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AF447 wreckage found

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AF447 wreckage found

Old 19th Jun 2011, 14:07
  #1781 (permalink)  
 
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Chris Scott;

this indeed helps a lot - thank you for clarification. apologies for my fundamental misunderstanding(s) in the first place and for inadvertently having produced some clutter - i appreciate the valuable contributions by pilots and other pros on this forum.
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Old 19th Jun 2011, 14:43
  #1782 (permalink)  
 
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@ jcjeant

Thanks for your post 1767 - nice to hear some once-familiar voices !
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Old 19th Jun 2011, 15:02
  #1783 (permalink)  
 
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RetiredF4 mm43
Good graphic, except that the profile is upside down. On the THS the curvature of the profile is on the downside.
I'm sorry to disagree, but I do believe that the mm43 profile is correct. The THS on almost heavy aircraft has a positive profile. That is why the "ground set" is minus 4, (zero lift). Remember that the A330/340 family (and others) carry 5 Ton of fuel in the trim tank (that is why THS, with trim tank full, flies at minus 3 - only 1 above ground zero set - and this is done) to save about 3% of fuel, due to less drag.
Also, the reason why the elevator flips 30 up and only 15 down is due to the fact that it needs a higher degree up in order to destroy positive lift created by the THS curved wing.

Last edited by Jetdriver; 19th Jun 2011 at 16:01.
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Old 19th Jun 2011, 15:11
  #1784 (permalink)  
 
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aguadalte,

I respectfully disagree!
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Old 19th Jun 2011, 15:38
  #1785 (permalink)  
 
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Chris Scott:
I know you do. When I made my studies, conventional stabilizers were designed to produce negative lift. At that time, there were no preoccupations with fuel saving or drag reductions...
I think (although not sure) that, with the advent of CGCC computers and in order to save fuel, that new big jets suffered a slight redesign of the stab profile. New jets are not designed to be stable. They are designed to fly with near zero trimming. It is therefore a waste of lift and fuel, to design stabs producing negative lift, together with 5 Ton of Fuel...
I will pay closer attention next time I fly and will take a closer look at the stab profile.

Last edited by aguadalte; 19th Jun 2011 at 16:42. Reason: grammar
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Old 19th Jun 2011, 16:58
  #1786 (permalink)  
 
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Originally Posted by Chris Scott View Post
Even a neutral (in pitch) stick would have resulted in full up-elevator, because the FBW would have still been trying to achieve (maintain) 1G, and the THS seems to have reached full nose-up trim (13NU) at around the apogee-point. Normally, Pitch-Alternate has a stall-protection function, based on AoA. But the AoA readings had been ruled invalid earlier because of the (false) low-airspeed data.

Although it was inevitably unsuccessful in maintaining 1G, and the aircraft's trajectory had become semi-ballistic, the system would have tried its best. In so doing, it would have selected full up-elevator. It no longer had recourse to the THS, which had reached full-travel.
Hi Chris,

I still find it hard to believe that given the amount of thought put into systems degradation upon loss of air data the "G-loading" system would still be in effect (after all, loss of air data will disconnect the FMC because it can no longer function reliably - I find it hard to believe they'd still have the FCU systems trying to work with spurious data). We'd need someone who worked on the system directly to clarify though, as I have no data. On the other thread we discussed the behaviour of the control laws as speed decays, where "stick-neutral" gradually transitions from "G-loading" to "zero pitch". There's also the BEA note which seems to be pretty sure that the PF was commanding a lot of nose-up.
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Old 19th Jun 2011, 17:38
  #1787 (permalink)  
 
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?

I assume the lower red vector is the Relative Wind? If so, wouldn't that make the AoA incorrect?
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Old 19th Jun 2011, 19:09
  #1788 (permalink)  
 
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camber of THS

aquadalte
Chris Scott:
I know you do. When I made my studies, conventional stabilizers were designed to produce negative lift. At that time, there were no preoccupations with fuel saving or drag reductions...
I think (although not sure) that, with the advent of CGCC computers and in order to save fuel, that new big jets suffered a slight redesign of the stab profile. New jets are not designed to be stable. They are designed to fly with near zero trimming. It is therefore a waste of lift and fuel, to design stabs producing negative lift, together with 5 Ton of Fuel...
I will pay closer attention next time I fly and will take a closer look at the stab profile.
No need to wait, look at the pictures Confiture posted in the other AF447 thread.

FBw could not change the laws of physics, if you use cambered airfoils, the lift of the wing has to be upward and that of the tail downward at 0 AOA to have some kind of positive stability, when the confusers go on leave.

Last edited by RetiredF4; 19th Jun 2011 at 20:19.
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Old 19th Jun 2011, 21:01
  #1789 (permalink)  
 
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Camber of THS and influence on CL-stall

In search for the correct profile of the THS to this post of mm43 (which turned out in vain) i stumbled on the following graphic:


positive and negative AOA

It depicts the point zero AOA Lift (there is lift due to the cambered profile) and max CL at 16 AOA (stalling AOA of that profile) in the positive AOA region and in the negative AOA region. In the negative AOA region the max CL is smaller by the amount of zero AOA CL.

When we transfer this information to the THS we must assume, that max AOA for max CL is the same for THS up (AND) or THS down (ANU), that the max CL for AND is smaller than max CL for ANU due to the cambered profile for ANU. The ANU function of the THS is more effective than the AND function.
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Old 19th Jun 2011, 21:08
  #1790 (permalink)  
 
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THS - Camber

I note there has been continuing discussion on whether the THS has top or bottom camber. As I said in a post earlier (which has been removed), I couldn't find any info on the profile and the drawing was a "best guess". Looking at the pics posted on the other thread, I am inclined to believe that the airfoil has a neutral camber.

A Flight Global cutaway of an A300 THS is the nearest I could find, and it appears to favour a slight positive top camber.



However, the general idea of how a high AoA would present itself to the THS was what I have attempted to portray, and in that respect I could have done a better job by showing the Angle of Incidence - which is:-

AoI = AoA + (-NU/+ND) or AoI = (61 - 13) = 48

Castle Don;

The attitude as depicted represents the nominal longitudinal baseline of the aircraft and an approximate chord line for the main wing. The THS is trimmable, and its angle of attack is represented by AoI in the formula above.

Last edited by mm43; 19th Jun 2011 at 22:25.
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Old 19th Jun 2011, 21:52
  #1791 (permalink)  
 
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About angles

Castle Don;

The AoA of most large airplanes is defined as the angle between the (undisturbed) airflow and the airplane's longitudinal reference axis, and is correctly shown in mm43's super graphic.

Just to confuse things a little, D.P. Davies' book uses the name "angle of incidence" for that same angle. However, for many others "angle of incidence" is the angle between the "wing chord" and the longitudinal reference axis. That angle may be meaningful for light aircraft with a straight, constant section, untwisted wing. It is not meaningful for the swept, twisted wings with variable section that are used on modern large transport aircraft. If you want to apply it to the airfoil section in mm43's drawing, the angle of incidence of the horizontal tail as drawn is -13 degrees.

When talking about two-dimensional flow about airfoil sections, it is customary to define AoA as the angle between the airflow and the section chord. However, when talking about the AoA of the tailplane, one should be aware that the airflow at the tail is not undisturbed. A lift-producing wing has a downwash behind it, and therefore the local AoA is less than the free-stream AoA.
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Old 19th Jun 2011, 22:09
  #1792 (permalink)  
 
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In the search for an answer for this camber question, I have found the document of the patented THS system invented by Airbus Industrie (Process for improving the maneuverability of an aircraft during a resource - AIRBUS France please open the PDF).
I know its not much but, the drawings of the THS show a profile with a "neutral wing" (sorry, can't remember the correct name of that wing type).
Please note the last claim of the "paper":
2. The process of claim 1, wherein said initial flight phase is a cruising flight and said resultant aerodynamic force is at least approximately zero.
I'm not sure if I'm right but, it seems understandable to me, that in view of the new flight controls systems and new aerodynamic designs, that a solution other than a down camber stabilizer wing would suite the needs for fuel efficiency.

P.S.- By the way, the pictures posted by Confiture were taken from below and do not give a correct idea of the profile of the wing/stab. So, I am still not convinced...
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Old 19th Jun 2011, 23:30
  #1793 (permalink)  
 
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THS

Well, lets look at some more pictures.
Is the A320 series different?

A330-200, see the curve on the underside, would be a very thick profile if symetric

close view of LE of THS A330

A330, see straight top and curved bottom

a319, definite camber on the underside

a330 air afrique, definite flat top

A320 stabilizer

Airbus A330-203 MRTT, copy and zoom in

Last edited by RetiredF4; 20th Jun 2011 at 19:06.
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Old 20th Jun 2011, 00:16
  #1794 (permalink)  
 
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RetiredF4;

I've reviewed the pics you posted and agree that the A330 THS has more camber on the underside. It follows that with an AoA of close to 0 degrees that a tail down moment is applied and least amount of drag is caused before NU trim is necessary.

Thanks to all those who have posted their comments, and I'll now redraw the THS with bottom camber.
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Old 20th Jun 2011, 00:16
  #1795 (permalink)  
 
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RetiredF4:
Is the A320 series different?
Yes it is. It doesn't carry fuel, doesn't act as a trim tank.

Me:
The THS on almost heavy aircraft has a positive profile. That is why the "ground set" is minus 4, (zero lift). Remember that the A330/340 family (and others) carry 5 Ton of fuel in the trim tank (that is why THS, with trim tank full, flies at minus 3 - only 1 above ground zero set - and this is done) to save about 3% of fuel, due to less drag.
Also, the reason why the elevator flips 30 up and only 15 down is due to the fact that it needs a higher degree up in order to destroy positive lift created by the THS curved wing.
I was speaking of heavy aircraft on the Airbus family.

HazelNuts39:
Just to confuse things a little, D.P. Davies' book uses the name "angle of incidence" for that same angle. However, for many others "angle of incidence" is the angle between the "wing chord" and the longitudinal reference axis. That angle may be meaningful for light aircraft with a straight, constant section, untwisted wing. It is not meaningful for the swept, twisted wings with variable section that are used on modern large transport aircraft. If you want to apply it to the airfoil section in mm43's drawing, the angle of incidence of the horizontal tail as drawn is -13 degrees
None of your pictures shows an undoubtedly lower camber on an A330/A340 aircraft. I've seen most of those pictures before you have posted them here. I've done my research also, and I have found no good picture to post also.

Remember these are twisted wings with variable section.
Would you say that this wing has an upper camber if you didn't know?
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Old 20th Jun 2011, 00:55
  #1796 (permalink)  
 
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I think (although not sure) that, with the advent of CGCC computers and in order to save fuel, that new big jets suffered a slight redesign of the stab profile. New jets are not designed to be stable. They are designed to fly with near zero trimming. It is therefore a waste of lift and fuel, to design stabs producing negative lift, together with 5 Ton of Fuel...
[my highlighting]

True that many jet fighters are fundamentally unstable, but it does not apply to public-transport jets (yet...).

The FBW-controlled A320 is not much less stable than a B737, as far as I know. And it is therefore flyable, and just about land-able, using the THS and rudder as the only means of control. (We practise the former on the aircraft, and the latter in the simulator, on initial conversion.)

The (up-to about) 5 tonnes of fuel in the A330 THS is moved there purely to relax the stability slightly in the cruise, and reduce the negative lift required of the THS (for the reason you stated).

Re the THS bottom-surface camber, I can only speak for the A320. The best view one gets is from a set of steps, when they are positioned at a rear door. The leading edge area is then clearly visible as having more bottom camber, What happens further aft is less clear, I must admit, but the top surface does not show very much convex camber.

Not sure all this makes much difference to mm43's graphic!

Last edited by Jetdriver; 20th Jun 2011 at 14:55.
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Old 20th Jun 2011, 10:09
  #1797 (permalink)  
 
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More terminology

From the Introduction of NACA Report No. 460, The Characteristics of 78 Related Airfoil Sections ...:
Airfoil profiles may be considered as made up of certain thickness forms disposed about certain mean lines. The major shape variables then become two, the thickness form is of particular importance from a structural standpoint. On the other hand, the form of the mean line determines almost independently some of the most important aerodynamic properties of the airfoil section, e.g., the angle of zero lift and the pitching moment characteristics.
Very old stuff, but still valid. The 'mean line' is equidistant between top and bottom surfaces of the airfoil, and 'camber' describes its curvature, i.e. a symmetrical airfoil has zero camber.
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Old 20th Jun 2011, 13:10
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HazelNuts39:

Just to confuse things a little, D.P. Davies' book uses the name "angle of incidence" for that same angle. However, for many others "angle of incidence" is the angle between the "wing chord" and the longitudinal reference axis. That angle may be meaningful for light aircraft with a straight, constant section, untwisted wing. It is not meaningful for the swept, twisted wings with variable section that are used on modern large transport aircraft. If you want to apply it to the airfoil section in mm43's drawing, the angle of incidence of the horizontal tail as drawn is -13 degrees

The inherent wing incidence in a transport aircraft, provides a level cabin in cruise. True with a straight wing, swept wing, any wing, in light, medium and heavy transports.

The vertical tail with its fixed and moving parts, has its angle of incidence set for optimal longitudinal stability and control. Mission requirements, aerodynamic analysis, wind tunnel testing and past company experience, determine which profile and every other parameter, would be optimal.

Saving aircraft operating expenses is affected in longitudinal channel design, by lightweight, low drag surfaces and CG location. Since installed in the back and designed to counter wing and fuselage pitching moments, one way to reduce drag is by reducing the pitching moments against which the horizontal tail must act, specifically by moving the CG backwards. This reduces the tail pitching moment required, and thereby its lift and drag. It does affect longitudinal stability characteristics, but with the use of augmented flight control systems, is mostly latent to the pilot. Having flown quite a bit in variable stability aircraft and rotorcraft for research purposes, I can attest that this design is mostly safely fliable. Not all designs are equal though, for example the MD-11, in which pilots had difficulty controling the aircraft to touchdowen during turbulence, crashing quite frequently because of that, e.g. Narita about two years ago.
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Old 20th Jun 2011, 23:46
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There are several factors going into angle of incidence decisions. The CC might like a level floor (zero body angle) in cruise, but then there's Vmu, and several other issues. Most airliners in fact cruise 2 or 3 degrees nose up.
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Old 21st Jun 2011, 03:10
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There is a lot of discussion on this thread regarding AOA and the efficiency of the THS.

It strikes me that both the main plane and tail plane were deeply stalled and providing significant (vertical) drag, rather than controllable lift.

Given that the drag from the main plane acts near the C of G and the drag from the tail plane acts well aft of the C of G this should have resulted in a nose down couple.

The only thing opposing that nose down couple would be the nose up couple provided by engine thrust.

Am I missing something, or was this aircraft unrecoverable until the throttle was closed?

Last edited by EGMA; 21st Jun 2011 at 03:24.
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