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Are Wings Shapes Made To Store Fuel?  
User currently offlineA380900 From France, joined Dec 2003, 1118 posts, RR: 1
Posted (9 years 9 months 1 week 4 days 19 hours ago) and read 3646 times:

To what extent is the shape of the wing determineds by the fact that it has to hold fuel? If the fuel was some place else, what shape would the wings have? Would they be much thinner?

16 replies: All unread, jump to last
 
User currently offlineLeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1
Reply 1, posted (9 years 9 months 1 week 4 days 18 hours ago) and read 3613 times:

The storage of fuel is one consideration among many in wing design.

Other factors that would keep the thickness of the wing closer to where it is now include:

Storage of landing gear.
Storage of mechanisms for flight control surfaces and high-lift devices.
Low-speed aerodynamics, for which a thicker section is generally beneficial, given comparable airfoil design technology.
Structural considerations; The thicker the airfoil section, the more structurally efficient the skin's resistance to twisting moment and flutter.

High-speed aerodynamics is close to alone in promoting a thinner wing.

Further, even if you were able to design a fuselage that could safely contain the required fuel, it would not be structurally optimal, even disregarding the above considerations, to do so. Placing the fuel in the wings achieves structural weight reduction because the weight of the fuel is distributed along the wing, countering the lift produced by the wing, and reducing the bending at the wing root compared to if you placed the fuel entirely within the fuselage. It could be compared, in a gross oversimplification, to spanning a yardstick across the gap between two desks. You can put more weight on the yardstick if you space it evenly across the span, but if you tried to place that much weight concentrated at the center of the span, you would probably break the yardstick.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3598 posts, RR: 66
Reply 2, posted (9 years 9 months 1 week 4 days 18 hours ago) and read 3604 times:

Airfoil sections are shaped to provide the best compromise between cruise efficiency (high speed/low drag) and structural efficiency. These two efficiency needs would drive airfoils in opposite directions if only one of them were being honored. High speed/low drag would make the airfoils thin (low thickness to chord ratio or t/c) while structural efficiency would make them thick (high t/c).

The success of supercritical airfoil sections used on current Airbus and Boeing airplanes is that they allow a successful compromise of these competing design requirements. Consider that the 737-300 has a wing sweep of 25 deg. and a .74M cruise. Thirty years of airfoil development yields the 737-700 still with 25 deg. wing sweep, but now with .8M cruise and a thicker wing to boot.

Fuel carriage tends to be a secondary consideration to these primary design criteria, but it is tied to structural efficiency. A thicker wing with wide spar spacing will be able to carry more fuel. And don't forget that the fuel itself can add to wing structural efficiency by providing wing bending moment relief. Wing structure can be lighter if it carries the fuel directly rather than having the same amount of fuel stored in body tanks.

In summary, airplanes look the way they do because 100 years of design trades have evolved very efficient designs. Configuration changes seldom yield better solutions unless special mission requirements come into play.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineL-188 From United States of America, joined Jul 1999, 29832 posts, RR: 58
Reply 3, posted (9 years 9 months 1 week 4 days 15 hours ago) and read 3526 times:

I am going to stick my head out and say, almost none.


I could go with in the following order for most aircraft

Airfoil Section. (low speed fits in this catagory)
Structural strength
Landing Gear Accomodation
Mechanical Accomodation.
Fuel tankage.

Usually by the time you get past the needs to create a strong enough wing section, you have enough room for fuel storage, else you go with external of fuselauge tanks...ala a learjet.



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 4, posted (9 years 9 months 1 week 4 days 11 hours ago) and read 3474 times:

To be honest, I don't know what is the order of the normal protocol of the design process. I would think there are loads of people doing work on different parts of a plane all at once but not independently, so some it trying to fit fuel into an ever-changing wing.

Personally I look at aerodynamics first; then structure, then weights and payloads, then engines and fuel last. I usually estimate how much fuel is needed by the type of engine and thrust needed to maintain a cruise, etc, etc. After that if the design needs to be tweeked, I do it in stages until I'm satisfied or I move onto another project. Inspiration screams shotgun.

Again, I don't know normal protocol, being the order in which stuff is figured out, I'm not in industry yet. I'm not getting paid for my research, it's career related hobby. Usually I worry about not having enough thrust than not knowing where to put the fuel.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
Reply 5, posted (9 years 9 months 1 week 4 days 10 hours ago) and read 3466 times:

Quoting OldAeroGuy (Reply 2):
The success of supercritical airfoil sections used on current Airbus and Boeing airplanes is that they allow a successful compromise of these competing design requirements.

How does a supercritical wing differ in profile from a normal wing?



Make the most of the available light ... a lesson of photography that applies to life
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 6, posted (9 years 9 months 1 week 4 days 10 hours ago) and read 3462 times:

Quoting Kukkudrill (Reply 5):
How does a supercritical wing differ in profile from a normal wing?

Thinner, like from top to bottom, or it can be longer. Some critical airfoils even have a symetrical shape up until the trailing edge where the tip curves downwards. These foils can push the critical mach number up, being the max subsonic speed you go before the air on top of the wing goes supersonic and thus wave drag fires up. That's why most airliners are stuck at under M0.90.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 7, posted (9 years 9 months 1 week 4 days 9 hours ago) and read 3454 times:

Quoting Lehpron (Reply 6):
Thinner, like from top to bottom, or it can be longer. Some critical airfoils even have a symetrical shape up until the trailing edge where the tip curves downwards.

Err, no. Actually they have a higher thickness/chord ratio than an equivalent non-supercritical wing (subcritical wing?). Or you can employ less sweepback. Airbus do both these things. A thicker wing can be made lighter and carry more fuel. Similarly less sweepback means a lighter wing. Or you can fly faster with less drag.

Previously transonic wings had to be highly swept and very thin to minimise drag and compressibility effects.

A supercritical wing tends to be flatter on the top surface, with a concave trailing edge undersurface. Forward of that the undersurface tends to be more curved than the top surface.

Take a look at the following links. A picture is worth a thousand words. The second link provides a very full description.

http://www.centennialofflight.gov/es..._Flight/Transonic_Wings/TH20G7.htm

http://www.centennialofflight.gov/es...of_Flight/Transonic_Wings/TH20.htm

They work by evening out the pressure distribution near the peak suction point so shock waves don't form. The local airflow can actually be supersonic without a shock wave, hence the term supercritical.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3598 posts, RR: 66
Reply 8, posted (9 years 9 months 1 week 4 days ago) and read 3380 times:

Quoting Jetlagged (Reply 7):
Err, no. Actually they have a higher thickness/chord ratio than an equivalent non-supercritical wing (subcritical wing?). Or you can employ less sweepback. Airbus do both these things. A thicker wing can be made lighter and carry more fuel. Similarly less sweepback means a lighter wing. Or you can fly faster with less drag.

I agree with Jetlagged's post.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 9, posted (9 years 9 months 1 week 3 days 21 hours ago) and read 3337 times:

Quoting Jetlagged (Reply 7):
Err, no. Actually they have a higher thickness/chord ratio than an equivalent non-supercritical wing (subcritical wing?)

Err, no. There are many typesof critical foils, there is not one defined shape, I'm sure you meant chord-to-thickness ratio? Also the links you have provided however show pictures with foil cross sections thinner than that of traditional foils, not thicker as you claim.

BTW, this:

Quoting Lehpron (Reply 6):
Thinner, like from top to bottom, or it can be longer. Some critical airfoils even have a symetrical shape up until the trailing edge where the tip curves downwards

And this:

Quoting Jetlagged (Reply 7):
A supercritical wing tends to be flatter on the top surface, with a concave trailing edge undersurface. Forward of that the undersurface tends to be more curved than the top surface.

...are the exact same thing explained differently.

Quoting Jetlagged (Reply 7):
The local airflow can actually be supersonic without a shock wave, hence the term supercritical.

Supersonic without a wave is impossible, the wave is simply weaker on a supercritical foil and has a lessened impact on drag divergence than a conventionally shaped counterpart. This is a poor scan (sorry) from "Introduction of flight" by John Anderson Jr. 4th edition, p259, Figure 5.19:



...clearly shows that a thinner wing is 1) lighter by being smaller and 2) increases the critical mach number. You can emply less wing sweep and thus increase L/D max, but thicker wing will encounter high transonic drag. Yes there will be less fuel volume, but less drag can improve on range.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineLeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1
Reply 10, posted (9 years 9 months 1 week 3 days 20 hours ago) and read 3324 times:

1) I don't see anything about wing weight in that picture. Structural design is a complicated thing, and a thicker wing very easily can reduce total wing weight. If the wings were machined out of billet, bigger = heavier might work, but that's not the way it's done.

2) That picture has nothing to do with a supercritical section either. It compares three wing sections that have the same camber line and thicknesses all the way down the chord, just scaled. And it correctly identifies that a given section, scaled to smaller thickness, will have a higher critical Mach number. However, that does not make that section a supercritical airfoil. The supercritical airfoil is an entirely different contour, not the same contour scaled to different thickness.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 11, posted (9 years 9 months 1 week 3 days 20 hours ago) and read 3308 times:

You said symmetrical. In fact supercritical wings are more convex on the undersurface than the top surface, hardly symmetrical. The opposite of a traditional aerofoil in fact.

The following link shows a better diagram of a real supercritical wing, even showing how the section changes and twists across the span.

http://www.centennialofflight.gov/es...hnology/supercritical/Tech12G2.htm

I stand by my comment that thickness chord ratio can by higher, or sweepback less. See the links and read the description.

To quote the centennial of flight site directly:

Quote:
There are two main advantages of the supercritical airfoil. First, by using the same thickness-chord ratio, the supercritical airfoil permits high subsonic cruise near Mach 1 before the transonic drag rise. Alternatively, at lower drag divergence Mach numbers, the supercritical airfoil permits a thicker wing section to be used without a drag penalty. This airfoil reduces structural weight and permits higher lift at lower speeds.

A non-supercritical wing was to be thin to reduce transonic wave drag, just as your diagram shows. But none of the wing sections in your diagram are supercritical. The whole point of the supercritical section is to avoid the thin wing solution. Why on earth would you want a thinner wing if you can avoid one? A thin wing has to have a heavier structure because it does not have the stiffness of a thicker wing. It also provides less lift than an equivalently sized "thick" wing, so needs more area or higher angle of attack.

You are correct about the shock wave, I was relying on memory too much.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1610 posts, RR: 52
Reply 12, posted (9 years 9 months 1 week 3 days 19 hours ago) and read 3301 times:
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Lehpron:

Jetlagged is right. The whole idea behind supercritical airfoils is to allow airfoils with higher t/c ratios to be used for a given amount of wave drag. Anderson's plot just demonstrates the impact of t/c on wave drag for a related family of non-supercritical airfoils.

Just to confuse things, I should add this. The original concept of supercritical airfoils used a region of very rapid curvature near the nose which created expansion waves that were tuned to weaken the shock wave. This is in Whitcomb's original patent. There was a symmetrical supercritical airfoil tested - it gots its name from having this feature. I have the NASA test report if anyone doubts me. As time went on, the name became applied to airfoils with limited camber in the mid region and lots of aft loading.

Also, to contradict Lehpron, there are such things as shockless airfoils. However, they are only shock free at a very narrow operating condition. Go a little faster and they get real bad shocks on them.

OldAeroGuy's post in the beginning told the story very well. However, I have one small clarification in light of where this discussion has wandered - neither the 737Classic or NG has supercritical airfoils, as they are now defined.


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 13, posted (9 years 9 months 1 week 3 days 17 hours ago) and read 3281 times:

Quoting LeanOfPeak (Reply 10):
I don't see anything about wing weight in that picture

Common sense 101: more internal volume, due to thick wings, equals more weight in terms of more skin weight and longer webs and flanges. Airliners have 3 going spanwise to 1/2 of the wing; that is an increase in mass, how is that lighter?

Quoting Jetlagged (Reply 11):
You said symmetrical. In fact supercritical wings are more convex on the undersurface than the top surface, hardly symmetrical.

I said some are symetrical. Of those that I have seen used on the X-29 forward swept wing, over 60% of the chord was symetrical. The last 40% trailing was curved downwards. Keep in mind there may have been hundreds of supercritical wing sections, do not expect them all to fit into the same profile.

Quoting Jetlagged (Reply 11):
The whole point of the supercritical section is to avoid the thin wing solution. Why on earth would you want a thinner wing if you can avoid one? A thin wing has to have a heavier structure because it does not have the stiffness of a thicker wing. It also provides less lift than an equivalently sized "thick" wing, so needs more area or higher angle of attack.

I'll buy that. I still don't see how a thicker wing is lighter...

Quoting AeroWeanie (Reply 12):
Also, to contradict Lehpron, there are such things as shockless airfoils. However, they are only shock free at a very narrow operating condition. Go a little faster and they get real bad shocks on them.

I never said there were no such things as shockless airfoils, just supersonic flow over a surface without a shock wave is impossible and I dare you to prove it. Supersonic flow is defined as flow moving faster than the speed of sound in that medium, not relative to some other arbitrary point away from the foil, or the ground. Oddly enough you then go on to say exactly what I said regarding critical mach.

Quoting AeroWeanie (Reply 12):
neither the 737Classic or NG has supercritical airfoils, as they are now defined.

Note bold.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1610 posts, RR: 52
Reply 14, posted (9 years 9 months 1 week 3 days 16 hours ago) and read 3262 times:
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> I never said there were no such things as shockless airfoils, just supersonic
> flow over a surface without a shock wave is impossible and I dare you to prove > it.

Go look up Helmut Sobieczky's papers - there are some on line. He published reams on shockless airfoils, that have supersonic flow on the top, with no terminating shock. Its called isentropic recompression and it can exist at a very narrow range of conditions. Joe Volpe at Grumman also published some inverse solutions for shockless airfoils. He demonstrated in the analysis of the airfoils that there were shocks, but they didn't terminate at the surface.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3598 posts, RR: 66
Reply 15, posted (9 years 9 months 1 week 3 days 3 hours ago) and read 3188 times:

Quoting AeroWeanie (Reply 12):
OldAeroGuy's post in the beginning told the story very well. However, I have one small clarification in light of where this discussion has wandered - neither the 737Classic or NG has supercritical airfoils, as they are now defined.

I agree that the 737Classic doesn't have a supercritical airfoil and my description doesn't say that it does. As for the NG, I think you're stretching the definition of supercritical to say that the NG airfoil is not supercritical. It employs aft camber and has the upper surface expansion characteristics of supercritical designs. Its basis is the 757 section which incorporated the same supercritical elements and had .8M cruise for a 25 deg. sweep wing. The 757 wing out performed its contemporary airplanes (727,767, A300/A310) in terms of wing sweep/cruise Mach. It still compares well with later wing sections incorporated on the 777 and A330/A340, especially since it was developed over a decade earlier.

Quoting Lehpron (Reply 13):
Common sense 101: more internal volume, due to thick wings, equals more weight in terms of more skin weight and longer webs and flanges. Airliners have 3 going spanwise to 1/2 of the wing; that is an increase in mass, how is that lighter?



Quoting Lehpron (Reply 13):
I'll buy that. I still don't see how a thicker wing is lighter...

Lehpron, talk to your Structures friends. If two wings need to carry a given bending moment, the one with the greater thickness will be lighter. This is easy to visualize as the wing skins, stringers and spar caps will be a greater distance from the elastic axis. Greater distance means that all these elements can be thinner (ie lighter). Think of it in terms of a lever arm.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 16, posted (9 years 9 months 1 week 2 days 23 hours ago) and read 3176 times:
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Quoting OldAeroGuy (Reply 15):
Lehpron, talk to your Structures friends. If two wings need to carry a given bending moment, the one with the greater thickness will be lighter. This is easy to visualize as the wing skins, stringers and spar caps will be a greater distance from the elastic axis. Greater distance means that all these elements can be thinner (ie lighter). Think of it in terms of a lever arm.

Another analogy would be bicycle frames. Large-diameter, thin-walled frames such as Klein and Cannondale are lighter and stiffer than small-diameter, thick-walled steel frames of the same size.


2H4





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