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Why Do DC-10's/MD-11's Have To Fly Nose-up?  
User currently offlineTupolev154B2 From United States of America, joined Jun 2000, 1332 posts, RR: 2
Posted (13 years 5 months 2 weeks 4 days 17 hours ago) and read 11602 times:

Why do DC-10's and MD-11's have to cruise with their nose pointing up?

14 replies: All unread, jump to last
 
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 1, posted (13 years 5 months 2 weeks 4 days 15 hours ago) and read 11509 times:

All airplanes cruise with a positive AoA (Angle of Attack). The camber of the wing alone doesn't produce enough lift to maintain altitude with 0 AoA.

 Big thumbs up



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User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 2, posted (13 years 5 months 2 weeks 4 days 5 hours ago) and read 11452 times:

Well......not really....This has to do with angle of incidence and not angle of attatck.

All jets fly nose up to utilize body lift. In cruise you will have a 1-2 degree body angle depending on loading

The angle of Incidence...the chord line relative to the longitudinal axis.....is designed so that under all instances of balance the jet cruises nose up and that the stabiliser always exerts a positive downward force.

JET


User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 3, posted (13 years 5 months 2 weeks 4 days ago) and read 11386 times:

1. While the pitch attitude is positive in most jet transports, the aircraft is actually pointing into the relative wind due to the way the aircraft "pulls" the air up in its path in subsonic flight (in reality there is a "wake" that moves ahead of the aircraft much like any other object moving through any fluid, and that "wake" moves forward at the speed that waves are transmitting through that medium, i.e. the speed of sound). So, actually, the fuselage is nicely aligned with the airflow for minimum drag.

As to Jet's second point about "the chord line relative to the longitudinal axis.....is designed so that under all instances of balance the jet cruises nose up and that the stabiliser always exerts a positive downward force. ", well, not sure what he meant by this, but you don't need the nose to be "up" to get a downforce on the horizontal stab at all. That force is a necessary balance to the forward pitching moment of the wing itself (which happens independent of CG, incidentally).

The camber of the wing DOES produce enough lift at zero and even at negative AoA, depending on the speed of the airflow across it's surface, so the other post was not quite right either.

The MD-11 and DC-10 do sit more "nose up" on the ground, I believe this is due to the size of the leading edge slats relative to other aircraft to achieve a near zero AoA during takeoff roll with flaps set to takeoff position for min drag.


User currently offlineB1C17L1011 From United States of America, joined Jun 2001, 96 posts, RR: 0
Reply 4, posted (13 years 5 months 2 weeks 4 days ago) and read 11362 times:

The previous post are correct and very nicely detailed. As an answer to the original question may I point out that the B52 cruises with the nose slightly down, but the wings have an angle of incidence of about 8 degrees.
On another note, have you noticed that the 747-100 & 200 land with a higher angle of attack than the 400 model. Could this be due to the winglets?


User currently offlineDuce50boom From , joined Dec 1969, posts, RR:
Reply 5, posted (13 years 5 months 2 weeks 3 days 23 hours ago) and read 11360 times:

the b-52 cruises nose down because, thanks to the sweep of it's wings, it's center of gravity is forward of it's center of lift, unless it's fully loaded with bombs and fuel, then it's nose high

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 6, posted (13 years 5 months 2 weeks 3 days 22 hours ago) and read 11354 times:

Duce50boom.....The Center of gravity is always foward of the center of lift or the airplane would be unrecoverable is a stall.

Seagull.....my post is right on the money.....maybe go back and read it again. A jets cruising body angle is in direct relstionship to the wings angle of incidence.

JET


User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 7, posted (13 years 5 months 2 weeks 3 days 22 hours ago) and read 11341 times:

Don't know why the 744 would be at a lower AoA, haven't noticed that. Are you sure it is? The longer and different fuselage shape could just be making it appear to be different. Winglets are unlikely to be making that much difference, but some other issue, such as flap config could.

The B-52 cruising nose low would not be due to any CG position particularly, but rather due to being lighter as opposed to heavier. Aircraft are not attached like a pendulum to anything, so the location of the weight doesn't affect them the same way that it would if they were attached to a tether.


Jet, you posted that "The angle of Incidence...the chord line relative to the longitudinal axis.....is designed so that under all instances of balance the jet cruises nose up and that the stabiliser always exerts a positive downward force. "

Perhaps read it again. While what you wrote is true taken as separate issues, there is not a relation per se. To say that the angle of incidence will affect the body angle in flight is true. It is true that the angle of incidence is designed so that the "under all instances of balance the jet cruises nose up" and it is also true that the "stabilizer always exerts a positive downward force". However, it would not be accurate that the angle of incidence is set to the others are necessarily true. There are many factors involved, including what the optimum mach cruise speed is, the altitude range for cruise flight, etc.

There are many factors here. I took issue with how you tied these factors together, an apparent confusion of causation with association.


User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3708 posts, RR: 34
Reply 8, posted (13 years 5 months 2 weeks 3 days 20 hours ago) and read 11325 times:
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When I worked on the DC10 the explanation I was told was that the DC10 spec was for it to fly at max cruise speed so the wing was attached at the appropriate angle of incidence. However in the years between design and actually operational flying fuel prices shot up, so now the a/c is operated at econ cruise speed which requires the nose up attitude to compensate for the "wrong" A of I.

User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4229 posts, RR: 37
Reply 9, posted (13 years 5 months 2 weeks 3 days 20 hours ago) and read 11324 times:

Cruise on the DC-10-30 at 519,000 pounds and 33,000 feet at STP has a +4 degree nose up attitude... higher than you would think.


Chicks dig winglets.
User currently offlineTupolev154B2 From United States of America, joined Jun 2000, 1332 posts, RR: 2
Reply 10, posted (13 years 5 months 2 weeks 3 days 19 hours ago) and read 11311 times:

Was on a DC-10-40 and it seemed as if I walked down the aisle faster than I walked up it.

User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 11, posted (13 years 5 months 2 weeks 3 days 18 hours ago) and read 11314 times:

Can't speak for the DC-10 really, but the difference in cruising M0.82 to M0.855 (that's all we can get out of the MD-11 due to the software restraints) is not much in terms of pitch attitude, and the deck angle appears to me to be about the same as the DC-10 when I've ridden on it, so I rather doubt the story of high speed cruise being a factor. I think that the issue is just pointing the nose into the relative wind. Haven't seen wind tunnel tests on it, but wouldn't be surprised if the flow is a bit different due to the farther aft wing than other widebodies. How does it compare to the L1011? The 727 and MD-80 series might be instructive as well.

User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6539 posts, RR: 54
Reply 12, posted (13 years 5 months 2 weeks 2 days ago) and read 11232 times:

There are only three factors involved:
1. weight
2. air density (altitude)
3. speed

The combination of these factors dictate at what angle of attack (AoA) the wing must be flying at level cruise. It simply has to produce a lift which is equal to the weight of the plane.

Nose heavy or tail heavy means practically nothing for the wing AoA, but it may mean a lot on the trim of the tailplane.

The fuselage is mounted on the wing at a certain incidence depending on plane type, mostly so it has a slight upward pointing attitude at normal cruise, which - as Seagull correctly points out - creates less drag since it alligns the fuselage better with the relative wind.

To create the least drag the fuselage should in fact be a little "banana shaped", but that would not be practical.

I read somewhere many years ago that the A300 has an almost horizontal fuselage attitude at cruise, at that should have been chosen for comfort reasons. Not passenger comfort, but FA comfort. It makes it much easier to pull the carts in the ailes.

Remember that originally the A300 (B-1 and B-2 versions) was a rather short range plane, so one or two percent more fuel burn wouldn't hurt that much. But it is quite some job for the FAs to serve 250 pax with meals, drinks etc. on a short flight.

I haven't noticed that myself since I have never been on an A300. Can somebody confirm or disaprove that?

Rgds, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineSkystar From Australia, joined Jan 2000, 1363 posts, RR: 2
Reply 13, posted (13 years 5 months 2 weeks 1 day 13 hours ago) and read 11211 times:

B1C17L1011,

Possibly some of the reasons for the 744s lower pitch on landing could be the fact that it has

i) a larger wing
ii) an extra leading edge flap panel

Flap configs as far as I know are the same on all 747 models.

In all honesty, I've never really noticed it, We don't get many 747 Classics at YMML. Are these aircraft of the same operator? Perhaps the difference in flaring technique is operator specific.

Cheers,

Justin


User currently offlineQantas737 From Australia, joined Jul 2000, 738 posts, RR: 4
Reply 14, posted (13 years 5 months 2 weeks 1 day 12 hours ago) and read 11209 times:

One 747 variant doesnt have the same triple slotted flaps as the others which is the 747SP. It has a single slotted flap rather than triple slotted flaps. I know im way off topic here, but I just thought i would point that out.

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