Airbus FBW - some technical q's
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Join Date: Jan 2004
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Obi,
The way I understand it now, is that the FBW system will maintain a constant vertical acceleration when the sidestick is neutral (in the Y axis) (as long as the bank angle is less than 33°)
Therefor, if you are climbing at 15° pitch up attitude, and your vertical speed is constant, the vertical acceleration will remain constant aswell (=0)
I think the whole concept of "1G" is a simplification Airbus made to ease transition to the FBW Airbus. 1G simply doesn't make any sense as soon as you have a bank angle...
So, at least as I understand it, the FBW will maintain a constant vertical speed when you let go of the stick. Does this make sense?
Regards,
Mark
The way I understand it now, is that the FBW system will maintain a constant vertical acceleration when the sidestick is neutral (in the Y axis) (as long as the bank angle is less than 33°)
Therefor, if you are climbing at 15° pitch up attitude, and your vertical speed is constant, the vertical acceleration will remain constant aswell (=0)
I think the whole concept of "1G" is a simplification Airbus made to ease transition to the FBW Airbus. 1G simply doesn't make any sense as soon as you have a bank angle...
So, at least as I understand it, the FBW will maintain a constant vertical speed when you let go of the stick. Does this make sense?
Regards,
Mark
Join Date: Jul 2006
Location: London
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Ka8 Flyer
I think I understand now.
Usually whenever a discription of the Airbus FCS is presented 'maintains 1g flight and pitch angle' is often times quoted in some form
, this is what I found confusing.
Considering that the FCS will maintain pitch attitude with neutral stick, while bank angle is < 33 degrees, it must vary the G load. It didn't help that I was observing pitch and roll separately.
My passion for aircraft mainly lies with military aviation and this has contributed to my confusion. My knowledge or perhaps more accurately, my lack of knowledge regarding FCS
is biased towards military fighter applications, this skewed my thinking.
With fighter aircraft the FCS/CAS is designed to provided constant stick force per g, so a pilot flying at 300kts would apply the same stick force to achieve a 4g pitch as would be done at 600kts. This feature I guess enables the pilot to fly by 'feel' and perform accurately as such, by keeping/feeling constant g on his/her body. Because stick force per g is constant hands-off stick force is usually trimmed for 1g i.e zero pitch in level flight, the most common occurance.
Unfortunately a side affect of this system is that in order to fly at any other pitch angle (velocity vector referenced), the pilot would have to hold a constant forward stick pressure, ranging from a g command equal to or greater than -1g, to a command less than 1g depending on pitch attitude. The only other option would be to re-trimmed for hands-off in said situation, but this would increase the pilots work load.
Trimmed for 1g level flight, if the pilot were to pitch up e.g to +1 degree, the aircraft would very slowly (initially) begin to pitch upwards. Pitch rate will increase as pitch angle increases and would be greatest at 180 degrees (inverted), thereafter pitch will begin to decrease until a pitch rate of 0 is reached at 0 degrees of pitch.
Airbus FCS logic demands that stick force/deflection is not constant per g and is dependent on pitch attitude. At 0 degrees of pitch a forward stick deflection could cause a 0.5g dive, whereas at 45 degrees of pitch (in theory) the same stick deflection would cause a 0.0g dive. Although the Airbus C* laws use inputs primarily from the accelerometer, it could be that the system infact acts as a pseudo or hybrid pitch-rate command system?.
With the Airbus FCS, the pilot will have to vary stick displacement to maintain a particular g load as pitch varies.
Airliners such as Airbus are limited regarding g loading and attitude ranges. Transtitioning between g loads in an airliner occurs relatively slowly, g-loads are low and changing g loads in pitch happens slowly, with rates that are not sustained for long periods, unlike fighter aircraft. Due to the limits mentioned, the smooth, comfortable and manoeuvre friendly aspect of airline flying, maintaining accurate flightpaths with little deviation and precise control of pitch should be easy to achieve.
In closing I guess my confusion was caused by a fighter plane bias, I failed to realise that two completely different aircraft, designed to accomplish two completely different tasks would have differences that also manifested in their flight control law design philosopies.
It seems to me, that while fighter aircraft perhaps have FCS's tailored to make precision flying less demanding while manoeuvring in a dynamic environment, Airbus has a FCS which is focused on reducing pilot work load for tasks such as cruise/climbs and descents.
Edited: After thinking some more about this, it would be wrong to label the Airbus FCS as a pitch rate command system (in normal law at least).
Obi
I think I understand now.
Usually whenever a discription of the Airbus FCS is presented 'maintains 1g flight and pitch angle' is often times quoted in some form
, this is what I found confusing.Considering that the FCS will maintain pitch attitude with neutral stick, while bank angle is < 33 degrees, it must vary the G load. It didn't help that I was observing pitch and roll separately.
My passion for aircraft mainly lies with military aviation and this has contributed to my confusion. My knowledge or perhaps more accurately, my lack of knowledge regarding FCS
is biased towards military fighter applications, this skewed my thinking.With fighter aircraft the FCS/CAS is designed to provided constant stick force per g, so a pilot flying at 300kts would apply the same stick force to achieve a 4g pitch as would be done at 600kts. This feature I guess enables the pilot to fly by 'feel' and perform accurately as such, by keeping/feeling constant g on his/her body. Because stick force per g is constant hands-off stick force is usually trimmed for 1g i.e zero pitch in level flight, the most common occurance.
Unfortunately a side affect of this system is that in order to fly at any other pitch angle (velocity vector referenced), the pilot would have to hold a constant forward stick pressure, ranging from a g command equal to or greater than -1g, to a command less than 1g depending on pitch attitude. The only other option would be to re-trimmed for hands-off in said situation, but this would increase the pilots work load.
Trimmed for 1g level flight, if the pilot were to pitch up e.g to +1 degree, the aircraft would very slowly (initially) begin to pitch upwards. Pitch rate will increase as pitch angle increases and would be greatest at 180 degrees (inverted), thereafter pitch will begin to decrease until a pitch rate of 0 is reached at 0 degrees of pitch.
Airbus FCS logic demands that stick force/deflection is not constant per g and is dependent on pitch attitude. At 0 degrees of pitch a forward stick deflection could cause a 0.5g dive, whereas at 45 degrees of pitch (in theory) the same stick deflection would cause a 0.0g dive. Although the Airbus C* laws use inputs primarily from the accelerometer, it could be that the system infact acts as a pseudo or hybrid pitch-rate command system?.
With the Airbus FCS, the pilot will have to vary stick displacement to maintain a particular g load as pitch varies.
Airliners such as Airbus are limited regarding g loading and attitude ranges. Transtitioning between g loads in an airliner occurs relatively slowly, g-loads are low and changing g loads in pitch happens slowly, with rates that are not sustained for long periods, unlike fighter aircraft. Due to the limits mentioned, the smooth, comfortable and manoeuvre friendly aspect of airline flying, maintaining accurate flightpaths with little deviation and precise control of pitch should be easy to achieve.
In closing I guess my confusion was caused by a fighter plane bias, I failed to realise that two completely different aircraft, designed to accomplish two completely different tasks would have differences that also manifested in their flight control law design philosopies.
It seems to me, that while fighter aircraft perhaps have FCS's tailored to make precision flying less demanding while manoeuvring in a dynamic environment, Airbus has a FCS which is focused on reducing pilot work load for tasks such as cruise/climbs and descents.
Edited: After thinking some more about this, it would be wrong to label the Airbus FCS as a pitch rate command system (in normal law at least).
Obi
Last edited by Obi Offiah; 20th Jul 2006 at 05:13.
