Flight Control System/CAS Inconsistencies
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Flight Control System/CAS Inconsistencies
Hello Everyone 
I found this forum a couple of days ago and have been enjoying reading through many posts particular in the technical sections, it seems like a great forum.
There are a few questions regarding aircraft flight control systems that have been bugging me for a couple of years now, I was wondering if anyone could shed some light on this oh so mysterious aspect, that has eluded me for so long
Now I wasn't sure whether to post here, in the Tech Log or the Flight Testing forum. Perhaps if this is the wrong area it could be moved? .
My aviation interests lie mainly with military aircraft, I understand that most of the posts in this forum are directed to civil aviation, however my questions below could apply to both I guess.
1) An aircraft such as an F-16, F/A-18 or F-15 is in 1g level flight at 1000 feet and at a constant speed of 300kts. The pilot suddenly selects maximum afterburner. By virtue of FCS and CAS descriptions, I would have expected the aircraft’s nose to drop, as the AOA necessary for 1g level flight decreases as speed increases and for the aircraft to maintain 1000ft. In reality from what pilots of different aircraft have said, what happens is the aircraft actually remains out of trim while its accelerating and this condition exists until the aircraft has stopped accelerating. The phenomenon is greatest during rapid accelerations and decelerations, but tapers off as the magnitude of accels and decels diminishes. Now all the aircraft mentioned have control augmentation systems that are supposed to keep stick force per g constant. In the example I described above the pilot did not touch the control stick, but was commanding 'hands-off' 1g level flight by having the aircraft trimmed as such, only throttle movements were made. Considering the pilot did not touch the control stick, but the aircraft began to climb as it accelerated i.e. g load was greater than 1g it diverted from constant stick force per g. Why does this happen? .
I can appreciate that there are regions in an aircraft’s flight envelope such as very slow speed flight, were there simply isn't enough dynamic pressure for response to meet command inputs, but I wouldn't expect this to be one of them.
2) Another situation, in which aircraft response is not consistent with stick force, is when lateral asymmetries arise. Some trapped fuel in a wing can cause this, aerodynamic imbalance or an asymmetric weapons load occurring as a weapon is dropped or fired. If an aircraft with two equal bombs (one per wing) was in level flight and trimmed for zero roll rates, I would have expected the flight control system to compensate when one bomb was dropped. Despite my preconception, the truth is that the plane will roll into the heavy wing, even though the pilot before weapons release had commanded zero roll rate and didn’t enter any subsequent roll rate commands after weapons release. Why is there no compensation? .
3) Rudder inputs command yaw rate but also have a natural tendency to induce roll. Again I would have thought the FCS/CAS would read this secondary roll as being uncommanded and apply lateral control surface movement to eliminate the event, this doesn’t happen, why is that? .
Thanks
Obi
I found this forum a couple of days ago and have been enjoying reading through many posts particular in the technical sections, it seems like a great forum.
There are a few questions regarding aircraft flight control systems that have been bugging me for a couple of years now, I was wondering if anyone could shed some light on this oh so mysterious aspect, that has eluded me for so long
Now I wasn't sure whether to post here, in the Tech Log or the Flight Testing forum. Perhaps if this is the wrong area it could be moved? .
My aviation interests lie mainly with military aircraft, I understand that most of the posts in this forum are directed to civil aviation, however my questions below could apply to both I guess.
1) An aircraft such as an F-16, F/A-18 or F-15 is in 1g level flight at 1000 feet and at a constant speed of 300kts. The pilot suddenly selects maximum afterburner. By virtue of FCS and CAS descriptions, I would have expected the aircraft’s nose to drop, as the AOA necessary for 1g level flight decreases as speed increases and for the aircraft to maintain 1000ft. In reality from what pilots of different aircraft have said, what happens is the aircraft actually remains out of trim while its accelerating and this condition exists until the aircraft has stopped accelerating. The phenomenon is greatest during rapid accelerations and decelerations, but tapers off as the magnitude of accels and decels diminishes. Now all the aircraft mentioned have control augmentation systems that are supposed to keep stick force per g constant. In the example I described above the pilot did not touch the control stick, but was commanding 'hands-off' 1g level flight by having the aircraft trimmed as such, only throttle movements were made. Considering the pilot did not touch the control stick, but the aircraft began to climb as it accelerated i.e. g load was greater than 1g it diverted from constant stick force per g. Why does this happen? .
I can appreciate that there are regions in an aircraft’s flight envelope such as very slow speed flight, were there simply isn't enough dynamic pressure for response to meet command inputs, but I wouldn't expect this to be one of them.
2) Another situation, in which aircraft response is not consistent with stick force, is when lateral asymmetries arise. Some trapped fuel in a wing can cause this, aerodynamic imbalance or an asymmetric weapons load occurring as a weapon is dropped or fired. If an aircraft with two equal bombs (one per wing) was in level flight and trimmed for zero roll rates, I would have expected the flight control system to compensate when one bomb was dropped. Despite my preconception, the truth is that the plane will roll into the heavy wing, even though the pilot before weapons release had commanded zero roll rate and didn’t enter any subsequent roll rate commands after weapons release. Why is there no compensation? .
3) Rudder inputs command yaw rate but also have a natural tendency to induce roll. Again I would have thought the FCS/CAS would read this secondary roll as being uncommanded and apply lateral control surface movement to eliminate the event, this doesn’t happen, why is that? .
Thanks
Obi
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Case 1: how well balanced is the stick in these aircraft to longitudinal accel?
If not perfectly balanced, the effect of the accel on the stick is to generate an apparent inertial force AFT (think of the stick as an upside-down pendulum, with most of the weight at the top of the stick: as the aircraft (pendulum pivot) accels either you have to push on the top of the stick to keep it vertical OR it moves back under the 'g'). That might cause out-of-trim behaviour during accel.
I know for certain that the BAe Hawk isn't balanced longitudinally and had to be specifically rebalanced as the T-45 due to the much higher accels on the cat (Hawk regular accel isn't big enough to matter). At 1'g;'-ish accels with lightweight fighters in AB, maybe it matters? Just a thought.
And for all three cases, unless the FCS is predictive, it can't counteract a roll, say, until it sees a roll. So once the bomb drops, the FCS won't (I suspect) apply roll control until the disturbance has started. It's technically impossible to completely zero out the controlled parameter in a simple feedback loop.
If not perfectly balanced, the effect of the accel on the stick is to generate an apparent inertial force AFT (think of the stick as an upside-down pendulum, with most of the weight at the top of the stick: as the aircraft (pendulum pivot) accels either you have to push on the top of the stick to keep it vertical OR it moves back under the 'g'). That might cause out-of-trim behaviour during accel.
I know for certain that the BAe Hawk isn't balanced longitudinally and had to be specifically rebalanced as the T-45 due to the much higher accels on the cat (Hawk regular accel isn't big enough to matter). At 1'g;'-ish accels with lightweight fighters in AB, maybe it matters? Just a thought.
And for all three cases, unless the FCS is predictive, it can't counteract a roll, say, until it sees a roll. So once the bomb drops, the FCS won't (I suspect) apply roll control until the disturbance has started. It's technically impossible to completely zero out the controlled parameter in a simple feedback loop.
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Thanks for your reply Mad (Flt) Scientist.
While accelerating or decelerating, the balance of the stick in the aircraft wasn't the problem. I thought another issue could be how the vertical accelerometer is referenced. These aircraft generally fly slightly nose-up with positive AOA particularly in the speed regime I mention previously.
If the accelerometer is referenced to the aircraft waterline/nose and not the velocity vector, then accelerating in level flight would cause the vertical accelerometer to sense a resultant <1g force.
However these devices are reference to the velocity vector so this isn't an issue.
Regarding case 3) I don't think the FCS needs to be predictive. If we consider unstable/RSS aircraft such as the F-117A, F-16 and Super Hornet, pilots talk about how smooth, responsive and easy the planes are to fly.
These aircraft I understand have a natural tendency to diverge from a current attitude when a small force is applied. So if the pilot entered a small pitch input, the aircraft will divert from its current attitude at an increasing rate without the aid of computers for stability.
Pilots flying these aircraft don't notice any divergence. A quick pitch up to ten degrees is performed accurately without the flightpath/nose overshooting or deviating. So the FCS and flight control surfaces are quick enough to stabilise any unwanted excurisons, not before they happen but before they are noticable to the pilot. I guess its like using a calculator, the delay before you get an answer is so very brief, it appears as if the calculations were performed instantaneously.
This is very confusing
Obi
While accelerating or decelerating, the balance of the stick in the aircraft wasn't the problem. I thought another issue could be how the vertical accelerometer is referenced. These aircraft generally fly slightly nose-up with positive AOA particularly in the speed regime I mention previously.
If the accelerometer is referenced to the aircraft waterline/nose and not the velocity vector, then accelerating in level flight would cause the vertical accelerometer to sense a resultant <1g force.
However these devices are reference to the velocity vector so this isn't an issue.
Regarding case 3) I don't think the FCS needs to be predictive. If we consider unstable/RSS aircraft such as the F-117A, F-16 and Super Hornet, pilots talk about how smooth, responsive and easy the planes are to fly.
These aircraft I understand have a natural tendency to diverge from a current attitude when a small force is applied. So if the pilot entered a small pitch input, the aircraft will divert from its current attitude at an increasing rate without the aid of computers for stability.
Pilots flying these aircraft don't notice any divergence. A quick pitch up to ten degrees is performed accurately without the flightpath/nose overshooting or deviating. So the FCS and flight control surfaces are quick enough to stabilise any unwanted excurisons, not before they happen but before they are noticable to the pilot. I guess its like using a calculator, the delay before you get an answer is so very brief, it appears as if the calculations were performed instantaneously.
This is very confusing
Obi
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A New.... Errr, Well, Actually.... A Semi-New Theory
Regarding the aircraft diverting from 1g while in level flight I have a theory.
I considered it a while back but not thoroughly enough and subsequently ended up with an unfavourable answer.
Basically it involves the vertical/normal accelerometer.
Most aircraft, B-52s withstanding, have their wings aligned with the longitudinal axis, so the nose and wings of most aircraft point in the same direction.
In order to generate lift the wings must have some AOA, but because they are aligned with the longitudinal axis, the aircraft's fuselage must also have an equal AOA.
The normal accelerometer is secured in the fuselage usually under the pilot. Let us assume again that an aircraft such as an F-16, F/A-18 or F-15 is in 1g level flight at 1000 feet and at a constant speed of 300kts. The pilot suddenly selects maximum afterburner.
In this situation, remembering that the aircraft even in level flight has some positive AOA e.g. 5 degrees, the normal accelerometer will sense a component of this longitudinal acceleration and read it as an uncommanded less than 1g excursion. To compensate, the FCS will command nose-up pitch until it establishes what it senses as 1g, but which in reality is a value greater than 1g.
This would explain why the phenomenon is most pronounced at lower speeds, (longitudinal acceleration is greatest) tapers off at higher speed (longitudinal acceleration is lowest) and stops when longitudinal acceleration ceases. However this is just a theory unfortunately .
I think the stability axis of an aircraft is its flight path/velocity vector? In order to calculate g at the velocity vector I believe the FCS uses the pitch rate gyro, as this should decouple g from the longitudinal axis. I previously disregarded the theory, because I pictured the decoupling as a physical procedure rather than a software/mathematical event, i.e. the actual vertical accelerometer being rotated.
To cure the pitch-up due to longitudinal acceleration if my theory is correct, a longitudinal accelerometer could be used. Apparently the Super Hornet has sensors with the capability of providing longitudinal acceleration, but they aren't utilised. The Super Hornet suffers from the same acceleration pitch-up phenomenon.
So, How'd ya like them apples?
Cheers
Obi
I considered it a while back but not thoroughly enough and subsequently ended up with an unfavourable answer.
Basically it involves the vertical/normal accelerometer.
Most aircraft, B-52s withstanding, have their wings aligned with the longitudinal axis, so the nose and wings of most aircraft point in the same direction.
In order to generate lift the wings must have some AOA, but because they are aligned with the longitudinal axis, the aircraft's fuselage must also have an equal AOA.
The normal accelerometer is secured in the fuselage usually under the pilot. Let us assume again that an aircraft such as an F-16, F/A-18 or F-15 is in 1g level flight at 1000 feet and at a constant speed of 300kts. The pilot suddenly selects maximum afterburner.
In this situation, remembering that the aircraft even in level flight has some positive AOA e.g. 5 degrees, the normal accelerometer will sense a component of this longitudinal acceleration and read it as an uncommanded less than 1g excursion. To compensate, the FCS will command nose-up pitch until it establishes what it senses as 1g, but which in reality is a value greater than 1g.
This would explain why the phenomenon is most pronounced at lower speeds, (longitudinal acceleration is greatest) tapers off at higher speed (longitudinal acceleration is lowest) and stops when longitudinal acceleration ceases. However this is just a theory unfortunately
.I think the stability axis of an aircraft is its flight path/velocity vector? In order to calculate g at the velocity vector I believe the FCS uses the pitch rate gyro, as this should decouple g from the longitudinal axis. I previously disregarded the theory, because I pictured the decoupling as a physical procedure rather than a software/mathematical event, i.e. the actual vertical accelerometer being rotated.
To cure the pitch-up due to longitudinal acceleration if my theory is correct, a longitudinal accelerometer could be used. Apparently the Super Hornet has sensors with the capability of providing longitudinal acceleration, but they aren't utilised. The Super Hornet suffers from the same acceleration pitch-up phenomenon.
So, How'd ya like them apples?
Cheers
Obi
