Glider Wings on a 747?

At 200,000kg, 217-237kts IAS depending on altitude. Average rate of descent 1,300fpm.

This makes them better performing than any sailplane I know of at this speed...

Ramy

A330:
Actually the angle of attack at cruise on an A330 is in the 2.5 degree range, so not that big of an AOA. But still close to stall. Because of the supercritical airfoil and compressibility effects, the stall angle of attack is quite dependent on the Mach number. Somewhere around 6° at M.82 (stall warning at 4°) and a higher AOA when slower (stall warning at 10° at .3M), becoming more constant at Mach numbers below .3.

For descent, we plan about a 3:1 (3 miles per thousand feet; 18:1 in glider terms) for idle descents, and that's at M.81/300 knots then 250 kts below 10,000. L/D speed being at landing weight usually around 210 kts, depending on weight.
Airbus quotes the same glide ratio (3:1) for dual engine flameout glide at best L/D (known as green dot speed due to the symbol used on the airspeed indicator for it). The exact speed varies with weight and altitude.

The 330's wing is a thing of beauty, with a 198' span, which is wider than the -200 is long (191') (which is actually quite similar to the 777's dimensions). However, it often flies higher than its widebody counterparts (767, 777) at the same mission stage (often by a significant margin). It's usually capable of FL410 at the last part of any ocean crossing.

In terms of "coffin corner", typical cruise numbers at FL 410, M.82 IAS:241kts Vmo/Mmo:257kts/M.84, minimum recommended speed (Vls)210kts., Which is really not that tight of a window. The Mach buffet is actually difficult to achieve due to the airfoil(according to Airbus test pilots), and the Mach buffet speed would be well beyond the given Mmo speed. Usually it's propulsion limited (the ability to be able to climb 300 ft/min) rather than aerodynamically limited (low and high speed limits converging).
The A330 is optimized for about M.82. which is slower than the 747, 777, & 787, but faster than the 767, 757, 737.


On Friday, November 23, 2012 6:59:52 PM UTC-8, Bruce Hoult wrote:
On Tuesday, October 23, 2012 6:03:13 AM UTC+13, jfitch wrote:

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.



Not really true of jet airliners. They fly so high that although they're going fast they're at a pretty big angle of attack and not all that far from the stall.

I have no idea! I do know that that the winglets on a 737 are the same as
those on an ASW27b and where designed by Afandi Darlington who is a UK
glider pilot.

I have no idea! I do know that that the winglets on a 737 are the same as
those on an ASW27b and where designed by Afandi Darlington who is a UK
glider pilot.

Take a look at the composite wing on the 787... much more glider-like
than traditional airliner wings. It's a higher aspect ratio and is
much cleaner than an aluminum wing.. If fact about 20% more efficient
through the speed range.

Bob

Hi, this is slightly off-topic but here's a piece I wrote for Pprune
earlier this year on airliner winglets. BTW Justin, I didn't have anything
to do with the B737 winglets as I worked for Airbus at the time! Cheers,
Afandi

---------

Before getting into specifics, let's first consider what the winglet is
doing, aerodynamically. So, a thought experiment - you're sitting in a
chair somehow suspended in space at 5,000' (the height is unimportant, but
let's say we're high enough to be out of ground effect, which really kicks
in a heights less than half the wingspan of the aircraft). The atmoshere is
still. An airliner flies by, before disappearing off into the distance. You
can't hear it anymore, but you can feel the air around you moving, having
been disturbed by the passage of the aircraft. What's going on?

If you could see the air, you'd be able to see a general downwards motion
in the area where the aircraft flew, and a gentle upwards movement to
either side of this area. You might notice that the velocity of the air
moving downwards is greater than the gentle upwards moving air at either
side - and in fact here is our first finding - the net vertical momentum
change of the air disturbed by the aircraft must equal the weight of the
aircraft. Newton's second law and all that. You will also have noticed,
with your air-x-ray specs, that there are two powerful horizontal tornadoes
roughly where the wingtips passed by - the 'tip vortices'. These were
caused by the airflow at the wingtip rolling up, moving from the higher
pressure side on the lower surface around the wingtip, to the lower
pressure side on the upper surface. They are an inevitable by-product of
generating lift.

Now, if we somehow knew the veolicty of every air molecule disturbed by
the aircraft, in both vertical and horizontal directions, we could do some
sums. We could calculate the net vertical momentum change - which will
equal the weight of the aircraft. The net horizontal momentum change,
assuming we've done our sums correctly, and the airliner wasn't
sideslipping, should be zero. Let's also work out the kinetic energy of the
air in this y-z plane (y being the horizontal direction, and z is upwards),
which is 1/2 * m * delta-v^2, summed over all the particles affected. It
will work out to be some value. Next, consider what happens if another
airliner of the same weight, but double the wingspan, passes by. The net
change in vertcial momentum of the air will be the same (same weight), but
a greater volume of air behind the wing is affected, because the wingspan
is larger, so the vertical velocity change ('downwash') is smaller.
Therefore when we calculate the kinetic energy in the y-z plane again, we
get a smaller number. Cool!

Any aeroplane that moves along through the air causing less kinetic energy
behind it, all other things being equal, must have lower DRAG. Specifically
induced drag. That's our second finding - and one that glider pilots have
known since the 1930s - that There Is No Substitute For Span. The greater
the wingspan, the lower the lift-induced drag. And considering that the
induced drag of an A340 cruising along at FL370 is 40% of the total drag,
it's an important item to minimise.

We have't mentioned winglets yet, have we? Don't worry, they're coming.

If span is so good, why don't airliners have very large spans, say of
100m? Well, we'd need bigger airport terminals and taxiway spacing for
starters, but the other reason is that as the wingspan goes up, so does the
wing weight, for a fixed wing area. So we have drag going down with span,
and weight going up - at some point there is a 'sweet spot', where things
are optimum. A slightly greater span means the induced drag is a little
less, but carrying around the additional wing weight isn't worth it in
terms of Direct Operating Cost, which is what the airlines care about.

It is common for this 'optimum span' to be larger than what the airports
can cope with, and for the A380, which was designed to fit within an 80m x
80m box, the optimum span was actually somewhere between 82 and 84m,
although the DOC-span curve was pretty flat between these values. So - how
do we 'involve' more air in the generation of lift as our aeroplane flies
along? What about bending up the wingtip? We could even call it a winglet!

Let's go back to our chair again. This time an airliner flies by, same
wingspan as the first one, same weight, but this one has winglets fitted.
The air still gets a net 'push' downwards to equal the weight of the
aircraft, but this time the tip vorticies are slightly weaker and in fact,
slightly larger. Same overall vertcial air momentum change, but lower
kinetic energy. Each winglet has 'diffused' the powerful tip vortex
vertically, along the height of the winglet, and in fact the designer of
that winglet probably tailored the aerofoil shapes, chord and twist along
the winglet to generate a particular level of lift at each station, (the
'lift distribution') in the cruise condition.

So, lift distributions then - every first year aeronautical engineer
undergraduate will be taught that for minimum induced drag of a flat wing
(no winglets), one must achieve an elliptcial lift distribution. Some
elegant maths will be produced with pi and other numbers, good stuff. But,
what's the optimum lift distribution for a wing with a winglet? Or a
'wingtip fence', like on the A320? We need to go and read two 1960's NASA
reports: (1) A 1962 NASA Tech Report R-139, by Mr. Clarence Cone 'The
Theory of Induced Lift and Minimum Induced Drag of Nonplanar Lifting
Systems', and (2) A 1968 NASA Contractor Report CR-1218 by Mr. J L Lundry
'A Numerical Solution for the Minimum Induced Drag, and the Corresponding
Loading, of Nonplanar Wings'. These give us these optimum distributions and
the induced drag reductions we can expect to gain, for any configuration of
'wingtip device'. No magic there, just sound maths and a brilliantly simple
way of finding the answer without today's computer programs.

The reports show that the optimum lift distribution for a wing with a
winglet, for the same overall lift, has slightly lower lift at the inboard
end and significantly more lift at the outboard end, plus some
inwards-pointing side load on the winglet itself. Interestingly the winglet
doesn't directly add to the lift very much - because it's close to
vertical, it's 'lift' has only a very small upwards component - but the
'blocking' effect out the outer wing increases the lift there considerably.
The overall effect is larger, less intense wingtip vorticies, for the same
overall lift - same vertical momentum change, lower kinetic energy.

Back to the original question regaring why some modern aircraft have
winglets, whilst others (B787, B777) don't. I see this as really two
questions:

1. Why do some new-design aircraft have winglets, whilst other new-designs
do not?
2. Why do some in-service aircraft sprout winglets?

I think the second question is easier to answer, so let's do that first.
Company A designs and builds a jet airliner, without winglets. Everybody
tries their best during the design and stressing of the wing, but when
designing details like stringer and skin thicknesses, some conservatism
inevitably creeps in. We can't have a LIMIT:ULTIMATE factor of less than
1.5, can we? So a bit of rounding up goes on here and there, with the
result that when the wing is tested, it breaks at say 156% of LIMIT load.
Everyone feels relieved and goes for a beer. The airliner goes into
production, and a few years later, some bright spark in the project office
works out that the extra 6% 'fat' in the wing might be used up by sticking
a winglet on, along with some local strengthening at the wingtip (and
whereever else is critical for those additional bending loads). Remember
that the increase in bending moment at the wingtip is infinite - we had
zero BM before, now we have some. There is a surge tank out there, with
minimum wall thicknesses, but it will need reinforcing. A few hundred lbs
of reinforcement goes in along with the winglet, the LIMIT:ULTIMATE is
still just above 150% - on paper - there's no need to break another wing
since we can clear this modification 'by analysis', and hey presto we've
reduced the induced drag by, say typically, 5%. Minus a bit of extra wetted
area for the winglet, and our overall drag standard is still 2% better than
before (bigger gains for bigger winglets). Everyone's still happy, and more
beer is drunk. Marketing types invent a name for their winglet, to
differentiate it and make it 'special', calling them 'sharklets' or
'advanced blended winglets' or whatever. This is my take on the BBJ/737-NG,
757WL, 767WL, A320NEO and others.

New design airliners are slightly different, because the winglet has to
'pay' for itself from day 1, in terms of direct operating cost. This means
that the additional bending, shear and torsional loads imposed on the wing
by the winglet have to be accurately calculated, as do the aeroelastic
effects - the wing bends and twists a little with the winglet flight loads,
not to mention flutter margins. And the weight prediction modelling has to
be REALLY GOOD. The specific load cases that design certain parts of the
wing structure can play a part - for example, if large areas of the wing
are designed by the 2.5g manoeuvre case, a winglet that moves the
aerodynamic centre at the wingtip further aft (wingtip fence for example)
can cause the wing to twist off more at the 2.5g manoeuvre point, washing
it out more. This shifts the spanswise centre of lift inboard at this
design point, reducing the bending loads; winglet-induced aerodynamic load
relief, if you like. This effect may not be the same from project to
project, short haul to long haul (short haul aircraft do more cycles, so
fatigue and damage tolerance may be the critical design case). The optimum
span for the airliner may not be constrained, like it was for the A380, so
there may be less gain to be had, if any, for the clean sheet design with
optimum wingspan. Of course, when the machine is built and tested, it
eventually becomes old, and tricks like question 2 can come into play.

I'll close by talking about winglet effects at different airspeeds.
Clearly the winglet has to pay for itself in the cruise, as that is the
point airliner wings are optimised for. For a long range aircraft, this is
especially important. But at lower speeds, the induced drag forms a greater
proportion of the total drag. So a winglet that cuts 5% from induced drag
in the cruise (say 2% net total drag reduction, if cruise induced drag is
40% of total drag), will cut say 4% of the aircraft drag at a low speed
point, where the induced drag is 80% of the total aircraft drag. So
winglets are great for low speed - takeoff lengths, first and second
segment climb. A poor winglet design with almost no cruise drag reduction
might still be worthwhile, if airport access - second segment climb for
example - is limiting.