Airbus laws: direct law
As FCeng84 commented, in flight mode 'roll demand' commands a rate of roll, not an angle of bank. A large sidestick input commands a large rate of roll, while a small sidestick input commands a small rate of roll. Zero sidestick commands zero rate of roll (ie a constant bank angle).
Also, in ground mode the aileron/spoiler movement for a given sidestick input reduces with speed, however, the elevator movement is not modified for speed - there is a direct relationship between the sidestick position and the elevator position.
Also, in ground mode the aileron/spoiler movement for a given sidestick input reduces with speed, however, the elevator movement is not modified for speed - there is a direct relationship between the sidestick position and the elevator position.
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Thanks to everyone!
Ok, so to complete this:
1) How do you express "roll of rate"? Like speed can be km/h, roll of rate????
@ Buzzbox: In ground mode, each SS deflexion has a aileron/spoilers movement. But are you sure that each SS deflexion has an aileron/spoiler movement at each speed? I had thought that speed was not counted.... So it gives:
2) In ground mode, when you take speed, the aileron/spoiler movement reduces with stick deflexion. True? So each side-stick deflexion at a given speed has a programmed aileron/spoilers movement. Right? So if you turn at 15kts or 25kts with same Side-stick deflexion, you will make a same turn because aileron/spoilers movement will reduce at 25kts. Am I right?
3) Direct law is same as Normal law's ground mode. Each SS deflexion has a programmed aileron/spoilers movement at each speed. But in direct law, this is true on ground AND flight. Right?
Thanks
1) How do you express "roll of rate"? Like speed can be km/h, roll of rate????
@ Buzzbox: In ground mode, each SS deflexion has a aileron/spoilers movement. But are you sure that each SS deflexion has an aileron/spoiler movement at each speed? I had thought that speed was not counted.... So it gives:
2) In ground mode, when you take speed, the aileron/spoiler movement reduces with stick deflexion. True? So each side-stick deflexion at a given speed has a programmed aileron/spoilers movement. Right? So if you turn at 15kts or 25kts with same Side-stick deflexion, you will make a same turn because aileron/spoilers movement will reduce at 25kts. Am I right?
3) Direct law is same as Normal law's ground mode. Each SS deflexion has a programmed aileron/spoilers movement at each speed. But in direct law, this is true on ground AND flight. Right?
Thanks
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Rate of roll would normally be expressed in degrees per second - if you are rolling at 15 degrees per second, it will take you 3 seconds to change the angle of bank by 45 degrees, for example.
Think of the Airbus acting like a non FBW aircraft on the ground, where the movement of the side-stick is directly reflected by movement of the associated control surfaces (this is not entirely true, as you've stated above it's mediated by airspeed, but close enough). Once the transition is made into flight mode, the movement in control surfaces is whatever is required to achieve the commanded maneuver - e.g. referring to roll, if a 5 degree per second rate of roll to the left is requested by the pilot (by displacing the SS slightly to the left), the related control surfaces (ailerons and flight spoilers if applicable) will move to the extent that a 5 degree per second rate of roll will result.
Think of the Airbus acting like a non FBW aircraft on the ground, where the movement of the side-stick is directly reflected by movement of the associated control surfaces (this is not entirely true, as you've stated above it's mediated by airspeed, but close enough). Once the transition is made into flight mode, the movement in control surfaces is whatever is required to achieve the commanded maneuver - e.g. referring to roll, if a 5 degree per second rate of roll to the left is requested by the pilot (by displacing the SS slightly to the left), the related control surfaces (ailerons and flight spoilers if applicable) will move to the extent that a 5 degree per second rate of roll will result.
1) How do you express "roll of rate"? Like speed can be km/h, roll of rate????
But are you sure that each SS deflexion has an aileron/spoiler movement at each speed?
'When the aircraft is on the ground (in “on ground” mode), the sidestick commands the aileron and roll spoiler surface deflection. The amount of control surface deflection that results from a given amount of sidestick deflection depends upon aircraft speed.'
2) In ground mode, when you take speed, the aileron/spoiler movement reduces with stick deflexion. True? So each side-stick deflexion at a given speed has a programmed aileron/spoilers movement. Right? So if you turn at 15kts or 25kts with same Side-stick deflexion, you will make a same turn because aileron/spoilers movement will reduce at 25kts.
3) Direct law is same as Normal law's ground mode. Each SS deflexion has a programmed aileron/spoilers movement at each speed. But in direct law, this is true on ground AND flight. Right?
To complicate things, in Direct Law the maximum elevator deflection varies as a function of CG, to ensure adequate controllability. With an aft CG, the maximum elevator deflection with full sidestick will be less than that with a forward CG. Similarly, the maximum aileron/spoiler deflection varies according to the slat/flap configuration. When the aircraft is clean, the maximum aileron/spoiler deflection with full sidestick will be less than that when the slats/flaps are extended.
Last edited by BuzzBox; 5th Feb 2015 at 01:15.
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Function of CG in Direct Mode?
I'm curious as to what signal is used in Direct Law to schedule the gain between stick and elevator. What is the source of CG data to effect this scheduling?
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Let's try to relate to conventional aircraft and talk about in air characteristics as it is more relevant to flying. In an oversimplifying attempt of explanation.
In air for conventional aircraft the amount of pitch in cockpit controls determine the amount of response. The more you pitch or roll, the more the aircraft pitch or roll. The faster you do it, the faster the aircraft does it.
On the Airbus in the air, the more you move the cockpit control, the FASTER the aircraft response. It does not matter whether you do it fast or slow, it is the amount you move your sidestick that matters.
Fundamentally this FBW algorithm will work in tandem with autotrim. Picture a turn, conventionally you turn an amount to get your bank angle and hold it there until you want to level off then you level off your control.
In Airbus you will neutralise your control when the bank angle is achieved. You DO NOT hold the displacement. Aircraft is trimmed in roll. When you want to level off you have to ask the aircraft to turn the opposite side, you need to input a roll in the opposite side to neutralise. However when the aircraft reverts back to Direct Law, the normal conventional behaviour of your ab-initio aircraft (as discussed earlier) prevails.
But it may not be conducive to think or analyse too much in flying. You wanna go left you turn left, wanna go right you turn right. Just remember there is autotrim and when you get what you desire, neutralise.
Having understand this fundamental philosophy then you can get into the protection (bank beyond 33deg, full deflection limitation of direct law during high speed take off roll regime...) part. Step by step.
In air for conventional aircraft the amount of pitch in cockpit controls determine the amount of response. The more you pitch or roll, the more the aircraft pitch or roll. The faster you do it, the faster the aircraft does it.
On the Airbus in the air, the more you move the cockpit control, the FASTER the aircraft response. It does not matter whether you do it fast or slow, it is the amount you move your sidestick that matters.
Fundamentally this FBW algorithm will work in tandem with autotrim. Picture a turn, conventionally you turn an amount to get your bank angle and hold it there until you want to level off then you level off your control.
In Airbus you will neutralise your control when the bank angle is achieved. You DO NOT hold the displacement. Aircraft is trimmed in roll. When you want to level off you have to ask the aircraft to turn the opposite side, you need to input a roll in the opposite side to neutralise. However when the aircraft reverts back to Direct Law, the normal conventional behaviour of your ab-initio aircraft (as discussed earlier) prevails.
But it may not be conducive to think or analyse too much in flying. You wanna go left you turn left, wanna go right you turn right. Just remember there is autotrim and when you get what you desire, neutralise.
Having understand this fundamental philosophy then you can get into the protection (bank beyond 33deg, full deflection limitation of direct law during high speed take off roll regime...) part. Step by step.
FCeng84:
I'm curious as to what signal is used in Direct Law to schedule the gain between stick and elevator. What is the source of CG data to effect this scheduling?
I'm curious as to what signal is used in Direct Law to schedule the gain between stick and elevator. What is the source of CG data to effect this scheduling?
The FMGEC normally uses the GW and CG information calculated by the FCMC. That information is passed on to the Flight Control Primary Computers (FCPC, or 'PRIM'). If both FCMCs fail, the Flight Envelope (FE) part of the FMGEC is also able to compute the GW and CG as a backup.
Simple really (not!!).
Last edited by BuzzBox; 6th Feb 2015 at 10:25.
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CG estimate dependent on pilot data input?
Thanks for the explanation of CG estimate calculation. It sounds like this is dependent on the crew entering the initial weight and CG and then the system monitoring fuel management during the flight. Is there anything in this process to guard against a data entry error? If the CG is actually 30% but the crew enters 10% does the system spend the whole flight thinking CG is near the forward limit when in fact it is much closer to the aft limit? How about if the initial ZFW entry is off by 30%?
I would imagine there are checks or limits to guard against an entry that is way out of bounds. I'm interested in whether or not the system is able to determine that the data entry, while within operational limits, is in error and if it can make such a determination what corrective action does it take?
I would imagine there are checks or limits to guard against an entry that is way out of bounds. I'm interested in whether or not the system is able to determine that the data entry, while within operational limits, is in error and if it can make such a determination what corrective action does it take?
I'm interested in whether or not the system is able to determine that the data entry, while within operational limits, is in error and if it can make such a determination what corrective action does it take?
The following Airbus document explains how the various Airbus types calculate GW and CG, and the effect of incorrect ZFW/CG entries:
Effect of ZFW/ZFWCG on Aircraft Operations
Last edited by BuzzBox; 6th Feb 2015 at 08:45.
I would imagine there are checks or limits to guard against an entry that is way out of bounds.
This includes ZFM, ZFM CG, Trim setting etc. etc.
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CG estimate for control laws independent of pilot input
BuzzBox - thanks for the reference document.
I see that there is an independent computation of CG and GW based on aerodynamics that is used for the control laws. My interest is mostly in that functionality as control law gains defining closed loop stability and handling qualities are impacted.
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.
I see that there is an independent computation of CG and GW based on aerodynamics that is used for the control laws. My interest is mostly in that functionality as control law gains defining closed loop stability and handling qualities are impacted.
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.
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Good evening,
No Amadis...I had said that I will answer on the 5th of feb, but I couldn't.
So I just wanted to excuse myself. I have started this thread so If I leave it...it doesn't look good (at least for me!). Hope you understand.
I first would like to thank everyone to have helped me to understand how the plane reacts. I also looked to F-COM, some questions were answered, some came!
Normal law
Ground mode
Everything is programmed. You are in direct link with surfaces. The turn will depend on the speed of the aircraft. Even if you turn the SS with Full deflection, everything depends on speed.Usually, small deflection = ailerons. Big/full SS deflection = spoiler + Aileron.
You don't have roll demand. It is simply programmed. But again, speed of the aircraft will make the turn.
So, for example, we know that:
At 0 kts - full side-stick deflexion = Aileron + spoiler
You can check it during check-list.
As you take speed, aileron/spoiler movement will reduce for each side-stick deflexion - for example:
If at 12kts, at full side-stick deflexion, you have a certain amount of aileron and, let's say spoilers X, Y and A (I am sorry, I don't know their real number - have to learn them!), at 18kts, you will get less amount of aileron and only X and A.
At take-off (after you have selected TO CONF?), you don't have lot of aileron/spoiler because as you have speed, even small amount of aileron can keep the wing to level.
Flight mode:
In flight mode/flare mode, the roll rate is calculated. If you turn the stick at a certain deflexion, the plane will get the roll rate (roll demand) by using aileron, spoilers and rudder at any speed.
Direct law
Direct Law is similar to Normal Law's Ground Mode, but it is not exactly the same. In Direct Law, the aileron/spoiler deflection is proportional to the sidestick deflection.
So basically it doesn't take the speed.
So if you turn the SS at full deflexion, on ground, in flight, you will get the same programmed aileron/spoiler movement.
So if you turn the SS to 3°/s, the computer has a programmed value (doesn't care of speed) of aileron/spoiler movement.
So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY...
Before going any further, do you all agree?
No Amadis...I had said that I will answer on the 5th of feb, but I couldn't.
So I just wanted to excuse myself. I have started this thread so If I leave it...it doesn't look good (at least for me!). Hope you understand.
I first would like to thank everyone to have helped me to understand how the plane reacts. I also looked to F-COM, some questions were answered, some came!
Normal law
Ground mode
Everything is programmed. You are in direct link with surfaces. The turn will depend on the speed of the aircraft. Even if you turn the SS with Full deflection, everything depends on speed.Usually, small deflection = ailerons. Big/full SS deflection = spoiler + Aileron.
You don't have roll demand. It is simply programmed. But again, speed of the aircraft will make the turn.
So, for example, we know that:
At 0 kts - full side-stick deflexion = Aileron + spoiler
You can check it during check-list.
As you take speed, aileron/spoiler movement will reduce for each side-stick deflexion - for example:
If at 12kts, at full side-stick deflexion, you have a certain amount of aileron and, let's say spoilers X, Y and A (I am sorry, I don't know their real number - have to learn them!), at 18kts, you will get less amount of aileron and only X and A.
At take-off (after you have selected TO CONF?), you don't have lot of aileron/spoiler because as you have speed, even small amount of aileron can keep the wing to level.
Flight mode:
In flight mode/flare mode, the roll rate is calculated. If you turn the stick at a certain deflexion, the plane will get the roll rate (roll demand) by using aileron, spoilers and rudder at any speed.
Direct law
Direct Law is similar to Normal Law's Ground Mode, but it is not exactly the same. In Direct Law, the aileron/spoiler deflection is proportional to the sidestick deflection.
So basically it doesn't take the speed.
So if you turn the SS at full deflexion, on ground, in flight, you will get the same programmed aileron/spoiler movement.
So if you turn the SS to 3°/s, the computer has a programmed value (doesn't care of speed) of aileron/spoiler movement.
So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY...
Before going any further, do you all agree?
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Lateral / Directional Control Inputs on Ground
For all airplanes lateral input (stick or wheel) is used on the ground only to keep the wings level when in cross winds. Without cross wind, no lateral input required or desired.
There are four ways to turn an airplane on ground:
1. Use the tiller to steer the nose gear
2. Use the rudder pedals to move the rudder thus generating yawing moment if speed is sufficient.
3. Use differential braking to generate yawing moment.
4. Use differential thrust to generate yawing moment. (This assumes you have wing mounted engines that can generate significant yawing moment.)
There are four ways to turn an airplane on ground:
1. Use the tiller to steer the nose gear
2. Use the rudder pedals to move the rudder thus generating yawing moment if speed is sufficient.
3. Use differential braking to generate yawing moment.
4. Use differential thrust to generate yawing moment. (This assumes you have wing mounted engines that can generate significant yawing moment.)