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777 Wing Design.  
User currently offlinePIA777 From United States of America, joined Dec 2003, 1738 posts, RR: 6
Posted (10 years 9 months 3 weeks 2 days 17 hours ago) and read 6054 times:

Hello everyone, I have some questions about the 777's wing design. First of all there is positive pressure, which is on the bottom of the plane, and negative pressure, on top of the plane. Negative pressure being greater than the positive. When the plane is in the air and the two pressures combine at the trailing edge of the wing as downwash, and making the plane stay in the air. My question is, wouldn't the two pressures crush the plane while its in the air? Because there are two pressures.

Sorry if some of these things don't make sense.


-PIA777


GO CUBS!!
26 replies: All unread, showing first 25:
 
User currently offlineAAnalyst From United States of America, joined Nov 2003, 136 posts, RR: 0
Reply 1, posted (10 years 9 months 3 weeks 2 days 17 hours ago) and read 6001 times:

Ummm, you're on the right track.

You should read the post on wake turbulence that was just posted, it explains some of the principals of flight.

Basically, when air hits the wing it is separated, and flows over the top and bottom of the wing. The curvature on the top of the wing causes the air that flows OVER the wing to both speed up, and lose pressure. In physics this is known as Bernoulli's Law. The pressure on the bottom of the wing stays the same.

So, you have high pressure below the wing, and low pressure above the wing. This is how the wing creates lift. The high pressure above the wing basically "lifts" the plane up. This explains why when you watch a plane like the 777 or 747 on takeoff you see the wings bow up.

There is no crushing of the wing, as you originally posted. The wing is basically being "pushed up" from below.



Knowledge is Power. Power Corrupts. - Study Hard, Be Evil
User currently offlineConcordeBoy From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 9 months 3 weeks 2 days 17 hours ago) and read 5987 times:

Here in MSY (4ft below sea level)... we're under 14lbs psi of air pressure... and yet, somehow we're not crushed. Amazing that biophysiology Big grin


New word for you to look up: "equilibrium"  Big grin


User currently offlinePIA777 From United States of America, joined Dec 2003, 1738 posts, RR: 6
Reply 3, posted (10 years 9 months 3 weeks 2 days 17 hours ago) and read 5932 times:

Ohh yea, now i get it. Tell me if i get this right, The area of low pressure on top of the wing is not a vacuum but reduced value of pressure relative to the surrounding air. The area of high pressure, under the wing is an increased value of pressure relative to the surrounding air. So since there is more low pressure, the high pressure is over come by the low. Then the low tries to go downwards. The two meet at the trailing edge of the wing as downwash, and from that the wing moves upward.

Again sorry if this doesn't make sense.

-PIA777



GO CUBS!!
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 4, posted (10 years 9 months 3 weeks 2 days 16 hours ago) and read 5802 times:

>Basically, when air hits the wing it is separated, and flows over the top and bottom of the wing. The curvature on the top of the wing causes the air that flows OVER the wing to both speed up, and lose pressure. In physics this is known as Bernoulli's Law. The pressure on the bottom of the wing stays the same.

This isn't really correct, but it's all right since this area often leads to misconceptions.. we just had a huge argument about it in the Tech/Ops forum. There are detailed explanations if you scroll down.


here: http://www.airliners.net/discussions/tech_ops/read.main/72270/4/


User currently offlineArtsyman From United States of America, joined Feb 2001, 4745 posts, RR: 33
Reply 5, posted (10 years 9 months 3 weeks 2 days 16 hours ago) and read 5784 times:

Here in MSY (4ft below sea level)... we're under 14lbs psi of air pressure... and yet, somehow we're not crushed
*******

Yes, but why give up hope...

Jeremy


User currently offlineEmiratesA345 From Canada, joined Jun 2003, 2123 posts, RR: 8
Reply 6, posted (10 years 9 months 3 weeks 2 days 16 hours ago) and read 5781 times:

Doesn't that apply to all aircraft? Not just the 777 operated by Pakistan International Airlines.

EmiratesA345 Confused



You and I were meant to fly, Air Canada!
User currently offlineZak From Greenland, joined Sep 2003, 1993 posts, RR: 8
Reply 7, posted (10 years 9 months 3 weeks 2 days 16 hours ago) and read 5747 times:

emiratesa345 you have obviously read my mind.
and regarding the physics involved in creating lift, the other thread quite sums it up.
talking about lift and wing designs, nothing beats insect wings.
i will hope to be alive when we have the first man made design incorporating the insectoidal wing concept, that will be a beast since the design allows far greater weights by wing(it isnt really a wing but doh) area and has unsurpassed mobility and vtol capability.



10=2
User currently offlineEmiratesA345 From Canada, joined Jun 2003, 2123 posts, RR: 8
Reply 8, posted (10 years 9 months 3 weeks 2 days 15 hours ago) and read 5733 times:

Well,

After reading many of his posts, I noticed that in some way or another Pakistan International Airlines and their soon to arrive 777's find a way into every post.

It's quite childish and boring really, but whatever.

EmiratesA345 Smile/happy/getting dizzy



You and I were meant to fly, Air Canada!
User currently offlineWindowSeat From United States of America, joined Sep 2003, 1312 posts, RR: 57
Reply 9, posted (10 years 9 months 3 weeks 2 days 12 hours ago) and read 5670 times:



EmiratesA345

Well said

cheers




I'm all in favour of keeping dangerous weapons out of the hands of fools. Let's start with keyboards.
User currently offlineRendezvous From New Zealand, joined May 2001, 516 posts, RR: 0
Reply 10, posted (10 years 9 months 3 weeks 2 days 9 hours ago) and read 5638 times:

The 777 (even PIA's!) also have curved under-surfaces on the wing. What advantage does this give?

User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 11, posted (10 years 9 months 3 weeks 2 days 9 hours ago) and read 5641 times:

Are you wondering why the wing (airfoil) has the shape it does? The bottom is curved downwards so that the wing turns airflow downwards. The reaction force created is lift. The way the airfoil is curved (camber) is carefully designed depending on cruise conditions for the plane, for example.

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 12, posted (10 years 9 months 3 weeks 2 days 7 hours ago) and read 5616 times:

MIT,
the top is curved downwards, and that's the important part. The bottom of the wing, well, it's not nearly as important. That's why we like to hang all the junk... er, external stores... there! Along with flap and actuator fairings and what not... putting the entire engine on top of the wing in a low wing design is necessary, but it is almost physically painful to see all that top surface wasted!

And, as the misconception about a high pressure under the wing lifting the wing seems to have propagated into this thread as well, I just thought I'd point out once more that often the pressure under the wing is lower than ambient... just less so than on top of the wing.

The wing is being sucked up rather than lifted.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLiamksa From Australia, joined Oct 2001, 308 posts, RR: 0
Reply 13, posted (10 years 9 months 3 weeks 2 days 4 hours ago) and read 5590 times:

What a strange thread this started out as!?  Big grin

Anyway... it's only at high angles of attack that the pressure below the wing actually becomes greater than ambient.

And for a rough figure, the upper surface of the wing will contribute about twice as much as the lower surface towards generating lift.

Rob.


User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 14, posted (10 years 9 months 3 weeks 2 days 2 hours ago) and read 5571 times:

Thanks, that makes sense to me.

BTW FredT - that BWB project I was talking about isn't that interesting because it's just an exercise really. The BWB will be fly-by-wire, but our project is to make it statically stable. Kind of useless, but still informative  Smile


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 15, posted (10 years 9 months 3 weeks 1 day 20 hours ago) and read 5506 times:

Dear MITaero, all transport planes need to be stable. FBW or not, they are all stable designs. BWB or not makes no difference.

Unstable design is only practical on military combat planes (such as F-16, F/A-18, B-2 etc.) where center of gravity can be kept within extremely narrow limits.

It simply doesn't work on a transport plane that the control software has to be altered every time a beer can is served in Y class from the forward galley, or when an F class passenger "returns" it to the backward lav.

Well, that was probably exaggerating a little. But you cannot just throw baggage from a few hundred pax into the baggage load of an unstable plane and hope that it will fly.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 16, posted (10 years 9 months 3 weeks 1 day 13 hours ago) and read 5451 times:

I'm not sure about that. The BWB design as it is now is statically unstable. I think it depends on the control system implemented. The BWB will not actually exhibit unstable behavior due to FBW. I might be wrong, but does anyone else know?

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 17, posted (10 years 9 months 2 weeks 6 days 20 hours ago) and read 5292 times:

MIT,
pressed on time to reply as I'm out travelling, but I agree. The control system can correct for CoG changes in an unstable aircraft. Just gotta have the authority and you must be willing to cope with the extra drag of the deflected rudder surfaces... but you'd cope with the drag in a stable design as well.

The CoG movements associated with sudden departures of external stores intended for the purpose of correction of structural and geographical inconveniences in the regional environment (i e bombs) aren't a problem for starters.

Typically, the CoG is very much more in control in pointy-noses than in transports though but hey... doesn't have to be that way.

F/A-18 unstable, BTW? Relaxed static stability I've heard... but still not even neutral.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Reply 18, posted (10 years 9 months 2 weeks 5 days 20 hours ago) and read 5232 times:

Please escuse my ignorance... what is BWB? Thanks in advance.

Regardless of that, I think that in any case, FBW is what can make an unstable aircraft stable: the computers manage the flight attitudes and make the corrections. If the aircraft were not FBW the pilot would have to correct continuously...

FredT,
I have a video collection on flight lessons, aircraft developments (B747 and B777), air competition, etc., and cannot tell for the F-18, neither can play the videos now to find, but the only ones having to do with military are on F-16s, Harriers, U-2s and F-22s. If memory serves me well, flight managing computers on board (most likely the Falcon or the U2) make 40 corrections per second. At least it is what the video says. And they all have been shooted in narrow collaboration with anyone involved, including pilots and manufacturers. Although not making a bible of htem, I tend to believe they say the truth...

Enjoy turbulences!!!


User currently offlineDw747400 From United States of America, joined Aug 2001, 1260 posts, RR: 1
Reply 19, posted (10 years 9 months 2 weeks 5 days 19 hours ago) and read 5223 times:

BWB is Boeing's Blended-Wing-Body, essentially a flying wing, but shaped a bit differently to able to carry passengers more efficiently and integrate better with airport facilities. I believe it’s estimated that it will carry 500-800 pax, if it is pursued.

Boeing has another interesting flying wing concept that’s less well known which could potentially drop fuel burn an additional 10% or so from the estimates of the BWB. Only problem is, it is so ugly many worry people would refuse to fly it (makes the Sonic Cruiser look as normal as a 737).

Even a conventional airframe stands to benefit from being unstable, as it eliminates the need for a downforce on the tail, and could possibly result in smaller, lighter control surfaces as well (less force needed to displace the aircraft).



CFI--Certfied Freakin Idiot
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 20, posted (10 years 9 months 2 weeks 5 days 17 hours ago) and read 5196 times:

>Even a conventional airframe stands to benefit from being unstable, as it eliminates the need for a downforce on the tail, and could possibly result in smaller, lighter control surfaces as well (less force needed to displace the aircraft).

Yes, this is what I'm saying. The project I'm working on now shows the tradeoffs between stability and other flight characteristics on the BWB (i.e. lift and drag coefficients), and there is a clear trade-off.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 21, posted (10 years 9 months 2 weeks 5 days 17 hours ago) and read 5184 times:

Believe me, in our lifetime there won't be any unstable commercial transports for safety and comfort reasons. Tailless, BWB or conventional, that doesn't matter, they just ain't gonna be built.

During the last 20 years all really new large airliner designs have had FBW controls. Therefore they could have been made unstable. But artificial stability hasn't even been discussed in the design offices.

Some planes (at least Concorde, A330/340 - probably more) can shift fuel between center- and tail tanks and optimize the stability factor for reduced drag, but they all fly with a positive pitch stability factor.

Every time a commercial transport takes off, then the flight crew only has a rough guess about the real center of gravity of the plane. That's very much opposite to a B-2 bomber. A B-2 drivers know exactly how loading of a bomb affects his CG.

In fact all commercial transports have a very generous size of horizontal stabilizer. It makes it possible to have the plane certified to fly with a wide CG span. The airlines want that since it makes them more flexible in daily operation. But sure it increases drag and fuel burn, and reduces range. It's one of thousand trade offs which goes into airliner design.

But it is totally incompatible with artificial pitch stability. With artificial stability you must know the CG very precisely before takeoff.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineDw747400 From United States of America, joined Aug 2001, 1260 posts, RR: 1
Reply 22, posted (10 years 9 months 2 weeks 5 days 16 hours ago) and read 5179 times:

I would argue that if it does not exist already, the technology to fly an unstable airliner will be available in the not to distant future. If a computer system can determine the motion of the plane accurately enough, it can effectively counter any sort motion rapidly... I'm not enough of an expert on the subject to say exactly how it would be done, but I definitely could see it being done. There are so many different concepts that could be used... Perhaps MITAero could tell us a bit more about systems he is familiar with, if its not confidential.

As for safety, I don't see a big problem. The 777 will likely be the last airliner that can be flown with the FBW system completely inoperative. The A380, at least as of a couple years ago when I read about is flight controls, does not have mechanical backups, but instead relies on quad-redundant FBW systems. If you are already putting all your eggs in the FBW basket, then it won't make much of a difference if you’re statically stable or not: in the unlikely event the FBW goes kaput, you have either a stable, uncontrollable airplane or an unstable, uncontrollable airplane.

The idea of making an unstable airliner has been tossed around for a while now, and there were some at Boeing who felt that the 777 should be unstable in pitch. With FBW a new concept in the traditionally conservative Boeing offices, an unstable plane didn't go too far.



CFI--Certfied Freakin Idiot
User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Reply 23, posted (10 years 9 months 2 weeks 5 days 16 hours ago) and read 5178 times:

dw747400,
Thanks for your explanation. I didn't know for the name abbreviation, but I had already seen that. Anyway, it was in a time (maybe more than 20 years ago) when FBW and computer systems were not as complex as today, to say the (very) least, and that try was quite hard.

As for existing technology, for sure it exists... but manufacturers may be quite "prudent" in applying some solutions to airliners... Otherwise we would have started seeing for a while pax transports radically different, with very small (if any) stabilizers, or variable geometry wings...

Anyway, despite the advantages of being unstable, for sure an airliner does not need in any way to be as unstable as fighters are, and some "too radical" designs could make potential customers "afraid"...

Enjoy turbulences!!!


User currently offlineGrandtheftaero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 24, posted (10 years 9 months 2 weeks 5 days 10 hours ago) and read 5146 times:

"The top is curved downwards, and that's the important part. The bottom of the wing, well, it's not nearly as important."

Actually most modern high-subsonic airliners have supercritical wings where most of the camber is located on the BOTTOM on the wing. Check out the "Supercritical Wings" thread. http://www.airliners.net/discussions/tech_ops/read.main/74664/

--Shane



25 FredT : Cannot happen and will not happen are two very different things. If you say that we will not see a statically unstable airliner, I'll agree that it is
26 Post contains images GrandTheftAero : Touche, FredT... I should have said, "where the maximum camber is located near the trailing edge with most of the curvature at the bottom surface of t
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