I have noticed, looking at the B73NG wing, the various portions of the leading edge have different sweep-back angles. The section inboard of the engine has a larger sweep back angle than the portion outboard of the engine. I’m wondering why this is designed in this way. Some other more recent Boeing aircraft have a constant sweep angle on the leading edge (B757/767/777), as does the direct competitor to the NG, the A319/320/321. Obviously there are no doubt reasons for both designs.I have noted in older aircraft (B707/727/737 Classics) that this dual sweep angle is also seen, but I would have imagined that with a redesign of the wing for the NG that it would have been eliminated if there was no reason to keep it. Does anyone have any information regarding this?Thanks, Bruce.

The inner section of the wing has Krueger flaps on the leading edge, and these are made more efficient if the wing is of a broader chord, a broad chord also reduces buffet as an additional benefit, and the easiest way to do that, is to alter the leading edge sweep angle a bit to widen the wing, the pylon mount providing a convenient point at which to alter the sweep angle for construction reasons. So the main reason for the alteration of the sweep angle was to give the aircraft more area in the region of the wing where the Kreuger flaps would deploy, since the 737 was originally designed to be operated at very basic airports with smaller runways, where short take off and landing runs were the big selling point.You see that kind of thing less on more modern airliners because they were mostly designed when engines provided faster acceleration, greater thrust and wing profiles had advanced in design somewhat. That is in fact why Boeing tried to partner up with the Airbus consortium prior to creating the B757, because they were after the British expertise in wing design.Boeing have played around with the 737's wing design quite a bit over the years, so much so that it actually does not have a NACA profile number on Classics, as it was a pure Boeing design. You will notice that the original 737's Krueger flaps do not extend in all the way to the fuselage like they do on the Classic 737s after the wing pylons were altered, so you can see how effective Krueger flaps are, given that Boeing were prepared to alter the design in order to extend their width only a small amount when creating the Classic 200ADV.The outboard wing had its chord broadened and the camber moved forward slightly too when Boeing designed the Classic version. This was to improve buffet characteristics, but they purposely did not change things too much because they wanted pilots to be able to transition easily from the 200 to the 300, 400 and 500, and they wanted to keep a lot of parts commonality too. This is why the Classics have a big dorsal fin extension in front of the vertical stabiliser, so that they handled like the earlier Originals as much as was possible.Because the 737 200ADV Classic is generally regarded as the best-handling 737 variant, Boeing were loathe to mess about with things too much when designing the NG. They again wanted to keep some commonality with the previous model and keep the handling much the same, as both these things are big plus points for airlines when considering upgrading their fleets to new models. Thus, the reason the 737 still has that wing shape is that Boeing know that if something isn't broken, you don't need to try and fix it, a point that is illustrated by the fact that airlines were still ordering the Classic variants even when the NG was available.You can see an early example of the same concept of broadening the wings inboard of the engine pods on the Messerschmitt 262 Schwalbe jet fighter (although it's the trailing edge in that case). The 262 only really had its wings swept in order to get the weight back a bit because of the heavy engines rather than for any special aerodynamic reasons, but if you look at a plan view of it, you can see the designers took the opportunity to widen the wings inboard of where the engine pods are for the same structural reasons, and the way the engine pods attach on a 262 is in fact very similar to how they attached to the Original 737's wing when it too had turbojets.Al

Thanks Al, much appreciated.Bruce.

Fantastic post Al!

Fantastic post Al!

Agreed. I don't think I knew a single thing he mentioned in his post, so it was a nice and informative read for me.

Agreed. I don't think I knew a single thing he mentioned in his post, so it was a nice and informative read for me.

Same here.

The inner section of the wing has Krueger flaps on the leading edge, and these are made more efficient if the wing is of a broader chord, a broad chord also reduces buffet as an additional benefit, and the easiest way to do that, is to alter the leading edge sweep angle a bit to widen the wing, the pylon mount providing a convenient point at which to alter the sweep angle for construction reasons. So the main reason for the alteration of the sweep angle was to give the aircraft more area in the region of the wing where the Kreuger flaps would deploy, since the 737 was originally designed to be operated at very basic airports with smaller runways, where short take off and landing runs were the big selling point.You see that kind of thing less on more modern airliners because they were mostly designed when engines provided faster acceleration, greater thrust and wing profiles had advanced in design somewhat. That is in fact why Boeing tried to partner up with the Airbus consortium prior to creating the B757, because they were after the British expertise in wing design.Boeing have played around with the 737's wing design quite a bit over the years, so much so that it actually does not have a NACA profile number on Classics, as it was a pure Boeing design. You will notice that the original 737's Krueger flaps do not extend in all the way to the fuselage like they do on the Classic 737s after the wing pylons were altered, so you can see how effective Krueger flaps are, given that Boeing were prepared to alter the design in order to extend their width only a small amount when creating the Classic 200ADV.The outboard wing had its chord broadened and the camber moved forward slightly too when Boeing designed the Classic version. This was to improve buffet characteristics, but they purposely did not change things too much because they wanted pilots to be able to transition easily from the 200 to the 300, 400 and 500, and they wanted to keep a lot of parts commonality too. This is why the Classics have a big dorsal fin extension in front of the vertical stabiliser, so that they handled like the earlier Originals as much as was possible.Because the 737 200ADV Classic is generally regarded as the best-handling 737 variant, Boeing were loathe to mess about with things too much when designing the NG. They again wanted to keep some commonality with the previous model and keep the handling much the same, as both these things are big plus points for airlines when considering upgrading their fleets to new models. Thus, the reason the 737 still has that wing shape is that Boeing know that if something isn't broken, you don't need to try and fix it, a point that is illustrated by the fact that airlines were still ordering the Classic variants even when the NG was available.You can see an early example of the same concept of broadening the wings inboard of the engine pods on the Messerschmitt 262 Schwalbe jet fighter (although it's the trailing edge in that case). The 262 only really had its wings swept in order to get the weight back a bit because of the heavy engines rather than for any special aerodynamic reasons, but if you look at a plan view of it, you can see the designers took the opportunity to widen the wings inboard of where the engine pods are for the same structural reasons, and the way the engine pods attach on a 262 is in fact very similar to how they attached to the Original 737's wing when it too had turbojets.Al

Jeez Al, A simple YES would have been just fine! :( Great awnser. I love how all of us aviation nuts are so detail addicted! Questions like the OP and awnsers like Al's really show how much of an impact these high end addons have on us true aviation addicts.

In that case: Yes.Al

That is in fact why Boeing tried to partner up with the Airbus consortium prior to creating the B757, because they were after the British expertise in wing design.

What did the British know that Boeing didn't?

It wasn't so much just what the Brits knew, as what they had extensive practical experience of, particularly with aircraft such as the Airbus A300, the Hawker Siddeley Trident, and to a lesser degree the Sud Aviation Caravelle, the Caravelle being an early co-operation precursor to the Airbus Consortium, since the French Sud Aviation shared some components used in the British de Havilland Comet. The practical implementation of widely dispersed production was also of interest to Boeing, since they were genuinely surprised how well Airbus had made that system work in producing the A300, and of course Boeing now do the same thing themselves with the Dreamlifter.To explain exactly why Boeing tried to strike up a co-operation deal with Airbus though, you have to look at the A300...The wing of the A300 is swept back to 28 degrees, but that choice had been as a result of the design having been worked on for nearly a decade by Hawker Siddeley. It used principles such as rear loading, which had also been used on the Trident, at the time this was a completely unknown and untried technique as far as Boeing were concerned. Rear loaded wings considerably increase lift over the rear portion of the wing before the onset of flow separation, by speeding up the airflow over the rear section of the aerofoil. In addition, these types of wing were also rather more advanced at the leading edges than the profiles Boeing had used - which Boeing did play around with on the 737 - so they were more efficient across a broader range of airspeeds, allowing the use of mechanical high lift devices to be simpler and therefore also more reliable and cheaper to produce.You can see that on the A300, which uses fowler flaps instead of the triple-slotted types on more complex wing designs. Hawker Siddeley had backed up all that production knowledge with thousands of hours of wind tunnel testing on various wing designs. The main plus point derived from all that research meant that the A300 could have a longer rear fuselage, so it could also have a smaller vertical stabiliser, which meant that in addition to being able to carry more passengers, it required less trimming and induced less yaw drag, and therefore it was more fuel efficient too. Faced with competing against a company that could produce a more fuel efficient aircraft that could also fit more people inside it, you can see why Boeing would want to get involved.On top of all this, the A300 also used a flight computer to make it more fuel efficient too, and it was a combination of all this which got Boeing interested in collaboratively producing an airliner with Airbus. Airbus bosses were suspicious of Boeing's motives though, fearing that it was merely a ruse to allow Boeing to gain access to the considerable research Hawker Siddeley had carried out, and they were probably right to think that too. As a result, Boeing instead built the 757, carrying out their own research and using the 737 in combination with NASA's Dryden Research Facility to come up with the concept of the FMC. It is worth noting however, that as good as the 757 is, it is known for leaving a massive amount of wake turbulence for an aircraft of its size, so they could maybe have done with getting their hands on some of that wing research info, as anything which is churning the air up that much, is obviously not passing through the air mass as efficiently as it could do.God that was boring, wasn't it? :( Al

God that was boring, wasn't it? :( Al

Nope. Very interesting! :(

Once again, thanks Al. Makes the wing on the C172 that I fly seem like a dinosaur. :)Years ago when I lived in Australia, TAA (one of the domestic airlines then) bought the A300, and I recall hearing the term "super-critical wing", is that the same as the rear loading that you wrote about?Thanks again, Bruce.

Nope, a supercritical wing is one that tells you your landing was crap if you don't grease it onto the piano keys :( Joking aside, yup, it's a similar sort of concept, not completely identical, but more or less the same - you shift the main lift area back a bit by altering the curve of the wing cross section. Nothing wrong with the C-172's wing, it gets the job done and it's probably put more pilots in the air than any other modern single.Al