Airbus Experts need to comment on Airbus details
AF330 - I am more familiar with the details for Boeing FBW so I will leave comments about Airbus system details to those experts.
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AF330:
Yes, if I understand you correctly then you seem to be more or less correct, with the following comments: Direct Law So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY... You would only encounter Direct Law after multiple in-flight systems failures. On the A330, the system reconfigures from Normal Law to Alternate 1 Law to Alternate 2 Law and finally to Direct Law, depending on the level of failure. The failures that would lead to Direct Law include:
FCeng84: From the reference it appears that for the single aisle models the CG and GW estimates used by the control laws are completely independent of pilot input of ZFW and ZFCG. For the long range models the picture does not look quite as clear with the stated impact on control laws characterized as "slightly affected". I realize that the aerodynamics based estimates will only be available in air and then only after a certain period of time to allow the associated estimation algorithms to converge. If the control laws are dependent on CG and GW during takeoff roll and/or initial climbout I suspect that either default values coded with the control laws or the data input by the pilots will have to be used. During the take-off roll, the flight control system is in Normal Law Ground Mode. There is a direct relationship between the sidestick deflection and elevator deflection, with full authority. As far as I'm aware the control law is not modified for GW/CG during take-off. The stab position is set manually (automatically on later aircraft) after engine start, according to the GWCG value (or loadsheet GWCG) and there is no auto-trim. Consequently, during take-off the pitch control operates exactly the same as it would in a conventional aircraft. It transitions to Flight Mode shortly after lift-off and it's only then that the FCMC derived CG value starts to affect the control laws. |
Where is A33Zab when we need him... :)
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In - read only - Law.:(
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Hi A33Zab!
Good to have you here! Can you post some definitive info on how/where from Flight Control Computers obtain C.G. data for Direct Law gains? |
See Buzzbox's reply, all I can add are some images.
http://i474.photobucket.com/albums/r...ps6a522af9.jpg http://i474.photobucket.com/albums/r...evatordefl.jpg |
:)
Good evening,
Interesting.... http://i474.photobucket.com/albums/r...ps6a522af9.jpg Ok so the center of gravity is the point at which the entire weight of the plane may be considered as concentrated so that if supported at this point the plane would remain in equilibrium in any position. 1) So does this point keep changing during the flight? Where is that point generally located? Ok, so let me take all the computers on your picture: FMGEC = Flight management, guidance and envelope computer FCMC = Fuel Control and Monitoring Computer FWC = Flight warning computer FCPC = Flight Control Primary computer 2) Ok, as I don't know all of these computers, I just would like to compare with the A320: a) I have heard about the FMGS/FMGC about the A320, but never about the FMGEC. Has the A320 a FE?? If not, who controls GW and C.G? b) Can we say that FCMC = FQI + FLSCU? c) Does an A320 have a FWC? So each time a computer detects a problem, it sends it to the FWC (and the FMGEC maybe, no?) d) FCPC...hmm... can we say that FCPC is kind of ELAC on Airbus? ELAC + SEC? ELAC + FAC? FAC + SEC? 3) Ok, so the FCMC is controlling the gross weight and the center of gravity. But is it calculating the fuel's GW? Is it calculating the plane's C.G? 4) So you have a AFT CG Monitor inside the FE. The FWD CG is calculated by getting the CG calculated by FCMC? CG calculated by FCMC - AFT CG calculated by FE = FWD CG? It's all for now! Thanks a lot A33Zab, BuzzBox, FCeng84, titaniumwings, Uplinker, vilas and everyone else! :) |
Are you asking if the -320 has a Flight Engineer?
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Ha ha Amadis! If required, yes! :) No just joking, of course.
But I think that A33Zab and others know everything about all this! :) |
Thanks A33Zab and Buzzbox. I am quite surprised that Direct Law, which is the most basic form of reversion (save the Mech Backup) still needs data from additional "boxes" like FCMC/FMGEC, or FAC in case of A320.
What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG? |
AF330:
Ok so the center of gravity is the point at which the entire weight of the plane may be considered as concentrated so that if supported at this point the plane would remain in equilibrium in any position. 1) So does this point keep changing during the flight? Where is that point generally located? To complicate matters, the long range Airbii have a trim tank in the tail. Fuel is transferred from the centre/inner tanks to the trim tank after take-off to move the CG aft. The fuel system aims to keep the CG close to an aft CG target, approximately 2% forward of the the aft CG limit, to reduce drag. Where is the CG located? It depends how the aircraft is loaded and what stage of flight you're talking about, so it could be anywhere between the forward limit (typically about 15% MAC) and the aft limit (typically about 40% MAC). The fuel system of the long range Airbii aims to keep the CG at about 38% MAC (roughly) during the cruise. 2) Ok, as I don't know all of these computers, I just would like to compare with the A320: a) I have heard about the FMGS/FMGC about the A320, but never about the FMGEC. Has the A320 a FE?? If not, who controls GW and C.G? b) Can we say that FCMC = FQI + FLSCU? c) Does an A320 have a FWC? So each time a computer detects a problem, it sends it to the FWC (and the FMGEC maybe, no?) d) FCPC...hmm... can we say that FCPC is kind of ELAC on Airbus? ELAC + SEC? ELAC + FAC? FAC + SEC? 3) Ok, so the FCMC is controlling the gross weight and the center of gravity. But is it calculating the fuel's GW? Is it calculating the plane's C.G? The FCMC calculates the weight of fuel on board by measuring the fuel volume and density, and displays that calculated figure on the ECAM. It also calculates the fuel on board by subtracting the fuel used from the fuel on board at engine start and generates a caution if there is a discrepancy between the two calculated figures. The FCMC also calculates the aircraft's CG using the ZFW and ZFWCG input by the pilots during the preflight, and the sensed fuel quantity & distribution - as discussed previously! 4) So you have a AFT CG Monitor inside the FE. The FWD CG is calculated by getting the CG calculated by FCMC? CG calculated by FCMC - AFT CG calculated by FE = FWD CG? The FE does a GW/CG computation independently of the FCMC, using various aerodynamic inputs (speed, thrust, altitude, THS position, elevator position). The GW/CG values computed by the FE are used as a backup in case of dual FCMC failure, and are also used to generate an 'Excess Aft CG' warning if the CG is found to be too far aft. |
Thanks a lot,
So I understand that during the flight, C.G moves AFT as fuel get's burned. 1) So does the plane tend to pitch-up as you burn fuel? But how do you reduce drag? If you pitch up, you reduce drag! 2) Sorry, what does MAC mean? Could you please explain? Ok, the FCMC needs the fuel's volume and density to get GW. But I didn't understand how is FOB calculated. 3) You know the weight of fuel. Why does it take fuel used during engine start??? What are the 2 calculated figures? I had XXX lbs at departure. Now, I know my fuel quantity by knowing the fuel density, no? Confused! ;) 4) The ZFWCG is given by the pilot. But does the plane calculate it again (and changes the value)? Like for fuel: when you enter "block", the FMGS can automatically change it If it wrong. 5) The FWD CG is also controlled by FE? |
What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG? (as you can see in the diagram there is NO GW/CG connection to FCSCs). Gains are defaulted at a safe value and depends on S/F position. The FM section memorizes the last GW/CG value for internal purposes in case of power shutdown of FMGEC but AFAIK this is not available to other equipment. |
@BuzzBox: Do you mran that it calcualtes FOB by subtracting weight of fuel before engine start - weight of fuel now?
It will then give the fuel's weight which has been burned.. Confused! |
AF330:
1) So does the plane tend to pitch-up as you burn fuel? But how do you reduce drag? If you pitch up, you reduce drag! As the CG moves aft, the amount of downforce the stab must produce reduces and so does the drag. The Airbus fuel system tries to keep the aircraft's CG as far aft as possible during the cruise to reduce drag and improve performance. The effect of CG on induced drag is discussed in the following link: http://www.pprune.org/tech-log/35785...uced-drag.html 2) Sorry, what does MAC mean? Could you please explain? The location of an aircraft's CG is usually expressed as a certain percentage of the MAC, e.g. 35% MAC means the CG is located 35% of the distance along the MAC. The following link has a simple explanation: Mean Aerodynamic Chord (MAC) - SKYbrary Aviation Safety Ok, the FCMC needs the fuel's volume and density to get GW. But I didn't understand how is FOB calculated. The FCMC measures the volume of fuel in the tanks and its density and calculates the FOB. The result is displayed on the ECAM and fuel system page. 3) You know the weight of fuel. Why does it take fuel used during engine start??? What are the 2 calculated figures? I had XXX lbs at departure. Now, I know my fuel quantity by knowing the fuel density, no? Confused! 4) The ZFWCG is given by the pilot. But does the plane calculate it again (and changes the value)? Like for fuel: when you enter "block", the FMGS can automatically change it If it wrong. 5) The FWD CG is also controlled by FE? Do you mran that it calcualtes FOB by subtracting weight of fuel before engine start - weight of fuel now? It will then give the fuel's weight which has been burned.. |
@A33Zab
Hi A33Zab, thanks for reply - that sorts it out :ok:
I undestand that the kind of failure required to lose the CG info on the A330 is fairly remote. OTOH, on the 320, FAC1+2 Fault and Gear Down should be enough if I understand the system correctly... What about FCPC gains variability with respect to S/F position? Why is that implemented? Is it due to additional pitch authority required co counter the pitching moments after S/F extension? (though this should be catered for by the THS if available) Or are there some other considerations? |
Thanks a lot BuzzBox! ;)
1) What do you mean by "balance"? If the plane is already pitching down, why do you want to create a down force with the horizontal stabilizer???? You want the plane to pitch even more down????!!!! 2) Yes, but is the ZFWCG RE-calculated by FCMC? FE? ...No? Just entered by pilots? 3) So FCMC calculates FOB and tries to figure out EFOB at a given WPT that it then sends to FMGEC? Thanks a lot! ;) PS: I am not able to get "balance"!!! COG is the point where the aircraft is balanced. So what is balance? Aircraft at level? Aircraft in which position? Thanks again! |
AF330,
You're not paying attention. If the H-Stab moves down, which way does the nose go? The elevators deflect upward, which creates downforce on the H-Stab, which then moves down, and the nose does what? |
Basic Physics
AF330 - If you want to better understand how airplanes fly and maneuver I suggest that you find a very basic book on the subject. There are many available on line.
For the very basics of trim and balance here are a few thoughts: In order for any object to be in steady state equalibrium the forces and rotational moments applied to that object must balance. The sum of the forces must be zero as must the sum of the rotation moments be zero. As for up/down forces on the tail, these generate rotational moments. A tail down force causes the airplane to rotate in the tail-down / nose-up direction. This is what BuzzBox is referring to. If the airplane has a tendency (due to other rotational moments) to rotate in the nose down direction, down load on the tail is needed to bring it into rotational equilibrium. |
Oh yes....forgive-me....
Got it now. But does balance = Lift + Drag + Weight + thrust = 0? So basically the plane flying at level and maintaining a speed?? Thanks |
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