufacturers don't have to worry about.

On Monday, October 22, 2012 9:11:38 AM UTC-4, 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?



Sign me "I ain't no AeroE".



Thanks, John

Keep in mind the glide ratios you site are at idle thrust which is still significant. Engines out glide would be somewhere around 12:1.

-karl

On Monday, October 22, 2012 7:21:46 AM UTC-7, Papa3 wrote:
Most "normal" people would probably be surprised that modern airliners have pretty good glide performance. I have a table in some textbook that quotes values of around 17:1 to 19:1 for various models that were current in the 1980s when I was in school. This is about what the Schweizer 2-22 I learned in could do. These glide ratios are typically at about 200kts or a little more, so they sure do penetrate! And in fact, they do tend to have long, high-aspect ratio wings with winglets (more and more of them). But, given the huge range of speeds they need to fly, the requirement to store lots of fuel, handle very heavy wingloading, etc, there are a range of compromises required. Sweep angle for high mach numbers, accomodating tons of lift augmenting devices (slats, fowler flaps), fuel tanks, etc. are all things that glider manufacturers don't have to worry about.



On Monday, October 22, 2012 9:11:38 AM UTC-4, 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?







Sign me "I ain't no AeroE".







Thanks, John

We always talked about a 12:1 glide ratio in the B-727-200. Who would care
about gliding with engines at idle, anyway?

In the T-33a, we were required to practice engine-out glides and approaches
down to short final. Engine-out performance was accomplished by setting the
throttle at 45% RPM and extending the speed brakes. With those settings,
the glide ratio was about 12:1, and the trick was to arrive over the numbers
on an upwind heading at 6,000 ft AGL, configure for landing, and perform a
360 deg spiral to short final where we'd initiate a go-around. Of course,
we'd modify heading/pattern entry point for winds and altitude.

IIRC, high key entry point for an engine-out F-106 was 18,000 ft AGL!


"Karl Kunz" wrote in message
...
Keep in mind the glide ratios you site are at idle thrust which is still
significant. Engines out glide would be somewhere around 12:1.

-karl

On Monday, October 22, 2012 7:21:46 AM UTC-7, Papa3 wrote:
Most "normal" people would probably be surprised that modern airliners
have pretty good glide performance. I have a table in some textbook that
quotes values of around 17:1 to 19:1 for various models that were current
in the 1980s when I was in school. This is about what the Schweizer 2-22
I learned in could do. These glide ratios are typically at about 200kts
or a little more, so they sure do penetrate! And in fact, they do tend
to have long, high-aspect ratio wings with winglets (more and more of
them). But, given the huge range of speeds they need to fly, the
requirement to store lots of fuel, handle very heavy wingloading, etc,
there are a range of compromises required. Sweep angle for high mach
numbers, accomodating tons of lift augmenting devices (slats, fowler
flaps), fuel tanks, etc. are all things that glider manufacturers don't
have to worry about.



On Monday, October 22, 2012 9:11:38 AM UTC-4, 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?







Sign me "I ain't no AeroE".







Thanks, John

On Oct 22, 8:00*am, Karl Kunz wrote:
Keep in mind the glide ratios you site are at idle thrust which is still significant. *Engines out glide would be somewhere around 12:1.

Just like our PW-2 GAPA! ;-)

B.

I have asked the same question, but with laminar profiles, to an airliner designer.

A few things are important:
- big wings are not made out of one piece, so gaps disrupt airflow
- unclean laminar wings (bugs) perform worse than the current non-laminar wings (google vortex lift)
- stall characteristics are less friendly on laminar wings

Roel

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?



Sign me "I ain't no AeroE".



Thanks, John

Nearly all powered aircraft cruise at speeds way above stall. That means the lift coefficients in cruise are low, therefore the induced drag (proportional to Cl ^2) is low, therefore aspect ratio is less important.

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.

On Oct 22, 12:15*pm, Bob Whelan wrote:

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, I don't think that there were any particular structural issues
with the HP-15. As I understand it, Dick built it while he was in a
phase of experimenting with honeycomb cores. So it had thick skins for
bending stiffness and milled honeycomb core to give it shape--but no
wing ribs or discrete spar caps. The carrythrough consisted of a set
of knuckles bolted or riveted to the skin that joined to their
counterparts on the opposite wing.

As I recall, you are spot-on regarding its performance
characteristics. It went like stink in a straight line, but had huge
sink rates when slowed down and compelled to circle.

Thanks, Bob K.

On Monday, October 22, 2012 3:12:22 PM UTC-7, Bob Kuykendall wrote:
On Oct 22, 12:15*pm, Bob Whelan wrote:



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, I don't think that there were any particular structural issues

with the HP-15. As I understand it, Dick built it while he was in a

phase of experimenting with honeycomb cores. So it had thick skins for

bending stiffness and milled honeycomb core to give it shape--but no

wing ribs or discrete spar caps. The carrythrough consisted of a set

of knuckles bolted or riveted to the skin that joined to their

counterparts on the opposite wing.



As I recall, you are spot-on regarding its performance

characteristics. It went like stink in a straight line, but had huge

sink rates when slowed down and compelled to circle.



Thanks, Bob K.

Dick was years ahead of his time on the HP-15. I did a quick comparison to the Duckhawk.

Wing Area: HP-15 75 sq.ft.
Duckhawk 80 sq.ft.

Aspect Ratio: HP-15 33
Duckhawk 30

Empty Wt. HP-15 330 lb.
Duckhawk 390 lb.

Gross Wt. HP-15 600 lb.
Duckhawk 960 lb.

I suspect the airfoil was a significant part of the problem for the HP-15. I don't have any information on the percentage thickness of the profile, but given the materials it's likely to have been thicker than the Duckhawk.

Dick did some amazing things during a time without sophisticated CFD and carbon fiber.

Cheers,
Craig